Symposium SF02—Additive Manufacturing—From Material Design to Emerging Applications

Digital light processing (DLP) based 3D printing is an additive manufacturing method that utilizes light patterns to photopolymerize a liquid resin. Typical DLP method uses a digital micromirror device (DMD) to control light pattern at the resolution around 20-50 microns and cure a thin resin in 1-5s. DLP thus has the advantage of high speed and high resolution. However, conventional DLP method is not suitable for multimaterial printing because it uses a single vat of resin. In recent years, our group has explored using light grayscale (or light intensity) to control local properties of printed parts to achieve multimaterial-like printed parts. The advantage of this grayscale DLP (g-DLP) method is that it still uses a single resin vat and thus retains the advantages of conventional DLP printing. In this talk, we will first present a few examples of how to use grayscale light together with novel resins to achieve multimaterial functional parts, including 4D printing of inflatable structures, 3D printing of cold-draw shape memory polymers, etc. Second, As the graded light intensity can create concentration gradients of the different chemical species during photocuring, the spatial resolution as well as the physical property resolution in g-DLP printing can be affected. We will briefly present a reaction-diffusion model coupled with radiative transfer to analyse the effects of component diffusion and its impact upon the resulting mechanical property resolution on a sub-pixel and macro scale. Third, the capability of spatially control mechanical properties offers a vast design space for 4D printing to create shape-changing devices. We present our recent efforts of using evolutionary algorithm and machine learning to rapidly explore the design space and to obtain optimized design. This new design paradigm seamless connects design concepts to the 3D printed shape changing devices. Finally, we will briefly discuss the future work in the g-DLP method.

Additive manufacturing of elastomeric parts has become increasingly important due to unique opportunities in biomedical and vibration isolation applications. Continuous liquid interface production (CLIP) is a recently developed technique that addresses some major limitations in this route by providing fast production of high-resolution features in a repeatable and reliable fashion. In this study, we report on the mechanical properties of CLIP-produced parts made of EPU 41, an elastomeric polyurethane developed specifically for CLIP.

Mechanical characterization studies employed a wide range of techniques, including tensile and compression testing and stress relaxation measurements. Tensile strength of EPU 41 exceeds 10 MPa with an elongation at break reaching 175%. A 2-term Ogden model accurately predicts the tensile response. Compressive stress relaxation measurements provided a compressive set of 13% after a 48-hour constant-strain loading. A three-term Prony series based on the generalized Maxwell model fits the experimental results accurately.

The study also considered the compressive response of a series of lattice parts, including octet, cubic, and BCC-based geometries. The structures fully recover after loading to a strain of 75%. The critical densification strain varies in the range of 0.5 – 0.9, and the corresponding plateau stress exhibits a wide range of values in the range of 0.01 – 0.30 MPa. Tuning of the geometry and the strut thickness enables structures to exhibit a wide range of load absorbing characteristics. The results show the great potential of the CLIP process in manufacturing new generation lattice structures and load-bearing components with optimized compliance and absorbance characteristics.

Photonic glasses may yield a new class of thermal barrier coatings, due to their broadband omnidirectional reflection capabilities. To apply such photonic structures, precise deposition even on complex substrate geometries is mandatory. However, conventional techniques, like spin coating, are not capable to form homogenous coatings on curved surfaces. Therefore, we combine an additive manufacturing technique, direct writing, with colloidal self-assembly (AMCA) to deposit polystyrene (PS) colloidal suspensions onto metallic and aluminum oxide surfaces, which subsequently assemble into PS-templates. The inverse opal photonic coatings are then formed via ALD infiltration and subsequent burn-out.
When printing low viscosity suspensions by direct writing, the contact line must remain pinned during solvent evaporation to prevent split up of the filament. However, such pinning also enhances capillary flow, usually leading to a coffee ring effect (CRE). In this work, we performed a parameter study varying both writing velocity v_w and dispense velocity v_d, as well as particle loading. We show that increasing the particle loading enhances the self pinning effect, thus enhancing printability, while the CRE can be suppressed by increasing the velocity ratio v_w/v_d. To ensure coverage of large areas, single lines must be connected, however the conventional line-by-line approach is insufficient for low viscosity colloidal suspensions due to capillary penetration of the previously printed line. Hereby, we present a “comb”-printing approach to overcome this issue and print coherent colloidal layers. Finally, we demonstrate that ceramic-based inverse opal photonic coatings are produced when the optimized templates are infiltrated via ALD, showing broadband omnidirectional reflection properties.

Novel lattice structure designs fabricated by additive manufacturing are promising in aerospace development in terms of lightweight alternatives to the current solutions. Cellular core sandwich structures offer many multifunctional advantages, such as high specific energy absorption capabilities and flexural stiffness, and can be further tailored to enhance specific properties. Fused deposition modeling (FDM) of functionally graded lattices shows the potential to be used as a lightweight cellular core for sandwich structures due to their high specific energy absorption in quasi-static compression. Variations in the structural configuration and material composition of an architected core structure are performed to observe the differences in compressive behavior. By FDM printing a lattice composed of repeating unit cells with rows differing in the wall thickness and, consequently, the filled cell volume, the structural grading effects in the direction of compression is explored. Both discrete and continuous grading of the wall thickening were applied from the top to bottom lattice rows along the direction of applied compressive force and display differences in energy absorption properties. The material grading effects are characterized by using two distinct filaments to vary composition of the core structure and hence the material density and tensile modulus properties between the layers. The stress-strain behavior and values of the specific energy absorbed were calculated by examining the printed samples during a quasi-static compression load test. Greater specific energy absorption (energy per mass) was seen in the graded layer by layer design compared to the non-graded octet lattices. The multi-material grading arrangements significantly modified the onset location of structural collapse to influence the layer compression order. These behaviors demonstrate the possibility of 3D printing a lightweight sandwich core with tailorable compressive properties using a combination of material and structural grading.

There is growing interest to develop next generation of additive manufacturing platform that can rapidly produce micro/nanofibers and precisely control of the fiber alignment in fibrous 3D complex structure, which allows biomimetic materials for next generation philologically assays or customized fiber reinforced composite materials. However, reproducing micro/nano sized fibrous structure is difficult using existing technologies. For instance, approaches such as 3D printing are generally considered too slow for producing fibers in micro/nano size, while high-throughput nonwoven fiber deposition methods provide poor control over structural alignment and 3D geometry. Here we report a new form of additive textile manufacturing platform termed focused rotary jet spinning (FRJS) where micro/nanofiber formation and patterning process are independently controlled. We show that these deposited fibers can conform to various 3D geometries with controllable fiber alignment. This technology has potential in a range of applications, reaching from biomedical applications, fashion design to next-generation food packaging.

The Razorblade Induced Fracture Test (RIFT) is a mechanical test method in which a sharp blade is pressed into a sample to initiate and propagate a crack and used to determine the energy release rates (G_C) of soft polymers such as gels or food products [1, 2]. Its use in measuring G_C of stiffer polymers has been limited as the frictional component of the measurement becomes significant [3-5]. In this study we develop a cyclic RIFT test that can easily and repeatably probe narrow regions of the sample and simultaneously account for the frictional contribution to the measurement. We use this method to determine the strength of polycarbonate (PC) welds made using Fused Filament Fabrication (FFF). In conjunction with geometry and thermal characterization using micro-computed tomography (μ-CT) and infrared imaging (IR), we undertake a parametric study of how the thermal and flow history of the print affects the measured strength of the welds in the printed material. This information is compared to bulk PC made by injection molding [6-8].

In-situ birefringence measurements of the sample have shown a concentration of plastic deformation at the crack tip, and a non-negligible strain due to bending which is accounted for through parallel FE modelling. Ex-situ birefringence measurements have shown a small region around the crack tip where the plastic deformation localizes. The width of this region is limited to a single layer on either side of the strongest tested welds, decreasing in size as the welds get weaker . The radius of the region is approximately 0.3 mm while the calculated elastofracture length is much smaller at 11.8 µm. Preliminary results suggest that the frictional force component ranges from 35% to 45% of the measured cutting force. As expected, measured values of G_C for PC welds vary with printing conditions and with the measured thermal and flow history of the print. Increasing the welding increases the measured strength of the weld. The G_C for samples with longer welding times (printed at higher temperatures), approach that of injection molding bulk PC.

[1] T. Brown, S.J. James, G.L. Purnell, Journal of food engineering 70 (2005) 165-170.
[2] T.T. Duncan, J.M. Sarapas, A.P. Defante, K.L. Beers, E.P. Chan, Soft Matter 16 (2020) 8826-8831.
[3] A. Gent, C. Wang, Journal of Polymer Science Part B: Polymer Physics 34 (1996) 2231-2237.
[4] H. Uete, R.M. Caddell, International Journal of Mechanical Sciences 25 (1983) 87-92.
[5] B. Zhang, C.-S. Shiang, S.J. Yang, S.B. Hutchens, Experimental Mechanics 59 (2019) 517-529.
[6] L. Fang, Y. Yan, O. Agarwal, J.E. Seppala, K. Migler, T.D. Nguyen, S.H. Kang, Additive Manufacturing (2021) 101983.
[7] J.E. Seppala, S.H. Han, K.E. Hillgartner, C.S. Davis, K.B. Migler, Soft Matter 13 (2017) 6761-6769.
[8] J.E. Seppala, K.D. Migler, Additive manufacturing 12 (2016) 71-76.

Additively manufactured (AM) metallic components have highly anisotropic microstructures spanning multiple length scales. The relationship between microstructure and mechanical properties of printed structures is complicated by the introduction of additional design parameters, such as build orientation and geometric variations. This study was undertaken to decouple the contribution of microstructure and geometry on the mechanical response of thin-walled Inconel 625 T-elements fabricated by laser powder bed fusion. The T-elements were printed at either 40^o or 90^o inclinations from the build plate resulting in a 10% difference in the ligament widths. Milli-scale tests indicate that the 90^o T-elements are stiffer in both the stress-relieved and the fully recrystallized states. However, the modulus along the loading direction determined from EBSD maps suggest the 40^o T-elements to be stiffer in the stress-relieved case. The measured yield load difference between the recrystallized samples was in better agreement with the values predicted from beam theory.

Consensus on the wear and friction performance of additively-versus-conventionally manufactured components has not been achieved; mainly because the tribological test set-ups thus far were not suited for investigating the underlying microstructure’s influence on the tribological properties. As a result, utilization of additive manufacturing techniques, such as selective laser melting (SLM), for sliding components remains questionable. Here, we investigate the anisotropic microtribology of SLM 316L stainless steel via in situ SEM reciprocating scratch testing to highlight the microstructure’s role. As-built 316L SLM specimens were compared against annealed wire-drawn 316L. We found that: (i) microgeometric conformity was the main driver for achieving steady-state friction, (ii) the anisotropic friction of the additively manufactured components is limited to the break-in and is caused by the lack of conformity, (iii) the cohesive bonds, whose strength is proportional with frictional forces, are stronger in the additively manufactured specimens likely due to the presence of dislocations and/or cellular structures, (iv) low Taylor-factor grains with large dimension stimulate microcutting in the form of long, thin sheets with serrated edges. These findings uncover some microstructurally driven tribological complexities when comparing additive to conventional manufacturing.

Additive manufacturing has the potential to make nanoscale electronics at a fraction of the cost required by current conventional fabrication. Through the directed assembly-based printing of nanomaterials, hundreds of steps of deposition and etching can be removed from the fabrication process. Previous research conducted at the NSF Center for High-rate Nanomanufacturing and other works have shown the effectiveness of the directed assembly processes that allow for high throughput and high precision at comparatively low cost. However, the assembly and sintering of semiconducting nanomaterials such as InP and GaAs poses a significant challenge to this endeavor. In this talk I will outline our recent results in the processing, assembly, and annealing of III-V semiconducting nanoparticles. Through high-speed ball milling, bulk particles of III-V semiconductors have been reduced to dimensions of less than 30 nm, after which they were formulated into a stable ink suitable for printing. A silicon wafer with a functionalized surface allows the precise assembly of nanoparticles through preferential adhesion of the ink. The particles are assembled as structures suitable for making transistors and measuring Hall and other electronics characteristics. Annealing the particles to make a continuous structure is critical for fabricating practical electronics, and it is this aspect which poses one of the greatest challenges due to the loss of the lighter elements (P and As) during annealing. This barrier is overcome through the use of small amounts of sacrificial particles during annealing, resulting in a continuous structure. Once the assembled particles are sintered, gold contacts (source and drain) are deposited for electronic characterization. The results from our work demonstrate the feasibility of printing III-V semiconductors for electronics applications.

Metal Additive Manufacturing (AM) is drawing attention to the application of biomedical implants because of its ability to fabricate customized precise structures readily than conventional casting methods. Powder Bed Fusion (PBF), which is implemented by irradiating powder particles with a heat source, provides sufficient mechanical property without post-processing in contrast to the other AM technologies. There are two types of heat sources that are used for PBF; laser and electron beam. The former is known as Selective Laser Melting (SLM) and the latter is Electron Beam Melting (EBM). Especially for SLM, a pulse laser can be used by modulating the power signal of a continuous-wave laser into a pulse shape. This process is called Pulse Width Modulation (PWM), which allows additional process control compared to a continuous-wave laser.

Volumetric Energy Density (VED), calculated by dividing laser power by the product of scan speed, layer thickness, and hatch spacing, is a concept that expresses the applied average energy on the unit volume of the specimen. In many cases, microstructures and mechanical properties of fabricated specimens with SLM can be described by VED, but there exist exceptions. This is because the degree that each process parameter contributes to the properties of the specimen is different.

In this study, we focus on the differences in microstructures and mechanical properties of the specimen fabricated using the same VED conditions, especially with PWM laser. Even if all the process parameters composing the concept of VED and the duty cycle (DC) of the PWM laser are the same, the microstructure and mechanical properties are still very different depending on the pulse width. The surface roughness is also changed according to the pulse width. Mechanical properties are measured by using the Vickers hardness test, and the porosity and roughness are compared using an optical micrograph. For microstructural analysis, electron back-scattered diffraction is measured on the transverse section of specimens. Based on the results from these measurements and the simulation of the thermal distribution obtained through the finite element method, the causes leading to the differences in microstructures and mechanical properties of specimens fabricated under the same VED and DC are analyzed.

This research was supported by the Ministry of Trade, Industry and Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the International Cooperative R&D program (N0002624) and was also supported by the National Research Foundation (NRF) of Korea under the “Korean-Swiss Science and Technology Program” (2019K1A3A1A1406720011).

An equiatomic AlCuFeNiTi high entropy alloy (HEA) was synthesized using pre-alloyed
powder by direct energy deposition (DED) on a Ti6Al4V substrate and characterized to provide
insight into its microstructural and nanoscale mechanical properties. Scanning electron
microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed to
characterize the alloy's microstructure. Also, electron backscatter diffraction (EBSD) and X-ray
diffraction (XRD) were carried out to understand the crystallographic information of the alloy.
Nanoindentation technique was used to determine the reduced modulus and hardness.
Microstructural characterization results show three distinct regions as Cu-rich, Fe-rich, and Al-
Ti-Ni rich regions. The presence of these regions shows the variance of local composition within
the alloy that influences its mechanical properties. The dendritic regions rich in Al, Ni, and Ti
presented a hard zone, whereas the inter-dendritic regions rich in Cu and Fe showed a soft zone.
XRD and EBSD analysis confirmed that the regions are a mixture of BCC and FCC phases. The
alloy was determined to have a reduced modulus and hardness of 181±14.26 GPa and 9.03±2.42
GPa, respectively.

At the time of the nuclear power plant accident in Fukushima of Japan, zirconium alloy (Zr-alloy) cladding reacted with high-temperature steam in Loss of Coolant Accident (LOCA) and a large amount of hydrogen gas was generated. [1] In order to solve these problems, the Accident-Tolerant Fuel (ATF) cladding is inevitably a hot topic, led to innovative designs, such as coated Zr-alloy, ceramic cladding and advanced steels. [2] Among others a variety of coating technology have been studied to satisfy the goal of improving ATF in a short term without replacing the existing Zr based alloys. However, it’s not easy to deposit coating materials on long cladding, almost 4-meters. The development of the multi-walled swaging & drawing process using Stainless Steel 316 (SS316) tube on Zr-alloy cladding is very simple and can be performed at room temperature which can be mass-produced. However, thermal conductivity of SS316 is low and the thermal neutron absorption cross-section is more than 10 times larger than Zr-alloy. [3] Therefore, it should be used as thin as possible in SS316 tube. The SS316 tube about 100µm thickness was overlapped inside and outside of the Zr-alloy cladding and adhered using drawing process. The multi-walled cladding specimen was oxidized at 1200°C for 600s, in conditions of LOCA, and test shown increase the resistance to oxidation compared to commercial Zr-based alloy cladding. The interface between SS316 tube and Zr-alloy cladding was investigated by Optical Microscope (OM) and Scanning Electron Microscope (SEM). The multi-walled cladding was successfully fabricated using a drawing process, and it was proved that oxidation resistance was excellent through Energy Dispersive Spectroscopy (EDS) and mass change results. Based on the results, it is expected that better Accident-Tolerant Fuel (ATF) can be derived with the thin SS316 tube swaging and drawing process.
[1]ZINKLE, Steven J., et al. Accident tolerant fuels for LWRs: A perspective. Journal of Nuclear Materials, 448.1-3: 374-379, 2014
[2]DUAN, Zhengang, et al. Current status of materials development of nuclear fuel cladding tubes for light water reactors. Nuclear Engineering and Design, 316: 131-150, 2017
[3]AZEVEDO, CRDF. Selection of fuel cladding material for nuclear fission reactors. Engineering Failure Analysis, 18.8: 1943-1962, 2011

Recently, 4^th generation nuclear power generator for Distributed Energy Resources (DERs) in Passivation Molten salt Fast Reactor (PMFR). However, the Interface of reactor vessel materials occurs corrosion by molten salt with Nuclear Fission Product (NFP) at operating temperature. Generators under conventional operating conditions have overcome their limitations and current models at 600 K to 700 K, 46 KCl-54 UCl (mol%) or 17.8 KCl-68.2 MgCl₂-14 NaCl (mol%) salts. Materials that overcome chemical stability, high corrosion, and temperature resistance for semi-permanent use should be selected. (NFP: Te, Nb, Zr). Penetrates into the grain boundary of the base material and acts fatally. Therefore, a non-reactive material, CrN, is coated to prevent corrosion decay accidents. The integrity behavior was confirmed through the combined Cr and CrN layer by PVD RF-Sputtering system. NFP adsorption and penetration analysis were performed by heating the specimen at the misch metal jig at 930.15K with 1atm. Through the nano-indenter-based scanning electron microscope (SEM) analysis, the adhesive strength of the 3μm thin film was confirmed, and the elastic modulus of 160GPa was confirmed. A corrosion current density plot was obtained through an immersion test to confirm the chemical stability of the grain boundary. Corrosion voltage section is improved over 0.5V. These results suggested that CrN as a Passivation Molten salt Fast Reactor (PMFR) base material with secured lifetime promises to be a suitable candidate.

Silicon carbide (SiC) is a desirable ceramic for a wide range of applications due to its ability to withstand very high temperatures (>1500K), resistance to thermal shock, and inertness in corrosive environments. These characteristics make the material ideally suited for heat transfer devices, which often require small feature sizes such as aviation engine components and microfluidic devices. For these types of applications, conventional manufacturing processes are primarily used and involve rigid tooling and expensive fabrication techniques. These methods have limited small component silicon carbide manufacturing to relatively simple geometries, with more intricate configurations requiring specialized modifications which can be cost-prohibitive for low volume production and one-off prototype manufacturing. Furthermore, these processes are not amenable to iterative design changes. Additive manufacturing for silicon carbide, while presenting a more flexible processing method compared to other orthodox manufacturing routes, continues to be challenged by smaller form factors or with high feature resolution due to constraints imposed by the slurry and powder composition. High aspect ratio configurations with small feature sizes in the range of 100 to 500 microns prove particularly difficult with these manufacturing methods, requiring numerous post-sinter processing steps or bespoke tooling implements.
To address these shortcomings, this work describes a novel processing route that is iterable, has a high throughput, and is inexpensive for manufacturing small, high aspect ratio silicon carbide geometries. This fabrication sequence utilizes a hybrid manufacturing technique that harnesses the rapid iterability of additive manufacturing and combines it with the knowledge base that exists for conventional ceramic processing. This novel method involves a unique SiC precursor composition that is tailored to be compatible with multiple compaction techniques. The flexibility of polymer and metallic additive manufacturing is utilized to create multiple variants of the tooling required to produce these intricate high aspect ratio geometries at scale and reliably. Specialized compaction tooling to achieve the required green-state strength for compatibility with these small volumes is detailed. The capability of this processing technique is extended to other high aspect ratio structures and ceramic types, showcasing its broad applicability. Material characterization studies are presented to compare features of the sintered SiC geometries constructed using this process to typical fabrication methods.
A use case for this manufacturing technique is presented in the development and production of microfluidic reactor devices. SiC reactors that are straight tubes with a 1mm inner diameter and 3cm in length featuring an internal constriction of 300 microns to stabilize the fluid flow were manufactured with our process, replacing the current tubular reactors of similar size produced using an extrusion process. An experimental evaluation of this new design in a combustion pyrolysis experiment is presented compared to the currently used SiC reactor geometry, highlighting improvements in material performance and the variation in results obtained due to the presence of an internal taper which stabilizes fluid flow within the reactor. This specific application showcases the adaptability of this method in producing small devices which can be iteratively modified based on the experimental requirement to meet the needs of small-scale experimental facilities.

A direct ink writing method for 3D printing low-viscosity inks has been developed and used to fabricate polyelectrolyte membranes. Poly(allylamine hydrochloride) and poly(acrylic acid) inks are deposited in a meandering pattern in a layer-by-layer fashion, forming films with thickness up to hundreds of nanometers. Additionally, films are formed using a traditional layer-by-layer dip coating methodology for comparison. The molecular weight of each polyelectrolyte and post-treatment is varied to change the properties of the resulting films. The films are characterized with scanning electron microscopy, atomic force microscopy, optical profilometry, contact angle, and water filtration.

Biominerals formed by organisms in the course of biomineralization often demonstrate complex morphologies despite their single-crystalline nature. This is achieved owing to the crystallization via a predeposited amorphous calcium carbonate (ACC) phase, a precursor that is particularly widespread in biominerals. Inspired by this natural strategy, we utilized robocasting, an additive manufacturing 3D-printing technique, for printing 3D objects from novel long-term, Mg-stabilized ACC pastes with high solids loading. We demonstrated, for the first time, that the ACC remains stable for at least a couple of months, even after printing. Crystallization, if desired, occurs only after the 3D object is already formed and at temperatures significantly lower than those of common post-printing sintering. We also examined the effects different organic binders have on the crystallization, the morphology, and the final amount of incorporated Mg. This novel bio-inspired method may pave the way for a new bio-inspired route to low-temperature 3D printing of ceramic materials for a multitude of applications.

Joining materials with dissimilar properties is a challenging but ubiquitous task for load-bearing engineering and biological materials. Bimaterial interfaces are prone to stress concentration, which may trigger failure upon loading. Nature’s materials feature several strategies to mitigate stresses at bimaterial junctions, making soft-to-hard attachments long lasting. Some biological materials, such as squid beak and byssus thread, display a gradual change in mechanical properties over a region spanning up to several millimeters, which is a length scale much larger than the soft and hard elementary constituents. Other biological interfaces, like the bone-tendon junction, lack long-range property gradients and the transition in mechanical behavior occurs over a micrometer-sized region. Here, failure resistance is obtained by introducing local interface patterning. Advances in multimaterial manufacturing are opening unprecedented possibilities to prototype new bioinspired solutions for bimaterial attachments. Using 3D printing is nowadays possible to assign location-specific material properties to individual voxels within a macroscopic object, enabling the fabrication of complex centimeter-sized heterogeneous structures, referred to as voxel-based materials, with properties tuned at the micrometer level. In this work, we combined voxel 3D printing and computer simulations to prototype bimaterial attachments based on local interface patterning at the mesoscale. We used a polyjet printer (Object 260, Stratasys), which deposits and UV-cures photopolymer droplets in a layer-by-layer fashion. Using different inks, the printer allows fabricating structures composed of individual stiff or compliant cuboid voxels having minimum dimensions of 40 x 80 x 30 µm. In a previous work we have shown that, owing to the printing process, the spatial transition in elastic properties across bimaterial interfaces can be fairly broad (around 150 μm) and larger than voxel size [1]. Here, we manufactured centimeter-sized bimaterial samples featuring voxels having side length of 420 µm. This dimension, higher than printer resolution, is chosen to minimize the impact of the interface, such that a voxel can retain its mechanical character. Samples were fabricated by assigning to the stiff voxels a rigid glassy polymer (Young’s modulus of 2 GPa) and to the soft voxels a rubber-like material (Young’s modulus of 40 MPa). We architectured bimaterial interfaces based on the general idea of introducing minimal interface patterning, i.e. the width of the perturbed region at the interface between stiff and complaint materials (mesoscale) was at least one order of magnitude smaller than sample length (macroscale). Stiff and compliant voxels were rearranged to form either geometrical patterns or compositional gradients. Starting from a flat interface, we considered chessboard, rectangular and triangular designs as well as gradients with different slopes. We performed failure tests to measure strength and fracture toughness. Our results indicated that minimal interface perturbation in the form of triangular patterns outperformed compositional gradients to enhance failure resistance. Finite element simulations were done to characterize stress concentration and spatial distribution of strain energy density (SED). Firstly, a mesh analysis underlined that at least 4 hexahedral elements were necessary to model individual voxels in case the two materials were assigned very dissimilar properties. More relevant, simulations highlighted that while a flat interface led to the smallest stress concentration in a defect-free case, in the presence of a locally damaged interface triangular patterns minimized stress localization and helped to redistribute SED away from the damaged region. Our results shall provide guidelines to improve failure resistance of bimaterial junctions using voxel-based interface patterning.
[1] Zorzetto et al., Scientific Reports 2020; 10: 22285.

Due to the unique ability to manipulate the local solidification conditions point by point, additive manufacturing makes it possible to design materials with spatially varying microstructures. By controlling the distribution of these microstructures, we can design components with enhanced structural and functional properties. In this work, we use our recently published layerwise engineering of grain orientation (LEGO) technique to design functionally graded materials using blocks of discrete near-single crystal and gradient crystallographic textures. We apply our LEGO technique during laser powder bed fusion of stainless steel 316L to demonstrate the potential of site-specific texture control with two applications. In the first, we leverage the anisotropic chemical properties of 316L to create a new, high-throughput method for encoding data in metals. In the second, macro-scale “metamaterial” specimens that mimic grain-scale structures such as grain boundaries, twin boundaries, and triple junctions are examined with in situ mechanical testing. The techniques presented in this work may be further extended to control any material properties affected by crystallographic texture.

The demand for metal based additive manufacturing (AM) is exponentially increasing in the field of aerospace, automotive, medical, and other sectors. Accordingly, the metal 3D printing market is also increasing, while steel is the most used material among them. In metal 3D printing, the most widely used technology is laser powder bed fusion (LPBF) for which typically expensive gas-atomized (GA) powders are used due to their high quality, flowability, and purity, which enables the production of high quality parts. On the other hand, water-atomized (WA) powders with their irregular shape causing low powder flowability, with poor layer deposition quality and forming defects in final parts, generally are not favorable in metal AM. However, due to its economic advantage and straightforward fabrication process, vigorous research has recently been carried out to prove its applicability to 3D printing.
In this study, low alloyed steel powder inspired by the ‘Distaloy series’ of Hoeganaes was fabricated for AM part production and for evaluating the resulting mechanical properties. Nano-sized Cu-Ni-Mo metal compound was mixed with WA iron powder for low alloy steel powder fabrication. Here, the nano-sized metal alloy elements were homogeneously milled and mixed, to be added to the concave part of the WA iron powder while the commercial powder (Distaloy series) uses only micro-sized alloy elements. It was found that the developed partially pre-alloyed powder showed better flow properties than those of commercial powder.
This powder was used to produce cuboids by applying LPBF with a varying energy density window from 90 to 124 J/mm³. The parts which had the relative density values of over 99 % can be obtained at a feasible range of laser power from 150 W to 225 W. Especially, the maximum relative density of 99.79% was achieved when producing parts at its laser power of 175 W, scan speed of 700 mm/s (108 J/mm³) while observing pore structure and the melt pool structure. Moreover, hardness of the parts was measured to find out that the 3D printed parts under that processing window show uniform hardness values (217.1±4.7 HV) among parts with no pores observed while other parts which were manufactured outside the processing window showed non-uniform hardness values (210.0±15.2 HV). Finally, the tensile strength measurement results compared the sample fabricated using partially pre-alloyed WA iron powder with the parts made by raw WA iron powder. The tensile bars made by pre-alloyed powder had two times higher yield stress values [points of yield (pre-alloyed powder: 372.4 MPa, raw WA powder: 167.0 MPa), two times higher ultimate tensile stress (pre-alloyed powder: 460.9 MPa, raw WA powder: 207.3 MPa)] and three times higher strain values [points of ultimate tensile strength (pre-alloyed powder: 7.0%, raw WA powder: 2.6%), three times higher elongation to fracture (pre-alloyed powder: 11.7%, raw WA powder: 3.0%)] compared to those made by raw WA powder. Finally, it was found that the fabricated parts achieved their tensile test results which are comparable to the values [the yield stress (357.4 MPa), the ultimate tensile stress (459.6 MPa), the strain at the ultimate tensile strength (6.5%), the elongation to fracture (7.0%)] of the sample proceeded by GA stainless-steel powder. Therefore, the results show that the pre-alloyed WA iron powder can potentially be a promising material suitable for the LPBF process used for various applications.

The ability for materials to adapt their shape and mechanical properties to their environment is useful in a variety of applications, such as soft robotics and deployable structures. Liquid crystal elastomers (LCEs) have been widely explored in recent years due to their ability to controllably actuate in response to multiple external stimuli, such as heat and light. In this work we integrate LCEs with multistable structures to allow autonomous reconfiguration in response to temperature changes. To accomplish this, we integrate LCE-silicone bilayers as hinges in a kirigami-inspired multistable material based on a rotating squares mechanism. The bilayer hinges bend and soften as the temperature increases. By choosing geometric parameters for the hinges such that bifurcation points in the stability exist, a transition from mono- or tristability to bistability can be triggered by a sufficient increase in temperature, forcing rearrangements of the structural configuration in the heat-effected area as minima in the energy landscape are removed. Depending on the choice of energy landscape, these local structural reconfigurations may or may not propagate to the rest of the structure, analogous to the propagation of a phase transformation. These effects are relevant to applications smart interfaces, reconfigurable structures, and soft robotics.

We present a novel platform based on capillary fluid flow in ordered three-dimensional open-cell lattices: cellular fluidics [1]. By designing the cell and lattice architecture, combined with additive manufacturing methods that provide access to length scales below 100 µm, we can fabricate complex polymeric 3D structures with deterministic porosity and advanced functionalities. This approach enables selective placement and direction of liquid flow into predetermined continuous paths through the structure and optimization of gas-liquid and liquid-liquid interfaces for multiphase processes. We demonstrate the application of such cellular fluidic devices for processes such as transpiration cooling and CO₂ capture. Further, we show that flowing the appropriate electroless plating solutions through the predetermined paths allows selective metallization in regions of interest in three dimensions, resulting in multimaterial heterostructures that can further be electroplated with different materials. Hence, this new platform is a promising tool for designing novel 3D microfluidic devices such as electrochemical reactors with control over multiphase transport processes.

[1] N.A. Dudukovic, E.J. Fong, H. B. Gemeda, J.R. DeOtte, M.R. Cerón, B.D. Moran, J.T. Davis, S.E. Baker, E.B. Duoss, "Cellular Fluidics", Nature 2021 (accepted)

*This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 within the LDRD program 19-SI-005. LLNL-ABS-810682.

Flexible and stretchable antennas are important for devices that use wireless communication to address mechanical mismatches at the tissue-device interface. To mechanically comply with the curvature of the organs and match their motion, such devices typically featured thin designs to ease bendability and elastomer encapsulations to enhance stretchability. Emerging technologies of liquid metal-based stretchable electronics are promising approaches to improve the flexibility and stretchability of conventional metal-based antennas. However, existing methods to encapsulate liquid metal requires monolithically thick (at least 100 µm) substrates and the resulting devices are limited in deformability and tissue-adhesiveness. To overcome this limitation, we demonstrated fabricating microchannels by direct ink writing on a 7 μm-thick elastomeric substrate, referred to as Ecoflex microsheet, to obtain liquid metal microfluidic antennas that exhibited unprecedented deformability. The fabricated wireless light-emitting device was powered by a standard near-field-communication system (13.56 MHz) and retained a consistent operation under deformations including stretching (> 200% uniaxial strain), twisting (180^○ twist), and bending (3.0-mm radius of curvature) while maintaining a high Q factor (> 20). We also demonstrated suture-free conformable adhesion of the polydopamine-coated device to ex vivo animal tissues under mechanical deformations. This technology offers a new capability for the design and fabrication of wireless biodevices, which require conformal tissue-device integration. Liquid metal antennas would be used as a platform for various multifunctional wireless biodevices. DIW 3D printing technology can also offer integration of other functional components such as physical sensors, electrochemical sensors, and drug delivery systems. This technology paves the way towards minimally invasive, imperceptible medical treatments.

Soft materials that exhibit self-sensing and programmable actuation are of great interest for applications ranging from robotics to adaptive structures. Liquid crystal elastomers (LCEs) are an emerging class of soft materials that exhibit reversible and large deformations with high energy density. These materials have been patterned into shape-morphing architectures with programmed director alignment via HOT-DIW, an extrusion-based 3D printing method (Kotikian et al, Adv. Mater. 2018; Kotikian & McMahan et al, Sci. Rob. 2019). Recently, we have further demonstrated the fabrication of innervated LCEs (iLCEs) composed of liquid metal (LM) cores surrounded by LCE shell features via core-shell 3D printing. iLCE actuation is programmed by Joule heating power and thermal response is measured and modeled. Importantly, iLCEs self-sense their own deformation during actuation, as strain correlates with resistance in unidirectional and 3D morphing structures. Due to their large concomitant changes in length and resistance and their high energy density, iLCE fibers and spiral architectures are capable of closed loop control, with regulation of actuation sequences even with large bias loads. Our multi-material 3D printing platform enables the rapid design and fabrication of soft functional materials with exquisite control over their composition, architecture, and performance.

This talk will discuss the use of liquid metals for 3D printing. Gallium-based metal alloys have interesting properties including low-viscosity, low melting point, low toxicity, and negligible volatility. Despite the large surface tension of the metal, it can be molded, patterned and printed into non-spherical shapes due to the presence of a native oxide skin that is only a few nanometers thick. We have harnessed these properties to form a number of electronic devices encased in polymer matrices. The resulting structures take on the mechanical properties of the polymer and can therefore be soft, self-healing, and ultra-stretchable. The metal can be printed and then withdrawn from 3D printed structures as a sacrificial, fugitive ink to create microvasculature in polymer monoliths. In addition, it is possible to 3D print vasculature within polymeric materials printed using conventional 3D printing, and then back-fill the structures with liquid metal to add function in a post-processing step. The metals can be used for adding electrical conductivity as well as toughening. In addition, liquid metal can be mixed with uncured elastomers to tune the rheology to create printable inks. These inks can be cured to create self-encapsulated, stretchable conductors. Finally, we have recently demonstrated ways to treat 3D printed polymeric structures so that liquid metal can be injected and withdrawn on demand. This allows for reconfigurable and dynamic structures in which the conductors can be fully recovered. The challenge is to create a coating that can be deposited on the walls inside 3D printed parts without damaging the part. To address this, we have create a solvent-based coating that uses fumed silica to create slightly roughened surface to which the liquid metal does not adhere. This talk will discuss the key processing parameters to enable printing of metal, the applications of such printing, as well as future opportunities and challenges.

Additive manufacturing (AM) represents a set of processes that enable layer-by-layer fabrication of complex 3D structures, with some advanced methods attaining nanometer resolution and the creation of unique, multifunctional materials and shapes derived from a photoinitiation-based chemical reaction of custom-synthesized resins and thermal post-processing. A type of AM, vat polymerization, has allowed for using hydrogels as precursors, and exploiting novel material properties, especially those that arise at the nano-scale and do not occur in conventional materials. The development of small-scale AM has revolutionized the production of complex parts for aerospace, military, automotive and medical applications, and is enabling major innovations in these areas. Shapes as complex as fractal trusses and as simple as cubes, with vastly multi-scale dimensions, from nanometers for nanophotonics to millimeters for sensors to centimeters for space-relevant technologies been demonstrated.
We present synthesis and fabrication of nano- and micro-architected materials using vat polymerization and 3D (two-photon and interference) lithography, and characterize their properties as a function of architecture, constituent materials, and microstructural detail. We focus on specific materials and architectures with unique properties: (1) Impact resistant nano-architected carbon, (2) Hydrogel-enabled AM of pure metals and alloys with micron and sub-micron dimensions, (3) Ligand exchange-based fabrication of 3D TiO₂ photonic crystals (PhCs), with critical feature dimensions between 150 and 600 nm, and (4) AM of Metallo-polyelectrolyte Complexes with stretchability up to 1000% and full recoverability, and (5) Hydrogels moisture-harvesting properties.

Stretchable electronics, tissue engineering, and soft robotics are a few of the many examples in which engineers require soft materials with higher toughness and better fatigue-resistance in order to prevent premature failure during demanding, repeated mechanical loading. One effective method to improve the toughness of soft materials is to embed stiff fibers in a soft matrix. We 3D print soft composites using direct ink writing (DIW). The composite material consists of polydimethylsiloxane (PDMS) reinforced with glass fibers (with diameter roughly 15μm). DIW processes result in alignment of fibers during printing, due to the shear field in the deposition nozzle. Hence, parts can be 3D printed with controlled spatial variations in fiber alignment, based on the print path and other parameters that affect the shear field in the nozzle. In this work, we first consider how the fiber-matrix bond influences the failure properties of the printed composite. We show that if the glass fibers are treated with acid prior to formulating the ink, the fiber-matrix bond is strengthened, and the toughness of the composite is greatly improved. Additional experiments are conducted to quantify the intrinsic toughness of the composite, and the effect of mechanical dissipation resulting from fiber-matrix debonding. We also measure the fatigue properties, which also show significant improvement after fiber treatment. Finally, we show that using bioinspired spatial variations in fiber alignment, such as in skin and artery, the composites are tougher and more fatigue resistant without further stiffening the material.

Embedded 3D printing (e3DP) has demonstrated fabricating freeform structures of curable polymer resins in microparticulate hydrogels. This method is, however, not compatible with thermoplastics extruded at high temperature. This communication presents a unique approach to 3D-print thermoplastics in embedding media, termed freeform polymer precipitation (FPP). FPP is based on spatially controlled immersion precipitation of polymer inks patterned in microparticulate gels for various thermoplastics with additives. The embedding media offers unique dual functions: Bingham plastic to maintain printed structures, and nonsolvent for in situ precipitation of polymer inks. Polymer inks with a wide range of vapor pressure (0.04 – 60 kPa) and viscosity (0.1 – 1000 Pa.s) were demonstrated for FPP. Using acrylonitrile butadiene styrene (ABS) dissolved in acetone (20 – 60 w/w%) as a model ink, we identified the printing conditions to ensure vertical and lateral attachments of printed inks. The fabricated 3D objects were porous due to rapid phase separation induced by nonsolvent, which was controlled by the concentration of the polymer and the porogens in the inks. FPP offers a simple route to fabricate 3D freeform structures of thermoplastics with controlled internal porosity and should serve as a useful toolkit to 3D-print multi-functional materials such as polymer nanocomposites.

Recent advances in additive manufacturing have allowed fabrication of high resolution, complex geometries with fine features from UV-curable acrylated polymers. Utilizing electroless deposition in conjunction with these manufacturing techniques allows us to leverage the aforementioned benefits and impart electrical and thermal conductivity, a basis for catalytic activity, and improved mechanical properties. Electroless deposition onto 3D-printed polymer acrylates is not well understood and frequently demands a trial-and-error approach to successful metallization of the surface. Plating of these polymer surfaces requires pretreatment steps; there is a lack of knowledge surrounding how these steps affect the coating adhesion and uniformity. Here, we present a systematic investigation into these pretreatment steps with the aim of leveraging quantifiable data to establish best practices. We characterize the effects of alkaline etching on surface roughness and surface chemistry through the use of optical surface topography measurements and ToF-SIMS, respectively. We utilized Quartz Crystal Microbalance to examine the absorption of Tin (sensitizer) and Palladium (activator) species on the polymer surface. The resulting adhesion strength of the plated metal was measured through peel testing and the coating uniformity was quantified with a coverage fraction parameter. We show that changes in surface roughness of the polymers during the etching step occur at several length scales and time scales, associated with uneven curing degrees in regions of oxygen-inhibited polymerization. We find that the surface concentration of adsorbed sensitizer and activator species is related to the surface concentration of hydrophilic groups produced in the alkaline etching step. We show that the adhesion and uniformity are a function of both surface roughness and surface chemistry. These findings facilitate design guidelines for electroless deposition on 3D-printed polymers, specifically UV-cured acrylates. Further, these design rules allow for a deeper understanding of the role mass transport plays in the metallization of multiscale three-dimensional architectures. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 within the LDRD program 20-ERD-056. LLNL-ABS-821864.

Additive manufacturing at the micro-/nano-scale has the potential of revolutionizing the conception of small-scale devices. Yet, a major challenge is currently encountered: combining high-resolution, geometry control, high-throughput together with multi-materials printing. Even more crucial towards real applications is the ability to synthesize device-grade materials [1].

In this contribution, we will show that electrohydrodynamic redox 3D printing (EHD-RP) can be used to locally optimize grain size in 3D printed wires, changing mechanical strength by a factor of 2. EHD-RP is an ink-free AM technique with multi-metal capabilities and offering sub-micron spatial and chemical resolution [2]. A unique feature of this synthesis technique (and all synthesis techniques in comparison to transfer techniques) is the possibility to locally tune microstructure by adjusting growth parameters. In this work, we show the different microstructures accessible by EHD-RP, showing potential for material optimization. To this end, we first map the microstructure and morphology of wires as a function of printing voltage. We then use detailed microstructure and elemental analysis by TEM and APT to elucidate the causal relationship between the printing voltage and the grain size, adding important understanding to the formation of the microstructure. The influence on materials properties is illustrated by micro-compression experiments showing the relationship between the grain size distribution and the materials' strength—the so-called Hall-Petch strengthening. Finally, we demonstrate the fast switching between the different regimes by fabricating nano-wires with sub-micrometer modulation in grain size, and thus local strength. Our work opens the door to the fabrication of small-scale metal 3D geometries with optimized microstructural distributions..

[1] A. Reiser, et al. Adv. Funct. Mater. 30, 1910491 (2020).
[2] A. Reiser, et al. Nat. Commun. 10, 1853 (2019).

Research in the field of additively manufactured (AM) architected materials has enabled engineered materials with mechanical properties that were previously thought unattainable. For instance, a large part of the research has focused on the linear properties of truss-based lattices, demonstrating high stiffness- and strength-to-weight ratios. However, most truss-based lattice structures suffer from a bending-dominated response and stress concentrations at the sharp joints, the latter of which results in initiation and propagation of fracture. Spinodoid metamaterials have been proposed as an alternative to combat these stress concentrations due to their finite (low) curvature and bicontinuous morphologies. Furthermore, the mechanical properties of these spinodoid metamaterials can be tuned by introducing preferential interface directions which results in tailorable anisotropy.
In this study, we explore the effect of curvature and anisotropy on the strength and toughness of spinodioid metamaterials. The spinodoid morphologies are generated using a computational spinodal decomposition framework where tunable anisotropy is introduced by energetically penalizing preferential directions for interface formation, resulting in a mean curvature and Gaussian curvature distribution that is dependent on the direction of the surface normal. Using a two-photon lithography process we fabricate prototypes of these 50%-relative-density (i.e., fill fraction) anisotropic spinodoid morphologies, containing spatially varying curvature distributions, and perform in-situ nanomechanical uniaxial compression and bending experiments to determine their curvature-dependent stiffness, strength, and toughness. Complemented by finite element models of the as-fabricated morphologies, we demonstrate a relation between curvature distributions, the stress distribution, and the toughness of these metamaterials. Our findings present the potential of architected spinodoid metamaterials as a new class of additively manufactured lightweight materials with widely tunable properties such as fracture resistance, which may expand the property space of classical lattice-based AM materials.

Current battery electrode manufacturing using slurry coating and drying to produce electrode films is energy and time-intensive, containing significant quantities of environmentally toxic solvents, and lacks processability that controls electrodes' thickness and geometric complexity. New manufacturing processes have been developed to produce electrodes with increased thicknesses and complex physical structures with improved performance and safety characteristics. A growing trend to rapidly create structured electrodes with programmable geometries, improved thickness, and multiscale structural details is using additive manufacturing, also known as 3D printing, as replacements for typical state-of-the-art ink casting. However, existing 3D printing strategies can only fabricate electrodes using ink-based slurries, which lack the capability of forming complex geometries or use a narrow range of UV-curable resins with insufficient ionic and electric transport capabilities. Therefore, a new electrode fabrication route is needed to improve structured electrode design and manufacturing flexibility.
In this talk, I will present a novel electrode processing route, Structured Electrode Additive Manufacturing (SEAM), which can rapidly fabricate ultra-thick structured electrodes with low tortuosity, mechanical robustness, and complex geometry, enabling LIBs to achieve high power and high energy at a low production cost. SEAM involves two innovations of dry electrode formation and the debinding/sintering process. The innovation in dry electrode formation is enabled by a high-loading dry-extrusion technique called Shear Assisted Particle Extrusion (SHAPE). SHAPE can manufacture thick electrodes using compositions close to commercial electrodes without using any solvents or the drying process, and it can orient geometrically anisotropic active materials (e.g., graphite flakes) along their longitudinal directions at high-pressure induced shear flow and deposition. The debinding/sintering process can remove polymer components in the electrode while retaining the original printed “green” geometry without structural collapse or failure. SEAM leverages the dry-extrusion additive manufacturing combined with a rapid debinding/sintering process to meet the geometric design and cost requirements for replacing slurry coating and drying techniques.

Three-dimensional (3D) ferroelectric materials are promising electromechanical building blocks for achieving human-machine interfacing, energy sustainability, and enhanced therapeutics. However, current natural or synthetic materials cannot offer both high piezoelectric responses and desired mechanical toughness at the same time to meet the practicality. Here, a nacre-mimetic ferroelectric metamaterial was created with a ceramic-like piezoelectric property and a bone-like fracture toughness through a novel low-voltage-assisted 3D printing technology. The one-step printed bulk structure, consisting of periodically intercalated soft ferroelectric and hard electrode intercalated layers, exhibited a significantly enhanced longitudinal piezoelectric charge coefficient (d₃₃) of over 150 pC N^-1, as well as a superior fracture resistance of ~ 5.5 MPa m^1/2, more than three times higher than conventional piezo-ceramics. The excellent printability together with the unique combination of both high piezoelectric and mechanical behaviors allowed us to create bone-like structure with tunable anisotropic piezoelectricity and bone-comparable mechanical properties, marking a cornerstone toward manufacturing practical, high-performance, and smart biological systems.

Recent advances in the additive manufacturing of trusses made from viscoelastic base materials have created new opportunities for the design of time-dependent metamaterial properties. Using compliant polymers as base material [1] to fabricate 3D periodic architected metamaterials facilitates high energy absorption capabilities and allows the engineering and tailoring of the topology to a specific impact situation. Such structures facilitate shock and impact absorbing capabilities that have great potential in blast-protection applications but also as protective sports gear. The dissipation of energy is induced solely by the viscous base material model instead of layer-wise plastic deformation and failure of beams [2], coming with the advantage of shape-recoverability [3] and thereby also reusability of the lattice after impact.

We use a linear viscoelastic constitutive model along with the co-rotational beam description by Crisfield [4], which captures large rotations. We solve linear momentum balance in a time-discrete fashion, based on a variational formulation employing variational constitutive updates for the viscoelastic material model, which results in a rate-dependent material response to be explored.
In addition to performing discrete numerical beam calculations, we include the viscoelastic beam model into the generalized continuum description of Glaesener et al. [5] and show a good match with the discrete formulation. The closed-form solution for the incremental update formulation, combined with the first-order homogenization approach at the microscale allows for an efficient computation of initial boundary value problems. Applying the linear viscoelastic material model to the investigation of non-catastrophic impact combines the study of history-dependent constituents with time-dependent load scenarios. We show that, for a given viscous base-material behavior, truss topology, and known impact velocity, the magnitude of damping (coefficient of restitution) upon impact can be computed. We present calculations for both the discrete and continuum framework, reporting good agreement with experimental observations.

[1] Gibson, I., D. W. Rosen, and B. Stucker,
2010. Additive Manufacturing Technologies: Rapid Prototyping to Direct Digital Manufacturing. Boston, MA: Springer Science+Business Media LLC.

[2] Mueller, J., Matlack, K.H., Shea, K. and Daraio, C., 2019. Energy absorption properties of periodic and stochastic 3D lattice materials. Adv. Theory Simul., 2 (10), 1900081.

[3] Meza, Lucas R and Zelhofer, Alex J and Clarke, Nigel and Mateos, Arturo J and Kochmann, Dennis M and Greer, Julia R,
2015. Resilient 3D hierarchical architected metamaterials. PNAS 112 (37), 11502 - 11507.

[4] Crisfield, M., 1990. A consistent co-rotational formulation for non-linear, three-dimensional, beam-elements. CMAME 81 (2), 131 – 150.

[5] Glaesener, R., C. Lestringant, B. Telgen, and D. M. Kochmann
2019. Continuum models for stretching- and bending-dominated periodic trusses undergoing
finite deformations. IJSS, 171, 117 - 134.

In this presentation, we will cover some of the latest work in advanced and custom additive manufacturing processes for the fabrication of architected materials and functional devices. The concept of architected materials revolves around the notion that material properties are traditionally governed by the chemical composition and spatial arrangement of constituent elements at multiple length-scales. This usually limits material properties with respect to each other creating trade-offs when selecting materials for specific applications. For example, strength and density are inherently linked so that, in general, the more dense the material, the stronger it is in bulk form. We will review the latest in design methods such as topology optimization, as well as advanced additive micro- and nanomanufacturing techniques to create new material systems with previously unachievable property combinations. These processes include projection microstereolithography (PuSL), direct ink writing (DIW), and electrophoretic deposition (EPD) as well as new advanced concepts such as volumetric additive manufacturing (VAM), computed axial lithography (CAL), parallel two-photon polymerization, liquid metal jetting, and diode-based additive manufacturing of metals (DiAM). The performance of the resulting material constructs is fundamentally controlled by geometry at multiple length-scales, from the nano- to the macroscale, rather than chemical composition alone. We have fabricated and will review the resulting properties of these mechanical metamaterials in polymers, metals, ceramics, and combinations thereof. These new processes and concepts can also be applied to functional materials such as those for supercapacitor electrodes, optical components, and even for fluidic system such as the recently published cellular fluidics concept. A review of these functional systems will be followed by a brief discussion of future directions.

Treating vascular grafts failure often requires complex surgery procedures and associates with a high mortality rate. Real-time monitoring vascular system could enable quick and reliable identification of complications and initiate safer treatments in the early stage. In this work, electric field-assisted 3D printing technology was developed to fabricate in situ-poled ferroelectric artificial arteries that offered battery-free real-time blood pressure sensing and occlusion monitoring capability. The complex functional artery architecture was made possible by the development of a printable ferroelectric bio-composite which could be quickly polarized during printing and reshaped into devised objects. Synergistic effect from the ferroelectric potassium sodium niobate (KNN) particles and the ferroelectric polyvinylidene fluoride (PVDF) polymer matrix yielded a superb piezoelectric performance (bulk-scale d₃₃ > 12 pC N^-1, confirmed by piezometer) on a par with that of commercial ferroelectric polymers. The sinusoidal architecture brought the mechanical modulus down to the same level of human blood vessels. The desired piezoelectric and mechanical properties of the 3D-printed artificial artery provided an excellent sensitivity to pressure change (0.306 mV/mmHg, R²> 0.99) within the range of human blood pressure (11.25 to 225.00 mmHg). The high pressure sensitivity and the ability to detect subtle vessel motion pattern change enabled early detection of partial occlusion (e.g., thrombosis), allowing for preventing grafts failure. This work demonstrated a promising strategy of incorporating multi-functionality to artificial biological systems for smart healthcare systems.

As the demand for smaller feature size and lower cost integrated circuits, printed circuit boards, and other micro and nano-scale devices increases, newer and faster printing methods that retain the accuracy of previously used methods must be developed in order to address such challenges. Previous research conducted at the NSF Center for High-rate Nanomanufacturing and others have shown the effectiveness of directed assembly processes which allow for highly precise and comparatively low cost manufacturing. However, the printing of single crystals using nanoparticles for making semiconducting devices, such as transistors and diodes, has proven to be a significant challenge. We report in this poster experimental efforts to print III-V semiconductors that show promising results. The particle size was significantly reduced to the nanoscale utilizing both physical milling and chemical synthesis techniques. Nanoparticle suspensions were formulated to be used in our fast fluidic directed assembly printing process. We have shown that indium phosphide and gallium arsenide nanoparticles can be reliably and accurately assembled. After printing, the printed structures are annealed and the crystalline structure, material properties, and electrical properties are measured. This poster shows the results of sintering the printed semiconductor devices along with their electrical characterization to demonstrate the effectiveness of the methods developed. Our directed assembly method allows for reliable and accurate assembly of these semiconducting devices. This will enable faster, lower cost, and more sustainable prototyping and production of micro and nanoscale electronic devices which could revolutionize semiconductor fabrication as we know it.

Carbon-nanotube thin-film transistors (CNT-TFTs) have the potential for a wide variety of applications, such as chemical sensors and digital logic circuits, due to their high field-effect mobility and compatibility with versatile fabrication approaches. Aerosol jet printing is an additive, direct-write approach for integrating CNT-TFTs onto various substrates. Among all the layers making up a CNT-TFT, the gate dielectric is the most difficult to additively print owing to challenges in thickness scaling, pinhole formation, and reliance on high sintering temperatures. The most commonly used printed dielectrics include electrolytes and ion gels due to their high specific capacitance and compatibility with solution-phase dispersion into inks. Recently, crystalline nanocellulose (CNC) has been utilized as an electrolyte gate dielectric to realize a fully recyclable, printed CNT-TFT. In this work, we present a detailed evaluation and benchmarking of three additively printed gate dielectrics for fully printed, top-gated CNT-TFTs that operate at low voltage: an electrolyte consisting of elastomer PVDF-HFP; CNC with varying salt concentrations from 0% up to .5%; and an ion gel consisting of triblock polymer PS-PMMA-PS, ionic liquid EMIM-TFSI, and ethyl acetate in a 1:9:90 w/w ratio. Using a surface map of the transistor on/off-current ratio, we evaluate the dependence of gate location on device performance. Because electrolyte dielectrics are performance-limited by ionic diffusion rates, the impact of voltage sweep rate on switching characteristics, such as threshold voltage, subthreshold slope, and hysteresis, is examined. Further, electrical bias and mechanical stress testing are performed to determine the resilience of these dielectrics. This extensive evaluation of three leading options for additively printed dielectrics will aid in moving the field of printed electronics closer to application-ready devices with stable, low-voltage operation TFTs.

For many industrial applications such as catalysis, sensing, optical elements and many more, complex three-dimensional micro-scale porous metal structures with pore sizes in the sub-micron scale are desirable. However the fabrication of such sophisticated small-scale structures is not an easy task. In recent years the field of additive manufacturing (AM) has received more and more interest as a very powerful tool for the production of complicated three-dimensional structures^[1] One AM technique that has been shown to be able to produce complex structures with sub-micron resolution is electrohydrodynamic redox printing (EHD-RP)^[2]. EHD-RP furthermore has multi-metal and multi-material capabilities.

In this contribution, we will describe a fabrication process that can be used to readily produce nano-porous Ag structures on the micro-scale utilising the multi-metal capabilities of EHD-RP. Aqueous Ag-Cu salt solutions were used as printing solutions to produce small-scale metallic structures. Porosity in these structures was then achieved by selectively etching away Cu. In this work we show that by varying the composition of the printing solutions used, precise control over the ratio of Ag to Cu in the as-printed structures was achieved. Utilising this control of the chemical composition we were able to tune the porosity as well as the pore size of the dealloyed structures. Being able to precisely tune the porosity of micro-scale three-dimensional structures, as demonstrated in this work, opens up many new possibilities in the field of optical meta-materials especially focused on sensing applications. The high local control given by EHD-RP combined with the porosity control described here could also be used to fabricate structures with locally varying mechanical properties.

[1] Hirt L. et al., Adv. Mater. 2017, 29, 1604211
[2] Reiser A. et al., Nat. Commun. 2019, 10, 1853

In 2015 NASA began issuing challenges for the development of innovative technologies that would support human colonization of the moon and Mars by 2050. Subsequently, our students began with concepts and images developed at MIT as part of the initial Habitat Challenge [1]. From there, the team, including middle and high school students, their advisors, MIT researchers, and grad students, hypothesized that the most efficient and sustainable means of structural development would be to use the indigenous materials: silicon-rich basaltic and calcium-rich plagioclase regolith with water-ice discovered at the poles. In addition, we assume NASA’s Challenge to biologically harvest solar energy to produce organic binders will be successful and provide additional structural constituents.
Our research focused on evaluating the structural capacity of a lunar regolith analogs [2] and their ability to withstand compression and impact forces. For this work we evaluated Lunar Highland Simulant (LHS-1) and Lunar Mare Simulant (LMS-1) prepared by Exolith Lab at the University of Central Florida. Commercially available stone-dust with similar geologic properties was also used as a lunar analog. Published works suggests that albumin may serve as an effective binder while other authors suggest chitin may be effective [3,4]. For this investigation commercially available chicken egg albumin (egg white) and chitosan (commercially available chitin) were among the materials evaluated. Water was used as a non-binding control. In addition, samples of fine fractions were vitrified using a glass-manufacturing kiln.
Each regolith sample was mixed with varying amounts of binding agent, formed in 5 cm cube molds, pressed, smoothed, and allowed to cure for 24 hours, 7 or 14 days. Afterward, each sample was non-destructively tested with commercial compression hammers. The results were compared to terrestrial construction standards and are being presented.
Based on our observations, we believe lunar regolith may become a viable construction material, but additional work will be necessary to produce a binding material that will be suitable for building sound, impact-resistant dwellings. In addition to reinforcing concepts developed during research, this activity dramatically demonstrates the links among elementary and secondary school learning, academic research, and the private sector that supports them.

References
[1] http://news.mit.edu/2017/mars-city-living-designing-for-the-red-planet-1031
[2] https://sciences.ucf.edu/class/exolithlab/
[3] https://chemrxiv.org/articles/preprint/Blood_Sweat_and_Tears_Extraterrestrial_Regolith_Biocomposites_with_in_Vivo_Binders/14769369/1
[4] [https://www.constructionspecifier.com/building-on-mars-chitin-may-help/]

As the demand for smaller and more reliable mass-produced electronic devices grows, the directed assembly of electronic devices as additive manufacturing technique provides a promising potential solution to achieve mass production with low fabrication costs. This technique is proved to be capable of directly printing conducting, semi-conducting, as well as insulating materials using variety of liquid dispersions within micro to nano-scale resolution. Moreover, this technique is additive, more efficient, and cheaper than many other conventional subtractive techniques. Conventionally, in order to print basic electronic components such as resistors, capacitors and inductors on different substrates, high vacuum vapor deposition, ink jet printing, and etching techniques are implemented. However, these techniques require high energy consumption, poor printing resolution, and incompatibility with many substrates. Here, we utilized the directed fluidic assembly technique to monolithically print basic electronic components on a large scale with high printing resolution and large production throughput. Using this technique, we tackled different obstacles simultaneously. First, the process can be applied to wide range of substrates, which allows these circuit components to be dynamically integrated into other systems that may have not been previously compatible. Second, unlike other traditional printing processes, this process is considered conservative, thus minimizing the fabrication cost significantly. Third, basic electronic components can be easily controlled by adjusting thickness and structure dimensions down to several microns. Our results, show two basic printed electronic components, these are thin film resistors and micro-scale capacitors, which’re printed monolithically on a flexible PET substrate. The resistors values showed wide resistance range of 1-10 KΩ using metallic conducting ink and 10 KΩ to 25 MΩ using ceramic composite dispersion. The achieved capacitance of the printed capacitors was between 80-100 pF which can be adjusted by varying the electrodes surface area and the spacing between the printed electrodes. We believe that this fluidic directed assembly will pave the way for the mass production of basic circuit components at lower time and energy costs than normally possible with conventional fabrication techniques.

Polyphenylene sulfide (PPS) is a high performance, semi-crystalline, thermoplastic polymer, demonstrating exceptional thermal stability, mechanical integrity, and chemical resistance. Properties such as Young’s modulus, thermal resistance, electrical conductivity, and toughness can be optimized by blending PPS with other materials (e.g. Nylon, PEEK, and carbon fiber). Such improvements motivate investigations for applications in areas ranging from the aerospace and automotive industries to electronic devices. This study aims to optimize the printing parameters for fused deposition modeling (FDM) and post-print thermal treatment, such that the aforementioned properties are enhanced for 3D printed PPS and PPS blends.

Various studies highlighted the failure of generically designed respiratory protective equipment (RPE) in fitting female and black, asian, and minority ethnic (BAME) users. As a result, many of those users’ lives were put at risk during the COVID-19 pandemic. Employing additive manufacturing (AM) techniques for producing custom-fit RPE may overcome this problem if the laborious barriers in computer-aided design (CAD) creation can be overcome. Here we propose a process for producing custom fit RPE based on a single input containing the user’s 3D facial scan captured via smartphone depth-sensitive camera. Through a combination of rigid and non-rigid iterative close point transformations, the coordinates of key landmarks were extracted to generate a custom-fit RPE via a CAD API. Subsequently, the goodness of fit was measured between each of the generated RPE and the raw facial scan using Euclidean distance. We evaluated the process for 207 submissions from users of different demographics, out of which 180 submissions were successfully processed (87.0%) with an average runtime of 3.33 minutes per scan. Furthermore, it was found that goodness of fit from the process-generated RPE was independent of age, race, gender, and Body Weight Index. The results demonstrate the viability of combining automated CAD generation with AM for producing inclusive wearable devices.

How to design metamaterials and soft robots to achieve a set of specific properties is an inverse problem facing the challenges of non-existent or non-unique solutions. The design of extreme properties such as the Hashin–Shtrikman (H-S) bounds of moduli and strengths makes the challenges even more daunting. Existing methods such as the trial-and-error approach have proven to be inefficient for such designs. Here we propose an initio approach to the design of metamaterials and soft robots to achieve multiple extreme properties simultaneously. The ab initio method is based on a combination of crystallography for structural symmetries, simulations to generate data, and machine-learning algorithms to exploit the data for designs. We demonstrate the concurrent designs of multiple extreme properties of architecture materials such as moduli, strengths, negative Poisson's ratios, and thermal conductivity. We will further discuss the implications of our ab initio method for material and structural designs in general.

With the growth of machine learning techniques and their more direct societal use, the need for interpretability and transparency of how these algorithms make their decisions also grows. This understanding instills trust and helps to engineer more robust algorithms and data treatments by ensuring that the algorithm is picking up on features that a human would deem important, as opposed to correlated or contextual features, or even machine-specific features that would not transfer across users and systems. Here, we use Layerwise Relevance Propagation (LRP) to examine the results of convolutional neural networks used to estimate the rheological properties of inks printed by direct ink write 3D printing. LRP is often used to assign relevance to pixels in image classification. In this study, we look at the changes in relevance for image regression analysis. In particular, ink properties such as the yield stress, rate index, and consistency index of an ink can be determined from the sharpness of printed corners and the width of deposited filaments. We determine how well these algorithms predict these properties and compare them to direct measures of width and curvature, while also drawing relations to the pixel relevancy values and locations. We bring a quantitative approach to LRP in understanding print morphologies in AM.

These days, topology optimization designs via pixelized/voxelized heterogenous material programming to achieve desired mechanical properties are of interest. Synergy between soft and stiff components in heterogenous materials leads to delocalization of stress field throughout the composite and thus advanced mechanical properties, such as high toughness and damage tolerance. The enormous possible combinations of two materials and design of crack path via displacing fracture-prone stress concentration areas make finding the optimum designs challenging, where advanced computer-aided and machine learning-aided designing strategies are needed. In this study, we investigate a genetic algorithm (GA) guided material programming strategy to get a robust model with optimum toughness to moderate catastrophic failure of bi-material systems. The GA is a stochastic global search algorithm mimicking biological evolution. In GA, the design space is pixelized and each pixel is treated as a gene by assigning one type of materials. An augmented finite element method (A-FEM), which is a numerical efficient cohesive zone model (CZM) type method, is used to perform the tensile and fracture simulations to evaluate fitness score of each design. We leverage the GA operator of crossover to create a new generation of designs by switching parts of pixels based on the objective function of previous generation. A mutation ratio, which randomly changes pixels, is used to avoid local maximum to find the global optimization. We develop a dynamic selection of the mutation ratio regarding the fitness scores by generations to further enhance the efficiency of GA searching. Finally, samples are fabricated by 3D printing and tested by tensile experiments to validate the materials designs. The GA-AFEM method can be further applied for different loading conditions (impact, bending) and combinations of other materials.

3D printing technology has been recently utilized to print microbial cells for biotechnological and biomedical applications. Typically, microbes are embedded in well-established bioinks to print 3D architectures. In contrast to existing approaches, we have designed and developed the world’s first bioink, termed as Microbial Ink that is produced entirely from the genetically engineered microbes by a bottom-up approach. The shear-thinning property of the Microbial Ink coupled with its high viscosity, high yield-stress and shape fidelity facilitated the extrusion-based 3D printing into multi-layered architectures. The Microbial Ink was further programmed by using rationally designed genetic circuits in microbial cells to produce; 1) Therapeutic Living Material – On demand secretion of an anticancer drug, 2) Sequestration Living Material – Selective removal of toxic moieties and 3) Regulatable Living Material – To control the number of cells by inducing cell death. This work enables unprecedented avenues to manufacture macroscopic Functional Living Architectures and showcases advanced capabilities of Living Materials Technology.

The correct balance of forces on weight-bearing joints such as the hip and knee during joint replacement surgeries is essential for implant longevity. Minimising implant failure and the corresponding need for expensive and difficult revision surgery is vital to both improve patient quality of life and lighten the burden on overstretched healthcare systems. This is increasingly important, for example, in total hip replacements, where the average age of patients is decreasing; these implants must survive greater activity levels and correspondingly higher stresses than those typically found in older, less active patients. However, force balancing during total hip replacements is presently entirely dependent on surgical skill, as there are no sensors capable of providing quantitative force feedback within the small and complex geometry of the hip joint. Here, we solve this clinical unmet need by presenting a thin, conformable and flexible microfluidic force sensor, whose use can be easily incorporated into the standard surgical process. Such sensors were fabricated using a combination of stereolithography 3D printing and aerosol-jet printing. Using precise mechanical testing and finite element modelling, we calibrate the sensors and optimise their design and materials choices. We incorporated our novel sensor into a model hip implant geometry and, with a bespoke mechanical testing rig, determined that these sensors are operational and highly sensitive at force ranges typically achieved during hip implant positioning in surgery. We anticipate that these sensors will be used in the clinic to guide implant positioning and thereby increase the lifetime of existing hip replacements, and more generally represent a powerful new surgical aid for a range of orthopaedic procedures where force balancing is crucial.

3D printing has been actively studied for patient-specific devices due to its advantage for low-cost customization and rapid prototyping. However, most 3D printed devices have fixed shapes and properties, which are not suitable for situations where we need dynamic changes, especially medical implants that require surgery for replacement. Right ventricle–to–pulmonary artery (RV-PA) conduits are used as a surgical palliative treatment for various congenital heart diseases. Due to the growth of the infant or child, these conduits require replacement as they cannot grow, which involves several major open-heart surgeries.

To address this issue, we have investigated 3D-printed RV-PA conduits that “grow” through self-unfolding mechanisms triggered by flow and pressure change associated with growth so that fewer surgeries are required from infancy to adulthood. We will present our results for design of self-adaptable implants, followed by experimental results of testing 3D printed implant devices using an in-vitro testing set-up. The results showed that our self-adaptable implants can autonomously change shapes to accommodate the growth of children. We anticipate that our approaches and findings can contribute to improving patient welfare by customized designs with growth potential based on patient anatomy, which can minimize the number of required surgeries and associated danger, trauma, and expenses.

Acknowledgments: The research was supported by the Eunice Kennedy Shriver National Institute of Child Health & Human Development of the National Institutes of Health under Award Number R21HD090663 and the Johns Hopkins University Whiting School of Engineering Start-Up Fund. E. O. Bachtiar was supported by the Indonesian Endowment Fund for Education. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agencies.

Recent innovations in additive manufacturing of liquid crystalline elastomer (LCE) inks via direct ink write (DIW) have produced actuators capable of programmable performance based on the tailorable actuation temperatures of their constituent inks. Approaches to date utilize polymer network optimization to control actuation onset temperatures and overall strain of DIW LCE. Here, we describe a molecular engineering approach to 3D printed LCE devices that are capable of producing high strains at or below body temperature and room temperature. To do so, we utilize the LCE mesogen C6BAPE in DIW LCE formulations, which we have previously shown to substantially reduce actuation temperature (T_NI) and increase rate of actuation. The rheological and phase transition properties of C6BAPE-based DIW LCE inks are explored to minimize smectic character conferred by C6BAPE and to maximize actuation output across a range of low temperatures. Resulting materials have been demonstrated with T_NI that are responsive across biological temperature ranges that may enable numerous biomaterial application opportunities.

Bioelectronic interfaces require intimate and prolonged functional communication with soft, wet, and dynamic biological tissues or organs. However, conventional bioelectronic devices based on engineering solids are dissimilar to biological tissues in their mechanical and chemical properties, and therefore, can face limitations in long-term functional interfacing and biocompatibility due to unfavorable foreign body responses. Clearly noticeable in the progressive advances in bioelectronic interfaces in the recent decades, bioelectronic interfaces have been continuously innovated in both designs and materials to achieve tissue-matching properties to minimize foreign body responses while providing desired electrophysiological functionalities such as recordings and stimulations. Owing to the unique tissue-matching properties, hydrogels have been one of the most promising materials to provide biocompatible long-term interfacing with biological tissues. In this talk, we introduce our recent advances on the 3D-printed hydrogel bioelectronic interface consists of highly stretchable and soft hydrogels. The unique set of 3D-printable hydrogel inks allows facile and flexible one-step additive manufacturing of bioelectronic interfaces with diverse designs. To validate tissue-matching properties and biocompatibility of the 3D-printed hydrogel bioelectronic interfaces, we perform systematic mechanical and electrical characterizations and in vivo evaluations based on rat models. The resulted hydrogel bioelectronic interfaces will not only provide a promising platform for bioelectronic devices and treatments but also inspire the future development of functional hydrogel devices and machines.

Anatomically and biologically relevant vascular models are critical to progress our understanding of cardiovascular diseases (CVDs) that can lead to effective therapeutics. Recent advances in 3D printing employ techniques such as sacrificial molding, direct ink writing, coaxial bioprinting, embedded bioprinting to 3D-print the intricate vascular architecture. Despite advances in the 3D-bioprinting, however, there is yet a single fabrication technique that (1) enables a wide array of anatomical mimicry (i.e., freestanding, branching, multilayered, perfusable, mechanically stretchable), and (2) enables customizability of the employed bioinks (i.e., improving compatibility with relevant cell types and mimicking the mechanical properties of native vessels). In this work, we highlight a technique capable of fabricating anatomical accurate vascular models (i.e., freestanding, branching, multilayered, perfusable, mechanically stretchable) with the option of customizing the bioink blend to improve compatibility with relevant cells and the mechanical properties. Briefly, this fabrication technique relies on a two-part mold that was 3D-printed using porous hydrogels (i.e., poly(ethylene glycol) diacrylate (PEGDA) hydrogel). These molds can be designed with desired shape (i.e., bifurcating, construction) or derived from computerized tomography (CT) scan of native vasculatures. Before casting, the molds were then soaked in calcium chloride solution to allow the calcium ions to diffuse into the PEGDA matrix. When alginate-containing precursor solution was perfused into the mold cavity, the calcium ions in the PEGDA matrix diffused radially inwards. The diffusion of calcium ions into the mold cavity prompted ionic crosslinking of the alginate in the precursor, forming a tubular construct. To prevent the further curing of alginate, a buffer solution was perfused through the mold to remove the uncrosslinked alginate-containing precursor. Finally, the vessel construct was removed from the mold and exposed to ultraviolet (UV) light to photo-crosslink the photocurable monomer (e.g., gelatin-methacryloyl (GelMA)) present in the precursor. Applying our proposed technique, vessel constructs with anatomically accurate geometries (i.e., constructions, bifurcation) and mechanical stress (i.e., cyclical motion) were readily fabricated. Our technique also permitted the fabrication of multilayered architecture mimicking native blood vessels. Importantly, we successfully fabricated cell-laden vessel constructs with smooth muscle cells and endothelial cells in a multilayer arrangement. Lastly, we demonstrate the capability of our platform to simulate the insertion of a drug-eluting stent, highlighting a possible preclinical model to test future percutaneous coronary interventions. Overall, we developed a fabrication technique capable of fabricating a freestanding cell-laden hydrogel vascular model with high anatomical relevance: freestanding, branching, multilayered, perfusable, mechanically stretchable. Furthermore, the hydrogel composition of the vessel wall can be specifically tuned for mechanical and biological functions. We believe that these vasculature models with increases biomimicry can benefit future research in mechanistic understanding of CVD progress and the development of therapeutic intervention.

The ability to three-dimensionally interweave biological and functional materials could enable the creation of devices possessing personalized geometries and functionalities. Indeed, interfacing active devices with biology in 3D could impact a variety of fields, including biomedical devices, regenerative biomedicines, bioelectronics, smart prosthetics, and human-machine interfaces. Biology, from the molecular scale of DNA and proteins, to the macroscopic scale of tissues and organs, is three-dimensional, often soft and stretchable, and temperature sensitive. This renders most biological platforms incompatible with the fabrication and material processing methods that have been developed and optimized for functional electronics, which are typically planar, rigid, and brittle. A number of strategies have been developed to overcome these dichotomies. Our approach is to utilize extrusion-based multi-material 3D printing, which is an additive manufacturing technology that offers freeform, autonomous fabrication. This approach addresses the challenges presented above by (1) using 3D printing and imaging for personalized device architectures; (2) employing ‘nano-inks’ as an enabling route for introducing a diverse palette of functionalities; and (3) combining 3D printing of biological and functional inks on a common platform to enable the interweaving of these two worlds, from biological to electronic. 3D printing is a multiscale platform, allowing for the incorporation of functional nanoscale inks, the printing of microscale features, and ultimately the creation of macroscale devices. This blending of 3D printing, functional materials, and ‘living’ inks may enable next-generation 3D printed devices.

Additive manufacturing (AM) of metal components has emerged as a uniquely powerful tool for rapid prototyping and creating parts with complex geometries. We explore AM enabled by vat polymerization, typically used to produce polymers and ceramics, for the fabrication of metals and metal alloys. This technique can simultaneously achieve high resolution and high throughput, making it a promising candidate for practical industrial use.

We developed a general method for the fabrication of a wide variety of metals and alloys with complex shapes and overall dimensions of several mm via vat polymerization AM and subsequent post-processing and thermal treatment. This streamlined technique makes use of a single resin composition and a single set of processing conditions during the vat polymerization process, followed by infusion of appropriate metal precursors into a hydrogel structure. Heat treatment in oxidizing followed by reducing atmospheres converts the polymer/precursor matrix into the target metal. Unlike previous vat polymerization strategies which have target materials or precursors incorporated into the resin during the printing process, this method does not require re-optimization of resins and resin curing parameters when the target material is changed.

We report photoresin design and post-processing strategies to show how to use this technique to easily fabricate a range of AM metal and alloy structures. As a proof of concept, we fabricated octet lattice architectures with beam diameters on the order of 50 µm from several materials including copper, nickel, silver, cobalt, cupronickel alloys, and tungsten. We characterize these materials to show the unique microstructures enabled by our technique and link them to mechanical properties measured by nanoindentation. Our approach represents a paradigm shift in vat polymerization AM of functional materials by reducing the experimental burden on resin design and allowing the target material to be selected after the structure is fabricated.

Patent Pending

Powder Bed Fusion (PBF) is an important process in polymeric additive manufacturing. However, there exist limited polymers available for PBF, where most efforts use nylon to create object using PBF. In an effort to expand the polymers available for PBF and to provide additional insight into the sintering process, we have utilized a relatively facile method, thermally induced phase separation (TIPS), to synthesize polypropylene (PP) powders for PBF. More specifically, using TIPS, polymeric microspheres from PP with molecular weights of 12k, 250k and 340k, and their blends, have been formed. Specific conditions of the TIPS process, including polymer concentration and precipitation temperature were controlled to fabricate spherical PP powders with smooth surface morphologies, and appropriate particle size that is suitable for PBF (between 45µm and 100µm). Interpretation of the temperature, solution concentration, and molecular weight dependence of particle size that emerge from the TIPS process provides important fundamental understanding into the phase separation processes that govern particle formation. This in turn offers insight that will enable the rational design of thermodynamic conditions required to create polymer powders that are suitable for powder bed fusion for a broad range of polymers using the TIPS process.
At the same time, coalescence in PBF occurs in stationary particles, and thus the rate of weld formation is related to the zero-shear viscosity of the polymer melt. Given the fundamental importance of polymer diffusion in particle coalescence in PBF, we are interested in examining the impact of polymer molecular weight, and thus zero-shear viscosity, on the success of the fabrication of structures by PBF. The production of PP powder that is suitable for PBF with varying molecular weight provides an experimental system to examine this relationship, which is underway. Thus, we will discuss how the TIPS process opens clear routes to fabricate PBF powders with controlled zero-shear viscosity, providing pathways to examine and more thoroughly optimize the complexities of the coalescence process in PBF printing.

Thin metal-dielectric-metal (MDM) structure, a simple Fabry-Perot cavity, is highly desirable for structural color applications due to its simple structure and ability to reflect strong visible colors. However, conventional fabrication methods typically require high vacuum processes such as physical vapor deposition, magnetron sputtering, which significantly limits their practical use, especially for large-scale production due to the high cost and long processing time. Herein, we developed a series of solution-based methods for thin layer deposition of MDM structural color aiming at similar quality to cost-ineffective vacuum processes. We chose Cu-SiO₂-Cu as our model system due to its easy accessibility and low cost. Electroless plating of copper is implemented for the top and bottom metal while an electrochemical-assisted sol-gel process is adopted for the middle silica deposition. A systematic investigation is carried out for each deposition layer to ensure good film quality as well as its compatibility with all previous layers. Precise control of the silica precursor concentration, deposition time, deposition potential, and pH allow for a wide dielectric (SiO₂) thickness range, from sub-100nm up to 400nm thickness. Due to the low refractive index of silica, attractive iridescent structural colors are produced. The spectroscopic and morphological qualities of these solution-deposited structural colors are then compared with conventional vacuum-deposited samples for color performance. Subsequent work will utilize solution-based thin film deposition methods to deposit conformal thin-film structural color on curved surfaces, which is considered difficult for conventional deposition methods. Further exploration of solution-based deposition of other thin metals and dielectrics depositions will be carried out to expand the material library. We believe that these solution-based thin film-based methods can advance practical implementations of structural color fabrication.

Additive manufacturing enables game-changing fabrication capabilities for customized, low-cost, and multi-functional devices. However, process-specific fabrication anomalies can result in non-intuitive variations in geometry and function, especially for electromagnetic applications. To address this challenge, we implemented a machine learning methodology to map between geometric variances and their impact on electromagnetic performance, and further characterized the underlying defect mechanisms of the print. Using a direct-write additive manufacturing approach with a conductive Ag thermoplastic polyurethane ink, we printed a frequency selective surface (FSS) consisting of ~4000 Archimedean spirals in a hexagonal-packed array. Each spiral was optically imaged and then simulated as a FSS in a computational electromagnetic solver, which resulted in a paired geometric and electromagnetic performance dataset. Dimensionality reduction was applied to both datasets, which revealed 6 distinct classes of geometry defects and a strong dependence of the electromagnetic data manifold on the resonance frequency of the FSS. To map between the two domains – geometry and electromagnetic performance – convolutional neural networks were used to predict electromagnetic performance from images of the spiral geometry. Two convolutional neural networks (CNN) were explored: one using the image-derived geometric description, and one using the geometric description plus surface current information from the simulation. Network latent space dimensional analysis showed that the neural networks organized the geometric input information according to both geometric and electromagnetic values even in the case with no seeded electromagnetic information. Collectively, these results highlight the potential of CNNs to serve as low-cost surrogate models for in-line feedback predictions of electromagnetic performance from as-printed geometry, and highlight the utility of manifold learning techniques to provide a lens into the mechanisms and distribution of defects in additive manufacturing processes.

Advances in additive manufacturing (AM) have enabled the fabrication of novel materials with high stiffness & strength and simultaneously low weight, but realizations of these lightweight materials beyond this quasi-static and small-deformation regime have remained scarce. For instance, few additively manufactured materials can achieve extreme deformations without accumulating permanent damage, and their stiff mechanical properties prevent their application in regimes where compliance is required (e.g., tissue engineering). At the other extreme, little is known about these materials’ performance under dynamic stimuli such as impact. To enable emerging applications of additively manufactured materials, new design and fabrication paradigms are required to achieve materials beyond the classical small-deformation parameter space.

To this end, we employ a two-photon lithography process to fabricate two types of material systems that independently achieve (i) extreme extensibility (and compliance), and (ii) extreme impact mitigation. First, we present a novel design paradigm for AM-materials that consists of ‘woven’ monolithic fibers which bypasses the need for nodes or joints between structural elements. Performing in situ nanomechanical experiments, we demonstrate that woven architecture gives rise to compliant mechanical properties, extensibility of up to twice the elastic strain limit of the constituent material, and cyclic compression to 35% strain with minimal damage accrual. Second, we present additively manufactured carbon materials with nanoscale features (consisting of 13,500 unit cells) that attain extreme energy dissipation upon supersonic microparticle impact. Here, we subject polymeric precursor micro-architected materials to a pyrolysis process of 900C in vacuum, resulting in isotropically shrunk carbon nano-architected materials. Using a laser-induced particle impact technique, we showcase the potential for architected materials at the nanoscale to serve as lightweight shielding materials, dissipating up to 70% more energy than Kevlar for the same mass-normalized impact conditions. Together, these works showcase new design paradigms and mechanical property spaces capable of opening new frontiers for additively manufactured materials.

Currently there is still a pressing need to develop new printable resins which are comparable with the state-of-the-art micro-/nano- printing techniques for a fast 3D fabrication of desired structures (for example, hierarchical nanoporous structure) with high shape-fidelity and improved properties. Herein, we report a new type of resin solution for the efficient fabrication of 3D metal-carbon hierarchical nanoporous structures which utilizes the atomically precise metal nanoclusters as two-photon initiators, nano-porogens and Reinforcing fillers for enhanced mechanical properties. Our resin shows high fabrication efficiency and high shape fidelity for various complex 3D metal structures with line widths down to ~200 nm can be obtained. Furthermore, 3D nanoporous metal-carbon structures are obtained after annealing and improved mechanical properties are obtained based on the nanoindentation and micro-compression testing. Overall, this work provides a new resin solution which utilizes the unique light-matter interactions and intrinsic chemicalphysics properties of ultrasmall metal nanoclusters for the efficient nanoscale 3D printing with high-fidelity and improved properties achieved in the final structures.

Cell viability, proliferation, and tissue formation are significantly influenced by the physical cues of local microenvironments, i.e., extracellular matrix (ECM) elasticity directs stem cell differentiation. Proper scaffold design would help identify the mechanisms of cell-ECM interactions in three-dimensional environments. A key unsolved challenge has been the lack of quantifiable mechanical cues that could guide the fabrication of physiologically relevant scaffolds with high architectural complexity and anisotropy. To date, matching the mechanical property landscape within a scaffold has relied on trial-and-error techniques and topology optimization, which are computationally and experimentally expensive, sensitive to the initial guess, and incapable to explore the entire design space.

In this work, we fabricated and characterized biomimetic 3D scaffolds with tunable anisotropic stiffness predicted by an artificial intelligence-enabled model. We generated spinodal-like topologies via a machine learning-based technique and fabricated the computed structures out of IP-Dip photoresist with critical dimensions of 2.3μm and overall sample dimensions of 70μm ×70μm ×70μm using two-photon lithography (TPL). The SEM images of the samples showed that they could geometrically resemble the porous structure of natural tissue environments, for example trabecular bone. Quasi-static uniaxial compression experiments, conducted at a strain rate of to a strain of 10^-3 s^-1, revealed the normalized elastic moduli of the samples in e₁, e_2, and e₃ directions as 1.000 ± 0.097, 0.354 ± 0.042, and 0.619 ± 0.100, respectively. The results matched closely with the numerically computed prediction that the normalized elastic moduli of the sample in e₁, e_2, and e₃ would be 1.000, 0.360, and 0.621.

Through these experiments and computations, we successfully demonstrate that this spinodoid model can accurately predict direction-dependent stiffnesses, with high tunability and flexibility, rendering it possible to systematically investigate ECM environments in various tissues. This approach could help expand the parameter space of mechanobiology research.

The objective of this research work is to design, develop and fabricate microchannel reactor with complex features using additive manufacturing (AM) for carbon capture and utilization. Fabrication parameters of selective laser melting were optimized to improve fabrication quality. The designs were optimized based on simulation results from computational fluid dynamics. AM technologies will provide an opportunity for enhanced design and superior heat transfer performance. In this study, innovative Manifold-microchannel reactor or heat exchanger (MMHX) are designed and fabricated starting from the study of fluid flow in single channel reactor to multichannel with complex features and configurations. An experimental instrument was designed to measure pressure drop and heat transfer in AM designed MMHXs with a wide range of Reynolds numbers. Measurement results demonstrated that in some scenarios adding more internal features, i.e. increasing design complexity, does not necessarily improve the MMHX performance. This fluid flow study provides a guidance for additively manufactured MMHXs design optimization from experimental perspective. The reactors were loaded with catalyst to hydrogenate carbon dioxide into methanol. The effect of twist angle of the channels on carbon utilization efficiency and selectivity were investigated preliminarily. Further study on balancing conversion efficiency with pressure drop due to complexity will be investigated.
MMHX studied here has become a favorite topic in various fields ranging from air conditioning to aircraft gas turbine engines. In essence, high manufacturing cost has emerged as the major drawback in broad applications of MMHXs. The possibility of cost-effective manufacturing has generated wide interest in applying additive manufacturing (AM) for fabricating MMHXs. The methodology established in this study will contribute to the development of other reactors using similar additive manufacturing methods.

The advances in additive manufacturing at the nanoscale drive the need to understand the role of microstructure and size effects in mechanical properties of constituent solids. Nanomechancial studies have shown the emergence of size effect in mechanical properties of materials, with small-is-stronger behavior in single crystals, smaller-is-weaker in nanocrystalline metals, and suppression of brittle failure in amorphous metallic glasses. In addition to this size effect that emerges in samples with sub-micron extrinsic dimensions, the intrinsic length scale of material microstructure also dictates the dominance of different deformation and failure mechanisms. While heavily studied in metallic systems, a dearth of studies exists on the contribution of nanocrystallinity and nanostructuring in ceramics.
Using a Photopolymer Complex Synthesis additive manufacturing technique, we created nanocrystalline zinc oxide (ZnO) pillars with diameters ranging from 250nm to 3μm and heights of 600nm to 9μm. We achieve these structures by first patterning zinc ion-containing aqueous photoresins using two-photon lithography and subsequently calcining them at 500°C to produce pure nanocrystalline ZnO. Through in-situ micro-compression experiments we show three distinct failure behaviors based on structure size: 1) splitting via crack propagation 2) shear off events and 3) non-catastrophic deformation of the structures. These three regions can be described through the energetic competition of different failure mechanisms, similar to those in metal and metallic glasses. TEM analysis of the microstructure and Molecular Dynamics simulations of nanocrystalline ZnO provide insights into the deformation mechanisms and help establish the link between the microstructural features and failure behavior. Investigation of this system highlights the interplay between traditional weakest link failure theory and stochastic localization of stress within the microstructure. Through these studies we show the effect of extrinsic size on failure behavior in nanocrystalline ZnO, reveal the role of intrinsic length scales in deformation and failure mechanisms, and lay the foundation for fundamental insight to designing metal oxide nano-architectures from a microstructural viewpoint, as well as emphasize the importance of microstructural investigation in micro- and nano-sized additively manufactured materials.

Shape memory alloy (SMA)-based smart structures have been long-awaited for comprehensive computational design tools. Generally, SMA-based smart structures are developed based on experimental or empirical designs. Their configurations vary drastically from application to application, and are limited in ability to achieve multifunctionalities with simple, generic geometries. Via the technique of topology optimization, we propose a computational design framework that enables programming the stimuli-responsive behaviors of SMA-based smart structures with multiple material candidates. Specifically, the proposed framework is able to program the target thermomechanical performance of the structure into its material distribution, by modeling nonlinear phase transformation of SMAs coupled with transient thermal conduction. The proposed design framework allows for achieving multifunctionalities such as bending, inchworm-like crawling, and twisting with simple geometries. Further, we study laser powder bed fusion (LPBF)-based 3D printing techniques to realize our designs. A series of 3D printed sample designs enabled by the framework including SMA-based stents and grippers are then discussed.

Additive manufacturing (AM) excels at fabricating structural materials in complex geometries, but electronics integration could transform these parts into sensors and systems, expanding 3D printing far beyond prototyping. Here we present a method for engineering conductive 3D mesostructures via low-temperature atomic layer deposition (ALD) and industrial microstereolithography (μSLA). μSLA was used to produce a range of 3D structures (cubes, spheres, etc.) composed of internal octet lattices (10 - 200 μm critical features) having ultrasmooth surface morphology, (r_q < 40 nm) enabling 3D deposition of ZnO, ZnO:Al, and SnO₂ films leveraging an Al₂O₃ seed layer to prevent growth inhibition.

Uniform growth of ultrathin layers (7 nm – 100 nm) was achieved over a wide range of temperatures up to the polymer T_g , allowing control of the films’ semiconducting or degenerate behavior and yielding resistance from 100 Ω to 100 kΩ for per dimensionless cubic volume. XRD studies of the 3D oxide coatings reveal the ability to grow both amorphous and crystalline films matching those of 2D ALD films. Similarly, XPS and EDS studies indicate the similar atomic composition of coatings on 3D polymers and 2D substrates. However, despite their similar material characteristics these 3D networks show significantly enhanced conduction vs. 2D films, stemming from their high surface area and parallel 3D conduction paths. This allows precision geometrical manipulation with μSLA to enhance volumetric conductance and engineer electronic transport in novel 3D devices. This process has significant and broad implications for designing 3D electronics that bridge the gap between nanoscale materials and 3D-printed mesostructures.

We have been investigating additive manufacturing across a range of applications in the consumer goods space at P&G. One commercial interest lies in creating tailored, personalized products. In this talk, I will review additive manufacturing technology approaches we have explored in the beauty products category, specifically, custom 3D beauty masks. I will cover scanning approaches, resolution requirements, a manufacturing path for creating custom shaped objects, application of custom chemistries and the benefits associated with custom shape applicators and chemistries. Our approach utilizes a combination of 3D scanning, additive manufacturing (3D printing) of disposable molds, thermoforming replicates, custom laser cutting, and custom application of chemistries to create a scaled approach of creating fully tailored, customized offerings. I also will cover business model considerations for offering such products and share some future directions of this research. I hope that this work serves as an example and case study of how additive manufacturing technologies can be applied in the consumer goods space and open the door for partnerships.

Natural materials are inherently constructed hierarchically from the molecular-level to the macroscale, which often possess light-weight porous scaffolds (e.g. bone), outstanding mechanical properties (e.g. shells and skins), as well as stimuli-responsive behaviors (e.g. muscles). My research group aims to develop 3D-printing materials that emulate natural systems’ multiscale and multifunctional integration across the nano-to-macroscale.
Currently, there is an emerging need to introduce smart molecular systems that lack the necessary molecular features for direct ink writing technique. This roadblock severely limits the number of smart materials available for 3D printing. To address this problem, my research group has developed a general method to integrate a variety of inorganic and organic molecules as functional 3D-printing inks through the introduction of micellar-based supramolecular templates.[1] In our hierarchical co-assembly enhanced direct ink writing design, molecular monomers and supramolecular templates co-assembled via multivalent noncovalent interactions to form a viscoelastic ink. After 3D printing, the molecular monomers and supramolecular templates in the 3D-printed monolith co-assembled in a holistic manner, affording monoliths with hierarchically ordered superstructures. In this process, a localized re-configuration of monomer and template was critical to maintaining the self-standing macroscale structure. Subsequent chemical crosslinking and template removal stapled these assembled molecular monomers together at the macroscale to afford the desired functional monoliths. 3D-printed mesoporous silica monoliths with a surface area of 304 m²/g were successfully fabricated. We further demonstrated that a molecular pillar insertion resulted in a macroscopic size expansion in the 3D-printed BTA monolith. The co-assembly process not only enhances the printing resolution by up to one order of magnitude but also enables precise spatial control of nanoscale features, such as molecular assembly, over a large scale. In the next step, we expanded this co-assembly-enabled 3D-printing method to fabricate highly ordered covalent organic frameworks (COFs) with imine and β-ketoenamine linkages.[2] The supramolecular template was introduced to coassemble with imine precursors in an aqueous environment. By limiting the degree of imine polycondensation during COF formation, co-assembled 3D-printable hydrogels were formed. After the removal of the template followed by an amorphous-to-crystalline transformation, the assemblies were converted to the first set of single- and multi-component 3D-printed covalent organic framework monoliths, possessing high crystallinity and large surface areas, as well as robust mechanical stability.

Reference:
[1] L. Li, P. Zhang, Z. Zhang, Q. Lin, Y. Wu, A. Cheng, Y. Lin, C. M. Thompson, R. A. Smaldone, and C. Ke, Angew. Chem. Int. Ed. 2018, 57, 5105 –5109.
[2] M. Zhang, L. Li, Q. Lin, M. Tang, Y. Wu, and C. Ke, J. Am. Chem. Soc., 2019, 141, 5154–5158.

Applications for polymer structures made using the two-photon polymerization additive manufacturing method, which produces structures with submicrometer resolution, are primarily optical or biological. Mechanical applications are less prevalent, partially because the structures’ properties depend on the processing parameters and structural design, and these are less extensively studied. This study investigates the performance of millimeter-size cellular auxetic structures (~0.2 g/cm³) subjected to impulse loadings at room and cryogenic temperatures (295 K and 130 K, respectively). Auxetic structures are an example of a metamaterial that is ideally suited for the deterministic production methods of additive manufacturing and can be purposely designed for specific applications.

The stress–strain relationship, creep behavior, and recovery response of structures made from a proprietary thermoset acrylate resin (IP-S^TM) under static and dynamic loading is reported. Different auxetic structures were designed using two finite-element programs, NX7.5 and ANSYS, to calculate the stress/strain distribution in the structure for different loads and to produce stereolithographic files for use with a commercial laser writer (Nanoscribe GmbH). These structures were tested using a thermal mechanical analyzer (PerkinElmer) at 295 K and 130 K, at loads ranging from 10⁵ to 10⁶ Pa, and at different cyclic frequencies. The compression modulus and strength of the best-performing structure were 2.5X and 2X higher, respectively, at 130 K and at 295 K. The maximum strain the structure could withstand under cyclic loading and recover was 2% and 0.8% at 295 K and 130 K, respectively; the maximum strain it could withstand before crushing irretrievably was 18%. The corresponding energy dissipated as strain is calculated to be ~86 J/kg.

This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and Development Authority.

Additive manufacturing technologies and techniques are becoming a more mainstream approach to prototyping, small-scale manufacturing, and one-off manufacturing especially in microelectronics and medical sciences. Two primary criteria are necessary for implementation of additive manufacturing techniques: resolution and scalability. One method of additive manufacturing that addresses these criteria is direct writing of three-dimensional structures by way of two-photon polymerization. Two-photon polymerization has the potential of being an incredibly useful tool for micro/nanofabrication due to the ability to fabricate with sub-wavelength resolution; however, the technique requires additional exploration. The equipment required to achieve two-photon polymerization provides the benefit of high resolution and is becoming more accessible for research groups lending to scalability through parallelization.

In this work, we demonstrate a process to additively fabricate microstructures on diced silicon chips with oxide. We describe the process of creating the polymerized resin as well as its application onto the target substrate. Parameters of the laser system are described. Pre-fabrication adhesion layers are also discussed and post-write procedures are described. Characterizations of the load shear failure mode at room temperature and post cryogenic thermal cycling at temperatures as low as 77 K are presented. Cryogenic thermal cycling shown in this work was accomplished by direct submersion in liquid nitrogen to assess the worst-case degradation due to thermal shock. A discussion of experimental results and a comparison of room temperature and thermal cycle data is provided. The reliability of the structures demonstrated in this work show promise for additional exploration for more complex structures at cryogenic temperatures.

Over years, additive manufacturing has gained growing interest and attention in applications where customization and cost-effectiveness are strategic features. In this frame, Ink-jet printing (IJP) has emerged as a potential process to be flanked by traditional microelectronics material deposition method, where less exigent restraints on resolutions are required. However, high resolution in the micrometric range can be achieved, making this technique attractive for microfabrication. Indeed, many devices feature microfabricated parts, as Micro-electro-mechanical systems (MEMS). Compared to more traditional surface patterning techniques, IJP is a contact-less, mask-less droplet-based technique able to deposit functional materials onto specific substrates. The possibility to finely control the thickness, by tuning the number of printed layers, combined with minimum material waste reveal the great versatility of this fabrication technique. However, major challenges of IJP are found on two levels. One is the long-term cartridges durability, as nozzle clogging is prone to occur. On the other hand, IJP requires precise ink rheological properties to guarantee flow under the application of a shear stress: low viscosity (i.e. 2-20 cP) and surface tension (i.e. 25-50 mN m^-1 ) ^[1,2].

IJP is also considered to be a key technology regarding polymers deposition. In particular, polymers are optimal candidate materials for vibration damping applications. Vibrations often represent a problem due to undesired motions and stresses, that can lead to fatigue, failure, degradation and reliability loss for a wide range of structures in microelectronics. As MEMS devices consist of mechanical parts, they are prone to experience harsh undesired vibrations too. There are two material classes for vibration control mechanism: active and passive damping. The most common passive damping material are Viscoelastic materials (VEMs), combining properties of elastic and viscous materials, where damping occurs by energy dissipation due to friction among oligomers chains under cyclic deformation. VEMs may be used as discrete dampers, absorbers or the surface of the structure can be fully or partially covered. As a matter of fact, a common viscoelastic treatment to control vibrations in a structure is the Free-layer damping (FLD) approach. The damping material is deposited on the structure surface, creating a damping layer. When the structure is deflected, vibrations are damped because of the energy dissipated following of the compression/extension of the damping material ^[3,4].

The aim of this work is to propose a simple VEM deposition through IJP on top of a spring in the MEMS device, exploiting the FLD approach. First, an extensive material selection has been performed to individuate an adequate polymeric material, with low elastic modulus that would not alter spring properties. The characterization of the optimized ink in terms of physical properties, to understand ink printability through considerations regarding the Ohnesorge diagram, will be presented. A jetting characterization will also be presented, evaluating the printability assessment as a function of the ink droplet speed. Eventually printed pattern characterization will be discussed either in terms of surface profilometry, microscopy and vibration damping ability.


[1] R. Bernasconi, M. C. Angeli, F. Mantica, D. Carniani, L. Magagnin, Polymer 2019, 185, 121933.
[2] E. P. Furlani, in Fundamentals of Inkjet Printing, John Wiley & Sons, Ltd, 2015, pp. 13–56.
[3] N. Choudhary, D. Kaur, J Nanosci Nanotechnol 2015, 15, 1907.
[4] R. Singh, A. Ghosh, R. Kant, M. Asfar, B. Bhattacharya, P. Panigrahi, S. Bhattacharya, Journal of Microelectromechanical Systems 2013, 22, 695.

Over the past two decades, single-walled carbon nanotubes (SWCNTs) attracted a lot of interests due to their outstanding electronic, physical, and structural properties. Which make them extremely promising candidates for high performance integrated circuits. On the other hand, the expanding of microelectronic large area sensors arrays, smart labels, and RFID tags enhanced the motivation of implementing these SWCNTs integrated circuits onto flexible and transparent substrates. However, large scale SWCNT development has been facing numerous challenges attributed to the difficulty of achieving homogeneously distributed nanotubes across large substrate surface area, as well as the complexity of the fabrication process, that in many cases affects the nanotubes stability as well as the maximum fabrication yield. Here, we report the fabrication of transparent large scale SWCNTs transistors and logic circuits on a four-inch scale wafer flexible substrate. Our approach implements directed fluidic assembly method to directly print SWCNTs over large scale on flexible and transparent substrates. This facilitates the fabrication of SWCNTs with high device density and yield without affecting the nanotubes stability compared to other fabrication techniques such as transfer printing or dry etching. Following this approach, the field effected transistors are fabricated with density of 1250 transistors per cm², with a yield of 97 %, whereas the on/off ratios for the fabricated devices was 0.5x10⁵ in average. This technique also allows the fabrication of various flexible integrated logic circuits using SWCNTs such as NOR gates, NAND gates, and inverters. We believe that this fabrication approach will be of a significant value in the development of flexible electronics for different application such as sensing, computing, and communications.

Ultralight mechanical metamaterials enabled by additive manufacturing (AM) have previously achieved density-normalized strength and stiffness properties that are inaccessible to monolithic materials, but the majority of this work has focused on static loading while the mechanical properties of these metamaterials under extreme dynamic loading conditions has remained largely unexplored. Properties such as energy absorption and the ballistic limit of these additively manufactured materials are of high interest for protective applications and recent works on their dynamic response have demonstrated the benefit of architecture for impact mitigation. Despite these promising results, limitations of current additive manufacturing processes at the macroscale have made thorough investigations of impact both time- and cost-inefficient, particularly if separation of scales between the periodic building block and the impactor is required.

Here, we use two-photon lithography as a rapid prototyping technique for microscopic polymeric microlattices, to systematically study their response to projectile impact. We fabricate lattice architectures of different morphologies (i.e., kinematically rigid and non-rigid architectures), with beam diameters ranging from ~1.0 to ~1.5 μm and unit cells of 7.5 μm, and we qualitatively and quantitatively characterize their response to microparticle impact. We employ the laser-induced particle impact test (LIPIT) method to accelerate ~30 μm-diameter microparticles to velocities of up to 800 m/s, and use ultra high-speed imaging of the impact process to measure impact energetics. To isolate the effect of architecture, we maintain a constant relative density (i.e., fill fraction) across prototypes and probe them with a range of impact energies. Lastly, we analyze our experiments in a dimensionless framework that allows application of these results to metamaterials at larger scales. This investigation provides a framework for the rapid design and characterization of future AM metamaterials for a variety of energy mitigation applications.

The objective of this research work is to design, develop and fabricate microchannel reactor with complex features using additive manufacturing (AM) for carbon capture and utilization. Fabrication parameters of selective laser melting were optimized to improve fabrication quality. The designs were optimized based on simulation results from computational fluid dynamics. AM technologies will provide an opportunity for enhanced design and superior heat transfer performance. In this study, innovative Manifold-microchannel reactor or heat exchanger (MMHX) are designed and fabricated starting from the study of fluid flow in single channel reactor to multichannel with complex features and configurations. An experimental instrument was designed to measure pressure drop and heat transfer in AM designed MMHXs with a wide range of Reynolds numbers. Measurement results demonstrated that in some scenarios adding more internal features, i.e. increasing design complexity, does not necessarily improve the MMHX performance. This fluid flow study provides a guidance for additively manufactured MMHXs design optimization from experimental perspective. The reactors were loaded with catalyst to hydrogenate carbon dioxide into methanol. The effect of twist angle of the channels on carbon utilization efficiency and selectivity were investigated preliminarily. Further study on balancing conversion efficiency with pressure drop due to complexity will be investigated.
MMHX studied here has become a favorite topic in various fields ranging from air conditioning to aircraft gas turbine engines. In essence, high manufacturing cost has emerged as the major drawback in broad applications of MMHXs. The possibility of cost-effective manufacturing has generated wide interest in applying additive manufacturing (AM) for fabricating MMHXs. The methodology established in this study will contribute to the development of other reactors using similar additive manufacturing methods.

The advances in additive manufacturing at the nanoscale drive the need to understand the role of microstructure and size effects in mechanical properties of constituent solids. Nanomechancial studies have shown the emergence of size effect in mechanical properties of materials, with small-is-stronger behavior in single crystals, smaller-is-weaker in nanocrystalline metals, and suppression of brittle failure in amorphous metallic glasses. In addition to this size effect that emerges in samples with sub-micron extrinsic dimensions, the intrinsic length scale of material microstructure also dictates the dominance of different deformation and failure mechanisms. While heavily studied in metallic systems, a dearth of studies exists on the contribution of nanocrystallinity and nanostructuring in ceramics.
Using a Photopolymer Complex Synthesis additive manufacturing technique, we created nanocrystalline zinc oxide (ZnO) pillars with diameters ranging from 250nm to 3μm and heights of 600nm to 9μm. We achieve these structures by first patterning zinc ion-containing aqueous photoresins using two-photon lithography and subsequently calcining them at 500°C to produce pure nanocrystalline ZnO. Through in-situ micro-compression experiments we show three distinct failure behaviors based on structure size: 1) splitting via crack propagation 2) shear off events and 3) non-catastrophic deformation of the structures. These three regions can be described through the energetic competition of different failure mechanisms, similar to those in metal and metallic glasses. TEM analysis of the microstructure and Molecular Dynamics simulations of nanocrystalline ZnO provide insights into the deformation mechanisms and help establish the link between the microstructural features and failure behavior. Investigation of this system highlights the interplay between traditional weakest link failure theory and stochastic localization of stress within the microstructure. Through these studies we show the effect of extrinsic size on failure behavior in nanocrystalline ZnO, reveal the role of intrinsic length scales in deformation and failure mechanisms, and lay the foundation for fundamental insight to designing metal oxide nano-architectures from a microstructural viewpoint, as well as emphasize the importance of microstructural investigation in micro- and nano-sized additively manufactured materials.

Shape memory alloy (SMA)-based smart structures have been long-awaited for comprehensive computational design tools. Generally, SMA-based smart structures are developed based on experimental or empirical designs. Their configurations vary drastically from application to application, and are limited in ability to achieve multifunctionalities with simple, generic geometries. Via the technique of topology optimization, we propose a computational design framework that enables programming the stimuli-responsive behaviors of SMA-based smart structures with multiple material candidates. Specifically, the proposed framework is able to program the target thermomechanical performance of the structure into its material distribution, by modeling nonlinear phase transformation of SMAs coupled with transient thermal conduction. The proposed design framework allows for achieving multifunctionalities such as bending, inchworm-like crawling, and twisting with simple geometries. Further, we study laser powder bed fusion (LPBF)-based 3D printing techniques to realize our designs. A series of 3D printed sample designs enabled by the framework including SMA-based stents and grippers are then discussed.

Additive manufacturing (AM) excels at fabricating structural materials in complex geometries, but electronics integration could transform these parts into sensors and systems, expanding 3D printing far beyond prototyping. Here we present a method for engineering conductive 3D mesostructures via low-temperature atomic layer deposition (ALD) and industrial microstereolithography (μSLA). μSLA was used to produce a range of 3D structures (cubes, spheres, etc.) composed of internal octet lattices (10 - 200 μm critical features) having ultrasmooth surface morphology, (r_q < 40 nm) enabling 3D deposition of ZnO, ZnO:Al, and SnO₂ films leveraging an Al₂O₃ seed layer to prevent growth inhibition.

Uniform growth of ultrathin layers (7 nm – 100 nm) was achieved over a wide range of temperatures up to the polymer T_g , allowing control of the films’ semiconducting or degenerate behavior and yielding resistance from 100 Ω to 100 kΩ for per dimensionless cubic volume. XRD studies of the 3D oxide coatings reveal the ability to grow both amorphous and crystalline films matching those of 2D ALD films. Similarly, XPS and EDS studies indicate the similar atomic composition of coatings on 3D polymers and 2D substrates. However, despite their similar material characteristics these 3D networks show significantly enhanced conduction vs. 2D films, stemming from their high surface area and parallel 3D conduction paths. This allows precision geometrical manipulation with μSLA to enhance volumetric conductance and engineer electronic transport in novel 3D devices. This process has significant and broad implications for designing 3D electronics that bridge the gap between nanoscale materials and 3D-printed mesostructures.

Over years, additive manufacturing has gained growing interest and attention in applications where customization and cost-effectiveness are strategic features. In this frame, Ink-jet printing (IJP) has emerged as a potential process to be flanked by traditional microelectronics material deposition method, where less exigent restraints on resolutions are required. However, high resolution in the micrometric range can be achieved, making this technique attractive for microfabrication. Indeed, many devices feature microfabricated parts, as Micro-electro-mechanical systems (MEMS). Compared to more traditional surface patterning techniques, IJP is a contact-less, mask-less droplet-based technique able to deposit functional materials onto specific substrates. The possibility to finely control the thickness, by tuning the number of printed layers, combined with minimum material waste reveal the great versatility of this fabrication technique. However, major challenges of IJP are found on two levels. One is the long-term cartridges durability, as nozzle clogging is prone to occur. On the other hand, IJP requires precise ink rheological properties to guarantee flow under the application of a shear stress: low viscosity (i.e. 2-20 cP) and surface tension (i.e. 25-50 mN m^-1 ) ^[1,2].

IJP is also considered to be a key technology regarding polymers deposition. In particular, polymers are optimal candidate materials for vibration damping applications. Vibrations often represent a problem due to undesired motions and stresses, that can lead to fatigue, failure, degradation and reliability loss for a wide range of structures in microelectronics. As MEMS devices consist of mechanical parts, they are prone to experience harsh undesired vibrations too. There are two material classes for vibration control mechanism: active and passive damping. The most common passive damping material are Viscoelastic materials (VEMs), combining properties of elastic and viscous materials, where damping occurs by energy dissipation due to friction among oligomers chains under cyclic deformation. VEMs may be used as discrete dampers, absorbers or the surface of the structure can be fully or partially covered. As a matter of fact, a common viscoelastic treatment to control vibrations in a structure is the Free-layer damping (FLD) approach. The damping material is deposited on the structure surface, creating a damping layer. When the structure is deflected, vibrations are damped because of the energy dissipated following of the compression/extension of the damping material ^[3,4].

The aim of this work is to propose a simple VEM deposition through IJP on top of a spring in the MEMS device, exploiting the FLD approach. First, an extensive material selection has been performed to individuate an adequate polymeric material, with low elastic modulus that would not alter spring properties. The characterization of the optimized ink in terms of physical properties, to understand ink printability through considerations regarding the Ohnesorge diagram, will be presented. A jetting characterization will also be presented, evaluating the printability assessment as a function of the ink droplet speed. Eventually printed pattern characterization will be discussed either in terms of surface profilometry, microscopy and vibration damping ability.


[1] R. Bernasconi, M. C. Angeli, F. Mantica, D. Carniani, L. Magagnin, Polymer 2019, 185, 121933.
[2] E. P. Furlani, in Fundamentals of Inkjet Printing, John Wiley & Sons, Ltd, 2015, pp. 13–56.
[3] N. Choudhary, D. Kaur, J Nanosci Nanotechnol 2015, 15, 1907.
[4] R. Singh, A. Ghosh, R. Kant, M. Asfar, B. Bhattacharya, P. Panigrahi, S. Bhattacharya, Journal of Microelectromechanical Systems 2013, 22, 695.

Over the past two decades, single-walled carbon nanotubes (SWCNTs) attracted a lot of interests due to their outstanding electronic, physical, and structural properties. Which make them extremely promising candidates for high performance integrated circuits. On the other hand, the expanding of microelectronic large area sensors arrays, smart labels, and RFID tags enhanced the motivation of implementing these SWCNTs integrated circuits onto flexible and transparent substrates. However, large scale SWCNT development has been facing numerous challenges attributed to the difficulty of achieving homogeneously distributed nanotubes across large substrate surface area, as well as the complexity of the fabrication process, that in many cases affects the nanotubes stability as well as the maximum fabrication yield. Here, we report the fabrication of transparent large scale SWCNTs transistors and logic circuits on a four-inch scale wafer flexible substrate. Our approach implements directed fluidic assembly method to directly print SWCNTs over large scale on flexible and transparent substrates. This facilitates the fabrication of SWCNTs with high device density and yield without affecting the nanotubes stability compared to other fabrication techniques such as transfer printing or dry etching. Following this approach, the field effected transistors are fabricated with density of 1250 transistors per cm², with a yield of 97 %, whereas the on/off ratios for the fabricated devices was 0.5x10⁵ in average. This technique also allows the fabrication of various flexible integrated logic circuits using SWCNTs such as NOR gates, NAND gates, and inverters. We believe that this fabrication approach will be of a significant value in the development of flexible electronics for different application such as sensing, computing, and communications.

Ultralight mechanical metamaterials enabled by additive manufacturing (AM) have previously achieved density-normalized strength and stiffness properties that are inaccessible to monolithic materials, but the majority of this work has focused on static loading while the mechanical properties of these metamaterials under extreme dynamic loading conditions has remained largely unexplored. Properties such as energy absorption and the ballistic limit of these additively manufactured materials are of high interest for protective applications and recent works on their dynamic response have demonstrated the benefit of architecture for impact mitigation. Despite these promising results, limitations of current additive manufacturing processes at the macroscale have made thorough investigations of impact both time- and cost-inefficient, particularly if separation of scales between the periodic building block and the impactor is required.

Here, we use two-photon lithography as a rapid prototyping technique for microscopic polymeric microlattices, to systematically study their response to projectile impact. We fabricate lattice architectures of different morphologies (i.e., kinematically rigid and non-rigid architectures), with beam diameters ranging from ~1.0 to ~1.5 μm and unit cells of 7.5 μm, and we qualitatively and quantitatively characterize their response to microparticle impact. We employ the laser-induced particle impact test (LIPIT) method to accelerate ~30 μm-diameter microparticles to velocities of up to 800 m/s, and use ultra high-speed imaging of the impact process to measure impact energetics. To isolate the effect of architecture, we maintain a constant relative density (i.e., fill fraction) across prototypes and probe them with a range of impact energies. Lastly, we analyze our experiments in a dimensionless framework that allows application of these results to metamaterials at larger scales. This investigation provides a framework for the rapid design and characterization of future AM metamaterials for a variety of energy mitigation applications.

Cellular silicone reinforced with silica filler prepared using additive manufacturing (AM) have been used widely for vibrational damping and shockwave mitigation. The two most commonly printed cellular silicone structures, simple cubic (SC) and face-centered tetragonal (FCT) display distinctly different static and dynamic mechanical responses dependent upon structure. In this work, the relationship between filler size and composition with mechanical response is investigated using polydimethylsiloxane-based silicones filled with aluminum oxide, graphite, or titanium dioxide. SC and FCT structures of porous, periodic silicone pads were printed using new direct ink write (DIW) resin formulations containing up to 25 wt.% of functional filler (TiO₂, Al₂O₃, or graphite). All AM pads were characterized using mechanical techniques (DMA, compression). Dynamic compression experiments coupled with time-resolved X-ray phase contrast imaging were performed to obtain insights into role of filler interactions in the in situ evolution of shockwave coupling in these functional, periodic porous polymers.

We report a stereolithographic three-dimensional printing approach for polymeric components that uses a mobile liquid interface (a fluorinated oil) to reduce the adhesive forces between the interface and the printed object, thereby allowing for a continuous and rapid print process, regardless of polymeric precursor. The bed area is not size-restricted by thermal limitations because the flowing oil enables direct cooling across the entire print area. Continuous vertical print rates exceeding 430 millimeters per hour with a volumetric throughput of 100 liters per hour have been demonstrated, and proof-of-concept structures made from hard plastics, ceramic precursors, and elastomers have been printed. This novel High-Area Rapid Printing (HARP) methodology works in the presence or absence of O₂, which expands the scope of resin materials that can be utilized for both prototyping and manufacturing.

This talk presents a method that uses simulation to explore the large design space of direct-ink-write (DIW) structures. Research has shown that we can design porous, elastomeric structures whose mechanical responses are specifically tailored to a given application; however, without complete information about the design space, we are limited in what mechanical responses we can achieve. We used this method to expand our knowledge of the DIW design space by examining helicoidal structures in compression. We defined a parameter set and performed 90 simulations. The results suggest that changing the angle of rotation between layers (while keeping other parameters fixed) can increase the stress in compression by up to 300%. This raises the question: what other parameters can we define, and what effect will they have on the stress response? Porous elastomers are used for stress mitigation, in medical devices, and in soft robotics, all of which could benefit from precisely tuned mechanical responses. This method can facilitate optimizing for such applications.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-823652

Selective Laser Melting (SLM) is one of the additive manufacturing processes (AM) that has gained increasing attention because of its ability to produce parts with added functionality and theoretically any shape. In addition, the SLM process can fabricate a wider spectrum of materials like Al-based, Cu-based, Co-based, Fe-based, Ni-based, W-based, Ti-based, Ta-based, Mo-based alloys, etc. Several of these alloys show age hardening or so-called precipitation hardening behavior. The traditional idea of improving the mechanical property of these parts with natural aging or with artificial thermal treatment was also employed in these alloys using the SLM process. However, not alloys show significant improvement in their mechanical properties with the aging/precipitation hardening treatments. The present talk will discuss the processing of age/precipitation-hardenable alloys by SLM and their merit and demerits. In addition, the alloy systems that show a significant response to age/precipitation hardening will be highlighted and the scientific reasons behind this class of materials will be discussed in detail.

Bio-polymers and bio-degradable plastics have attracted a considerable attention in scientific and industrial fields as a solution for alleviating pending issues regarding petrochemical-based plastic wastes, carbon dioxide emissions, and depletion of fossil resources. Poly(lactic acid) (PLA), an aliphatic polyester generally derived from bio-resources such as plant starches including corn, cassava, and sugarcane, is one of the most essential bio-polymers for various industrial applications. However, it is well known that the applications of PLA are considerably limited when compared to the typical petrochemical-based polymers owing to its inherent characteristics such as brittleness. Accordingly, increasing the toughness of PLA, i.e., concurrently increasing both the tensile stress and elongation at break, remains as a major challenge. In this present study, the fully bio-based PLA blends with polydecalactone (PDL)-grafted cellulose copolymer (CgPD) were fabricated to enhance the tensile toughness of the PLA matrix. As a toughness enhancer, the CgPD was synthesized via the direct ring-opening polymerization (ROP) of ε-decalactone (ε-DL), which is known as renewable and bio-degradable alkyl-branched lactone derivatives. The blending ratio was controlled by varying the amount of CgPD from 1 wt% to 10 wt% relative to that of PLA matrix, and the formation of thermoplastic film was performed by solution casting and subsequent melt-quenching methods under 4500 psi at 190 ^oC. The prepared bio-blends were found by FT-IR and solid-state ¹H NMR to be physically intact and miscible at the sub-twenty-nanometer scale. The WXRD and DSC analyses indicated that the addition of the alkyl-branched CgPD imparts a more structurally disordered PLA mesophase state to the prepared PLA_CgPD bio-blends. UTM analysis was used to characterize the macroscopic mechanical properties of the PLA_CgPD bio-blends. Both the tensile strength and elongation properties were simultaneously improved with the addition of 1 wt% CgPD loading amount to PLA (PLA_CgPD1). This study experimentally demonstrates that the enhanced mechanical properties of PLA_CgPD1 are closely related to the existence of more ordered PLA mesophases induced by the introduction of an optimal amount of CgPD additive into the PLA matrix. Consequently, the proposed strategy in this study presents the possibility of designing a fully bio-based polymeric blend materials with exceptionally improved mechanical properties.

As demand for high-performance composite materials rises, their management at end-of-life (EOL) must be considered more carefully. Bio-based polymers and composites have gained interest as global environmental awareness has heightened and governments and industries alike have put forth efforts to lower their carbon footprint. Identifying efficient routes for recycling of bio-based composites and establishing their application space after reprocessing will make these materials even more environmentally friendly. Additive manufacturing (AM) has also gained attention as a low-waste and highly flexible technique in which bio-based polymers and composites can be readily used. High-throughput, large area AM is a relatively new and fast-growing area of research, and the recyclability of these parts is important to establish considering the scale of parts and amount of material used. As such, the effect of recycling on additively manufactured poly(lactic acid) (PLA)/wood flour (WF) composites was studied. Parts were printed on the Big Area Additive Manufacturing (BAAM) system at Oak Ridge National Laboratory. The parts were size reduced using a water jet cutter and fed into a shredder/granulator. The obtained granulate was then fed directly back into the BAAM for reprinting without the need for repelletization. The thermomechanical performance of virgin and recycled parts was then characterized and compared in order to determine the impact of mechanical recycling on PLA/WF parts produced by AM. Mechanical recycling and direct reprinting of EOL AM parts was demonstrated as a cost-effective pathway for converting used bio-based composites into new products.

In recent years, environmental issues regarding the disposal of conventional synthetic plastics derived from petrochemical resources have emerged as a major issue. Thus, the development of bio-based plastics has received significant scientific interest over the past decades. Among the various natural resources, cellulose, which constitutes the main skeletal component of plants, is the most abundant natural polymer on the planet. However, it is well known that cellulose has a low solubility in most organic solvents, in addition to a poor melt processability owing to their intrinsic supramolecular hydrogen bonding properties and chain rigidity. Therefore, it is necessary to the develop of methodology capable of controlling molecular structure, flexibility, and stretchability of cellulose by imparting an effective plasticizing structure.
In this study, we developed a highly stretchable internally- and externally-plasticized (i.e., dually-plasticized) cellulose, which is composed of a polydecalactone-grafted cellulose copolymer (CgPD) and a hyperbranched polycaprolactone external plasticizing additive (HPC). To investigate the effect of branching structures of polycaprolactone-based external plasticizing additives on the plasticization capability, a linear polycaprolactone (LPC) and a star-shaped polycaprolactone (SPC) plasticizing additives were prepared as control groups to HPC. It is observed that dipolar interactions existed between CgPD and external plasticizing additives in the dually-plasticized cellulosic materials (i.e., CgPD_LPC, CgPD_SPC, and CgPD_HPC). Especially, among the dually-plasticized cellulose samples, the CgPD_HPC exhibited the lowest apparent activation energy (E_a= ~128.8 kJ mol^-1) required to achieve the sufficient plasticizing behavior, which resulted in an enhanced elongation at break value (ε = ~136%) compared to those of the CgPD (ε = ~75%), whereas CgPD_LPC and CgPD_SPC displayed severely deteriorated values (ε = ~25 and ~59%). This deterioration of mechanical properties may be attributed to the induction of structural discontinuity derived from the recrystallized polycaprolactone in the dually-plasticized celluloses. Therefore, it can be concluded that the highly-branched architectures as well as the structural uniformity between the internal- and external-plasticizing components play a critical role in determining the plasticization behavior.

This work introduces a morphological simulation of rapid solidification process deposited structures on 2-D planar sections, modeled as Apollonian packs of generalized ellipsoidal domains growing out of nuclei and dendrite arm fragments. The model employs descriptions of the dynamic thermal field based on superposed Green’s/Rosenthal functions with source images for initial/boundary effects, along with alloy phase diagrams and classical solidification theory for nucleation and fragmentation rates. Initiation of grains is followed by their free and constrained growth by adjacent domains, represented via potential fields of level set methods, for effective mapping of the solidified topology and its metrics, i.e. apparent grain size and fractal dimension of densely-packed domains. The model parameters are calibrated and its predictions are validated against micrographs of three Mg-Zn-Y alloys deposited by uniform droplet spraying for additive manufacturing.
Further evolution of this real-time computational model and its application as a process observer for feedback control in 3D printing processes, as well as for off-line material design and optimization, is also summarized.

Post-processing of additively manufactured (AM) 316L stainless steel parts printed via laser powder bed fusion (L-PBF) is required for removing support structures and to improve surface finish. Various novel processes have been recently developed to remove supports and/or improve surface finish on the entire part without requiring intensive manual labor. One technique, a self-terminating etching process, removes supports and smooths surfaces; another technique that improves surface finish involves a stepwise application of a chemical conversion coating and subsequent mechanical removal of material. The corrosion response of AM 316L stainless steel parts having undergone these novel processes is not fully understood. This puts into question their ability to be reliably used in real-world environments and limits their practical application. To date, most corrosion studies of L-PBF 316L materials have been limited to parts in the as-built condition, with preliminary data showing that L-PBF 316L stainless steel's corrosion response differs significantly from conventionally manufactured counterparts. Additionally, the corrosion response of L-PBF specimens has been found to change based on print orientation with respect to the build platform and also vary with post-processing heat treatments that impact microstructure. This work evaluates AM 316L stainless steel samples in the as-built condition and after they have undergone post-processing techniques to show how printing parameters, build orientation, and novel post-processing treatments impact corrosion response. Electrochemical data was combined with metallographic analysis to connect corrosion mechanisms to the underlying microstructure and, ultimately, the processing conditions.

Rapid prototyping using additive manufacturing techniques has increased the need for faster methods to characterize the fatigue behavior of 3D-printed alloys. Fatigue testing instruments utilizing ultra-sonic (20kHz) sample resonance enable testing to 10 ⁷ cycles in about 10 minutes versus over 10 hours using standard instrument designs. Ultra-sonic fatigue measurements also allow sample testing to extremely high cycles (10¹⁰) in a reasonable timeframe allowing insight to material failure behavior beyond the traditional “fatigue limit” of 10⁷ cycles.
This paper reviews the experimental considerations necessary for successful USF measurements and provides examples of USF measurements of 3D printed alloys.

There is high interest in smart and nanostructured materials based on the use of chemical and nanoparticle additives that enable structure-composition-property correlation towards their targeted application. 3D printing enables a convergence of optimized materials and geometrical design which can be further fine-tuned by the effective use of the 3D printing parameters: flow, slicing, temperature, adhesion, etc. There is the possibility of using artificial intelligence and machine learning (AI/ML) methods including simulation to optimize these properties such that valuable feedback can be obtained for optimized processing both during and post-printing. Herein, we focus on developing thermoset polymer materials to enable high performance for thermo-mechanical properties and eventual light-weighting applications. These include the following projects: 1) Thermoset elastomers based on VSP process emulsions, 2) Shape-memory polybenzoxazine materials that are highly stable, and 3) High-performance epoxy-carbon fiber materials and ideal light-weighting properties. The focus on thermosets enables materials development independent of melt-processability and using of photo-curing methods enabling the use of both chemical and physical crosslinking for curing modes. The VSP methods when applied at room temperature followed by post-printing curing methods including thermal treatment enable a more versatile method compared to high-performance polymer (HPP) FDM 3D printing methods.

Disordered or irregular microstructures may demonstrate extraordinary mechanical characteristics when short-range phenomena, including interfacial (2D) and nanoscale confined (3D) interactions, are tailored to govern the overall collective characteristics. Since the lack of lattice symmetry and statistical isotropy can allow scalable additive manufacturing as well as defect insensitivity in manufacturing, irregular metamaterials have been gaining greater attention in recent years. Because the range of mechanical exchanges is highly localized for the mechanical stimuli of high strain rates (HSRs), the irregular mechanical metamaterials can be more effective compared to ordinary regular mechanical metamaterials.
In this respect, we have investigated irregular solid-liquid hierarchical structures having characteristic lengths ranging from micro to nanometers. In their two-step additive manufacturing processes, first, polystyrene spherical nano-shells are stacked with uniform thicknesses using self-limiting electrospray deposition (SLED). Second, the nanoscale interstitial space of the PS structure are subsequently backfilled with weakly-crosslinked polydimethylsiloxane to introduce nanoscale interactions found between solid and viscous liquids to the system. To observe HSR viscoelastic responses of the microstructures, a single silica microsphere (approximately 20 µm diameter) is accelerated up to 500 m/s incident to the sample using laser-induced projectile impact test (LIPIT). Kinetic energy dissipation of the speeding microsphere during its collision with the metamaterial is quantified from the ultrafast stroboscopic microscopy. The correlations of the energy dissipation with a projectile’s impact energy and viscoelasticity of the PDMS filler are demonstrated and compared with low-strain-rate indentation results. This study provides an understanding of the role of solid-liquid interactions within irregular solid nano-scaffolds materials under HSR inelastic deformation conditions.

Cold spray is a deposition process of solid powders and is employed in the additive manufacturing process to fabricate parts and structures below the powders’ melting temperatures. Supersonic velocity impacts of microparticles to target substrates in this process results in the microparticles experiencing high-strain-rate microscopic ballistic collisions. At these high speeds, the kinetic energy of the particles leads to extreme plastic deformation of the interface of the microparticle and the target substrate, which eventually enables solid-state consolidation. Study of impact dynamics, bonding mechanism, and critical velocities of these microparticles helps us better understand the cold spray process.
It has been generally understood that the parameters influencing the critical velocity and bonding mechanism include the presence of an oxide layer and the temperature at which the collision events take place. In this study, as a model system, the dynamic behavior of polycrystalline aluminum 606l T6 alloy (Al 6061) microparticles (~ 20 µm diameter) after impact on a target substrate at ultra-high strain rates are quantified by using a micro-ballistic method: advanced laser induced projectile impact test (α-LIPIT). The controlled parameters in these highly controlled experiments are impact speed (0-1,000 m/s), temperature (room temperature-300°C), and substrate oxide film thickness (10-20 nm additional oxide layer prepared using atomic layer deposition). Individual particle’s deformation characteristics are systematically studied with electron microscopy and focused ion beam milling. The precisely simulated microscopic collision events establish a quantitative relationship between the high-strain-rate dynamic response of the microparticles and the control parameters.
*This work was sponsored by the U.S. Department of Defense under Cooperative Agreement Award No. HQ0034-15-2-0007 and W911NF-15-2-0024.

Three-dimensional direct laser writing (3D-DLW) is an indispensable tool to high accuracy structuring and fabricating of 3D micro- and nano-objects via polymerization processes induced by direct laser writing. Over the past decade, this technology has become a well-established lithography tool for fabricating 3D geometries directly on functional substrates. This capability provided the possibility for a wide range of applications in photonics, microfluidics, mechanical microstructures, and cell scaffolds.
As the dimensions of the printed structures shrink, it remains a challenge to control the stiffness and resolution of the fabricated structure by changing the many conditions influencing direct laser writing. We, therefore, developed a nanoscale simulation approach as an alternative to physical experiments to optimize the 3D writing process and reduce the number of experiments. This approach enables a systematic study of how various writing parameters affect the structure and stiffness of the fabricated polymer network. We employ a coarse-grained molecular dynamics (CG-MD) approach to simulate direct laser writing of 3D polymer networks from a monomer pool, where monomers react irreversibly according to reaction rate constants, which results in 3D objects being printed voxel by voxel.
Simulations show that the degree of monomer conversion increases with higher laser power or, correspondingly, higher exposure times and saturates towards high writing powers/ high exposure times. Nonetheless, the laser power has a more pronounced effect on the degree of monomer conversion and mechanical properties than the exposure time. Furthermore, we study the relationship between mechanical strength, the porosity of the polymer network, and the mixture ratio of the radical quencher into the resins. We propose the optimum writing condition including the oxygen quenching effect to fabricate polymer networks of high resolution with acceptable mechanical stability. This approach allows us to tune the writing conditions of the 3D printed objects exploiting the fine interplay between laser power, exposure time, oxygen quenching to meet the requirements for different applications.

Polypropylene (PP) is a widely used plastic, due to its superior mechanical properties and resistance to corrosion. Processing of PP using classical extrusion is well established but using PP as filaments for 3D printing has proven challenging. One problem, common in general to semi-crystalline polymers, is warpage which is exacerbated by the lack of adhesion to the base plate and makes printing of larger structures difficult. Polystyrene is another common polymer with good chemical corrosion resistance, which is easy to extrude, but very difficult to print. Its high melting point, makes it brittle, resulting in filament breakage and clogging of the nozzle mechanism during printing. Here we demonstrate that blends of PP and PS can overcome these problems to produce filaments that are ductile and print structures that resist warping and maintain structural accuracy. Furthermore, it has been shown that PP allows high loading of graphene nanoplatelets, GNP, while still retaining ductility [kai]. Printing with PP/GNP produced highly oriented layers, with high electrical conductivity, but the warpage persisted, and only narrow interconnects were easily produced.
Here we show that blends of these polymers could be made, extruded into filaments, which are then easily printed and could produce large multilayer structures. Blends of PP and PS, with 80/20, 70/30, and 60/40 ratios, were produced via melt mixing in a Brabender twin-screw extruder, where torque was monitored, and drawn into filament using a Filabot extruder. Samples were printed for tensile and Izod impact testing, as well as temperature and frequency rheological measurements. Measurement of G’ and G’’at T=200C, showed shear thinning behavior, which is advantageous for nozzle extrusion followed by rapid thickening to maintain accuracy. Tensile data indicates that the impact toughness of 80/20 PP/PS was significantly higher than the rest. Filaments were also produced where the PP phase was loaded with up to 20 weight percent GNP, and the electrical and thermal conductivity properties were compared to those obtained from pure PP with comparable loading, where large differences were observed between the perpendicular and parallel orientations relative to the extruded control samples. Correlation with TEM images of the blend morphology and in operando x-ray scattering of the printing process will be presented.

Volumetric additive manufacturing (VAM) describes a developing class of 3D printing methods in which a photosensitive precursor material is transformed into a solid object via highly parallelized illumination. Computed axial lithography (CAL) is a specific VAM process in which a rotating volume of material is illuminated with a time-evolving light pattern such that the cumulative light dose is tomographically synthesized and can be controlled in three dimensions. Where the light dose exceeds a material-dependent threshold, the material is transformed into a solid object. Unlike conventional photopolymer additive manufacturing, CAL is layer-free, which brings several advantages. Here, we describe two ways in which the unique characteristics of CAL are enabling multimaterial objects to be prototyped.

Firstly, because the entire volume of the target object is exposed simultaneously, pre-existing solid objects can be immersed in the precursor material and “overprinted” with a polymeric geometry, analogously to overmolding or insert molding in thermoplastic injection molding. We have evaluated ways of strengthening the bond between CAL-printed polymer geometries and metallic objects, including silane-based coupling agents and cross-pattern knurling, the latter of which was found to increase the shear strength of the printed bond by approximately a factor of two. We also describe a specialized algorithm to generate illumination patterns that compensate for the occlusion of light by the object. This algorithm is shown to improve the accuracy of 3D dose reconstruction during overprinting, compared to ignoring the occlusion of light. We discuss the application of overprinting to microfluidic devices incorporating CAL-printed hydrogels.

Our second multimaterial process innovation exploits the fact that in CAL there is no relative motion between the precursor material and the fabricated object during printing. Thus freed of the limitations of hydrodynamic stresses, CAL can access a wider range of higher-viscosity or thermally pre-gelled precursor materials without sacrificing processing speed. We can harness this advantage to process dispersions of solid particles with exceptionally high volume fractions and associated high viscosities. We discuss approaches to modeling the propagation of light through these precursor materials, and the prospects for processing dispersions of fibers using CAL.

Lightweight materials with ballistic resistance and load-bearing capabilities have been the object of intense research in the last years. There are many examples of these materials in nature, but mimicking natural materials is difficult due to challenges in the necessary synthesis methods. However, recent advances in additive manufacturing techniques (such as 3D printing) have created a new perspective in creating complex structures, which will be very difficult to obtain using standard methods.
In this work, we have translated the atomic structures of tubulanes from atomic to macroscale scale using 3D printing techniques. Tubulanes [1] are 3D porous carbon allotrope structures obtained from the cross-linked carbon nanotubes. The 3D printed versions of tubulanes are topologically identical in structure to the atomic structures scaled to be 7 orders of magnitude larger (cm size). The tubulane 3D models were generated with the open-source code Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [3] and 3D printed with high-resolution using standard polylactic acid polymer filaments.
We investigated the mechanical behavior under a compressive load and the hypervelocity impact of the atomic and 3D printed models. The atomic models were investigated using fully atomistic reactive molecular dynamics simulations and the 3D print models using room temperature standard tensile testing machine and the hypervelocity impact tests were carried out with a specially designed two-stage gas gun.
Our results [2] show that both atomic and 3D printed models are remarkable high load-bearing materials. They also exhibit hypervelocity impact-resistance behavior. Surprisingly, some of the observed mechanical properties are scale-independent, manifested along with multiple length scales, from atomic to macro dimensions.

[1] R. H. Baughman, D. S. Galvao, Chem. Phys. Lett. v211, 110 (1993).
[2] S. M. Sajadi, C. F. Woellner, P. Ramesh, S. L. Eichmann, Q. Sun, P. J. Boul, C. J. Thaemlitz, M. M. Rahman, R. H. Baughman, D. S. Galvao, C. S. Tiwary, P. M. Ajayan, Small v15, 1904747 (2019).
[3] S. Plimpton, J. Comput. Phys. v117, 19 (1995).

To meet the increasing needs of high-precision glass micro-optics and address the major limitations of current three-dimensional (3D) printing optics, we have developed a liquid, solvent-free, silica precursor and two-photon 3D printing process. The printed optical elements can be fully converted to transparent inorganic glass at the temperature as low as 600 °C with the shrinkage of 17%. We demonstrate the complete process chain, from material development, printing process, and performance evaluation of the printed glass micro-optics. 3D printing of glass micro-optics with isotropic shrinkage, micrometer resolution, low deviation peak-to-valley value (<100 nm), and low surface roughness (< 6 nm) is achieved. The reported technique will enable the rapid fabrication of complex glass micro-optics previously impossible using conventional glass optics fabrication processes.

New high temperature materials exceeding Ni-base superalloys are highly demanded because it can realize higher burning efficiency in turbines or internal combustion engines by improving temperature resistance of heat resistant parts. Although Ni-base superalloys has excellent high temperature properties up to 1000°C, improvement is needed for their use at higher temperatures due to their melting temperatures and γ’-solvus. As a breakthrough solution, high entropy alloys(HEAs), which are composed of multi-principal elements, are highly attracted because of their several unique properties [1]. Among them, refractory high entropy alloys(RHEAs), which are based on refractory metals such as WNbMoTaV and WNbMoTa, are known as alloys with excellent high-temperature strength because of high melting temperature [2]. Also, additive manufacturing for RHEAs has been reported, and thus they are promising for new structure materials [3]. However, most of RHEAs with outstanding high-temperature strength are low ductility at room temperature and they have higher density than conventional Ni-based alloys. They are large obstacles to make RHEAs in practical use.
The purpose of this study is development of alloys with both high strength and low density for additive manufacturing. We focused on the elements with both high melting temperature and low density compared with Ni, and the properties of VNbMoCrTi alloys and VNbMoCrTiC alloys were investigated.
VNbMoCrTi alloys and VNbMoCrTiC alloys ingots were prepared by arc-melting under an Ar atmosphere. Specimens cut from the ingots were solution treated at 1300°C for 24 hours under vacuum followed by furnace cooling. All ingots were characterized by scanning electron microscopy(SEM) and energy dispersive X-ray spectroscopy(EDX). The phase structure was analyzed by X-ray diffractometer(XRD). Density of each specimen were measured by Archimedes method. The high-temperature strength was evaluated by the compressive test.
From the SEM observation of VNbMoCrTiC alloys, homogenized body centered cubic(BCC) matrix phase with carbide were confirmed after heat treatment. Also, EDX results indicated that carbide is mainly composed of TiC. XRD pattern showed the peak derived from BCC phase and carbide, and these results agree with EDX results Besides, VTiNbMoCrC alloy showed the yield stress of 780MPa at 1000°C regardless of lower density compared with Ni-based alloys.

Selective Laser Melting (SLM) is an emerging manufacturing technique that enables fabrication of parts with complex geometries and excellent mechanical properties. However, metal SLM components require expensive and difficult post-processing steps to remove supports and improve surface finish. To address this issue, this work presents a new self-terminating etching process to simplify the post-processing of copper alloy components fabricated using SLM. This process was performed on two advanced copper alloys; GRCop-84 and GRCop-42 that are interesting materials in aerospace applications due to their excellent thermal and mechanical properties. The self-terminating etching process contains two main steps: 1) altering the chemical composition of the outer surface of a metal by a proper sensitizing agent, and 2) dissolution of the resultant corrosive region by wet chemical etching. We demonstrate that iodine can be used to “sensitize” the surface of copper alloy components by forming soluble metal iodide salts that can be then dissolved in acetonitrile. Using this technique, a pre-defined amount of copper is removed from all interior and exterior surfaces resulting in separation of the component from the support. In addition, the surface roughness measurement shows an improvement of surface finish of GRCop by 70 % after 4 iodization-dissolution cycles.

Additive Manufacturing (AM) technologies have evolved significantly. The applications of AM have expanded from rapid prototyping to a larger scale, such as the construction of buildings and the mass production of spare parts. The world’s first 3D-printed laboratory has been constructed in Dubai in 2017, aligned with Dubai’s strategy to be a leading hub of 3D printing technologies by 2030. AM has various advantages, such as reduced material usage, geometric freedom, and automation that will shape the future of the manufacturing industries.

AM technologies can be classified into material extrusion, binder jetting, and powder bed fusion (PBF), direct energy deposition (DED), and other technologies. Each of the above classifications can be sub-categorized based on feedstock material and sub-processes. With the rapid expansion of AM applications, feedstock materials have developed noticeably: from polymers and ceramics to metals and composites. Selective Laser Melting (SLM) is the most common method within PBF to fabricate metal parts. Electron Beam Additive Manufacturing (EBAM) is adopted for DED of metals.

The progress in developing new AM feedstock material has empowered the exploration of other technologies to manufacture metal parts, such as Material Extrusion. The flexibility in choosing the feedstock material is an advantage for the technology that facilitated its growth by users around the globe in research and industry. Fused Filament Fabrication (FFF) is one of the most common and cost-effective material extrusion AM technologies. FFF applications include rapid prototyping and the production of high-performance functional parts. The feedstock material is shaped in a wire form that is utilized through a filament. It passes through a nozzle that heats and extrudes it layer by layer over the print bed, following a predefined path determined by the G-code. Different terms have been used in the industry and literature that refer to metal fabrication through FFF, such as metal FFF, Bound Powder Extrusion (BPE), Atomic Diffusion Additive Manufacturing (ADAM), and Metal Fused Filament Fabrication (MF³). The process of manufacturing metal parts with FFF consists of 3 main steps: printing, de-binding, and sintering.

It is critical to understand how different printing parameters and other factors along the process of metal FFF can affect the mechanical properties of the parts. Understanding the different characteristics of such parts is considered essential to expand the applications of this process and understand the economic and environmental impacts.

This paper attempts to explore the different characteristics of the parts produced via metal FFF, which includes the mechanical properties of two metals: 17-PH stainless steel and copper. The study will also investigate the porosity of these parts, cost analysis, weight, and environmental impact.

Two comparison models are designed to validate the results: experimental 3D Printed and numerical Finite Element Analysis (FEA) for the same. For the experimental test, two sets of each metal material are manufacture using FFF through the Markforged Metal X system. One set is fabricated with solid infill, while the other has triangular infill. Each set consists of four identical “dog bone” specimens. The dimensions and test conditions were set according to ASTM F3122 and ASTM-E8 standard. A uniaxial tension test will be conducted using Instron 3367 tensile testing machine. For the numerical FEA Analysis, the Digital Twin reconstruction method will be used where an explicitly generated CAD Model is made using an Additive Reconstruction Tool with the G-Code as input to simulate the solid layer by layer and obtain a digital equivalent of the 3D Printed part in CAD Software. A Scanning Electron Microscope (SEM) will be used to investigate the parts’ internal structure and average pore size. The porosity will be characterized through density comparison and analyzing pore distribution.

Additive Manufacturing (AM) is an emerging and highly promising technology for manufacturing metal-based complex geometries for specialized applications such as aerospace, defense, medical implants, and in-space manufacturing, among others. However, the structural and property impact of additive manufacturing processes on manufactured parts remains to be fully explored. For example, the repeated thermal cycles during layer-by-layer deposition can create uneven material properties and residual stresses. These localized residual stresses may result in material failure during use. Also, minor changes in input parameters such as powder size, laser power, and laser scan speed can significantly impact the final quality of the deposit. A critical study of the relationship of input parameters with residual stresses and part performance is necessary for designing post-processing heat treatment and broader acceptance of AM process.
The finite element analysis (FEA) is an effective tool to analyze the impact of AM input parameters on the final performance of manufactured parts, resulting in a significant saving on cost and time compared to production and experimental analysis. Hence, to generate design guidelines for AM processes, we analyze residual stress patterns on the directed energy deposited (DED) thin-wall structure in as-built and post-treated conditions. The computational results are compared and contrasted against the X-ray diffraction (XRD) study of AM. The simulation is performed in ABAQUS (Dassault Systemes, USA) using an integrated thermomechanical analysis step with temperature-dependent material properties. The materials properties for the simulation were used from tensile and fatigue tests data available in literature for AM copper alloy (GRCop-42), a material developed for NASA rocket propulsion application. A plot of the effect of input parameters such as laser power and scan speed on the residual stress changes is presented, along with simulation validation through comparison with the experimental residual stresses XRD data. The simulation stress distribution implicates the agreement with experimental analysis. Also, it exemplifies the use of the thermomechanical model to guide parameter space for the AM.

Common housings for Internet of Things (IoT) sensors consist of a polymer like acrylonitrile butadiene styrene (ABS). The primary function of these objects is to protect the electronics from environmental influences e.g., water. Due to the exposed location of IoT sensors, especially in alpine regions, the housings are important for a proper and long-term function of the sensor. Beside protection, additional features like insulation properties are important too, for ensuring sensor functionality. The insulation aspect of common IoT housings has been realized by adding insulation material in addition to the housings. Additive manufacturing may enable a combined design approach in fulfilling mechanical and thermodynamical requirements at the same time.
The aim of the paper is to describe and investigate different designs for additive manufacturing which combine insulation properties without affecting structural stability of housings for IoT devices in alpine regions.
Finite element analysis simulation models for IoT housings are used to determine the thermodynamical and structural properties of different materials and designs. Selected designs are fabricated and tested in a climate chamber regarding insulation and mechanical properties.
The approach of design for additive manufacturing enables an integration of different functionality (e.g., thermal and mechanical function) into the designed part. The insulation property of additive manufactured housings for IoT devices exhibit a significantly higher insulation ability in comparison to common housings of IoT devices. This approach enables highly innovative housings which can withstand the harsh environment in alpine region.

Direct Ink Writing (DIW) is an extrusion-based additive manufacturing method where the print medium is a liquid-phase ‘ink’ dispensed out of small nozzles and deposited along digitally defined paths. Conventional DIW methods for thermosetting resins rely on the use of viscosifiers, novel crosslinking chemistries, and/or long curing schedules in an oven. Here, we demonstrate the use of a co-planar radio frequency applicator to generate an electric field which can be used to rapidly heat and cure DIW-printed, nano-filled composite resins. This method avoids the need of an oven or post-curing step. The proposed process consists of a layer-by-layer, print-and-cure cycle which allows for printing of high-resolution, multi-layer structures. Every extruded layer is partially cured using RF before depositing the next layer; this allows the printed part to maintain structural integrity without buckling under its own weight. The process enables both increased throughput and decreased touch time relative to traditional part manufacturing. Commercial epoxy resin with varied nano-filler loadings were examined as DIW candidates. Rheological characterization was used to assess both curing kinetics and printing behavior. After printing, the thermo-mechanical properties, surface finish, and shape retention of RF-cured samples were evaluated and found to be comparable against samples conventionally cured in an oven. This method of manufacturing establishes RF heating as a suitable alternative to conventional methods, facilitating rapid, free-form processing of thermosetting resins without a mold.

Modern propellants hope to employ nano fuel-oxidizer components. Unfortunately fabrication is problematic at high nanoparticle loadings but a variety of AM methods are showing promise. These methods include electrospray, ink printing, and laser writing. Multiple components can be employed in both molecular and particulate form to fabricate a variety of structures including multicomponent nanostructured mesoparticles, laminates and fiber composites. Ultimately even materials comprising molecular clusters. One significant benefit of AM is the ability to fabricate small quantities in a well defined manner to enable characterization. In this talk we demonstrate how using AM methods coupled to ultra-fast microscopy and imaging pyrometry we can probe the reaction front and the thermal field at the microsecond and micron meter scale to extract structure-function relationships [1-4].
Using this approach we are able to explore the thickness of the reaction front, the role of heat transport mechanism on the reaction front surface area, velocity distributions in the reaction front and how corrugating the reaction front can change the over all energy production rate.

H. Wang, D. J. Kline, M. R. Zachariah. In-operando high-speed microscopy and thermometry of reaction propagation and sintering in a nanocomposite, Nat. Commun. 10 (2019) 1-8.
H. Wang, B. Julien, D.J. Kline, Z. Alibay, M.C. Rehwoldt, C. Rossi, M.R. Zachariah, Probing the reaction front of nanolaminates at ~µs time and ~µm spatial resolution. J. Phys. Chem. C 124 (2020) 13679-13687.
[H. Wang, J. Shen, D.J. Kline, N. Eckman, N.R. Agrawal, T. Wu, P. Wang, M.R. Zachariah, Direct writing of a 90 wt% particle loading nanothermite. Adv. Mater. 23 (2019) 1806575.
D.J. Kline, Z. Alibay, M.C. Rehwoldt, A. Idrogo-Lam, S.G. Hamilton, P. Biswas, F. Xu, M.R. Zachariah, Experimental observation of the heat transfer mechanisms that drive propagation in additively manufactured energetic materials. Combust. Flame (2020) 417-424.

Lucas L. Kilpatrick^a,b, Eugene B. Caldona^a,b, Zane J. Smith^b,c, and Rigoberto C. Advincula<sup>a,b,d,e

a</sup>Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA
^bJoint Institute for Advanced Materials, University of Tennessee, Knoxville, TN 37996, USA
^cDepartment of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA
^dDepartment of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA
^eCenter for Nanophase Materials and Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA

Abstract

This study focuses on the fabrication of a porous superhydrophobic membrane for oil/water separation. The membrane, made from nanosilica-containing poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), was prepared by direct ink writing. The as-prepared membrane displayed anti-wetting property and superoleophilicity, allowing oil to pass, but not water, through its openings. Such surface characteristics are mainly attributed to the synergistic effect of hierarchical surface roughness created by the silica nanoparticles and low surface energy inherent to PVDF-HFP. Combined with mechanical robustness and high thermal stability, the nanosilica-filled fluoropolymer membrane is practically useful in solving industrial problems concerning water contamination and oil spill response.

Keywords: superhydrophobicity; superoleophilicity; fluoropolymer; silica; oil/water separation

The high temperatures and rapid cooling rates during laser powder bed fusion make it difficult to characterize the thermal cycles the material undergoes during printing. Such information is however crucial for understanding the evolving microstructures, especially, when the material undergoes phase transformations during thermal cycling. We recently demonstrated an operando X-ray diffraction technique to measure temperature and phase evolutions in a newly deposited layer during laser powder bed fusion (L-PBF) of Ti-6Al-4V (Hocine et al., Materials Today, 34, 2020). Here, the operando X-ray diffraction experiments are carried out on thin walls of Ti-6Al-4V-3Fe and Ti-6Al-4V. Temperature profiles and cooling rates are derived from X-ray diffraction patterns measured in transmission mode for multiple layers below the surface while the laser rescans the surface. Microstructures are further characterized by transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) spectroscopy. The powders used were prepared by decorating a pre-alloyed Ti-6Al-4V powder with pure Fe particles using a simple mixing method (Simonelli et al., Materials Characterization, 143, 2018). It is known that minor addition of Fe can stabilize the β-Ti phase at ambient temperature. The present study shows that Fe is heterogeneously distributed after printing and that the β-Ti phase is stabilized where the Fe concentration is highest. During remelting the surface layer, temperatures above the β transus (1252 K) are reached in at least 3 layers below that newly printed layer. Furthermore, during remelting of the upper layer, the Fe distribution is homogenized. The evolution of the β phase during thermal cycling is discussed in terms of the temperature profiles and cooling rates in multiple layers along the building direction.

The increased demand for wearable devices such as smartwatches, shoes, and even masks, amid the pandemic, has led to an increased demand for customized and adaptive materials, specifically for use on the human body. One such material is smart materials with an intrinsic shape memory effect to morph into another shape when an external stimulus is applied. The fabrication of such materials through additive manufacturing is known as 4D printing. However, such 4D printing is limited to materials with intrinsic shape memory effects, which are usually expensive and limited. Thus, herein, we proposed a new smart material platform where a conventional non-smart thermoplastic material with pre-programmed structure induces reversible 4D effect through local heat generation associated with selective Joule heating mechanisms. Through multi-material printing capability of a Multi-Jet Fusion printer, we successfully fabricated polyamide-12 (PA-12) with embedded heat channels. Through a designed 3D structure and silver-polymer composition variation, resistance gradient was induced at selective local regions where Joule heating considerably occurred. 3D finite element analysis (FEA) modeling was conducted in parallel to experiments, and the simulation results showed good agreement with the experimental observation, validating the underlying mechanism of this 4D system. Through parametric studies with various structural and material parameters, those 4D systems can be tuned depending on operation conditions. We also demonstrated the applicability of this system through a proof-of-concept product, adaptive mask frames that could be conformed depending on the contour of a human face. This 4D system was also durable and reversible, showing consistent performance over multiple loading cycles. In conclusion, this study demonstrated that common thermoplastics with embedded heat channels can be used as a new type of smart material systems, creating a new material category, “4D plastics”. We envisage that this system can be applied to build wearable wearable devices, customized clothing and shoes, military weapons, and medical instruments where adaptability, conformity and rigidity are required.

Laser direct metal deposition (DMD) is an efficient and flexibility additive manufacturing technique which has broad application prospects, but limited due to varieties of defects and anisotropy. Laser remelting (LR) is a process that the same slice data is scanned secondly without powder deliver after each layer is deposited, and it is often used during selective laser melting. Herein, LR process has been applied during the DMD process of a 17-4 PH steel to enhance the densification level and relieve the mechanical anisotropy. It is found that thermal history, porosity and microstructural evolution are shown to be dependent on the LR energy density. Moreover, the roughness of top surface of the deposited layer and intralayer porosity decrease with increase of laser remelting energy density. While for interlayer defects, there is an optimal LR energy density corresponding to the lowest interlayer porosity. Furthermore, LR process can enhance the holding time at high temperature, even sometimes heat the sample above Ac₁, resulting in change in contents of austenite and carbide. In addition, LR process greatly dilutes the pre-solidified texture. It was also proved that the flat defects in-plane to the interlayer and the anisotropy of the crystallographic orientations are extremely harmful for the mechanical isotropy of the DMDed samples. Finally, sample manufactured with laser remelting at 15.0 J/mm² has the lowest porosity and random crystallographic orientations resulting in near isotropous tensile strength, but the contribution of laser remelting to eliminate anisotropy in elongation is limited because of the interlayer defects.

In recent years, additive manufacturing has become an attractive technology that offers many advantages over the other widespread subtractive methods. The most evident advantage is the possibility to make any object, virtually without any geometrical limitations.[1] Among the additive manufacturing processes, stereolithography enables the manufacturing of small items with miniaturized features and extraordinary precision. For these reasons, the production of MEMS (microelectromechanical system)-based sensors by stereolithography could be a valid alternative to the existing silicon-based technologies, even though it is diametrically opposite to current technologies, which allow the production of huge amounts of identical objects, i.e. mass production. Oppositely, stereolithography enables mass customization thanks to the production of objects which can be different from each other and completely customizable.[2]
In this work, a commercial resin was used and fully characterized, because its formulation, as well as its physical and mechanical properties, were unknown. The high precision of stereolithography, together with the thermal stability of the resin, were exploited to produce customized MEMS-like sensors. However, stereolithography is suitable only for photo-curable materials called photopolymers. They are usually insulating materials, which should be coated with metallic layers either by inkjet printing or by electroless deposition and subsequent wet metallization, to add electrically conductive patterns to the 3D printed objects.[3] In this work, carbon nanotubes (CNTs) were added to the photocurable resin to obtain a photocurable conductive nanocomposite aiming to create electrically active parts in MEMS-like sensors. Furthermore, different types of CNTs were investigated: (i) pristine CNTs; (ii) amino-functionalized CNTs; and (iii) silver-decorated ones. The addition of CNTs to obtain an electrically conductive photocurable material has been revealed to be effective. The enrichment of the matrix with a CNT content greater than 0.1 wt% led to improve the conductivity of the material by 5-to-6 orders of magnitude. Using a 0.5 wt% CNT concentration, the material showed a conductivity equal to 3.9 10^-4 S/cm. This result could be increased by 2.4 times using CNTs functionalized with NH₂ due to their more homogenous dispersion in the matrix and by 10.8 times by using CNTs decorated with silver nanoparticles. Even higher values could be obtained by better controlling the synthesis of Ag-MWCNTs to deposit much smaller nanoparticles. However, a custom-made formulation for the resin can be developed in the future to tune some properties such as stiffness and viscosity.


[1] I. Gibson, D. Rosen, B. Stucker. Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping and Direct Digital Manufacturing, 2015, Springer, New York.

[2] P.F. Jacobs, Rapid Prototyping & Manufacturing, 1999, Society of Manufacturing Engineers, Dearborn.

[3] L. Gaffuri Pagani, P. Carulli, V. Zega, R. Suriano, R. Bernasconi, A. Frangi, M. Levi, L. Magagnin, G. Langfelder, "The First Three-Dimensional Printed and Wet-Metallized Coriolis Mass Flowmeter," in IEEE Sensors Letters, vol. 4, no. 6, pp. 1-4, June 2020, Art no. 2500604, doi: 10.1109/LSENS.2020.2995570.

Ceramics are characterized with high hardness, brittleness and heat resistance, the applications of which are restricted by traditional fabrication processes. Additive Manufacture (AM) can be innovatively applied to direct manufacture of ceramic structures, hence the relatively labour-saving fabrication of 3D ceramic parts in contrast with the conventional methods. DLP (Digital Lighting Process), as one of the AM techniques, is capable to fabricate alumina ceramics with complex structures through mask-image-projection, which ensures the precision and improves the efficiency of manufacture.
In this research, alumina ceramic with complex structures was obtained by DLP with a high resolution and applicable mechanical properties. A long-time stable ceramic suspension with high ceramic powder content was firstly obtained, which achieved the mass fraction of 70%. Then optimized parameters of the UV-curing process ensured the resolution of the complex structure. The two-step sintering method was adopted to optimize the sintering process in order to reduce defects and improve density. The complex ceramic structure with high resolution showed an average flexural strength of 220MPa and a relative high density, with potential applications of aerospace, tissue engineering, machinery manufacture and so on.

Yuxiang Zhang and Yiran Ma contributed equally to this work.

Additively manufactured metallic materials exhibit excellent mechanical strength. However, they often fail prematurely owing to external defects that act as sites for crack initiation. Cracks then propagate through grain boundaries and/or cellular boundaries that may contain brittle second phases. In this work, the premature failure mechanisms in selective laser melted (SLM) materials are studied. A submicron structure is introduced containing semicoherent precipitates distributed in a discontinuous but periodic fashion along the cellular boundaries. Alternatively, martensitic phases are used to create alloys with modulation of phases and different strengthening mechanisms. The observed hierarchical microstructure combining different phases spanning over different length scales, consisting of remelted zones and track core regions at the macro-scale, regions with elongated columnar grains and equiaxed grains at the micro-scale, martensitic laths at the submicron-scale, and a high density of dislocations and stacking faults at the nano- and atomic scale leads to outstanding tensile properties of as-prepared and annealed samples. The mechanisms determining the strength and ductility are discussed in terms of the hierarchical microstructure and internal defects. The cellular structure with a high density of dislocations along their boundaries and the martensitic structure are the reasons for strengthening, while the hierarchical microstructure helps to obtain appropriate ductility. Furthermore, martensitic transformation induced plastic bending of residual FCC plates helps to balance the different plastic strains within the heterogeneous structure. This work provides a material design approach for creating hierarchically structured additively manufactured materials with improved strength-ductility tradeoff provided that premature failure is alleviated.

A challenge of combined traumata is, that these situations may require contradictory interventions. Open fractures or fractures in combination with burns requires on one hand a stabilization of the fracture and on the other hand a treatment of the superficial injury. Common therapeutical approaches may lack the ability, to fulfill both requirements at the same time.
The aim of the paper is to develop and characterize a 3D-printable bioactive splint. This bioactive device ensures splintage in combination with the healing stimulation of superficial injuries (e.g. skin abrasion, burn). By creating a digital model of the affected body part, an individualized splint can be realized. Stimulants for healing support are transported through a permeable membrane. This membrane enables an exchange of stimulants to the affected region.
The University of Applied Sciences Kufstein develop the design of the splints. A digital model of the affected body part is generated by 3D-scanning. The resulting point cloud of the surface is used for designing the splint. Reservoirs for healing stimulants are added at the individual positions. Standardized connectors are used for enabling the application of standardized medical devices. Polymeric filament blends are fabricated by Technical University of Clausthal. The filaments are processed on a material extrusion machine.
The fabricated bioactive splint enables a personalized design. This approach can support the therapy of combined traumata by resolving contradictory requirements, thus, the approach may have a positive effect on the healing duration.

Electron beam melting (EBM) is a powder bed fusion (PBF) based additive manufacturing technology which has been gaining increased popularity for manufacturing complex customized parts for various sectors such as aerospace, medical etc. An interesting candidate to process using EBM technique is Alloy 718, which is a nickel-based superalloy extensively used in aerospace sector. Irrespective of the choice of processing conditions, EBM processed Alloy 718 is almost always subjected to thermal post-treatments to resolve challenges associated with formation of detrimental defects and/or phases, microstructural inhomogeneity, as well as to achieve the required mechanical response for demanding applications. However, the post-treatment (hot isostatic pressing, HIP and heat treatment, HT) procedures employed are those typically used for castings. This is also reflected in the ASTM F3055 standard for PBF Alloy 718, which appears to adopt the long time-temperature schedules applied for castings. However, this procedure might not be most appropriate and perhaps excessive for EBM Alloy 718 builds.
Therefore, the objective of this study was to investigate the possibility of shortening the duration of post-treatment from the typically long protocols suggested in the above ASTM standard. Results from a detailed study of microstructure evolution during post-treatment revealed possibility of significantly reducing the time for post-treatment. A pathway for further shortening the post-treatment by combining different post-treatment steps into one uninterrupted HIP cycle was also explored. The above results were further substantiated by the mechanical response of EBM Alloy 718 builds subjected to varied post-treatment protocols. Furthermore, such a shortened protocol was found to be robust when applied to EBM builds with distinctly different starting microstructures.

Quantitative percussion diagnostics (QPD) is a has been used successfully in a number of applications for non-destructive evaluation of structural integrity. This technique involves a rod that is actuated to tap on a specimen with a given kinetic energy. The resulting mechanical response of the specimen as a function of time is measured using a piezoelectric force sensor and the mechanical energy returned to the rod is then analyzed. Because contact with the specimen is on the order of a few hundred microseconds, QPD can be used in situations and for specimen conditions (e.g. high temperature) that are not feasible for other nondestructive testing methods. The objective of the present study was to evaluate the use of QPD for characterization of defects in specimens fabricated using two additive manufacturing methods, i.e. cold spray deposition and laser powder directed energy deposition (LPDED). Cold spray specimens were produced using commercially pure nickel with varied process conditions including gas type (i.e. helium and nitrogen) and deposition rate. In addition, a Ti-6Al-4V specimen was manufactured using LPDED with varying porosity content. All specimens were characterized using both QPD and destructive methods (i.e. cross-sectional metallography) to compare results for observed defect characteristics. Cold spray specimens exhibited a lower energy return when they contained more porosity and/or surface roughness. Microscopic plastic deformation at highly porous surfaces was indicated during percussion testing that reached a saturation level after repeated percussion. Overall, the results showed that QPD can effectively evaluate cold spray and LPDED specimens for porosity and surface roughness.

Laser powder bed fusion (L-PBF) is a versatile additive manufacturing process that can print geometrically complex metal parts for a variety of applications. However, poor control of defect formation during processing hampers its widespread industrial adoption. Many materials suffer from a high crack susceptibility during L-PBF, which results in degraded mechanical properties, and is an obstacle to the certification of critical parts.

The CM247LC nickel-based superalloy is a gamma-prime-strengthened Ni superalloy, which is of particular interest due to its excellent mechanical, creep, wear, and oxidation properties at both ambient and elevated temperatures. However, this alloy contains high amounts of Al and Ti and is considered vulnerable to cracking due to L1₂ Ni₃ (Al, Ti) precipitation. In literature, several mechanisms are reported to be responsible for cracking during the manufacturing of CM247LC alloy.

In order to unveil those cracking mechanisms, we employ, for the first time, high-speed synchrotron X-ray imaging in combination with a miniaturized L-PBF set-up that reproduces real processing conditions (Hocine et al. Materials Today 34 (2020) 30-40). This unique setup provides operando imaging of crack formation during L-PBF. These measurements are complemented by post-mortem inspection of crack morphology and analysis of the chemistry around the cracks. Furthermore, finite elements simulations were performed to obtain the evolution of the local temperature during printing. The simulations are validated by operando X-ray diffraction-based measurements (Hocine et al. Additive Manufacturing 24 (2020) 101194) . The results provide, for the first time, direct evidence of the occurrence of solidification and liquation cracking in CM247LC.

Heat flow direction during Additive manufacturing plays an important role while shaping the textural properties of the build samples. The grain structure of nickel-base alloys fabricated using powder bed fusion additive manufacturing processes display a preferred orientation along heat flow direction that usually overlaps with building direction. These highly oriented grains with strong orientational dependencies can cause anisotropy in the properties of the final components. Age-hardenable nickel base alloys, such as IN718, undergo a series of high-temperature post-process heat treatments in order to obtain their strength. However, the effectiveness of these high-temperature heat treatments in altering the grain structures and removing the preferential grain orientations is not well established. Through a series of high-temperature heat treatments at temperatures between 1050C and 1200C, the evolution of the as-deposited grain structure across different orientations with respect to the build direction was evaluated. Surface textures along the build direction rotated from a prominent [001] to [011] direction while the texture perpendicular to build direction remained unchanged along the [011] with increasing temperatures and times. At the same time, the grain sizes and morphologies changed along this same direction transitioned from elongated to equiaxed grains, matching those observed on the plane normal to this orientation.

The biomimetic superhydrophobic and superoleophobic surfaces require the microscale and nanoscale hierarchical surface textures, combined with coatings with low surface energy. While it has always been difficult and expensive to fabricate the superwetting surfaces with well-controlled textures via conventional manufacturing techniques, 3D-printing serves as a time-efficient and cost-efficient alternative. Here, a novel microscale re-entrant surface topography is successfully fabricated by two-photon polymerization (2PP) 3D-printing. On-demand wettability of various liquids has been achieved on the single re-entrant surface. For a given liquid, the re-entrant angle can be precisely manipulated to be either smaller than the liquid CA on the flat surface to achieve dewetting, or larger than the liquid CA on the flat surface to achieve wetting. The 3D-printable microscopic repeating re-entrant surface, which is difficult to make by conventional techniques, has been demonstrated to further enhance the desired on-demand surface wettability. Moreover, efficient on-demand separations of liquid mixtures with high flux are enabled by the 3D-printed separation membranes with the repeating re-entrant structures, which is critical for wastewater treatment in the chemical industry.

In order for 3d printing to realize its potential for distributed manufacturing, it must be possible to make parts with acceptable mech. properties, use local materials, and recycle or rework printed materials. Fused Filament Fabrication (FFF) printing of thermoplastics is generally limited by poor adhesion between layers in prints. Composite filaments for FFF are difficult to make and use due to the challenges in mixing highly viscous molten thermoplastics with filler and the inflexibility of the resulting filled filaments. Soln. processing of thermoplastics facilitates homogeneous incorporation of filler, but introduces addnl. process steps, creates a solvent waste stream and potentially degrades composite performance through plasticization by residual solvent. Thermoreversible thermosets provides a soln. to all of these problems. These highly cross-linked resins have thermal weak-links that break apart at higher temps. and can reform to re-establish crosslinks once the temp. is reduced. The most common weak-link is the Diels-Alder cycloadduct of maleimides and furans that form at room temp. and exhibit a reversion temp. of 100°C. In our study, we have prepd. and mech. tested composites based on PLA, ABS, TPU, and bismaleimide-tetrafurfuryl thermoreversible thermosets with silica, silicate minerals, and tricalcium phosphate.

This talk will present the design of a new class of smart elastomeric architectures that undergo complex reconfiguration and shape change in applied magnetic fields. These magnetoactive soft actuators are fabricated by 3D printing with Homocomposite Thixotropic Pastes (HTPs) of capillary ink consisting of silicone microbeads, liquid silicone and water [1,2] Superparamagnetic and ferromagnetic nanoparticles are embedded inside the microbeads before curing to render the resulting ink magnetically active without compromising its mechanical properties. The mechanical stability of the printed and cured structures as well as their extensibility to attain maximum possible actuation distance is achieved by optimizing the HTPs material moduli. We will discuss how the magnetic particle loading into the microbeads, volume of liquid polymer precursor used for capillary bridging and controlled level of polymer crosslinking, can be adjusted to fabricate 3D printed structures which are very soft, yet strong enough to withstand many cycles when actuated by external magnetic fields. We will show that the printed meshes and strands are much softer and more flexible than those fabricated by merely reducing crosslink density of bulk PDMS. This ultrasoft material is biocompatible and has tunable mechanical characteristics, which make it suitable for biomedical and soft robotics applications. The HTP 3D printing method makes possible the fabrication of soft architectures with different actuation modes, such as isotropic/anisotropic contraction and multiple shape changes [3]. The cyclic actuation of these objects can be modulated to mimic the pumping mechanism like that seen in heart and lung muscles, while the moduli can be tuned by design to match that of live tissues. Specifically, magneto-capillary actuators can be reversibly laterally contracted and expanded by controlling the normal magnetic field applied from below the vessel. Similar architectures that reconfigure in magnetic fields in response to external stimuli could find a broad range of applications, such as making 2D auxetic materials, soft actuators and soft robots with non-contact actuation.

[1] S. Roh, D. P. Parekh, B. Bharti, S. D. Stoyanov and O. D. Velev, Adv. Mater. 29, 1701554, 1 (2017).
[2] S. Roh and O. D. Velev, AIChE J. 64, 3558-3564 (2018).
[3] S. Roh, L. B. Okello, N. Golbasi, J. P. Hankwitz, J. A.-C. Liu, J. B. Tracy and O. D. Velev, Adv. Mater. Technol, 1800528, 1 (2019).

Multilayered polymer nanoparticle composite (PNC) has shown potential in enhancing thermal and electrical properties. Most widely used manufacturing techniques include layer-by-layer coating, filtration, and ice-templating. Here, we introduce a new scalable manufacturing technique including both forced assembly and dry-jet-wet fiber spinning processes. Multipliers were designed and fabricated with metal 3D printing. The resulting continuous fiber, approximately 100 µm in diameter, consists of 4 to 512 alternating layers of composite A and composite B. Depending on the fabrication process, each layer's thickness can be controlled from a few micrometers to hundreds of nanometers. To demonstrate the fiber’s potential as a thermally regulated textile, boron nitride (BN) and polyvinyl alcohol (PVA) were chosen as materials A and B. The alternating structure enabled continuous pathways of confined BN for high thermal conduction within flexible PVA matrices. By changing A and B materials, alternating layers of porous structure also showed excellent mechanical stability and thermal insulting behavior. Through varying the material selection for A and B, including nanoparticle and polymer, this fabrication technique can be applied to numerous applications with multilayered structures.

In the 4th industrial revolution, the market size of wearable devices is growing dramatically with the recent advances in the smart healthcare system. Users receive the notification from the device by sound or vibration when the real-time sensor detects a sudden change in vital signal or danger from the surrounding. In the wearable device market, wristwear and eye-headwear take more than 60% of the whole market of wearable devices. Among them, the market size of the smart contact lens can be extended with the advantage of important and diverse vital signs from eye and tear. Examples of those elements involve ions, such as sodium and potassium, organic molecules, such as glucose, and biomolecules. However, in the past research, wireless communication was used to transfer the data from the sensor in the smart contact lens. Also, it required a complex circuit containing an antenna and receiver inside and outside. In previous our group's research, the only single channel was suggested with a limited amount of data transmission for electrochromic data transmission.
Here, a multi-channel electrochromic data transmission system on the 3D printed template is suggested. 3D printer was used to manufacture the template having eight channels in one procedure instead of a complicated and multi-step process. A 3D printed template provided more strength to sustain unintended bending or folding of contact lenses. The device was made with eight electrochromic channels and one reference electrode. The color of each electrode was changed between blue and transparent. Electrochromic channels were used to transfer the diverse and complex signals to the user. 256 (28) different signals can be expressed theoretically at least, and the number of signals has the potential to be increased by controlling the color change in three or four steps. Also, the Prussian Blue (PB)'s electrochromic performance was characterized, and various color-changing combinations were realized as an 8-bit barcode pattern by controlling each channel's voltage. For example, 10110001 was changed to 11110001 for transferring different signals to the user. Furthermore, the video of eight-channel color change was taken, and color change patterns were recognized by video processing using Python with OpenCV, one of the libraries in Python to solve the computer vision problems.

As the key component of a magnetic refrigerator, magnetocaloric regenerators (also known as heat exchangers) consist of magnetocaloric materials (MCM) with voids and channels to enable efficient heat transfer to a heat-exchanger fluid. To date, research efforts to produce magnetocaloric regenerators are mainly focused on packed particle beds and plane plates that can be stacked and separated by spacers to provide channels for the heat-exchanger fluid. Both structures possess inherent processing challenges that reduce their performance once integrated into a device. Powder bed fusion-based additive manufacturing methods, such as selective laser sintering, on the other hand, are expensive; and the melting and re-crystallizing of the magnetic particles changes the magnetic and crystalline phases and induces defects in the printed structures.

To address these unique challenges, in this work, we have fabricated magnetocaloric cooling devices using a polymer-assisted 3D printing process followed by a two-stage sintering post-treatment. The printing ink consists of magnetocaloric particles, a sacrificial polymer binder, and a tri-solvent system. Preliminary experiments were conducted using magnetocaloric nanoparticles of nominal composition, La_0.6Ca_0.4MnO₃. The solvents modulate the rheology of the printing ink to facilitate the 3D printing process. The polymer binder (polyethylene glycol or PEO) holds the 3D printed magnetic materials in place after solvent removal. A low-temperature sintering process removes the polymer binder, and a high-temperature sintering process fuses/bonds the magnetic particles inside the printed 3D architectures. Thermogravimetric analysis (TGA) is used to determine the processing condition of the sintering. Results indicate that the magnetocaloric response, i.e., the magnetic entropy change upon application or removal of a magnetic field, exhibits a small difference between the unprocessed MCM powders and the 3D printed devices, which demonstrates the effectiveness of this fabrication process. Compared to other metal additive manufacturing methods, this approach eliminates the complexity of powder handling, expensive laser or e-beam systems, and extremely delicate green parts for powder removal (e.g., binder jet 3D printing). With the inherent advantage of creating spatially designed channels and structures, this polymer-assisted 3D printing scheme provides a promising additive manufacturing platform for high-performance magnetocaloric cooling devices.

Understanding the dynamic solidification behavior during metal additive manufacturing (AM) is essential as it directly influences final microstructures, defects and therefore mechanical properties of the part. Many efforts in this domain are based on numerical and computational approaches. Some studies focus on experimental determination of these parameters using in operando monitoring including pyrometry or IR imaging, which fail to provide sub-surface information.
Novel in-operando synchrotron x-ray experiments at different spatial and temporal resolutions provide an unprecedented opportunity to develop a new understanding of the underlying physics of the AM process and solidification in cooling rates applicable to beam-based AM. Here we demonstrate a novel approach using in operando synchrotron x-ray diffraction to analyze the solidification process by tracking the evolution of the mushy zone during a single line scan of Inconel 625. Using this approach, we calculate the melt pool and mushy zone dimensions, along with real-time cooling rates, all by virtue of tracking real-time changes in lattice parameters. Finally, our work aims to establish a link between the transient ‘mushy zone’ formed during solidification for prediction of defects inside the final microstructure of the AM parts. Results are corroborated with microscopy analysis of the printed structures and show good agreement. Future experiments to study material behavior during extreme heating and cooling rates of AM process will be discussed as a tool to develop new materials for AM.

Significant advances in battery technology requires electrochemistry that goes beyond conventional Li-ion batteries to further boost energy density and increase safety. All-solid-state lithium battery (ASSLB) technology, with the integration of solid-state electrolytes (SSEs), has the potential to provide higher energy density, better safety, and longer cycle life. Significant progress has been made for ASSLBs by the development and optimization of SSEs that can provide efficient Li-ion conduction while mitigating the problems of leakage and high flammability associated with the organic liquid electrolytes currently used in commercial Li-ion batteries. Among the different chemistries employed as inorganic SSEs, e.g., sulfides and halides, oxides are considered promising SSE candidates for ASSLBs due to their chemical stability and ionic transport properties. Among the reported inorganic solid Li-ion conductive oxides, the garnet-like Li_6.4La₃Z_1.4Ta_0.6O₁₂ (LLZTO) cubic structure has received increasing attention due to its chemical stability against lithium metal, high lithium ion conductivity (10^-3 S/cm at 25°C) and wide electrochemical window.

ASSLBs are difficult to process by traditional manufacturing strategies due to the brittleness of SSE ceramic materials, poor solid-solid contact (SSE/electrodes), and electrolyte-electrode stability issues; as a result, battery performance is far from ideal. The advancement in novel manufacturing techniques such as 3D printing has enabled the assembly of electrodes and SSE materials in more complex geometry configurations, with intimate solid-solid contact and lower interfacial resistance. Particularly, direct ink writing (DIW), is the most widely used additive manufacturing method due to its low cost, easy operation, and broad applicability. Here, we developed different ink formulations with desired rheological properties that enable DIW 3D printing of the LLZTO solid electrolyte with varying features. Different additives such as Al₂O₃, SiO₂, and Li₃BO₃ and post-sintering approaches were investigated to boost the LLZTO’s density and improve battery performance. These inks were used to 3D-print different patterns, which after sintering revealed thin, nonplanar, intricate architectures composed of LLZTO SSE. Using the herein developed 3D-printing ink formulations to further study and optimize electrolyte structure and properties could lead to the advance of batteries with low cell resistance and higher energy and power density.

The advent of 3D printing has widened the design space and complexity of parts/assemblies manufactured using Ti-6Al-4V (Ti64) alloy. The complex designs manufactured using additive manufacturing (AM) inevitably have sacrificial support structures that are challenging or inaccessible to remove using traditional methods, i.e., milling, electrical discharge machining, or using a rotary tool. The removal of supports accounts for almost 50 percent of the post-processing cost. With a high demand for one-of-a-kind parts in the medical and aerospace industries, there is a need to develop faster, cheaper, and scalable ways to remove supports. This work introduces a novel, self-terminating etching process for support removal for AM Ti64 parts. During heat treatment, a corrosion agent is diffused into the part to a depth equal to at least half of the support thickness. The corrosion products formed during the heat treatment are then dissolved chemically. The etching solution dissolves only the corrosion products and not the base metal. The effect of the process on the surface finish and the mechanical properties is studied in this work, along with the process kinetics. This work also describes a COMSOL Multiphysics simulation developed to study the impact of part geometry and position on the support removal process. In summary, a novel approach to remove supports from additively manufactured Ti64 parts is demonstrated to decrease costs and further expand the design space for additively manufactured Ti64 parts.

Nanoplastics contamination has increasingly become one important environmental problem
because of their presence in rivers and oceans, toxicity and difficulty in separation and removal.
Here we report the development of a 3D printed water filter system [1], which is topologically
based on schwarzites, which can perform as an efficient nanoplastic scavenger. Schwarzites are
mathematical, 3D porous solids with periodic minimal surfaces having negative Gaussian
curvatures. The atomic-scale models obtained from molecular dynamics simulations are
converted into .stl format. This approach has been proved effective to other classes of 3D
printed materials [2-4]. The FlashForge Adventure 3 Printer (Fused Deposition Modeling) was
used to fabricate the water filters. The ABS (acrylonitrile butadiene styrene) filament used to
print the filter was melted in the extruder at 210 °C and extruded through a nozzle onto the
heated bed. The proposed filtration mechanism of surface-charge-based water cleaning was
further validated using ab initio and molecular dynamics simulations. The fabricated filters
proved to be effective in removing nanoplastics from samples from various sources, such as
sea, river, and pond. These new classes of 3D printed water filters represent a significant low-
cost, energy-efficient, and feasible advanced system to address the problem of nanoplastics
water remediation.
[1] B. Gupta, R. S. Ambekar, R. M. Tromer, P. S. Ghosal, R. Sinha, A. Majumder, P. Kumbhakar, P.
M. Ajayan, D. S. Galvao, A. K. Guptaf, C. S. Tiwary, RSC Advances 11, 19788 (2021).
[2] S. M. Sajadi, P. S. Owuor, S. Schara, C. F. Woellner, V. Rodrigues, R. Vajtai, J. Lou, D. S.
Galvão, C. S. Tiwary, and P. M. Ajayan, Adv. Mater. 30, 1704820 (2018).
[3] S. M. Sajadi, C. F. Woellner, P. Ramesh, S. L. Eichmann, Q. Sun, P. J. Boul, C. J. Thaemlitz, M.
M. Rahman, R. H. Baughman, D. S Galvao, C. S. Tiwary, and P. M. Ajayan, Small 15, 1904747
(2019).
[4] R. S. Ambekar, E. F. Oliveira, B. Kushwaha, V. Pal, L. D. Machado, M. Sajadi, R. H. Baughman,
P. M. Ajayan, A. K. Roy, D. S. Galvao and C. S. Tiwary, Addit. Manuf. 101628 (2020).

Pressure sensors for wearable healthcare devices, particularly force sensitive resistors (FSRs) are widely used to monitor physiological signals and human motions. However, current FSRs are not suitable for integration into wearable platforms and are expensive to process. This work presents a novel technique for developing textile force sensitive resistors (TFSRs) using a combination of inkjet printing of metal-organic decomposition silver inks and heat pressing for facile integration into textiles. The TSFR structure consists of a thermoplastic polyurethane (TPU) membrane sandwiched between two nonwoven textiles with inkjet printed electrodes. The insulating void between the top and bottom electrodes creates an architectured piezoresistive structure. The structure functions as a simple logic switch where under a threshold pressure the electrodes make contact to create conductive paths (on-state) and without pressure returns to the prior insulated condition (off-state). The TFSR can be controlled by arranging the number of layers and hole diameters of the TPU spacer to specify a wide range of activation pressures from 4.9 kPa to 7.1 MPa. For a use-case scenario in wearable healthcare technologies, the TFSR connected with a readout circuit and a mobile app shows highly stable signal acquisition from finger movement. According to the on/off state of the TFSR with LED bulbs by different weights, it can be utilized as a textile switch showing tactile feedback. When integrated with a physical device or object, the sensor design has a high potential for registering tactile interaction as a means to quantify health-related behaviors such as medication adherence or use of a self-monitoring device.

Material properties are governed by their chemical composition and intrinsic crystalline structures. This fundamentally limits material properties and their applicability creating trade-offs for selecting materials for product applications, from structural components to antennas and robots.
Metamaterials represent the concept of utilizing artificial material building blocks to create desirable properties and behaviors derived from three-dimensional architectures and compositions. Additive manufacturing is widely used to construct complex 3D objects and architectures made of metal, plastic, or ceramics from a digital computer model. However, it is presently difficult to combine different materials (structural, dielectric, conducting and ferroelectrics) to create a complex device with multiple functionalities.
In this talk, I will outline a suite of new material processing and additive fabrication routes to rapidly assemble complex topology, multi-scale features and multi-material attributes. I will present high speed printing techniques that enable the rapid patterning of a variety of structural, electronic and ferroelectric materials into complex architectures, not limited by tool-path, embedding and wiring requirement. Attention is focused on how these additive manufacturing techniques, combined with computations and artificial intelligence, unlock new material discovery and design freedoms, which leads to new classes of structural and ferroelectric materials, where presently unachievable mechanical and energy conversion behaviors can be created. Next, I will present future device architectures derived from these artificial building blocks, from materials for energy absorption, wave guiding, to smart transducers and autonomous micro-robots.

Ceramics are extensively used in applications due to their excellent mechanical and chemical performance, such as outstanding compressive mechanical strength, high thermal stability, and corrosion resistance. Extrusion-based 3D printing methods are promising for digitally manufacture ceramics due to ease of use, good adaptability to different materials, and high solid loading of slurries. The auger extrusion method, extruding the slurry by a screw, not only provides accurate control of the slurry flow including consistent flow rate and precise start and stop, but also possesses the ability of mixing multi feedstocks into a homogeneous slurry before being extruded. The viscosity of the slurry determines the printability of the slurry in the auger extrusion-based 3D printing process. However, little research has focused on the in-line measurement of the slurry viscosity and a feedback system to self-control the viscosity of the slurry. In this work, we fabricate alumina parts using an adaptive auger extrusion-based 3D printing system with an in-line viscosity monitoring and feedback system. A high viscous alumina feedstock and a low viscous ink are fed into the barrel and mixed by the auger screw. The viscosity of the mixed slurry is calculated with an in-line capillary rheometer using measured pressure drop after extrusion. Based on the measured in-line viscosity, the slurry is adaptively controlled by adjusting the feeding ratio of two ceramic feedstocks with different rheological properties. This work enables a speedy development of printable ceramic slurries and provides a quality control in the extrusion-based 3D printing of ceramics.

Energy storage devices have a spreading application in various fields, such as soft robotics, human motion monitoring, and smart textiles. Fiber-shaped supercapacitors are especially attractive as an energy storage unit in these applications due to their excellent flexibility. However, fabricating robust fibers with large yields remains a challenge.
In this work, we prepare flexible core-shell fibers via coaxial extrusion printing. Specifically, an extrusion system with a custom-designed core-shell nozzle is used to extrude two separate slurries at the same time. Carboxymethylcellulose sodium salt (CMC) slurry with controlled rheological properties is extruded from the outer channel, while the graphene oxide (GO) slurry is extruded from the inner channel simultaneously. The freeze-drying processing protects GO sheets from agglomeration, providing more efficient chemical reduction. The reduced GO (rGO) sheets are separated and expanded to fill in the CMC sheath, which eliminates the shrinkage of the shell structure and delamination between the CMC sheath and rGO core after drying. CMC is designed as a sheath to package the inner conductive rGO core, as well as a separator to avoid short circuits between two electrodes. We study the influences of the freeze-drying process on the fiber microstructures, and explore the slurry design, fiber quality, reduction condition, and electrochemical performance. The core-sheath structure endows the fiber-shaped supercapacitors with robustness and flexibility. The fabrication method realizes more efficient conducting networks, allowing scalable manufacturing of the core-shell microfiber electrodes.

Rapid cooling rates and stochastic interactions between the heat source and feedstock in additive manufacturing (AM) result in strong anisotropy and process-induced defects deteriorating the tensile ductility and fatigue resistance of printed parts. We show that by deliberately introducing a high density of lack of fusion (LoF) defects, a processing regime that has been avoided so far, followed by hot isostatic pressing, we can print Ti-6Al-4V with reduced texture and exceptional properties surpassing that of wrought, cast, forged, annealed, and solution-treated and aged counterparts. Such improvement is achieved through the formation of low aspect ratio α-grains around LoF defects upon healing, surrounded by α-laths. This occurrence is attributed to surface energy reduction and recrystallization events taking place during healing of LoF defects. Using an adaptive domain misorientation approach of electron backscattered diffraction, we resolve the heterogenous distribution of strains in the duplex microstructure in the form of complex dislocation cells, and sub-grains. Our approach to design duplex microstructures is applicable to a wide range of AM processes and alloys and can be used in the design of damage tolerant microstructures.

The ability to control the nanoscale internal structures of soft polymers networks during fabrication is challenging. When done, these materials have interesting mechanical, optical, and physical properties. Herein, using direct ink writing, phase separated liquid crystalline networks are 3D printed that exhibit oriented, phase-separated molecular structure. The liquid crystalline domains within the printed material are aligned in the print direction and feature a nanoscale lamellar structure that impacts the materials mechanical and optical properties. At temperatures above the phase transition of the liquid crystal, its nanostructure deteriorates resulting in material softening and loss of optical birefringence. Upon cooling, the nanostructure reforms and its properties return. As a result of the oriented structure, when the printed material is submerged in solvents that swell the liquid crystalline phase, the material expands preferentially in its printed direction. This expansion results in a reconfiguration of the materials structure that can be controlled by adjusting its print direction and orientation during fabrication. The results and strategies presented herein, will be of interest to those studying how additive manufacturing can be used to design polymers with aligned and oriented structures.

With the increasing interest in long-term human habitation of space on the lunar and Martian surfaces for both research and commercial interests, there is a growing need for cutting-edge, versatile materials that can be modified to be used for more than a single purpose. TiO₂ based foam and microemulsion inks have been shown to have many advantageous properties, including: photocatalyzation with the UV spectra, filtration, habitat repair/reinforcement, and solar cell creation.
In this report, the first successful 3D printed metal oxide nanoparticles aqueous-based foams performed in microgravity are recorded. While undergoing parabolic flight and simulating microgravity in an aircraft, a Direct Foam Writing (DFW) method was used to print TiO₂ foam in line patterns onto glass substrates. Initial characterization of the foam’s properties is stated. The printed foam’s cross-sectional area, contact angle of DI water, and surface roughness is affected by microgravity as compared to Earth gravity, displaying less deflation and lower amount of spread after being deposited on the substrate.
As a result of the experiment’s success, the WVU Microgravity Research Team plans to test the viability of two additive manufacturing techniques; Cold Spray Additive Manufacturing (CSAM) and spray atomization of a TiO₂-based foaming microemulsion.