Symposium SF03—3D Printing of Functional Materials and Devices

Halloysite nanotubes (HNTs) are naturally occurring clay nanotubes having 50 nm external diameters, 15 nm luminal diameters, and 500-1500 nm lengths on average. Halloysite is a two-layered aluminosilicate, chemically similar to kaolin, and has a predominantly hollow nanotubular structure in the submicron range. HNTs typically display an inner diameter ranging from 15–50 nm, an outer diameter from 30–50 nm, and a length between 100–2000 nm. HNTs exhibit vigorous activity, especially when coated with metal, and have found industrial uses due to their properties as a filler leading to strength reinforcements. We created a new nanocomposite 3D printer filament designed to manufacture replacement parts in a matter of hours, not days, using a 3D scanner and a commercially available 3D printer. Our composite consists of HNTs and nylon (6 and 12). Our lab analysis (3-point bed test, crystallinity measurements, elasticity, hardness tests) of this printer filament products showed significant increases in mechanical strength. In addition, DSC analyses showed enhancement of the nanocomposite filament's thermal resistance. Fused depositional modeling (FDM) or high-intensity laser-powered projector printing (LAPS) were used to design and print high-end, high-performance devices, parts, and tools. Regardless of which nylon type or a blend of nylon, the addition of HNTs significantly reinforced the material properties of 3D printed parts and tools. 3D printed constructs showed a higher bend capacity, increased hardness, and elasticity while keeping the inherent native properties of nylon. The filaments are the most costly to produce. The application of nylon with additives 3D printing will enable 3D printed devices, and parts produced on-site. Examples include anti-icing gasket, O-rings, sump cover, non-critical components, and standard requirements such as surface toilet cover, door handles, and light switches. These devices can be custom-fitted, produced on-demand, and in a rapid manner. The complexity of design, internal or external architectures, does not significantly challenge the fabrication of high-quality finished parts. Accordingly, this technology can move beyond its original low-volume, high-value market niches toward economical and sustainable manufacturing on an industrial scale.

Programmable Materials are physical materials that can be embedded with information and capabilities like logic, actuation, or sensing.We are now able to design and fabricate these material compositions through new forms of 3D and 4D printing. In this talk we will present the fundamental principles for programmable materials through material properties, geometry, activation energy and physical transformation. We will then explore various material properties and fabrication processes that enable new programmable behaviors and functionality in our built environment. By taking advantage of these new material behaviors and emerging fabrication processes we are now able to create higher performing products which tap into natural forces like moisture, light, temperature, or pressure to become highly active and tuned to their environment.

Soft electrostatic actuators stand out from other classes of compliant transducers thanks to their combination of fast response, high strain, and high power density. Hydraulically-Amplified Taxels (HAXELs) consist of planar polymer pouches filled with dielectric oil, with electrodes patterned on the periphery of both surfaces, and a central stretchable region. Applying a voltage between the electrodes generates an electrostatic pressure that pulls the electrodes together in a zipping motion, thus displacing the fluid towards the center of the actuator, creating a raised bump.
We present the design, fabrication and characterization of stretchable, ink-jet printed, HAXEL arrays. Using additive manufacturing (AM) allows the fabrication of fully-integrated actuators whose design can be rapidly changed. Three inks were developed for a commercially available printer (Jetlab 4XL from MicroFab Inc.). A solution of polydimethylsiloxane (PDMS) and solvent is dispensed in order to deposit 30 µm thick stretchable dielectric layers. 5 µm thick electrodes are patterned by using a suspension of carbon black particles combined with silicone polyglucoside. Fluidic channels and cavities are obtained by printing sub-3 µm thick sacrificial layers from ethyl cellulose. Printing HAXELs requires patterning a stack of at least 6 layers. The sacrificial layer, which defines the fluidic circuit, extends to the edge of the printed structure, where PDMS tubes are temporarily connected. The sacrificial material is removed by injecting ethanol, which is then replaced by a dielectric oil (Envirotemp FR3) at the desired pressure. The HAXELs are finally sealed, and the filling tubes removed.
We printed and tested 2x2 arrays of 5 mm wide square HAXELs. Each actuator is obtained by printing a 100 µm thick stack of the functional materials on a 150 µm thick PDMS substrate. Actuators are less than 1 mm thick after filling with dielectric fluid. We observe 250 µm of free displacement and 30 mN blocked force at 4 kV voltage. These actuators will be integrated into stretchable haptic feedback systems. Inkjet printing enables easily tailoring the geometry of the device to adapt to different location on the body, and to different morphologies between users.

Designing soft sensorized robots with distributed sensing capabilities is essential for addressing long-standing challenges in soft robot control. However, progress in designing soft sensorized robots has been limited due to the inherent multi-material manufacturing challenges of fabricating such devices and the disadvantages posed by many sensing strategies with soft conductive materials. We present a strategy for creating soft sensorized robots through the fabrication of fluidically innervated, architected materials. These structures are 3D printed with distributed, embedded networks of open fluidic channels. The fluidic channels are connected to pressure transducers, which provide an analog voltage corresponding to changes of pressure within the deforming networks. Importantly, our manufacturing method involves one single material: structure, actuation, and sensing capabilities are seamlessly integrated through printed architecture. Here, we first demonstrate our methods for fluidic sensing on well-studied lattices printed from elastomeric and flexible polyurethanes. We then design and characterize fluidically innervated handed shearing auxetics (HSAs), which we use in examples of electrically-driven soft sensorized robots. Finally, we showcase the importance of robust, reliable sensory feedback in an HSA-based soft robotic system that learns its configuration via deep learning. Overall, our methods represent a simple, single-material strategy for sensorizing soft robotic materials that we anticipate will help expedite efforts in the design, fabrication, and control of autonomous soft machines.

Micrometer-scale robots capable of navigating enclosed spaces and remote locations are approaching reality, with a rich literature emerging on externally actuated and supervised agents. To accelerate the advancement of micron-sized robots with greater autonomy, alternatives to conventional top-down lithography, particularly those amenable to assembly by additive manufacturing, are desired for rapid prototyping and access to novel materials. Building upon experimental findings that memristive networks can be rapidly printed on substrates and lifted-off as single, intact microparticles, we herein introduce an alternative design paradigm based on arrays of two-terminal memristive elements that enables autonomous use of memory, sensing, and actuation in robots at the micron scale. The design’s simple circuit topology permits the use of facile technologies – such as printing, stamping, and colloidal self-assembly – as well as diverse materials including polymers, bio- and nanomaterials. We validate several memristor-based designs representing key building blocks towards robot autonomy: tracking elapsed time, detecting and timestamping a rare event, continuously cataloguing time-indexed data, as well as accessing the collected information for a feedback-controlled response as in a robotic glucose-responsive insulin for diabetes treatment, as a test problem. Our computational results establish a novel actionable framework for microrobotic design whereby tasks that normally require complex microelectronic circuits can be achieved with self-assembled and printed memristor arrays within single microparticles.

Soft silicone structures are difficult to fabricate using the conventional FDM approach to 3D printing. Without adding rheological modifiers silicone inks, printed features will sag and spread upon deposition from the nozzle. Embedded printing into jammed support materials helps to overcome this challenge by trapping the silicone ink in space as it is printed, eliminating the need for ink additives. However, previously developed support materials for 3D silicone printing exhibit a large interfacial tension against silicone inks, creating instabilities that break up fine features and setting a minimum feature size. These instability-driven breakups can be prevented by developing a jammed support material that is chemically similar to silicone oil. Such a material would enable 3D printing of high-resolution structures having fine feature sizes. In the work presented here, we developed a silicone oil-based support material having tunable rheological properties that can be optimized for embedded 3D printing of soft silicone. We show that by practically eliminating interfacial tension between silicone ink and the silicone-based support material, features as small as 8 microns in diameter can be fabricated; these fine features are indefinitely stable over time. Using this new material we are able to fabricate robust, accurate, and complex structures like a model brain aneurysm for surgical simulation and a functional tri-leaflet heart valve model for potential biomedical applications.

Soft robots have garnered interest due to their potential ability to change shape in response to changing tasks or environments, be robust to impacts and falls, conform to the human body without restriction on the natural mechanics of motion, and grasp delicate and diverse objects. Fluidic actuators, which are controlled by the influx of circulating fluids, are the most common artificial muscle technology employed in the soft robotics literature. However, fluidic actuators typically require actuator-source tethering, limiting the mobility of the robot, especially for micro-scale systems. Alternatively, stimuli-responsive materials have been proposed to realize untethered, micro-scale actuation. Such stimuli-responsive materials include hydrogels and shape-memory polymers, among others. However, these material-based actuators are often slow or only expand or contract axially, preventing use in applications that require volumetric expansions.
To address such limitations of existing actuating materials, actuators based on the liquid-to-gas phase change of solvent inclusions encapsulated in a hyperelastic matrix have been realized. These phase-changing inclusion composites fill a gap in the field of material-based actuators by inducing rapid volumetric expansion similar to fluidic actuators, without being tethered to an external fluid source. However, phase-changing inclusion composites have never been demonstrated in micro-scale actuation spaces and generally are not amenable to printing, given the high content of low-viscosity liquid solvent in the mixtures.
In this talk, we will present granular actuators composed of volumetrically expanding grains as a new class of actuator to unlock scalable and printable actuation. A single grain consists of multiple solvent cores encapsulated in a hyperelastic silicone shell (Ecoflex 00-30). At elevated temperatures, the encapsulated solvent vaporizes and increases the internal pressure of the hyperelastic shell, inducing volumetric expansion of the grain. The grains are independently capable of rapid, high-force actuation, and are also easily arranged into granular assemblies to form larger-scale bulk actuators. Finally, the use of grains suspended in a carrier solvent or resin enables compatibility with granular self-assembly and 3D printing techniques, offering the potential to print volumetrically expanding actuators into freeform patterns across scales.

Ductile all-ceramics composite materials are at the forefront of the next generation of materials. Typically, engineering ceramics are brittle across working temperatures and do not allow for any deformation, let alone extreme ductility. The ability to have an all-ceramic, with typical ceramic characteristics, that can have ductile behavior, under relatively low temperatures, and maintain structural integrity without catastrophic failure is a unique characteristic that is experimentally explored in this work. Further, thermally conductive filler particles in a ceramic matrix has the potential to provide unique thermal management opportunities for high-powered electronics or hypersonic aircraft. In order to fabricate these materials, a previously developed 3D printing technique is used to align the micro-particles to produce the green-body part, and then run through a sintering process to produce the all-ceramic composite. This work also explores the materials thermal shock resistance that is believed to be directly correlated to the alignment patterns of the microstructure, and the thermal conductivity of the platelets distributed throughout the dominant ceramic matrix. Therefore, the work presented here demonstrates the feasibility of a highly-ductile all-ceramic composite, at relatively low temperatures, that demonstrates good thermal shock resistance.

Direct ink writing of 3D structures has many advantages, specifically in this study is the ability to print multiple materials with tailored functionalities in a controllable and single-step process. It further allows for net shape printing in ambient conditions of a wide range of materials normally incompatible with each other or with other printing systems. In this work, custom-designed coaxial nozzles have been 3D printed using a stereolithography printer. These nozzles were then used to co-extrude conductive ink cores within shells of ambient or UV-cured elastomeric shells. Initially, conductive fibers were printed to study the interfaces, curing, and electro-mechanical properties of the dual ink system. Further, the core-shell fibers were printed into strain gages and incorporated within direct ink written 3D soft robotic pneumatic actuators. Infrared spectroscopy, nuclear magnetic resonance, and optical and electron microscopy were performed to further characterize the printed structures.

There is a technological need for materials with improved functionalities to propel new biomedical applications. Some of the outstanding challenges could be solved using three dimensional (3D) printing. However, there is a pressing need for new generation of biomaterials that have unique properties. Combining nanomaterials with natural and synthetic polymers could yield biomaterials with improved propertesi including the ability to better recapitulate complex architecture, better match native mechanical properties of tisses and even guide the formation of functional tissues. To accomplish some of these goals we have engieered dual nanocoated (Zinc and Silver) filaments using polybutylene succinate (PBS) and lignin as the base polymer blend. We show that this combination of polymers and coating provides strain specific antibacterial activity and antioxidant properties while improving the rheological and physical properties of the composites filaments. Taken together these nanocoated filaments could propel several biomedical applications including aiding design of bracing materials.

Electrochemical reactors and separation devices are predominantly planar in nature and limited to basic design space and inability to optimize complex reactor designs. An explicit limiting component are ion exchange membranes (IEMs) that control transport of selective ions across a physical boundary. IEMs are typically manufactured by evaporation casting ionomers into thin flat sheets; however, three-dimensional ion exchange membranes have the potential for significant improvement in optimized mass transport properties, antifouling capabilities, as well as increasing active interfacial surface areas for membrane electrode assemblies. Here, we present a photocurable anion exchange membrane (AEM) formulation that can be 3D printed with ~100 um resolution using micro-stereolithography. We optimize the printable formulation by characterizing the ion exchange capacity, ion conductivity, water uptake and dimensional stability as a function of the composition. Printed 3D structures are investigated and operated in diffusion dialysis acid recovery flow cells for continuous recycling of metal contaminated acid. This technology has the potential to enable customizable manufacturing of membrane electrode assemblies for CO₂ electrolysis, unlocking control over structures across multiple length scales to optimize for desired reduction products. This work is 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-823498.

The ability to capture, manipulate and release microscale objects using autonomous systems can enable widespread applications—from microsurgery and selective cell extraction to the assembly of complex microdevices. Microelectromechanical (MEMs) grippers fabricated using semiconductor techniques provide precise and reliable gripping mechanisms but are intrinsically planar and require direct electrical contacts for operation. With continuing development of smart and environmentally responsive materials compatible with 3D printing, cue-driven micro-grippers with high spatial and temporal precision can give rise to biocompatible devices. In this work, we investigate 3D printing of microgrippers based on traditional MEMs designs as well as non-planar, bioinspired architectures. Our aim is to develop robust micro-grippers integrated into imaging/optogenetic devices at the scale of single fiber optics and individual biological cells to enable micro-surgical applications. Using multiphoton lithography, we fabricate multimaterial actuators that incorporate inert skeletal components with hydrogel actuators. We evaluate the response time, accuracy and cyclability of grippers that use temperature and pH responsive hydrogels. Following this optimization, we designed microgrippers that incorporate a photothermal transducer within a thermally responsive gel actuator fabricated on the tip of a 200-micron diameter core fiber. By incorporating 3D printed micro-optics (lenses and reflectors), we show that gripper components can be actuated individually via site-specific photothermal transduction. This work provides a foundation to integrate stimuli responsive mechanical functions with micro-scale optical devices.

New fabrication approaches for mechanically flexible materials hold the key to advancing the applications of bioelectronics in fundamental neuroscience and the clinic. By combining the high precision of thin film microfabrication with the versatility of additive manufacturing, we demonstrate a straight-forward approach for the prototyping of intracranial probes capable of multichannel local field recordings and convection-enhanced drug delivery. The process makes use parylene-based arrays of neural recording electrodes and a commercial vat polymerizable elastomer, combining to produce a flexible implant. The mechanical and electrical properties of the implant are characterised, and its function is validated in an in vivo rodent model. We show that the implant can modulate neuronal activity in the hippocampus through local drug delivery, while simultaneously recording brain activity by its electrodes. Chronic implantation tests show good implant stability and minimal tissue response one week post-implantation. Our work shows the potential of hybrid neuronal probes combining different manufacturing technologies and paves the way for a new approaches to the development of multimodal probes.

Recent advancements in the field of flexible electronics, for instance, smart wearables, soft robotics and health monitoring devices has garnered immense attention in the domain of stretchable chemical and physical sensing methods. Amongst the plethora of sensors available, temperature and pressure sensors have been extensively investigated for fulfilling the demand of flexible, light-weight and disposable sensors demonstrating high sensitivities. The sensors of the former category can be utilized for structural-health monitoring, whereas sensors from the latter category can be utilized for pulse monitoring and provide vital information regarding the arterial physical situation of the body in a non-invasive way. A printed and built-in geometry with a capability of detecting stimuli such as temperature, pressure, etc. over a wide range are highly sought-after characteristics for any sensor. These smart components are manufactured using a combination of conductive copper inks and an extrusion-based direct-ink writing technique, onto flexible substrates. The sensors exhibit high sensitivities and show resistance to mechanical stimuli of bending. These demonstrations enunciate the capability of additive manufacturing techniques for flexible electronics.

Combining the unprecedented design freedom in microscale additive manufacturing with the ability to control the chemical nature of each printed voxel could unlock unique possibilities for tailoring mechanical, chemical, electrical, optical and magnetic properties of inorganic microstructures.
A current challenge in the field of micro- and nanoscale additive manufacturing is to provide what microfabrication needs: the fast and reliable deposition of a wide range of device-grade materials, from insulators to semiconductors and metals [1]. In addition, on-the-fly switching between different materials classes remains a crucial, yet unsolved problem. Electrohydrodynamic redox 3D printing (EHD-RP) is a novel small-scale additive manufacturing technique, that offers a resolution of 250 nm with feature sizes down to 120 nm [2]. The ink-free approach, based on the on-demand generation of metal ions, enables the local modification of the chemical composition in a voxel-by-voxel fashion. However, so far only the deposition of metals was demonstrated.
We will focus on the challenges involved in expanding the materials range of EHD-RP from Cu and Ag to a wide range of metals, semiconductors and insulators. In particular, the deposition of Zn structures and their subsequent thermal oxidation to semiconducting ZnO will be presented. Together with an analysis of its structure, its optical properties will be assessed by photoluminescence spectroscopy. In addition, the direct deposition of insulating structures will be demonstrated on the example of MgO and first results towards their integration with printed metal conductors will be shown.
In summary, we outline a route towards establishing electrohydrodynamic redox 3D as a toolbox for the manufacturing of multi-material microelectronic devices.

[1] L. Hirt et al., Additive Manufacturing of Metal Structures at the Micrometer Scale, Adv. Mater., 29(17), 2017.
[2] A. Reiser et al., Multi-metal electrohydrodynamic redox 3D printing at the submicron scale, Nat. Comm., 10(1):1-8, 2019.

The Internet of things (IoT) market is a new groundbreaking field that has gained great attention both from the academic and industrial point of view. Progress in this field is deeply interconnected to the possibility of integrating smart devices with everyday objects and instruments, so that they are able to collect process and elaborate data without the need of human input. Therefore, always more demanding energy storage devices able to power such smart objects are needed. However, rigid, conventional batteries, as well as their respective fabrication processes, find limited applications when dealing with wearable electronics. Specifically, one of the major limitations of conventional batteries is, indeed, their rigidity and inadequate mechanical. When patterned electrodes or unconventional substrates are involved, new fabrication strategies are required to overcome limitations of traditional slurry coating electrode processing ^[1,2].

In this context, printing of batteries can be framed as a potentially innovative fabrication technique able to combine the benefits of thin-film technology, as for the lightness, mechanical flexibility, easiness of integration, and those typical of Additive manufacturing (AM), as for the long-term production low costs, scalability and versatility, easy processability and reproducibility. Among the technologies belonging to the AM class, Inkjet printing (IJP) is a solution-based mask-less additive technique able to deposit layers of different materials by propelling droplets of ink onto various substrates. When dealing with thin-film batteries, IJP is preferable over other printing techniques for several reasons, above all the limited material waste, as it is possible to finely control the position of the droplets, and the higher resolution this technique can provide ^[3]. However, a challenging aspect is related to the material processing, as specific ink physical properties need to finely be tuned to achieve a printable suspension.

Despite being a strategic solution for patterned flexible electrodes fabrication, to date poor attention has been driven toward a systematic study of how IJP could be possibly applied to Lithium-ion batteries (LIB) fabrication. In addition to this aspect, printing current collectors represents the main obstacle to obtain a fully inkjet printed LIB. An emerging class of material with outstanding electrical, electrochemical and rheological properties is represented by 2D transition metal carbides/nitrides, known as MXenes. Ti₃C₂T_x is the most studied, due to its extremely high conductivity (> 15 kS cm^-1) and hydrophilicity, allowing to produce a safe, non-toxic aqueous ink, without the need of any surfactant or additive to control the stability ^[4]. The combination of these properties allows to easily print an extremely conductive current collector.

The aim of the work is the fabrication of a thin-film flexible inkjet printed battery exploiting aqueous-based LTO and LMO inks, used as anode and cathode, respectively, and MXene-based current collector. An extensive inks characterization will be presented in terms of viscosity, surface tension and mean zeta potential, to understand the inks stability and printability. Morphological and electrical analysis of the printed substrates, combined with an in-depth electrochemical characterization in order to understand the battery performance, will be discussed.

[1] K.-H. Choi, D. B. Ahn, S.-Y. Lee, ACS Energy Letters 2018, 3, 220.
[2] W. B. Hawley, J. Li, Journal of Energy Storage 2019, 25, 100862.
[3] S. Lawes, Q. Sun, A. Lushington, B. Xiao, Y. Liu, X. Sun, Nano Energy 2017, 36, 313.
[4] S. Uzun, M. Schelling, K. Hantanasirisakul, T. S. Mathis, R. Askeland, G. Dion, Y. Gogotsi, Small 2021, 17, 2006376.

Owing to its unique properties such as tunable bandgap and strong optical absorption, perovskite material holds great potential and has drawn extensive research interest for a wide range of optoelectronic applications including solar cells, photodetectors (PDs), and light-emitting diodes (LEDs). In most reported work, the perovskite-based optoelectronic devices are constructed in a multilayer structure on rigid indium-tin-oxide (ITO) glass substrate with a vacuum-evaporated metal top electrode. In this work, we report a unique approach for direct handwriting of high-performance hybrid organic-inorganic perovskite LEDs and PDs on unconventional substrates (paper, textile, three-dimensional surfaces) using a regular ballpoint pen filled with formulated electronic inks. This novel and alternative fabrication strategy allows high-performance LEDs and PDs to be easily fabricated by any untrained individuals in a time-efficient (the whole device can be made within a few minutes) and cost-effective manner. This is especially useful for early-stage demonstrations or application with less stringent requirement on resolution, as well as science popularization and education purposes.

More specifically, the fully-handwritten perovskite LEDs and PDs employs a simple sandwich structure, consisting of a composite transparent bottom electrode of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and poly(ethylene oxide) (PEO), a composite active layer of methylammonium lead tribromide (CH₃NH₃PbBr₃) and PEO, and a silver nanowire (AgNW) top electrode. With the addition of polymer into the perovskite precursor, the voids between crystals can be eliminated to obtain a uniform and compacted functional layer. The multicolor perovskite-LEDs exhibit emission covering the whole visible range, especially the green emission with a low turn-on voltage of 2.3 V and high brightness of greater than 600,000 cd m^-2. Furthermore, this direct handwriting approach is also extremely versatile for use on various cellulose and fabric substrates and on curved and irregular surfaces that cannot be accomplished via conventional coating and deposition approaches. By integrating the perovskite-based LEDs and PDs, a handwritten sensor patch capable of measuring photoplethysmogram (PPG) waveforms from humans has also been demonstrated. This work demonstrates the great promise of using handwritten perovskite optoelectronic devices for emerging wearable health-monitoring applications.

The rapid growth of the electronics industry has resulted in a sustained need for portable electronics. Significant effort has been directed towards developing miniaturized devices for applications in wireless sensor networks, biomedical sensors, actuators, and wearables. Due to a rise in the popularity of the Internet of Things (IoT), demand for “autonomous wireless sensors” has increased. These devices can be used for smart building control, industrial process automation, factory automation, and many other applications. Such devices require integrated power sources to provide stable current supply and to deliver high peak currents on the order of ~1-10 mA while occupying small areas (< 1 cm2). Recent work has shown that except for the power supply, all the components needed for standards-compatible wireless communication can be integrated onto a single silicon chip with a wirebond antenna. Printing the power supply directly on such a chip at wafer-scale would be a low-cost, small-volume solution.

For miniature or scaled-down Li-ion batteries, there are three key battery configurations that have been adopted, thin-film, thick-film, and 3D architectures. Typically, thin-film batteries suffer from insufficient power and capacity per unit footprint area to allow the operation of microelectronic devices. Thick-film battery configurations employ printing processes such as stencil printing[1], laser-direct write technique (LDW)[2,3] etc. to allow the fabrication of porous structured thick-film electrodes without requiring any lithographic patterning. 3D battery architectures involve 3D electrodes in several forms such as arrays of microrods/microtubes synthesized by templating method, honeycomb-like and sponge-like. However, the fabrication of 3D battery architectures is often quite complex and restricted to very few cathode and anode materials. One promising approach to develop high capacity miniature/micro Li-ion batteries is to employ printing methods such as stencil and screen printing to deposit thick, high capacity electrodes[1]. Printing methods also provide the flexibility to customize battery active area as per the device layout and size requirements.

This work demonstrates stencil-printed high-capacity Li-ion micro-batteries fabricated in Silicon for IoT applications. The battery consists of synthetic graphite and lithium cobalt oxide (LCO) as the respective anode and cathode layers printed on thin evaporated current collectors. For the fabrication of batteries, LCO is printed in a 225 um deep, 25 mm² Si cavity and the graphite electrode is printed on a glass substrate. Once printed, the batteries are assembled and packaged using an airtight seal (Torr Seal) in a glovebox and tested in air. This work investigates the electrochemical limitations of Li-ion batteries as the active areas are scaled-down. The influence of the electrode film thickness on the battery performance is studied. The batteries demonstrate an areal capacity as high as ~7.2 mAh/cm2, capacity retention >80% and columbic efficiency > 97% under 30 charge/discharge cycles. Our experiments show that a 25 mm2 battery can power a wireless transceiver chip, SCµM [7] that consumes a baseline current of 650 μA with a peak current as high as ~4 mA.

[1] R. Kumar, K. M. Johnson, N. X. Williams, V. Subramanian, Adv. Energy Mater. 2019.
[2] H. Kim, J. Proell, R. Kohler, W. Pfleging, A. Pique, J. Laser Micro Nanoeng. 2012.
[3] J. Pröll, H. Kim, A. Piqué, H. J. Seifert, W. Pfleging, J. Power Sources 2014.
[4] A. M. Gaikwad, A. C. Arias, D. A. Steingart, Energy Technol. 2014.
[5] R. E. Sousa, C. M. Costa, S. Lanceros-Méndez, ChemSusChem 2015.
[6] K. H. Choi, D. B. Ahn, S. Y. Lee, ACS Energy Lett. 2018.
[7] F. Maksimovic, B. Wheeler, et al, in IEEE Symp. VLSI Circuits, Dig. Tech. Pap., 2019.

Polymeric adhesive materials are widely used in the electronic industry for applications such as wafer bonding, 3D integration and packaging. One of the most used materials is benzocyclobutene (BCB). BCB is a thermosetting polymer that can be processed as polymer solution, also called ink. Usually, BCB is processed with spin-coating. However, spin-coating alone does not allow to fabricate complex patterns and it needs to be coupled with a lithographic process. Moreover, lithography is challenging when high topographies need to be fabricated ¹.

To overcome this issue, jet printing technologies can be used. Jet printing is a direct ink writing additive manufacturing technology based on the emission of material in form of droplets ². It is a non-contact and maskless technology. Two of the most used jet printing technologies are drop-on-demand inkjet printing and aerosol jet printing. Inkjet printing is characterized by the selective emission of droplets obtained by a thermal or a piezoelectric actuator ³. Aerosol jet printing, instead, is based on the emission of a focused nebulized ink, i.e., the aerosol, onto a substrate ⁴. Both inkjet and aerosol jet printing present common advantages, such as the high versatility, high scalability, and the possibility of printing a wide variety of functional inks, e.g., polymeric, metallic and ceramic inks ⁵. The main advantage of aerosol jet printing over inkjet printing is the possibility of using inks with a much wider range of viscosity, i.e., up to 1000 mPas instead of 20-30 mPas. However, aerosol jet printing has some drawbacks such as overspray and ink stability ⁶.

Here, we present the fabrication of BCB-based inks by inkjet printing and aerosol jet printing. For inkjet printing, we use a drop-on-demand system with a piezoelectric actuator. For aerosol jet printing, a pneumatic system is used allowing to print inks with a relatively high viscosity. Careful ink formulation is carried out for inkjet printing to have printable inks. Indeed, inks having a viscosity of ~5 mPas exhibit a stable jetting. For aerosol jet printing, instead, we evaluate the effect of different polymer dilutions. We show aerosol formation and printing of BCB-based inks with a viscosity up to ~400 mPas. Moreover, the other physical properties of the inks that have an influence on the printing process are characterized. Then, we print simple geometries to perform an in-depth characterization of the printing parameters. The feasibility of printing complex patterns is also presented. A comparison between inkjet printing and aerosol jet printing is proposed, with particular interest in the difference of the morphology of the printed patterns. On one hand, we observe that aerosol jet printing exhibit a higher versatility being possible to print pattern having a thickness ranging from 1 μm to 30 μm. On the other hand, inkjet printing shows a better reproducibility for complex geometries and better edge sharpness. Finally, adhesive properties of the printed materials are characterized.

1. Iervolino, F. et al. Inkjet Printing of a Benzocyclobutene-Based Polymer as a Low-k Material for Electronic Applications. ACS Omega 6, 15892–15902 (2021).
2. Shao, F. & Wan, Q. Recent progress on jet printing of oxide-based thin film transistors. J. Phys. Appl. Phys. 52, (2019).
3. Nayak, L., et al. A review on inkjet printing of nanoparticle inks for flexible electronics. J. Mater. Chem. C 7, 8771–8795 (2019).
4. Secor, E. B. Principles of aerosol jet printing. Flex. Print. Electron. 3, (2018).
5. Seifert, T. et al. Additive Manufacturing Technologies Compared: Morphology of Deposits of Silver Ink Using Inkjet and Aerosol Jet Printing. Ind. Eng. Chem. Res. 54, 769–779 (2015).
6. Abdolmaleki, H. et al. S. Droplet-Based Techniques for Printing of Functional Inks for Flexible Physical Sensors. Adv. Mater. 33, 2006792.

Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronics [1] and quantum technologies [2]. With properties ranging from insulating (e.g., hexagonal boron nitride) to semiconducting (e.g., transition metal dichalcogenides) to conducting (e.g., graphene), nearly any electronic device can be fabricated by properly assembling 2D materials into heterostructures [3]. While device prototypes have been demonstrated on idealized research-scale samples, scalable manufacturing remains a challenge for 2D materials. In parallel, the field of printed electronics has made significant progress towards roll-to-roll additive manufacturing based on organic and nanoparticle inks. This talk will explore recent work aimed at uniting these efforts by designing electronic inks that combine the superlative electronic properties of 2D materials with the scalable manufacturing of printed electronics. To achieve this objective, multiple design goals have been concurrently pursued including exfoliation (i.e., optimizing yield, throughput, and flake size), ink formulation (i.e., engineering solvents and stabilizing surfactants and polymers for aerosol, inkjet, gravure, screen, and 3D printing), printed structure morphology (i.e., controlling film morphology and microstructure following printing via solvent evaporation, solvent additives, and curing conditions), and control of interfacial properties (i.e., minimizing interfacial resistance in conductive inks, maximizing capacitance in dielectric inks, and maximizing mobility for semiconductor inks). By achieving high levels of 2D material ink homogeneity and printing fidelity, a variety of high-performance applications have been achieved including photodetectors, optical emitters, supercapacitors, batteries, and sensors [4-7].

[1] V. K. Sangwan and M. C. Hersam, Nature Nanotechnology, 15, 517 (2020).
[2] X. Liu and M. C. Hersam, Nature Reviews Materials, 4, 669 (2019).
[3] M. E. Beck and M. C. Hersam, ACS Nano, 14, 6498 (2020).
[4] J.-W. T. Seo, et al., ACS Appl. Mater. Interfaces, 11, 5675 (2019).
[5] K. Parate, et al., 2D Materials, 7, 034002 (2020).
[6] K.-Y. Park, et al., Advanced Energy Materials, 10, 2001216 (2020).
[7] W. J. Hyun et al., Advanced Materials, 33, 2007864 (2021).

Additive manufacturing of glass allows the fabrication of complex structures and geometries that traditional approaches cannot achieve. However, current strategies for 3D printing glass require thermal processes over 1000 °C to produce functional parts. We describe the development of low temperature process to 3D print glass and multimaterial composite glasses based on metastable silicate chemistry. The direct-write deposition process occurs at room temperature and the curing process only requires 250 °C to achieve a stable glass structure. The properties of the printed glass can further be tailored in a plug-and-play fashion by introducing functional filler materials such as conductive particles. This straight-forward strategy will enable the fabrication of a wide variety of microfluidic, electronic, and radio frequency devices with higher thermal stability without the need for extensive thermal processing.

We present a strategy for transforming 3D-printed polymer lattices into a versatile technology platform for multimodal electronic sensing. This method exploits nanometric ALD coatings of ZnO and ZnO:Al deposited directly onto the 3D printed polymer scaffold structures. We demonstrate that these conductive octet lattices can be designed for electronic multifunctionality, demonstrating 3D-enhanced chemical, thermal, mechanical, and fluid sensing. These applications leverage the synergistic effects of ultrathin semiconducting and conducting coatings with an increased surface-to-volume ratio due to the microscale pores of the 3D lattice.
The 3D sensors show a 100X enhanced response over 2D films when applied as low power, (10-50 nW) room temperature gas sensors for volatile organic compounds (VOCs). Leveraging the ultrathin deposition capability, 3D sensors with varying ZnO coatings (11 - 60 nm) were fabricated to enhance sensitivity, achieving thicknesses near the Debye Length to induce surface-dominated electrostatic interactions with physiosorbed gas molecules. The tunable electrical conductivity of these structures also facilitates a variety of thermophysical response mechanisms, including rapid air temperature sensing enabled by the low thermal mass resulting in millisecond response time. We explore self-heating functionality at the mW level and utilize geometric control of convective heat transfer to perform thermal anemometry over a 100X range of air velocities. Finally, the 3D scaffolds were also implemented using deformable polymer lattices that allow these oxide-coated structures to function as kPa-level mechanical pressure sensors operating up to 3% overall strain, overcoming the inherent mechanical limitations of ceramic materials.
This ability to engineer electronic transport in additive manufactured mesoscale structures offers huge potential for microelectronics integration and provides a glimpse of how 3D design can fundamentally enhance the performance of biosensors, actuators, and microelectromechanical systems.

The unique structural and electronic properties of 2D materials can be exploited for use in transformative optoelectronic applications. However, devices are usually bespoke structures that require the sequential deposition of exfoliated 2D layers. Therefore, there is a need for scalable manufacturing techniques capable of producing high-quality large-area devices comprising multiple 2D materials. The 3D printing of inks containing 2D material flakes provides a promising solution. It has been demonstrated that inkjet-printing can be used to manufacture devices incorporating 2D materials. However, a greater understanding of the electronic transport phenomena and structural properties of such devices is still needed. Here, detailed electrical and structural characterization is reported and explained by comparison with charge transport modeling which includes inter-flake tunneling transport and percolation network dynamics. Results reveal that the electrical properties are strongly influenced by flake density and complex meandering electron trajectories, which form in the presence of disorder and can traverse several printed layers. Controlling such trajectories is essential for printing high-quality devices that exploit the properties of 2D materials. We use inkjet-printed graphene to form the conductive channel of a fully inkjet-printed field effect transistor and to provide Ohmic contacts on an InSe phototransistor. This is the first time that inkjet-printed graphene has successfully replaced single layer graphene as a contact material for 2D metal chalcogenides.

Photopolymerization additive manufacturing (AM), particularly stereolithography (SLA) and Digital light processing (DLP) is widely adopted due to its high resolution, simplicity, and the broad range of mechanical properties available from photopolymer resins. However, the technique is currently restricted to crosslinked polymers, resulting in challenges in both post-processing and recycling of printed parts that are absent when considering thermoplastic AM techniques. Here, we present a new process called interfacial photopolymerization (IPP) which enables photopolymerization AM of linear chain polymers. In IPP, a polymer film is formed at the interface between two immiscible liquids, each containing one type of reactive species. We demonstrate the integration of IPP in a proof-of-concept projection system and discuss how the reaction parameters affect printing resolution and final polymer properties. We also present a macrokinetics model to deepen understanding of the transport and reaction rates involved in IPP and evaluate the resolution of the printing process.

The development of miniaturized 3D devices with programmability and high performance is key in paving the way towards building next-generation smart micro-systems to revolutionize healthcare, data storage, and human-machine interface. Integrating advanced functionalities, heterogeneous properties and complex 3D geometries is challenging in the micro-scale. Among all the fabrication techniques, 3D printing is the most advantageous in shaping 3D geometries with automated processes of large flexibility and design freedom. Two photon polymerization direct laser writing (2PP-based DLW) has risen as a promising technique for printing arbitrary 3D architectures with a resolution down to hundreds of nanometers.
However, functional composite inks that are compatible with 2PP-based DLW usually suffer from poor performance, due to their low loading fractions of the functional dopants. Because of the intense near-infrared laser beam needed for the 2PP 3D lithography, highly light absorptive materials are intrinsically not suitable for the DLW process—bubbles generated locally will destruct the printed structure, and not enough transmitted light can reach the desired printing depth. Such exemplary materials are composites doped with micrometer-size particles and non-transparent particles (such as carbon nanomaterials, magnetic particles, metallic particles) and highly photoactive materials (such as gold nanoparticles, photothermal dyes). Researchers have used different methods to incorporate DLW-incompatible materials with printed 3D architectures, such as coating, electroplating, surface modification, and molding. Every fabrication technique has its own advantages to meet the requirements of specific applications, but it also has its own limitations. For example, the coating method functionalizes the architectures in a 2D manner, lacking 3D space selectivity; electroplating and surface modification have a limited selection of compatible materials; molding a single material gives an isotropic property lacking advanced programmability.
We present here multistep-molding-integrated direct laser writing that is able to integrate those incompatible functional materials into high-resolution and complex 3D geometries, to create various microdevices with unique functionalities. Metachronal wave motion, fluid mixing, function reprogramming, structural reconfiguring, multiple degrees-of-freedom rotations, and stiffness-tunable double-layer mechanical bits are exemplary demonstrations of the versatility of this fabrication routine.

3D printing or additive manufacturing is the process for building complex geometries via successive addition of materials based on a digital model. However, the traditional 3D printing processes employing planar-layer printing strategy have rather limited flexibility in customization and multi-materials integration. Originally developed for the sole purpose of rapid prototyping, the planar-layer printing process slices the digital models along the z-axis to generate a set of horizontal build layers in x-y plane. Those layers are sequentially stacked from bottom-up to create the physical replica of the digital model. After more than three decades, this standard procedure remains the would-be choice in most of the commercial 3D printers possessing 3-Degree of freedom (DOF) Cartesian motion. Despite its popularity, the limited 1-DOF stage motion along z-axis imposes serious constraints in 3D printing complex geometries. Recognizing how multi-axis machines are capable of producing freeform geometries with better quality and efficiency as compared to the 3-axis machines, there is a growing interest in exploring the new slicing and printing strategy beyond the 3-DOF Cartesian motion. Methods that employed high-DOF stage motion have also investigated in enabling conformal 3D printing on non-planar surfaces and support-free 3D printing. Recently reported computed axial lithography (CAL) method prints entire complex objects via angular accumulation of dynamically evolving light pattern in a cylindric coordinate system. All these methods demonstrated the ability to handle complex geometries with improved speed and surface finishing. Inspired by these most recent studies, we went a step further by allowing freeform manipulation of each build layer during the layered manufacturing process.

We show here untethering the build layer fabrication from unidirectional stage motion unlocks the means to reconstitute the 3D objects with added manufacturability such as custom design transformations, conformal geometry, and multi-material integration. The newfound fabrication flexibility allows us to executes full degree-of-freedom (DOF) transformation (translating, rotating, and scaling) of each individual building layer while utilizing continuous fabrication techniques. Transforming individual building layers within the sequential layered manufacturing process enables dynamic transformation of the 3D printed parts on-the-fly, eliminating the time-consuming redesign steps. Preserving the locality of the transformation to each layer further enables the discrete conformal transformation, allowing objects such as vascular scaffolds to be optimally fabricated to properly fit within specific patient anatomy obtained from the Magnetic Resonance Imaging (MRI) measurements. The manufacturability of the 3D printing process is further augmented with the use of the high-precision 6-axis robotic arm. It allows us to perform manufacturing tasks other than 3D printing, such as rinsing the samples mid-printing process. Finally, exploiting the freedom to control the orientation of each individual building layer, we further establish multi-materials, multi-axis 3D printing capability for integrating functional modules made of dissimilar materials in 3D printed devices. This final capability we demonstrate through 3D printing a soft pneumatic gripper via heterogenous integration of rigid base and soft actuating limbs. Although the reported method primarily focusing on the continuous stereolithography process, the underlying principle can be applied to other 3D printing process, with the potential to ease the burden towards the envisioned multi-process 3D printing.

Spatial Atomic Layer Deposition (SALD) is a recent approach 100 times faster than conventional ALD, even at atmospheric pressure. Previous works exploited these assets focussing on high-rate, large-area deposition for scaling-up into mass production. Conversely, this work shows that SALD indeed represents an ideal platform for area-selective deposition of functional materials by proper design and miniaturization of close-proximity SALD heads. In particular, the potential offered by 3D printing is used to fabricate low-cost customized close-proximity heads, which can be easily designed and modified to obtain different deposition areas, free-form patterns, and even complex multimaterial structures. Finally, by designing a miniaturized head with circular concentric gas channels, 3D printing of functional materials can be performed with nanometric resolution in Z. This constitutes a new gas-phase 3D printing approach. Because the process is based on ALD reactions, conformal and continuous thin films of functional materials can be printed at low temperatures and with high deposition rate in the open air. Our approach represents a new versatile way of printing functional materials and devices with spatial and topological control, thus extending the potential of SALD and ALD in general, and opening a new avenue in the field of area-selective deposition of functional materials.

Heat is omnipresent in natural and artificial environments, more than 60% of which is dissipated. Thermoelectric (TE) power generation can provide a unique solution to convert this dissipated, wasted heat into useful energy, that is, electricity. Generally, TE conversion efficiency depends on the material properties and design of the module structure. However, despite the development of highly efficient TE materials, module engineering is rather less advanced and is still fabricated by the traditional multi-step process of materials synthesis, dicing, and assembly, restricting the available design of modules to that of planar structures. At this moment, three-dimensional (3D) printing technology can maximize the flexibility in the design and fabrication of TE modules into more efficient structures. Furthermore, the printing process can significantly reduce the processing cost for the fabrication of TE modules owing to lower energy input and a simplified assembly process. Herein, I present the development of the 3D printing process applied to a range of different TE materials of Bi₂Te₃, BiSbTe, PbTe, and Cu₂Se-based inorganic alloys. Surface states of TE particles were precisely optimized with the controllable charge states in the presence of anionic inorganic binders, achieving the suitable viscoelasticity of the TE inks to the extrusion-based 3D printing. The geometrically designed 3D-printed TE materials were assembled into power generating systems, in which high efficiencies of energy conversion were achieved by the optimization of heat transfer.

The discovery of new materials and their compositional/microstructural optimization holds the key to next-generation technologies in areas such as sustainable energy and environmental mitigation. Highly universal, fast, and controllable manufacturing method with the ability to generate a large number of sample features is therefore becoming progressively more important. Here we report a high-throughput combinatorial printing (HTCP) technique capable of integrating multiple nanoscale materials in a single nozzle and varying the material compositions on the fly, generating chemically heterogeneous films in a gradient manner (compositional density>10⁴/cm²). Leveraging aerosol-based mixing and deposition, the proposed HTCP approach demonstrates the on-demand control on the compositional/structural arrangements with programmable accuracy. Due to the vast tolerance of ink particle size and morphology, the HTCP successfully fabricates intriguing matter with gradient compositions and complex structures by integrating a wide range of nanoparticles, including 0D nanosphere, 1D nanowire/nanorods, and 2D nanoplates/nanosheets. Furthermore, we demonstrate a fascinating variety of printing strategies including combinatorial doping, alloying, chemical reaction, and compositional microstructuring, expanding the application space for the HTCP method. As a proof of concept, HTCP has been used to investigate the role of doping level in thermoelectric (TE) materials, which helps to identify a high-performance n-type TE composite. The ability to combine the top-down designing freedom of additive manufacturing with bottom-up molecular control over material composition and structure promises almost infinite possibility of heterogeneous material library which is inaccessible from conventional manufacturing approaches, offering a new pathway toward the design and discovery of next-generation materials and composites for broad applications.

Inkjetprinting of organic electronics is an emerging field of interest in current research. The flexibility of the production process allows the production of organic photovoltaic with different shapes, colors and on several different 2D substrates. Here we show the first time all inkjet printed organic solar cells and OLEDs on a 3D object using a 5-Achs automated robot system. The ink engineering process was guided by the optimization towards stable inks that can be deposited on curved substrates with different printing distances. To overcome shunting issues a method for planarizing different substrates on 3D was developed using different polymer coatings. A successive study on performance dependence of print distance was performed in order to achieve full automation of the printing process. On top live time studies of fully printed solar cells on 2D and 3D were performed.

Additive manufacturing (AM) is one of the most powerful manufacturing tools available today. Its layer-by-layer approach to fabrication enables greater flexibility in the use of topology to design functional materials with significant improvements in performances and/or previously impossible functionalities. Vat photopolymerization (VP) stands out amongst the various AM techniques as its ability to achieve high volumetric throughputs and resolutions allows it to be applied in a variety of different settings, from fabricating nanophotonic devices to shoe soles. However, a key limitation of VP is the limited number of functional inorganic materials that are compatible with it. While composite resins with inorganic fillers and hybrid inorganic-organic resins have been developed to allow VP of these inorganic materials, they are often challenging to use, have poor resolutions, or are limited in the compositions achievable.

In this presentation, we will discuss the use of in-situ materials synthesis as an alternative materials development approach for VP that can address some of the current challenges associated with VP of functional inorganic materials. By printing a 3D object that contains the necessary reagents for the in-situ synthesis of the desired material, i.e. printing a “chemical reactor”, we can adapt the extensive materials synthesis reactions already developed for use with VP. As a demonstration of this “chemical reactor” framework, we show how in-situ combustion synthesis can be used with various VP processes for the fabrication of various functional metal oxides and metals.

Central to this approach is the fabrication of metal-ion containing 3D hydrogels which are then thermally treated to yield either 3D metal oxide or metal structures. These homogenous hydrogels are facile to prepare and compositionally versatile, circumventing the issues associated with the state of the art in metal oxide and metal VP. Using this system, we fabricated zinc oxide structures with sub-micron features using two photon lithography and demonstrated their piezoelectric properties via in-situ compression experiments. Lithium cobalt oxide materials were also fabricated using digital light processing (DLP) printing and showed efficient performance as a lithium ion battery cathode over greater than 100 cycles. An array of micro-architected metals such as copper, silver, and cupronickel alloys were also fabricated with DLP printing, which could be used for thermal management applications.

This work highlights the use of polymer chemistry and materials science in expanding the range of materials that can be made via additive manufacturing. The ability to fabricate these functional 3D materials in a facile and versatile way has the potential to open up the field of smart devices and change how we design them.

The continuous development of thermoelectric materials in the past decades led to the improvement of the dimensionless figure of merit and ultimately the overall device efficiency. However, synthesis and manufacturing of electronic materials in general, and thermoelectric materials particularly, requires delicate attention to attain the planned electronic and thermal properties. Furthermore, most of the conventional synthesis techniques, such as the hot pressing, limit the shape of the synthesized material; restricting them from being used in wide range of applications, including wearable devices. The literature of 3D printing of thermoelectric materials, so far, is limited to binary materials ( Bi₂Te₃,PbTe,..). We are showing here the case for 3D printing of the half-Heusler alloy Nb_1-xCoSb, in which we tackle the problems of secondary phases formation, porosity control and the relation of all that to the transport properties. This should bridge the gap towards printing other ternary and quaternary thermoelectric materials.

Soft electronics and soft robotics aim to bridge the gap between functional, high performance, electronic devices & systems and our compliant, organic, human-centric environment. It involves the development of active and responsive electronic materials, devices, and systems in a form factor that is soft, flexible, stretchable, and mechanically robust. The use of 3D printing and direct write techniques offers some distinct advantages with respect to the fabrication of soft robotic components of increasing size and complexity, as well as in the heterogeneous integration of functional components such as distributed power sources and electronic components and interconnects. In particular, the development of flexible, printable, and mechanically robust energy storage devices is a key and oftentimes limiting requisite for many of these applications due to the fact that common battery materials and manufacturing methods are often incompatible with 3D printing and soft electronics. We demonstrate control of 3D printed batteries and energy storage components using a variety of printing approaches, including aerosol jet and direct ink write, combined with a mixed solvent, phase inversion approach to deliver porosity control, high resolution feature sizes, and high temperature performance. Additionally, advances in materials and design approaches are propelling the field of soft robotics at an incredible rate, however current methods of fabrication remain cumbersome and struggle to incorporate desirable geometric complexity at large scale. By developing a 3D printable, tunable, self-healing elastomer system, we demonstrate a soft robotic manufacturing approach that combines the advantages of 3D printing (e.g. vat photopolymerization) together with modular soft robotics whereby smaller subcomponents can be printed and subsequently self-healed together to form integrated parts.

The manufacture of pixels is a core task in optoelectronics and hybrid perovskites are emerging as a promising material candidate due to their strong, tunable, and high-color-purity luminescence. A number of lithographic or printing methods have been devised to fabricate perovskite pixels in thin-film forms. However, downsizing a thin-film pixel to the nanoscale loses its brightness, which remains a longstanding issue.
Here, we show that 3D layout of perovskite nanopixels offers an effective route to improve the brightness without sacrificing the lateral resolution [1]. Our nanoscale 3D printing approach that is based on femtoliter meniscus-guided crystallization [2] enables the fabrication of freestanding metal halide perovskite color nanopixels with programmed dimensions and placements. This approach provides outstanding lateral pixel dimensions such as a diameter of ~ 550 nm and a pitch of ~ 1.3 µm (pixel density > 19,500 ppi). Furthermore, on-demand control of the vertical pixel dimension by our 3D printing enables a couple of remarkable achievements: (1) enhancement and balancing of nanopixel brightness (tens of times brighter than a thin-film pixel) and (2) data encryption into nanopixel height for multi-level anticounterfeiting which is not easily accessible by high-magnification optical microscopy with limited depth-of-field. This 3D printed nanopixel scheme is general and could be applied to diverse optoelectronic materials. In this talk, we will present our results and discuss the prospects of our work for potential applications in smart optoelectronic devices.

Corresponding author. Email: [email protected]

References
[1] Chen M., Hu S., Zhou Z., Huang N., Lee S., Zhang Y., Cheng R., Yang J., Xu Z., Liu Y., Lee H., Huan X., Feng S.-P., Shum H.C., Chan B.P., Seol S.K., Pyo J.*, Kim J.T.*, Three-Dimensional Perovskite Nanopixels for Ultra-High-Resolution Color Displays and Multi-Level Anticounterfeiting. Nano Letters (2021) https://pubs.acs.org/doi/10.1021/acs.nanolett.1c01261
[2] Chen M., Yang J., Wang Z., Xu Z., Lee H., Lee H., Zhou Z., Feng S.-P., Lee S., Pyo J, Seol S.K., Ki D.-K., Kim J.T.*, 3D Nanoprinting of Perovskites. Advanced Materials 31, 1904073 (2019)

Solution processing of 2D materials [1] allows simple and low-cost techniques, such as ink-jet printing, to be used for fabrication of heterostructure-based devices of arbitrary complexity. However, the success of this technology is determined by the nature and quality of the inks used.
Our group has developed highly concentrated, defect-free, printable and water-based 2D crystal formulations, designed to provide optimal film formation for multi-stack fabrication [2]. I will give examples of all-inkjet printed heterostructures, such as large area arrays of photosensors on plastic [2], programmable logic memory devices [2], capacitors [3] and transistors on paper [3,4]. Furthermore, inkjet printing can be easily combined with materials produced by chemical vapor deposition, allowing simple and quick fabrication of complex circuits on paper, such as high-gain inverters, logic gates, and current mirrors [5].

[1] Coleman et al, Science 331, 568 (2011)
[2] McManus et al, Nature Nano, 12, 343 (2017)
[3] Worsley et al, ACS Nano, 2018, DOI: 10.1021/acsnano.8b06464
[4] Lu et al, ACS Nano, ACS Nano, 13, 11263 (2019)
[5] Conti et al, Nature comms, 11 (1), 1-9 (2020)

3D printing of conductive inks is poised to revolutionize the microelectronics industry. However, some new technologies use thermally sensitive substrates that are not compatible with the high sintering temperatures (~ 200 °C) of metal nano-particle conductive inks. Reactive inks are an emerging technology that can solve this problem due to their ability to form highly conductive features at low temperatures (~ 90 °C). The underlying kinetics of reactive inks systems is crucial for designing high-performance inks but is not well understood. This work elucidates the impact of reaction kinetics on the morphology and the media resistivity of reactive silver inks (RSI) printed via direct ink writing. We introduce a new RSI system consisting of silver acetate, formic acid, and ethylamine, whose silver reduction mechanism is predominantly thermal. Comparing this new ink to an ink whose reduction mechanism is predominantly evaporative shows that thermal reduction produces dense bottom-up silver growth, and evaporative reduction produces highly porous silver morphologies. Decreasing the vapor pressure of the complexing agent or increasing the substrate temperature leads to more thermal reduction and, therefore, denser morphologies with lower media resistivities.

Recent advances in Inkjet-printing of advanced materials have provided a versatile platform for the rapid development and prototyping of sensor devices. We have recently demonstrated inkjet-printed surface enhanced Raman scattering (SERS) sensors on flexible substrates for the detection of small chemical molecules. (Tay et al., J Raman Spectrosc., 2021, 52, 563-572) These flexible SERS sensors have good batch-to-batch uniformity and many advantages for performing point-of-sampling testing such as liquid or aerosol filtration and swabbing capabilities. These simple sampling and separation attributes make these inkjet-printed paper-based sensors ideal for field applications. The SERS effect relies upon the electromagnetic field enhancement that results from coupling of plasmonic nanostructures (e.g. silver or gold nanoparticles). SERS exploits the unique vibrational signature of an analyte molecule and in practice, it is often coupled with spectral recognition software for the detection of molecules of interest. Most SERS sensors are fabricated as non-functionalized plasmonic nanostructures in a single device. Because SERS is an optical near-field effect, it works best with molecules that have good binding affinity towards the plasmonic surfaces (such as thiols). SERS detection of non-binding molecules such as cannabinoids or other narcotics with poor affinity towards the plasmonic surfaces performs poorly. This can be improved through the use of surface functionality to help sequester or attract the molecules of interest to the plasmonic surfaces. The functionalization concept has been applied in SERS detection for a number of years. However, its applications have been limited typically to optimization of functional groups to target one particular chemical analyte, for example, the use of alkylthiol to improve glucose SERS sensing. (Lyandres et al., Anal. Chem., 2005, 77, 6134-6139) Once the surface has been functionalized and tailored for one specific analyte, it can often render the entire surface in-accessible for other molecular analytes. For example, a gold nanoparticle surface functionalized by halide ions (Cl- or I-) is often no longer active for detection of thiol species, despite the strong affinity of thiolated species towards gold surfaces. This problem can be overcome through the use of inkjet-printed SERS devices. With inkjet printing, multi-functionalized SERS active surfaces can be prepared in a single device thus retaining the performance of multiple types of functionalization in a single device. In this presentation, we will demonstrate the optimization of functionalized plasmonic ink formulation, inkjet-printing conditions and integration of multi-functional SERS active sites in a single sensing device. We will discuss the performance of differently functionalized SERS devices for the detection of narcotics.

As human spaceflight pushes beyond Low Earth Orbit (LEO), resupply of consumables becomes a significant challenge. One solution to this problem is In-Space Manufacturing (ISM), the capability to perform on-demand manufacturing and repair of consumables in an in-space environment. ISM offers significant flexibility to missions as it allows for a high degree of tailorability and reduction in launch mass. Leveraging advancements in fabrication, repair and recycling, ISM provides a highly sustainable and affordable solution to exploration mission operations and logistics. In this talk, advances in printed electronics and sensors, ranging from nanomaterial ink development and hands-free fabrication methodologies to devices will be presented. Applications presented will include sensors for crew health monitoring along with supporting electronics. In the future, these devices will be fabricated and characterized on the International Space Station and the approach will be evaluated for future in-space manufacturing to support human spaceflight.

3D printing has become one of the most attractive fabrication techniques in materials science and engineering, offering its immense degree of freedom in designing complicated architectures with feature sizes smaller than millimeters or even micrometers. As the designed architectures can control the physical and chemical properties of the materials, 3D printing thrusts creating innovative multifunctional materials.
In this presentation, we employed stereolithography (SLA) 3D printing to fabricate hierarchically porous carbon microarchitectures that integrate structural robustness and energy storing functionality. By using photocurable resin mixed with magnesium oxide (MgO) nanoparticles as a porogen, an LCD (liquid crystal display)-SLA 3D printer successfully replicated the 3D-modeled microarchitectures. After pyrolysis in vacuum at 1000degC and removal of MgO nanoparticles in heated hydrochloric acid, we obtained fully-carbonized simple cubic lattices that retained the architected micropores (<200µm) and stochastic nanopores (<40nm) developed in the individual structural elements. While showing ~300m²/g of the specific surface area, supercritical liquid or other complicated setups were not required for drying. The obtained structures demonstrated a combination of mechanical robustness and functionality as a supercapacitor in a single body. Further optimization of both microarchitectures and nanoporosity will lead to a highpower energy device embedded in structural components.
Out results can present a novel configuration of structural capacitors and/or batteries different than ones using carbon fibers, benefitting from the capability of 3D printing.

Over the past 40 years, the scanning probe microscope (SPM), a class of Nobel-Prize-winning techniques, has become a routine and essential imaging tool for exploring not only surface topography but also material properties at the nanoscale. The core feature of SPM is a “nanoprobe” typically made up of a sharp tip mounted on a cantilever.

Recently, high-aspect-ratio (HAR) nanoprobes have ushered in a new era in SPM characterization. First, the HAR probe enables full-profile topography of deep and narrow 3D nanodevices, e.g., fin-based multigate transistors (FinFETs), photonic metamaterials, and neuromorphic circuitries. Second, the HAR probe geometry is quite effective in amplifying the chemical imaging contrast in tip-enhanced-Raman spectroscopy (TERS), as it provides enough space to separate the light excitation region from the Raman scattered region on the probe. However, the technological challenges associated with the fabrication of HAR nanoprobes such as high-cost, labor-intensive, and low- throughput remain unresolved.

Here, we have developed a one-step, on-demand, material-saving benchtop 3D printing method to fabricate metallic HAR atomic force microscope (AFM) probes for deep 3D imaging [1]. This method exploits electrohydrodynamic (EHD) nano-dripping to fabricate a freestanding silver nanowire (AgNW) with an aspect ratio over 30 directly on a cantilever. Our 3D printing approach offers the benefits associated with unprecedent simplicity in the probe’s dimension control, material selection, and regeneration, which are not easily attainable in the traditional lithographic approach. Moreover, our 3D printed tip delivers a high fidelity in deep-trench imaging better than a standard pyramidal tip and comparable to commercial HAR probes, yet at a fraction of their cost.

Furthermore, 3D printing has the potential to produce/customize diverse classes of functional nanoprobes, as recently demonstrated scanning 3D thermocouple networks for mapping microscale temperature fields in three-dimensions [2]. In this talk, we will present the experimental results including 3D printing of functional nanoprobes and discuss prospects of our work for potential applications in nanomaterial characterization and sensing.

KEYWORDS:3D printing; electrohydrodynamic nano-dripping; nanowire probe; 3D deep AFM imaging

[1] Lee H., Gan Z., Chen M., Min S., Yang J., Xu Z., Shao X., Lin Y., Li W.-D., Kim J.T., On-Demand 3D Printing of Nanowire Probes for High-Aspect-Ratio Atomic Force Microscopy Imaging. ACS Appl. Mater. Interfaces, 12, 46571 (2020).
[2] Lee H. and Kim J.T. (in preparation)

Backswimmers (family Notonectidae) are aquatic insects known for their ability to hang upside-down at the water-air interface, due to their superhydrophobic legs.¹ They can, however, also regulate their buoyancy underwater through controlling the size of a bubble attached to their abdomen.^2,3 They do that by controlling the amount of oxygen attached to haemoglobin cells. In this way, they reach neutral buoyancy without the need to swim actively.
With the miniaturization of robots, interfacial phenomena become prominent in terms of stability, adhesion and arts of locomotion.⁴ While interfacial phenomena may hinder locomotion and complicate the development of small robots, it can also be used in order to facilitate the performance of specific tasks, as exhibited in nature, and promote the development of robotic insects and insect-inspired devices. This is especially important in underwater robotics, where energy consumption and hydrodynamics challenge the miniaturization of robots.
In this study, a 3D printed backswimmer-inspired miniature robot is developed. A mat of gas bubbles is created and autonomously controlled, in order to regulate the buoyancy of the robot. The bubbles nucleate and grow using a catalytic mechanism underwater. Water from the environment split into oxygen and hydrogen micron-sized bubbles on small (2 cm) comb-like electrodes, in response to voltage. In order to release bubbles and regulate the buoyant forces, three mechanisms for bubble release are investigated: rotational, linear and pulse mechanism. An undesirable outcome of the bubble release rotational mechanism is rotational motion of the robot to the opposite direction, due to conservation of angular momentum. In the case of the pulse driven release mechanism, high-energy consumption is recorded, in comparison to the other release mechanisms. Therefore, the linear mechanism is the most useful for bubble release in terms of power consumption and motion control. Through a control loop, the nucleation, growth and release of bubbles are self-regulated, allowing the robot to achieve neutral buoyancy without any further energy consumption, in an autonomous fashion. The development of this device lays the ground for investigating interactions between autonomously moving underwater miniature robotic swimmers.

3D printing is an emerging additive manufacturing technology poised to transform both design and manufacturing due to its ability to customize design and generate structural complexity not possible using traditional manufacturing processes. Commercial examples of 3D printed objects using monolithic materials, such as GE’s fuel nozzle, illustrate early success of this technology by enabling new designs and significantly reducing the number of parts and supply chain required for manufacturing. However, only marginal progress has been made to demonstrate multi-material printing and as such it remains a low technology readiness level research endeavour at the moment with enormous potential for innovation and commercial impact. Multi-material 3D-printing would increase the complexity and enable the fabrication of complete devices in a single printing process combining materials with different properties (structural, functional, conductive, etc.). Through novel chemistries, and the exploitation of controlled self-assembly, various functions (e.g. electrical, optical, magnetic, mechanical) may be introduced into objects as they are being constructed/printed. 3D spatial segregation of materials within the object will be controlled from the nano-scale to the macro-scale, carefully tuning bulk and surface properties, yet breakthroughs in many aspects of material science will be required to achieve this level of control. These “smart multifunctional objects” are intended to be adaptive and responsive (i.e. intelligent) and enable applications that cannot be easily addressed using other forms of fabrication. This is a leap beyond traditional assembly. Success will provide breakthroughs in robotics and IoT via new form factors with optimized mechanical properties, embedded sensors and antennas for wireless communication. In this presentation, we demonstrate a strategy to generate functional objects by vat polymerization 3D printing that relies on controlled phase separation. Using photoresins comprising of judiciously selected components, the diffusion of phase separating components are modulated via the kinetics of gelation and network entrapment to yield spatial control over material placement. This methodology yields a rich selection of morphologies with distinct functional surfaces that provide direct access to 3D printed smart objects such as sensors and wireless applications.

Additively manufactured custom-form batteries can maximize practical energy densities in applications where power sources must be more efficiently packed into a given space. Conversion cathodes paired with lithium metal further enable high energy density batteries. Carbon monofluoride (CF_x) and iron disulfide (FeS₂) are two conversion cathode materials of broad interest where little to no work has been carried out on the development of microextrusion methodologies for these materials. Meanwhile, silica sol based ionogels have been shown as a promising solution-deposited solid-state electrolyte, though no work has been carried out on the printing of this material for custom-form cells. In this work we demonstrate and optimize the printing of CF_x and FeS₂ cathode systems in custom-form lithium batteries, where the impact of ink solid composition on rheology, custom-form shape retention, cell capacity, rate, and cyclability is investigated. Additionally, the microextrusion of silica-based ionogels is demonstrated for the first time, where it is used with conversion or intercalation chemistries in custom-form cells. For the FeS₂ material system, it is found that ink solid compositions between 40-50% w/w are needed to adequately retain shape in custom-form configurations while maintaining up to 450 mAh/g cell capacity over 10 charge/discharge cycles. For the CF_x material system, ink solid compositions between 30-40% w/w are needed for shape retention in custom-forms and low process variance while cells discharge at capacities above 600 mAh/g at rates greater than C/25. Overall, this work demonstrates the microextrusion of energy storage materials for custom-form lithium batteries, which are of broad interest generally and for Sandia related applications where practical energy densities in a given system must be maximized. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

In this research, we develop a nature-inspired droplet-based nanoparticle 3D printing method that creates highly complex three-dimensional architected structures and use them to make a new class of biomedical devices. Fluid dynamics of aerosol microdroplets is used to stack nanoparticles in 3D space without any support materials to create such structures. First, we use this technique to realize a regenerable biosensor that detects COVID-19 antibodies and antigens within 10 seconds! This is the fastest detection time yet reported in literature and will have a significant positive impact on the course of the current pandemic. Several fundamental problems related to deformation during nanoparticle sintering were solved during this research. Second, we use 3D printing to create fully customizable brain-computer interfaces (BCIs) having recording densities of thousands of electrodes/cm2, which is 5-10x higher than the current state-of-the-art BCI technologies. Recording of action potentials from the brain of anesthetized mice was achieved with a high signal to noise ratio. Lastly, we also demonstrate highly flexible 3D printed cardiac biomonitors that lead to minimal patient discomfort. Our research thus focuses on advancing microfabrication via 3D printing and using the same to realize next-generation biomedical devices for the benefit of public health.

Poly (methyl methacrylate) (PMMA) is a synthetic polymer commonly used for medical implants in cranioplasty and orthopaedic surgery owing to its good mechanical properties, optical transparency, and minimal inflammatory responses. Recently, the development of 3D printing opens new avenues in fabrication of patient-specific PMMA implants for personalized medicine. However, challenges are confronted when introduction of medical-grade PMMA resins to a conventional 3D printing process due to its dynamic viscosity and poor self-supportability during printing. In addition, the intrinsically exothermic polymerization of PMMA creates voids and pores within the printed product due to locally trapped MMA gas to which is often attributed poor mechanical performance and resultant opaqueness. Therefore, in this study, a freeform embedded 3D printing platform is proposed, followed by the pressurized thermo-curing for fabrication of customized PMMA implants with improved strength and transparency. An alginate-based supporting matrix is used to structurally support the extruded liquid acrylic resin during both 3D printing and post-curing processes; moreover, the autoclave reactor enclosing the alginate matrix and as-printed PMMA is utilized to generate temperature-dependent hydrostatic pressure, which serves for suppressing bubble formation and densifying polymerised PMMA during the post-curing. The 3D printed PMMA shows almost comparable mechanical behavior to that of PMMA from a conventional casting method because of well-densified uniform microstructures. Also, in vitro cytotoxicity results confirm that 3D-printed PMMA is as biocompatible as casted PMMA. As proof-of-concepts, various complex structures are also fabricated through this 3D printing and post-curing platform. We envisage that the proposed freeform 3D printing platform in conjunction with the pressurized post-curing method can also allow fabrication of various liquid resins such as epoxy resins and their composites besides acrylic resins.

Introduction
Skeletal muscle comprises the majority of tissue in the human body. It is essential to keeping posture, balance and mobility but also has crucial functions in many physiological and metabolic processes. Gene mutations leading to muscular dystrophies, such as Duchenne Muscular Dystrophy (DMD), have a major impact on patients’ lives causing progressive disability and, ultimately, death due to cardiac or respiratory failure. Current therapeutic methods against DMD are very limited, including corticosteroids with severe side effects and experimental stem cell based therapies [1]. Despite great promise, this latter method has been restricted by the absence of 3D matrices that promote myofiber alignment and, importantly, support and stimulate muscle contractile force during regeneration. Furthermore, magnetic fillers embedded into 3D matrices can be used to externally induce cyclical deformation over implanted scaffolds to activate mechanotransduction pathways that will improve tissue regeneration. [2] In this work, we hypothesized that iron-modified reduced graphene oxide (rGO-Fe) with magnetic responsive properties can be processed by melt electrowriting (MEW) – a unique technique for the fabrication of highly ordered scaffolds with ultra-fine fibers – once mixed with polycaprolactone (PCL), and that well-organized fiber scaffold architectures can guide myofiber growth and remotely stimulate muscle contraction.
Materials and Methods
Composites of medical grade PCL and rGO-Fe were prepared by melt blending (rGO-Fe loadings between 2-20 wt%), and their processing compatibility was systematically investigated according to key MEW parameters. Fabrication of well-organized fibrous scaffolds with different laydown microstructures (square, hexagonal and auxetic) and macrostructures (straight and circular) was explored. The effect of rGO-Fe content and MEW processing parameters on fiber scaffold morphology, magnetic properties and mechanical performance was studied. Finally, fiber scaffolds were seeded with an immortalized mouse myoblast cell line (C2C12), encapsulated in an extracellular matrix-like hydrogel, to ensure uniform cell distribution within the fiber scaffold. Cell survival, morphology, tissue structure and functionality were studied after 7 and 14 days in culture.
Results and discussion
Well-organized scaffolds (microfiber diameter=10-15 µm) composed of rGO-Fe/PCL blends with square, hexagonal and auxetic microstructures were successfully manufactured for the first time using MEW. The smallest pore resolution achievable was 300 μm, for a scaffold thickness of 300 μm and 2 wt% rGO-Fe. Both hexagonal and auxetic architectures allowed for better deformation recoverability and approximately a 10-fold increase in applied strain with respect to conventional square fiber architectures. Additionally, above a rGO-Fe content of 10 wt%, scaffolds allowed for in-plane and out-of-plane deformation of near to 10% in the presence of magnetic stimulation. All scaffolds were cytocompatible, and hexagonal constructs showed enhanced myofiber alignment and organization with respect to square fiber architectures.
Conclusion
In summary, the present study demonstrated for the first time that highly ordered microfiber scaffolds based on rGO-Fe/PCL blends can be reproducibly produced and that these organized constructs are promising candidates to guide skeletal cell growth, stimulate contraction and (potential) regeneration under externally controllable magnetic triggering.
References
[1] Sun et al., Exp Neurol 323 (2020) 113086.
[2] Vining & Mooney, D. Nat Rev Mol Cell Biol 18 (2017) 728–742.

Drug-eluting polymer devices play a pivotal role in combating foreign body reactions and infections associated with the functional devices. We report here a one-step 3D printing strategy to create drug-eluting polymer devices with a drug-loaded bulk and a drug-free coating. The spontaneously formed drug-free coating is crucial to biomedical applications because it drastically reduces the surface roughness and also functions as a protective layer to suppress the burst release of drugs. A moisture cured silicone is used as the printing ink that can be extruded based on its shear-thinning property and quickly vulcanize upon exposure to ambient moisture to retain the fidelity of the 3D printed patterns. S-nitrosothiol type nitric oxide (NO) donors in their crystalline forms are selected as model drugs because of the potent antimicrobial, antithrombotic, and anti-inflammatory properties of NO. Direct ink writing of the homogenized polymer-drug mixtures results in devices with very rough and ill-defined surfaces because of the exposed S-nitrosothiol microparticles. By further addition of a low-viscosity silicone (polydimethylsiloxane) into the ink, the low-viscosity silicone diffuses outwards upon deposition to form a drug-free outermost layer without compromising the printing integrity. S-nitroso-N-acetylpenicillamine (SNAP) embedded in the printed silicone matrix releases NO under physiological conditions from days to about one month. The micro-sized drug crystals are well preserved during the ink preparation and printing processes, which is one reason of the sustained NO release. Preliminary biofilm and cytotoxicity experiments confirm the antibacterial property and safety of the printed NO-releasing devices. This additive manufacturing platform does not require dissolution of drugs and involve no thermal or UV curing processes, and therefore, offers unique opportunities to produce drug-eluting silicone devices in a customized manner.

Rapid development of wearable electronics, accessible and environmentally sustainable energy generation has attracted an extensive attention in next generations. In this context, piezoelectric materials have the ability of generating electric signal when it is subjected to certain mechanical stimuli, such as stress, elongation, tension, vibration, etc.^[1] It has been found that different types of piezoelectric materials are reported so far, for example, as a semiconductor (ZnO, ZnS, GaN, etc.), ceramics (PZT, BaTiO₃, NaNbO₃, KNbO₃, KNN, ZnSnO₃, etc.), polymers (PVDF and its copolymers, polyamides, etc.) and some natural materials (bones, hairs, collagen fibrils, peptide, cellulose, sugar cane, etc.).^[2] Among them, natural piezoelectric materials possess several advantages over the other class of piezoelectric materials. For example, cellulose, a novel piezoelectric polymer is almost endlessly available polymeric raw material that exhibits fascinating natural structure and also treated as eco-friendly and biocompatible product. In addition, cellulose is also utilized for versatile sensing applications and easily obtained from various plants as micro-fibrils consist of glucose−glucose linkages with linear chains.
On the other hand, the advancement of three-dimensional (3D) printing technology has recently attracted a great deal of attention as the 3D printing enables the fabrication of materials with intricate macroscopic shapes and controlled microstructures and properties.^[3] Cellulose nanocrystals (CNCs) have recently received significant attention due to its high Young’s modulus, high strength, light weight, low density, sustainability, biocompatibility, biodegradability, recyclability, and abundance nature. As a result, it is conceivable and advantageous to substitute traditional 3D printing thermoplastics with cellulosic materials such as CNCs. In addition, the choice of electrodes and their compatibility with 3D printed surface remain challenging tasks, particularly when device performance, durability and implant ability are considered.
Here we present an all-3D printed piezoelectric nanogenerator (PNG) based on CNCs by direct-ink write (DIW) process for energy harvesting from various types of mechanical vibrations. We achieved the all-3D printed multilayer structure where active piezoelectric ink CNC is printed in between two printed electrodes (top and bottom). For electrode printing, the conductive ink is developed using the carbon nanotube (CNT) into CNCs matrix. Interestingly, we address the electrode compatibility issue by keeping the same polymer matrix (CNCs here) through the whole structure and subsequently enhancing the interface compatibility of the piezoelectric active layer and the electrode layers. Noteworthy that, for distinct electrode and piezoelectric material, mechanical pressure imparting leads to the delamination of the interfacial layer due to the mismatch of Young’s modulus, and results in the degradation of PNG performance. The crystallinity of the printed CNC was determined from X-ray diffractometry (XRD) spectra. The Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy are used to examine the vibrational bands of CNC and CNT. The surface morphology is characterized by Field emission scanning electron microscope (FE-SEM) images. The mechanical properties are done by stress-strain testing. Owing to excellent electrode contact-adherence, the PNG exhibits an effective conversion of mechanical energy of human finger movements into electrical energy (~10 V). Importantly, the fatigue test under continuous mechanical impact (over 1000 cycles) shows great promise as a highly durable mechanical energy harvester. Thus the all-3D printed PNG will play an essential role in future self-powered electronics.

References:
[1] Wang et al., Science 2006, 312, 242.
[2] Maity et al., ACS Appl. Mater. Interfaces 2018, 10, 44018.
[3] Davidson et al., Adv. Mater. 2019, 1905682.

Exploiting microorganisms to fabricate engineered living materials has gained impulse in recent years due to the sustainable nature of biological processes and the possibility to add living functionalities to synthetic materials, including self-healing, self-regeneration and adaptation. In this talk, I will show how 3D printing can be used to leverage the metabolic activity of microorganisms and create a new generation of complex-shaped living materials. To this end, biocompatible hydrogels are thoroughly designed to enable loading with selected microorganisms and printing using the extrusion-based Direct Ink Writing technique. By formulating inks with suitable rheological properties and appropriate culture media, it is possible to control the geometry of the printed object and promote the proliferation and growth of material-forming microorganisms within the deposited structures. Using selected examples, I aim to demonstrate that the achieved shape programmability and tailored spatial distribution of microorganisms makes 3D printing a powerful tool for the manufacturing of functional living materials.

Ferroelectrets with internal polarized pores are an appealing solution for self-powered sensors and wearable energy harvesting devices due to its flexibility and strong piezoelectric response. Additive manufacturing, or 3D printing, enables the designability of pore structures and the piezoelectric performance of ferrroelectrets to be optimized further than traditional manufacturing methods. Our previous investigation found that the compressible fabric-based electrodes and the outer surface of the ferroelectret film can form an additional electro-electret and can significantly contribute to the overall piezoelectric performance. In this work, polypropylene (PP) ferroelectrets with different kinds of porous structures (ellipsoid, hexagon, triangle, spherical protrusions) were fabricated by 3D printing and polarized through corona discharge. The all-organic ferroelectret nanogenerator (FENG) was then assembled after applying the electrodes of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-coated Spandex fabric. It was found that the PP ferroelectret with spherical protrusions on the surface, which is the optimal balancing in film thickness, porosity and elastic modulus, shows the best short-circuit current and open-circuit voltage under the same compressive condition. The performance of the all-organic FENG was demonstrated by charging several capacitors and lighting up the light emitting diodes (LEDs) successfully. The 3D-printed porous structures would be conducive for studying the charge storage mechanism of ferroelectrets due to the pores’ regularity and designability. Moreover, because of its easy integration with clothing, shoes, carpets, etc., the all-organic FENG with fabric electrodes would be a competitive candidate in wearable electronics field.

A hernia is a common soft tissue injury resulting from a weak or defective abdominal wall, causing protrusion of the intra-abdominal contents. Surgical implantation of a prosthetic mesh is the most common hernia repair technique to support and reinforce the abdominal wall. Over 400,000 incisional hernia repair surgeries are performed annually in the US. However, surgeries involving prosthetic meshes induce local inflammatory responses leading to the formation of visceral adhesions, hardening, and gradual shrinking of the implanted meshes, which require additional surgeries. Hence, there is a critical need for a soft tissue repair scaffold to minimize post-surgical complications.
This study presents the development of a 3D-bioprinted functional biomaterial-based surgical mesh (Biomesh) that modulates local inflammation. We designed and fabricated Biomesh to yield a soft, pliable, and mechanically strong mesh by in situ phosphate crosslinking of polyvinyl alcohol. The crosslinking of hydroxyl groups with phosphate groups gives a negative surface zeta potential of Biomesh. We showed that the negatively charged Biomesh “captured” positively charged pro-inflammatory cytokines such as IL-1β, IL-6, and TNF-α via electrostatic interactions. We also reported that in a rat ventral hernia model, the implantation of Biomesh modulated inflammation in the surgical site and prevented visceral adhesions even after 8 weeks compared to the polypropylene mesh implant. Thus, this study successfully demonstrated the therapeutic efficacy of Biomesh as an intrinsically inflammation-modulating surgical mesh without the need for anti-inflammatory agents for soft tissue repair surgery.

The concept of digital medicine, which broadly deals with streams continuous information from the body to gain insight into health status, manage disease and predict onset health problems, is currently only gradually developing.^[1][2] Key technological hurdles that slow the proliferation of this approach are means by which clinical grade biosingals are continuously obtained without frequent user interaction.^[3] To overcome these hurdles, solutions in power supply and interface strategies that maintain high fidelity readouts function chronically are critical. Current approaches for high fidelity recordings typically rely on adhesive interfaces that are subject to epidermal turnover, limiting sensor lifetime. Additionally, they rely on electrochemical power supplies which are subject to frequent recharge, add bulk and weight, require user interaction and introduce motion artefacts. Here we introduce a new class of devices that overcomes the limitations of current approaches by utilizing digital manufacturing to tailor geometry, mechanics, electromagnetics, electronics, and fluidics to create unique personalized devices optimized to the wearer. These elastomeric, 3D printed and laser structured constructs, called biosymbiotic devices, enable adhesive-free interfaces and the inclusion of high performance, far field energy harvesting to facilitate continuous wireless and battery-free operation of multimodal and multi device, high-fidelity biosensing in an at-home setting without user interaction.

References
[1] T. R. Ray, J. Choi, A. J. Bandodkar, S. Krishnan, P. Gutruf, L. Tian, R. Ghaffari, J. A. Rogers, Chem. Rev. 2019, 119, 5461.
[2] J. Heikenfeld, A. Jajack, J. Rogers, P. Gutruf, L. Tian, T. Pan, R. Li, M. Khine, J. Kim, J. Wang, Lab Chip 2018, 18, 217.
[3] T. Stuart, L. Cai, A. Burton, P. Gutruf, Biosens. Bioelectron. 2021, 113007.

Elastomer composites with embedded liquid phase fillers are an emerging material architecture for creating highly functional elastomer composites that are soft, elastically deformable, and exhibit unique combinations of electrical, thermal, and mechanical properties. The material architecture consists of a functional emulsion of liquid phase fillers, such as liquid metal microdroplets, dispersed in an uncured hyperelastic polymer that can be cured into an elastomer composite. These functional emulsions are typically manufactured in bulk using replica molding or soft lithography techniques that results in primarily spherical inclusions with little control of shape, orientation, or spatial placement. Here, we will introduce a new additive manufacturing process to actively tailor the liquid inclusion microstructure of functional emulsions throughout a manufactured part to control the electrical, thermal, and mechanical properties of soft elastomeric composites of complex geometry. The material microstructure is actively tailored through a direct ink write alignment processing technique and the creation of a new emulsion ink that is suitable for additive manufacturing. The programmed inclusion shape and orientation is characterized as a function of material properties and processing parameters. Individual printed filaments and printed sheets are experimentally characterized for mechanical and functional properties. Finally, the ability to print sheets and multilayer parts with varying material microstructure is demonstrated. This manufacturing method for processing functional emulsions provides new opportunities to create soft elastomer composites with directly tunable and addressable material properties and performance.

Conventional synthesis and manufacturing of thermoelectric materials is often tedious, time-intensive, and expensive with severe limitations in terms of resultant sample geometry and size. While traditional growth techniques have been used for many years in the field, as the interest in thermoelectric devices and environmental efficiency has increased, the wastefulness of the process has become abruptly apparent. Recently, popularity has grown for using additive manufacturing methods in materials science fields, offering the potential to decrease fabrication time and cost for both samples and devices. Nevertheless, success in additive manufacturing has been majorly limited to metals and polymers, while the manufacture of thermoelectric semiconductors has identified current limitations [1]. These limitations include the need for post-processing that often leads to pore formation and decreased adhesion [1]. Additionally, current work has not extensively studied how additive manufacturing techniques alter thermoelectric transport. In this work, we approach the problem starting with Ni, whose thermoelectric properties are well known [2], albeit less compelling than those of Bi2Te3 and other conventional thermoelectric materials. Specifically, we focus on the particle-laden ink extrusion printing process, which has proven successful with producing bulk Ni samples [3]. This process contributes to the formation of micro and macro cracks and pores in bulk samples. Through altering printing parameters and sintering techniques, we study the microstructure of printed samples with a specific focus on densification, composition, and pore and crack concentration. Then through comparison of the temperature dependence of thermopower, electrical resistivity, and thermal conductivity in the printed samples to those made using traditional crystal growth techniques, including dense and porous polycrystalline samples, the effects of the particle-laden ink extrusion printing processing on thermoelectric transport properties can begin to be made clear. This information is then used to inform the extension of the particle-laden ink extrusion printing technique to more efficient thermoelectric materials, in this case starting with Bi, which are more demanding in their manufacturing requirements. Bi makes an ideal starting point due to the well-known transverse thermoelectric transport properties. Pure Bi has also not previously been printed using extrusion based additive manufacturing; therefore, we present a newly-developed low-temperature method.

[1] A. El-Desouky et al. Mater. Lett. 185, 598-602 (2016).
[2] S. J. Watzman et al. Phys. Rev. B 94, 144407 (2016).
[3] S. L. Taylor et al. Adv. Eng. Mater. 19 (11), 1600365 (2017).

3D Printing methods have enabled rapid prototyping and production of parts and OEMs that have been considered a paradigm shift in several industries. In the area of polymers, thermoplastics, thermosets, and elastomers based on their thermo-mechanical properties have been matched with their processability. However, it is not as simple as the classification into these groups when it comes to their intended applications and the development of new properties. There are a number of choices for 3D printing methodologies (FDM, SLA, SLS, VSP)
which can make use of blended or formulated compositions. However, this talk will focus on a class of viscoelastic materials that can be 3D printed even at room temperature. This will be exhibited in the following examples: 1) Elastomeric foam materials from plastics based on polyurethane, 2) Silicone materials with high electrical and thermal conductivity, and 3) Metals from bio-based sustainable viscoelastic materials. The design of the materials and the geometry are very important and must be matched with the 3D printing parameters. The utilization of rheological and viscoelastic materials studies is important prior to the study as these emphasize shear thinning behavior, sufficient yield stress, and thixotropic behavior to achieve high fidelity with the CAD design.

Wearable sensors are increasingly prevalent in health monitoring and human–machine interface applications as a result of improvements in their size and comfort. Flexible and printed electronics have enabled the fabrication of wearable sensors that can mechanically bend and conform to non-planar and dynamic surfaces of the human body, allowing physiological signals of low bandwidth to be measured. Hybrid systems that combine flexible sensors with rigid computational components on a separate substrate have also been developed. To provide a real-time analysis of physiological signals, wearable biosensors can implement machine-learning models for signal processing. Traditionally, skin bioelectrodes have relied on wet electrodes, metal or composite electrodes coated in an electrolytic gel, applied by trained professionals. These wet electrodes ensure consistent electrode-skin contact but are susceptible to impedance mismatch related noise and interference as they dry out. In addition, the skin preparation required for wet electrodes (often lead to skin-irritation and hair loss. We have developing methods for 3D printed dry electrodes that are optimized for different sensing locations. In this talk I will discuss several fabrication methodologies and materials used for skin conformal sensors, including dry electrodes. These electrodes exhibit high effective surface areas and when compared to wet electrodes of the same area, they express comparable electrode-skin impedances. Applications in electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG) will also be discussed.

Layer-structured composites have broad applications in structural systems, thermal insulation, microelectronics, optics, and biomedical devices. The design and assembly of layered structures containing polymer and particle materials have been challenging in sub-microscale. The bottleneck is the lack of appropriate manufacturing compatible with soft matter and nanomaterials. This research will demonstrate three examples in 1D, 2D and 3D structures containing polymer and nanoparticle layers for different applications.

The first example was the use of polymer-nanoparticle interphase design to achieve 1D fibers. As well known that polymer fibers of microscale with continuous nanoparticle distribution are difficult to process due to the stiffness mismatching between polymers and nanoparticles. This example will demonstrate unique fiber spinning that blends polymers and nanoparticles at different layers. The soft macromolecules and rigid particles will be stacked along the radial or the fiber axis for enhanced mechanical, electrical, and thermal properties. The unique material system of composite fibers were used as piezo- and chemi-resistive sensors.

The second example exhibited a thin composite film on compliant structures for thermoelectric generators (TEGs). The stacking of 2D films contained conjugating polymers in-situ polymerized at the presence of nanoparticles. During the processing, the polyaniline molecules were found to coat evenly on carbon nanotubes and led to high electrical conductivity, power factor, and enhanced mechanics. Thus, these compressed films were exhibited on compliant structures and used as wearable devices for human body heat collection. The generated electricity was for powering wearable sensors in detecting human movement and health conditions.

The third demonstration utilized layer-by-layer-based deposition techniques. Both additive manufacturing (e.g., 3D printing) and coating methods were used on the same processing platform. One dimensional carbon nanofibers (CNF) and 2D MXene particles were used as examples to be selectively deposited on polymer surfaces with pre-printed patterns. The control of the surface patterns and the nanoparticle assembly conditions (e.g., thermodynamic parameters, nanoparticle interactions, solid-liquid-air contact lines, etc.) led to selective deposition and preferential alignment of nanoparticles. As a result, the conductive paths on the substrate were developed to be anisotropic; following this characterization, the multifunctional sensitivity to strain, temperature, chemical liquids and volatile organic compounds (VOCs) were also displayed.

Piezoelectricity provides an ideal electromechanical mechanism with emerging applications in wearable devices due to its simplicity and self-powered nature. However, the 3D printing of piezoelectric devices still faces many challenges, including material printability, high energy poling process, and low dimensional accuracy. This study demonstrates, for the first time, a tellurium nanowire-based poling-free piezoelectric device fabricated by a hybrid printing method. The home-built hybrid printer consists of an extrusion printing head for structural material fabrication and an aerosol jet printing head for functional material fabrication. The aerosol jet printed tellurium nanowire demonstrates piezoelectric properties without the need for any poling or post-printing processing due to the unique properties of the tellurium nanowires coupled with the aerosol jet printing process. The sliver nanowire electrodes printed by aerosol jet printing demonstrate excellent conductivity and stretchability without the need of sintering. The extrusion method is employed to print the silicone films which serve as the stretchable substrate and the electrical insulation layers between the printed tellurium and silver. The fully printed, sintering-free and poling-free, and stretchable piezoelectric device opens enormous opportunities for facile integration with a broad range of printed electronics and wearable devices.