Symposium SF04—New Types of Polymers, Composites and Hybrid Materials for Additive Manufacturing

Natural materials and composites are receiving increased attention for sustainable manufacturing. They are renewable, biodegradable, and inexpensive. Manufacturing using natural resources into mechanically-robust materials alleviates environmental concerns associated with the use of petroleum-based materials. Cellulose fibers have been used as fillers in polymer matrices to make composites. However, the hydrophilic cellulose fiber are incompatible with synthetic hydrophobic polymers¹, leading to poor interfaces between fillers and matrix. In addition to traditional manufacturing, additive manufacturing enables the customized and precision fabrication of composites with the control of internal microstructures^2,3. Although there are emerging researches using natural materials into 3D printing^4,5, limitations on printable materials and their mechanical robustness exist.
In this study, we develop 3D printable biocomposites entirely from corn byproducts and waste. We extract cellulose microfibers from corn husks by alkalization, delignification and high-speed homogenization. FTIR spectrum reports the effective removal of hemicellulose and lignin from corn husk. The extracted cellulose fibers (60 um in length and 10 um in diameter) are distributed into compatible matrices derived from corn starch to formulate 3D printable biocomposites. Ink preparation is tailored with water content and thermal treatment to achieve desired rheological properties for direct ink writing (DIW) 3D printing and tunable mechanical properties for the printed samples. The printed triangular cellular architectures using composites (10 wt% fiber) show 2 and 1.5 times increase in stiffness and strength, respectively, over the pure starch. Besides, preheating time substantially affects the mechanical properties of the printed biocomposites. The specific young’s modulus and specific strength increase 159.8% and 160% for 14 minutes preheating at 120⁰C compared to 6 minutes preheated samples, correlating to the microstructures evolution at different preheating times. The results indicate a comparable mechanical performance compared with other 3D printed thermoplastics. The manufactured biocomposites consist of purely natural components will have potential applications in medical and food area.
References:
1. Mohanty, A. K., Misra, M. & Hinrichsen, G. Biofibres, biodegradable polymers and biocomposites: An overview. Macromol. Mater. Eng. 276–277, 1–24 (2000).
2. Compton, B. G. & Lewis, J. A. 3D-printing of lightweight cellular composites. Adv. Mater. 26, 5930–5935 (2014).
3. Raney, J. R. et al. Rotational 3D printing of damage-tolerant composites with programmable mechanics. Proc. Natl. Acad. Sci. U. S. A. 115, 1198–1203 (2018).
4. Nguyen, N. A. et al. A path for lignin valorization via additive manufacturing of high-performance sustainable composites with enhanced 3D printability. Sci. Adv. 4, 1–16 (2018).
5. Jiang, Y. & Raney, J. R. 3D Printing of Amylopectin-Based Natural Fiber Composites. Adv. Mater. Technol. 1900521, 1–8 (2019).

To enable the planned rapid growth of both government and private operators in space, including satellites, space tourism and manned missions to the Moon and to Mars, a realistic and holistic approach to radiation risk reduction is needed. In deep space, ionizing radiation from Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs) from the sun pose a critical threat to both humans and electronic equipment. GCRs provide a chronic, slowly varying, highly energetic background source of High-Z high-Energy (HZE) particles, while the Sun's activity varies with an 11-year cycle during which the Sun produces Solar Wind (SW) at varying intensities and unpredictable bursts of Solar Particle Events (SPEs). SPEs can be a nightmare for the astronauts and cause acute radiation damage, while GCRs can cause long term damage including cancer, cataracts, central nervous system and cardiovascular system damage, fibrosis, neurodegeneration, digestive diseases, and immunological, endocrine, hereditary effects, and cognitive impairment.

The GCRs and SPEs can also cause degradation of micro-electronics, optical components and solar cells. Spacecraft electronics are especially susceptible to radiation effects that emerge from interactions with HZE particles, but highly energetic gamma photons, neutrons and protons can also damage electrical components. Since the exposure of humans and electronics to GCRs and SPEs in deep space, and on the surface of the Moon and Mars, will cause huge radiation risks, it is very important to apply the best possible protection for both humans and electronics. Currently, the only proven and practical countermeasure to reduce the exposure to GCRs and SEPs is passive shielding. It is well known that low atomic number (Z) materials are most effective for shielding in space, and liquid hydrogen has the maximum theoretical performance as a shielding material. Hydrogen is not, however, a practical shielding material, being a low temperature liquid associated with practical handling problems and explosion risks. Hydrogen concentrated in specially engineered and doped polyethylene based composites, however, is ideal for stopping primary cosmic and solar radiation, as well as secondary neutrons created when the primary particles are impinging on the spacecraft. Low atomic number (Z) materials also produce less electron-positron pairs and Bremsstrahlung than materials with higher Z, such as aluminum alloys, which are used in conventional satellites and spacecraft.
Additives can improve the flame retardancy and inhibit the release of toxic gases as well as absorption of neutrons created when HZE particles hit the shielding and other components of the spacecraft. The material can also be produced for protection against micrometeoroids, debris, extreme temperature variations and protection against atomic oxygen present at low Earth orbit (LEO).

We will present the basic physics, as well as simulated and experimental properties of a 3D printed specially engineered and doped polyethylene based composite, which can be manufactured with different degrees of flexibility, stiffness and thermal conductivity. We have demonstrated that hierarchical engineering of PE materials and composites enables drastic modification of their light absorption, thermal emission, heat, and moisture transport properties [1,2]. The material can therefore be used for many different applications, ranging from radiation shielding of components inside a satellite or spacecraft, to a construction material for satellites, to spacecraft and space habitats, and components in intravehicular (IVA) and extravehicular (EVA) spacesuits.

1. S.V. Boriskina, An ode to polyethylene, MRS Energy & Sustainability, 6, E14, 2019.
2. M. Alberghini, et al, Sustainable polyethylene fabrics with engineered moisture transport for passive cooling, Nature Sustainability, 2021, doi: 10.1038/s41893-021-00688-5.

Thermosetting polymeric composites which are ubiquitous in structural applications due to their superior strength and stiffness compared to their thermoplastic counterparts. These superior properties are principally imparted by dense and permanent crosslinking present at the molecular network level. However, as thermosets are subjected to fatigue loading, they accumulate damage in terms of ruptured crosslinks which eventually result in catastrophic failure. Vitrimers provide an advantage relative to the standard thermosets in that the network is transient and damage introduced by excessive loading may heal. In this work we explore the ability of the vitrimer to repair damage introduce during fatigue loading. We work with a vitrimeric system based on epoxy resin and adipic acid, with a suitable catalyst. The system has stiffness and strength comparable to those of conventional epoxies. We demonstrate that the vitrimer can be periodically healed by heating at temperature between Tg and Tv. Further, we develop and test a carbon fiber vitrimer composite which can be similarly healed by periodic healing. Such composites can pave the way for next generation of structural components which are recyclable and have longer service lifespans.

Direct writing describes any technique or process capable of depositing, dispensing, or processing, including removal of, different types of materials over various surfaces following a present computer-generated pattern. Indeed, 3D assembly involves the confluence of information, energy, and materials in different and often novel platforms. This presentation will describe the unique role that materials play in present day pulsed laser and photonic processing of 3D hierarchical assemblies ranging from mesoscopic passive and active electronics and engineered tissue constructs to nanometric control of surfaces for energy storage and conversion. In all cases there is a need to refine and optimize the aforementioned confluence to have better control over the function of the final product. The energy supplied by pulsed lasers and photonic processing can be used to simply heat materials to high temperatures almost instantaneously allowing for rapid synthetic transformations or to initiate photolytic processing such as in Click Chemistry. In all cases there is the absorption of light, electronic excitation, and followed by the relaxation of excited states into the desired material phases as well as shapes and sizes. Therein the photon-material interaction plays a paramount role and the energy attributes of intensity, wavelength, pulse width, and repetition rate make this route to processing very powerful. The distinctive control of pulsed lasers and photonic processing make this approach to processing materials for 3D assembly especially enabling and amenable to roll-to-roll manufacturing.

Often, the time required to regenerate bone in nonunion bone defects using tissue-engineered scaffolds is too long for practical clinical relevance. Hence we have designed novel interlocking blocks using 3D printed miniature molds to fill large bone defects. The scaffold blocks are fabricated using nanocomposite polymer materials with amino acid-modified nanoclays. In addition, these blocks are coated with bone morphogenic proteins (BMP-2 and BMP-7) to initiate and speed up bone growth. On seeding the scaffolds with a combination of human mesenchymal stem cells and osteoblasts, the BMP-coated scaffolds exhibit a much higher formation of the extracellular matrix formed within three days. We also observe enhanced osteogenic differentiation of MSCs at the interfaces between the blocks and the formation of bone mimetic fibril-like collagen. We also observe that the interlocked assembly of the scaffold blocks maintains the mechanical integrity of a single cylindrical scaffold. We also report long-term bone growth over a period of up to 9 weeks on the scaffolds. The coating of scaffolds with BMP-2 and BMP-7 increases cell viability on the scaffold interlocking blocks. We also observe increased intracellular ALP levels in interlocking blocks coated with BMPs indicating enhanced osteogenic differentiation of MSCs. Further immunocytochemistry and FTIR experiments indicate the formation of bone mimetic fiber-like collagen formation and increased ECM formation in the BMP-coated scaffolds. The role of BMPs on long-term bone growth is significant. The degradation in material properties of a hydrated scaffold over a period of 0 to 9 weeks is compared with increasing mechanical properties of the cell-seeded scaffold with the formation of ECM using the nanomechanical investigation of elastic modulus of the scaffolds. The changes in mechanical properties are consistent with the gene expressions indicative of enhanced osteogenesis in the scaffold blocks. Thus, here a novel BMP-coated interlocking blocks system is presented for accelerated bone growth.

3D printable materials reinforcement and fabrication processes have attracted many scientists and researchers, as most industries and consumers require tailored materials specifically suitable for their needs. These materials are usually chosen for their specific characteristics and attributes, which can enhance the desired characteristics or reinforce the polymer, while fabricating the materials by physical blending then by extruding it into filaments with a specific dimensions. In this study a method for the preparation of 3D printing wire rods of Poly (lactic acid) (PLA) / polypropylene (PP)/ Tungsten tri-oxide (WO₃) nanocomposite with up to 10wt% of WO₃ constitution was developed. The results showed that the interfacial compatibility of WO₃ and PLA was improved by adding 20wt% of PP. The addition of nano sized WO₃ with PLA greatly increased the tensile strength of the PLA composites and addition of PP has enhanced the elasticity of the composites. Moreover, morphological studies revealed the brittle behaviour of the PLA/WO₃ composites has shifted to ductile behaviour by the influence of PP phase in the nanocomposite.

Keywords: Poly (lactic acid) (PLA), Tungsten-trioxide (WO₃), polypropylene (PP), nanocomposites, 3D printing (3DP)

Rice husk ceramic (RHC) can be used as a reinforcing agent in metal-matrix composites (MMC) to improve the tribological properties because of their remarkable mechanical and tribological properties. This research scrutinizes the role of RHC to enhance the tribological properties of Ni-MMC. Ni-MMC with additives RHC, Al, Cu, MoS₂, and graphite in different content were prepared by the powder metallurgy method. The wear and friction mechanism of Ni-MMC was investigated using a ball-disk tribometer. SEM, EDS, and optical microscope were used to examine the wear properties, distribution of reinforcement, and microstructural phases, respectively. The study concludes that the addition of RHC particles can strengthen the anti-wear and friction reduction properties of Ni-MMC. The addition of 3wt% RHC results in the lowest value for the coefficient of friction and wear rate. Further expansion of MoS₂ and graphite along 3wt% RHC, both the wear rate and coefficient of friction decreases. At the same time, the value depends on the content of elements like aluminum and copper in MMCs.

Magnetorheological elastomers are smart materials that are magnetoactive, meaning they can deform and change shape based on the application of a magnetic field. These magnetic elastomers perform best in soft robotics and biomedical applications when they have increased anisotropy, or different properties along different axes. Magnetic annealing and the use of hard magnetic particulate are common ways to increase the anisotropy- and thus performance- of magnetic elastomers. To further increase this anisotropy, one can utilize fused deposition modeling (FDM), also known as 3D printing. FDM generates different underlying infill patterns within the sample, creating meso-scale anisotropy [1]. FDM of magnetic elastomers has recently been shown to control the magnetic anisotropy within these materials [2]. To create added micro-scale anisotropy by aligning magnetic particulate within the sample, magnetic annealing is used during extrusion of the elastomer filament. Hard or permanent magnets can be oriented using an external alignment field, and strontium ferrite is particularly interesting because it costs less than other rare-earth magnets (or materials). Previous studies in this group have focused on carbonyl iron, a soft or impermanent magnet that showed a large amount of magneto-action, though small changes in its properties after magnetic annealing. Strontium ferrite shows a smaller magnitude of magneto-action in comparison to carbonyl iron, but it has more distinct anisotropy effects after magnetic annealing. Combining these two magnetic particulates into one hybrid composite elastomer filament leverages these trade-offs, creating innovative and emergent mechanical, magnetic, and magnetoactive properties. Our thermoplastic magnetic elastomer composite is created using solvent-casting techniques with 15 vol% SrFe₁₂O₁₉ particulate, 10 vol% SrFe₁₂O₁₉/5 vol% carbonyl iron, and 5 vol% SrFe₁₂O₁₉/10 vol% carbonyl iron. The material is then extruded into FDM filaments. During the extrusion process, some filament of each different chemical composition is magnetically annealed in an axial applied field created by NdFeB magnets, while other filament extruded without magnetic annealing acts as the control. Mechanical stress vs. strain curves of the extruded filaments are acquired using an MTS tensile testing machine. Magnetic hysteresis loops are measured using a vibrating sample magnetometer (VSM). In order to determine the relative level of magnetic anisotropy within the samples due to these factors, hysteresis loops with the field applied parallel and perpendicular to the axis of extrusion are compared to one another. Magnetoactive testing is used to measure the mechanical deflection angle as a function of the strength of a transverse applied magnetic field. Understanding the mechanical, magnetic, and magnetoactive properties of these magnetorheological elastomers provides insight into how magnetic anisotropy can be controlled by a combination of hard and soft magnetic particulate, magnetic annealing, and FDM for optimized performance.

We use molecular dynamics simulations to characterize the mechanical behavior and failure mechanisms of Cu₆₄Zr₃₆ metallic glass-nanoglass (MG-NG) composites. The composites are designed with a brick-and-mortar arrangement consisting of a soft and highly ductile NG matrix and a strong MG second phase. The results show that the best compromise of mechanical properties is reached at a 30/70% volume fraction of the MG/NG phases, when the ultimate strength of the NG is distinctly improved from ~1.8 to ~2.0 GPa without inducing localized plasticity. In addition, MG bricks arranged in a staggered way can further synergize the mechanical properties of the two phases by effectively distributing the stress load, hindering the buildup of local stress hotspots, and preventing the generation of critical shear bands.

Cellulose is the most abundant natural polymer on the planet. Despite its high heat resistance and high toughness, it is insoluble in many solvents, making its fabrication process difficult. Poly(lactic acid) (PLA), a biodegradable polymer made from corn or sugar cane, is made by polycondensation of lactic acid or ring-opening polymerization of lactide. PLA has poor foam processability because of its low melt viscosity. In this study, a cellulose/PLA composite is prepared with wood-like vertically aligned pore structures to overcome the weak mechanical properties of foam. Cellulose was dissolved in an alkaline solution of NaOH and urea at a low temperature. This solution is frozen by a liquid nitrogen reservoir followed by ice removal with cryogenic ethanol. Cellulose aerogel is made by exchanging solvents from ethanol to water and then freeze drying. Finally, PLA solution is infiltrated into the dried cellulose aerogel to create PLA-cellulose aerogel composite. NaOH-urea solution can be frozen at low temperatures, especially using the liquid nitrogen reservoir. Etching ice with ethanol retains the cellulose structure created during freezing. The temperature gradient freezing method creates honeycomb-shaped holes aligned vertically like a tree. The aerogel composite is made of cellulose and PLA using infiltration method. The porosity of aerogel is calculated by the ethanol replacement method. In this way, the porosity of cellulose and cellulose/PLA is about 80% similar. Cellulose/PLA aerogel composite has a higher modulus and stress-to-break than the control cellulose aerogel in compression tests, due to the thin PLA continuous phase. Because both PLA and cellulose are biodegradable, cellulose/PLA composites have great potential to be used in future eco-friendly applications.

The injuries and defects of uterus, especially impaired endometrium, can cause abnormal menstruation, recurrent miscarriage and pregnancy complications and may lead to absolute uterine factor infertility (AUFI). The inability to achieve pregnancy usually has a severe impact on couple’s physical and mental health. Conventional gynecological treatments including hysteroscopic adhesiolysis, intrauterine devices (IUDs) intervention and hormone therapy can relieve patients’ symptoms to some extent and partially restore uterine structure but they do not recover the function of fertility for patients. The anatomical structure of the uterus consists of three layers: perimetrium, myometrium and endometrium. The myometrium comprising smooth muscle cells is very elastic and the endometrium has a significant regeneration capability. Current studies start to employ tissue engineering scaffolds to repair uterine defects. For example, natural polymers such as collagen, hyaluronic acid and gelatin and synthetic polymers such as poly(lactic acid) (PLA) and poly(lactic-co-glycolic acid) (PLGA) have been used for uterine scaffolds. Although, those biomaterials are biocompatible and biodegradable and have been widely used in the tissue engineering field, their lack of good elasticity restricts their ability to simulate myometrium functions. In this study, a multilayered scaffold with high elasticity and good shaping-morphing ability was designed to mimic the anatomical structure of the uterus and was fabricated by combining 3D printing and electrospinning techniques. First, highly elastic thermoplastic urethane (TPU) and poly(D,L-lactide-co-trimethylene carbonate) (PDLLA-co-TMC) were melted and blended at high temperature. The blends were then 3D printed into scaffolds with different hierarchical structures in accordance with a CAD model. The TPU/PDLLA-co-TMC scaffolds were prepared to as a substitute for the uterine myometrium layer. Therefore, the elastic properties of printed scaffolds were studies through tensile testing. Additionally, the glassy transition temperature of TPU/PDLLA-co-TMC blends was investigated. The shape-morphing ability of 3D printed TPU/PDLLA-co-TMC scaffolds was investigated by immersing the scaffolds in a water bath at 37 degrees Celsius. Subsequently, a layer of electrospun PLGA/polydopamine (PDA) hybrid fibers was fabricated on the surface of 3D printed TPU/PDLLA-co-TMC. In this electrospun nanofibrous layer, 17beta-estradiol (E2) was loaded in PDA particles. The incorporation of E2 in PDA particles could solve the E2 insoluble problem and also preserve its bioactivity. Previous studies showed that E2 could promote endometrium regeneration by augmenting the function of angiogenetic growth factors, e.g., vascular endothelial growth factor (VEGF) and human basic fibroblast growth factor (hFGF). The controlled and sustained E2 release was studied in different pH solutions (pH4.5, 7.4 and 9.0) and under different NIR laser (808nm) irradiation energy. Finally, the last layer from a gelatin methacryloyl (GelMA)/gelatin (Gel) bioink encapsulated with human bone marrow derived mesenchymal stem cells (hBMSCs) was printed on the electrospun PLGA/PDA layer. In vitro biological behaviors such as cell viability and cell proliferation on 3D printed GelMA/Gel were evaluated. The effects of released E2 on cell survival and differentiation were also studied. The multilayered tissue engineering scaffolds made by 4D printing and electrospinning possess hierarchical structures to bio-mimic native structures of the uterus and have the potential for uterine regeneration.

Surfing is an iconic sport that is extremely popular in coastal regions. Current surfboard fin manufacturers produce high end products using an expensive injection moulding process to create hydro-foil shaped fins. This process, however, does not allow for easy customisation or rapid prototyping.

I will discuss the development of surfboard fins (a hard material) using a performance feedback loop. This loop involves the unique combination of computational fluid dynamics, computer aided design, 3D printing of hard materials, stiffness/flex testing, ocean testing (surfing the waves), embedded sensors / wearables and surfers’ perceptive experiences.

I will finish my talk with our progress on measuring the mechanical flex behaviour of surfboards, including modal (vibration) analysis and (what we call) ‘fingerprint’ analysis.

The use of 2D nanomaterials including graphene for improving electrical and thermal conductivity properties in thin-film devices and sensors is well-studied. There is also high interest in using these materials for anti-microbial properties in coatings and formulations. However, there is high interest in the use of these materials in fabrication methods based on 3D printing. The 2D and high aspect nature of the nanomaterials enable nanostructuring of the nanocomposites for improved transport and stress or residual stress mitigation. Other than their known contribution to electrical and thermal conductivity they also enable stimuli-responsive properties in 3D fabricated objects and function. To this goal, this talk will focus on emphasizing the unique properties of graphene materials and their incorporation in 3D printed materials using FDM, VSP, SLA, and SLS Methods. In this talk, we will demonstrate the nanostructuring of graphene materials for 3D printing in the following projects: 1) FDM of polyurethane nanocomposites, 2) SLA 3D printing, and the use of post-processing thermal curing for highly improved thermo-mechanical properties, 3) Plastic motor based on emulsions coating on SLS 3D printed powders. On these demonstrated projects it is important to emphasize the use of integrated spectroscopic and microscopic characterization to correlate with structure-composition-property relationships.

The free surfaces of liquids are often used to demonstrate optical reflection with laser beams due to their mirror-like flatness. Here we first report the discovery and development of a new type of liquids: their surfaces can be depressed dramatically and molded by light or laser beams. We then present a counterintuitive and unprecedented phenomenon: instead of a valley, a spike above the liquid surface can be created by the same laser beam. Ferrofluid is identified as the first of such novel optocapillary fluids. A full scale fluid simulation model is developed to combine optical absorption, heat transfer, surface tension and fluid dynamics and to handle fluid with arbitrary depth without any approximation. Both simulation and experiments revealed a dramatic effect of fluid thickness on the deformation. Below a critical thickness, the depression depth roughly follows Landau’s prediction of inverse dependence on the fluid thickness and eventually leads to the rupture of the fluid; while above the critical thickness, a spike emerges in the center of depression and eventually dominates the deformation with increasing thickness. To demonstrate our understanding and scientific accessibility and entertainment, we created a new series of optocapillary fluids by mixing transparent lamp oil with different candle dyes. These colorful fluids can be cut open by sunlight, and be patterned to all kinds of shapes and sizes using an ordinary laser show projector. The discovery and creation of novel optocapillary fluids open up a new field of interdisciplinary research and a wide range of applications.

Commercial thermoset elastomers are widely available at lower costs compared to prepolymer formulations. They are classified as do-it-yourself (DIY) home improvement and repair materials. They are based on a wide array of chemistries and have been found to have rheologies very suitable for AM. We hereby investigated their 3D printability using direct ink writing (DIW) methods. The materials are divided into two main classes: silicone (three materials) and non-silicone (six materials) with hybrid chemistries and composites.They were characterized by: functional identification using GC-MS pyrolysis, rheology, cure behavior, cured Shore A hardness, 3D printability, TGA, thermal conductivity, dielectric, and mechanical properties. Corresponding ASTM standard test methods were also used and the results are reported. We also investigated the addition of nanoparticles to develop new properties out of the typical DIY function. Dispersion of nanoparticles is achieved using high-frequency ultrasonication techniques. Methods to incorporate nanoparticles into commercial thermoset elastomers is described. Nanoparticles include carbon nanotubes (CNT), graphene oxide (GO) and a 50/50 composite blend of CNT and GO up to 2% by weight. Reported results highlight the advantage in thermal and electrical conductivity.

3D printing has significantly participated to a general acceleration of research activities by providing a rapid and economical source of completely personalized parts. Besides being the central focus of some publications thanks to the novel design strategies enabled by additive manufacturing technologies, the holders, adaptors and other mechanical supports are innumerable. However, printed parts are often relegated to purely mechanical functions due to the difficulties to print multi-materials and the restrictions associated with functional (and in particular conductive) material.
In this work, we aim at developing a hybrid 3D printer and a new composite material to print both conventional Fused Filament Deposition (FFD) filaments and a highly conductive material. FFD of conductive materials is limited by the necessarily thermoplastic properties of the filament used. Since no currently existing material is at the same time highly conductive and thermoplastic, a trade-off is obtained with composite materials using conductive particles inside a thermoplastic matrix. The composites use high aspect ratio particles to form conductive path by percolation, but the maximal particle loading is limited by the mechanical properties of the filament. As a result, the high loading leads to embrittlement and lowers the printability of the filament. Furthermore, the matrix material present at the junction between particles strongly impacts the contact resistance. Electrify, the most conductive filament currently available on the market still has a conductivity of 0.006 Ω cm (too high for advanced PCB applications) and suffers from high contact resistance and high price (196USD/100g).
To remedy this limitation of the maximal loading of particles in the printed material, here we propose a two-step printing process comprised of the FFD of a composite material, followed by a step of laser sintering to remove (evaporate) the thermoplastic matrix material and sinter the conductive particles together. This process is repeated for each layer to produce fully 3D PCB-like structures. To achieve this, we developed both a hybrid 3D printer based on economical desktop 3D printer and laser engraver parts, and a composite filament based on PLA and copper microparticles. The detailed electrical and mechanical characterizations of the printed conductive materials will be presented.

Two photon polymerization involves the use of ultrashort (e.g., femtosecond) laser pulses to selectively polymerize photosensitive resins; this technique has been used for 3D printing of medical devices with microscale and/or nanoscale features. The two photon polymerization technique involves near simultaneous absorption of two or more photons from a Ti:sapphire or fiber laser within a small volume over a short time period. The nonlinear nature of two photon absorption enables 3D printing of structures, including medical devices, which contain features below the diffraction limit. Several kinds of photosensitive resins, including polymers and polymer-ceramic hybrid materials, may be processed into medical devices with two photon polymerization. Recent medical uses of two photon polymerization have included processing of drug delivery devices, scaffolds for tissue engineering, and transdermal sensors. In vitro and in vivo evaluation of two photon polymerization-fabricated medical devices will be considered. Our results indicate that two photon polymerization is an important technique for creating microstructured and nanostructured medical devices.

3D printing of thermoplastics has become a mainstay of modern engineering research and development, enabling industry, universities, and even individuals the ability to rapidly manufacture and iterate upon computer-aided designs. 3D printing of metals, however, has continued to remain out of reach except for those able to bear the exceptional start-up and operational costs of existing solutions, such as powder bed fusion or metal injection molding (MIM). One solution that has the potential to reduce costs is the MIM-like process of using an organic binder mixed with powdered metal to produce a feedstock (filament) that can be printed using any conventional fused-deposition-modeling (FDM) 3D printer; the result after debinding and sintering is a pure metal part. We report here an even more accessible variant of this process we have improved upon to create 3D-printed copper parts. The process utilizes a purely thermal, as opposed to chemical debinding step, and the addition of carbon produces a locally favorable gaseous environment during sintering without the use of vacuum or outside injection of protective gas. The influence of printing orientation and sintering temperature on the microstructure and mechanical properties of printed and sintered copper parts was investigated. Higher sintering temperatures resulted in stronger and denser parts but with increasingly anisotropic shrinkage. Parts sintered at 1074 C showed comparable tensile strength and ductility as copper produced using the powder bed fusion process. Printing orientation was found to play a major role in the strength of unsintered parts, but this effect diminished to undetectable levels after sintering. Microstructural analysis showed the presence of carbon in both the printed part interior and exterior; this presence of carbon and its role in the densification process was discussed. The process shows promise for rapid, inexpensive turnaround of 3D printed copper parts for applications without the strict requirements of both strength and 3D intricacy.

Nanoparticles are known to play a crucial role in helping achieve excellent mechanical properties in advanced metal matrix composites fabricated by emerging selective laser melting (SLM) technology. Despite this, the understanding of their impacts on the evolution of microstructure and mechanical properties remains nebulous. In the present study, we adopted the SLM process to fabricate in-situ nano-TiB₂ decorated AlSi10Mg composites with alternative TiB₂ nanoparticle contents (0, 0.5, 2, 5, 8 wt.%) to investigate the effects of introduced nanoparticles on the SLM processability, microstructures, texture evolution and mechanical properties. Results show that nearly fully-dense TiB₂/AlSi10Mg composite samples can be manufactured at the optimized SLM processing parameter due to the enhancing SLM processability by nano-TiB₂ particles. Besides, increasing the nano-TiB₂ addition can gradually transform the coarse columnar grain structure with <100> fiber orientation texture to fine equiaxed grain structure without preferred crystallographic texture due to the heterogeneous nucleation effect of nano-TiB₂ particles. The elongated cellular sub-structure transforms to the equiaxed cellular sub-structure without obvious directionality. The dislocation density of the TiB₂/AlSi10Mg composites is higher than that of the alloy due to the mismatch of the thermal expansion coefficient between TiB₂ particles and a-Al matrix. Additionally, the microhardness, tensile strength and ductility can be improved simultaneously by the addition of nano-TiB₂ particles. With increasing TiB₂ nanoparticles content, the tensile strength and microhardness increased in a stepwise manner while the ductility increased first and then decreased. Moreover, the SLMed TiB₂/AlSi10Mg composites have superior tensile properties comparing to the previous SLMed AlSi10Mg alloy and their composites with the addition of other particles. The underlying mechanisms of strengthening are mainly attributed to grain boundary strengthening, dislocation density strengthening, load-bearing transformation strengthening, and Orowan strengthening. Meanwhile, the enhancing ductility in the composites is mainly attributed to higher relative density due to better SLM processability, less strain localization due to modified grain structure, high dislocation plasticity due to the intragranular nanoparticles. This study sheds light on SLM-produced nanoparticles decorated aluminium composites for the production of advanced materials.

Adhesive bonding using epoxy resin is a key industrial manufacturing process of joining materials. Recently, advanced technologies requiring extensive use of adhesive bonding, such as multi-material design with proper mixing of metals and polymers for lightweight automobile bodies, tight packing of electronic components requiring rapid heat dissipation, and additive manufacturing for novel materials, have become increasingly popular. However, a significant challenge facing adhesive bonding is the weakening or lost of adhesion under damp conditions. For instance, the adhesion strength between aluminum and epoxy resin decreases by 30–50% under wet conditions. This study proposes, for the first time, a proton-related mechanism for the moisture-induced breaking of epoxy resin from the extensive first-principles calculations.

We investigated the cohesive failure of epoxy resin prepared using amine-based curing agents by DFT-based free energy calculations. To this end, we performed the thermodynamic integration of the DFT energy difference between the deprotonated and original states through the artificial process of gradually transformation of the target proton into a dummy atom having zero charge to calculate the free energy of protonation. We thereby found that the amine group of the epoxy resin is protonated at equilibrium depending on the location of the amine group when the epoxy resin is embedded in water under standard conditions.

Next, we calculated the bond breaking barrier energies of the principal groups of the epoxy resin under both dry and wet conditions. Concerning the ether group, the energy barrier to C-O bond breaking is reduced from 2.86 eV under dry conditions to 2.30 eV under wet conditions because of the dissociation of H2O. The energy barrier for breaking the amine group decreases from 2.30 to 1.94 eV under dry and wet conditions, respectively, because of the protonation of the amine group, but the dissociation of H2O does not lower the transition state barrier energy further. We thereby show that the bond breakage of the protonated amine groups is the principal process causing the weakening of epoxy resins under wet conditions.

3D printing provides a powerful manufacturing platform for different industries and has been increasingly used in the tissue engineering field. 3D printing technologies includes an array of technologies: liquid-based, filament- or paste-based, and powder-based technologies. Using smart materials and with innovative designs, 4D printing produces dynamic structures that can change their shape, property, and/or function under external stimulus/stimuli. Using inks that contain living cells, 3D/4D bioprinting creates living structures for different purposes (cancer tissue models, tissue engineering, organ-on-a-chip, etc.) in the biomedical field. 3D printing technologies greatly improve our ability to fabricate a variety of complex and customized biomedical products accurately, efficiently, economically and with high reproducibility. However, finding or developing suitable biomaterials appears to be a bottleneck for the advancement of 3D/4D printing in biomedical engineering (J.Lai, C.Wang, M.Wang, “3D Printing in Biomedical Engineering: Processes, Materials and Applications”, Applied Physics Review, Vol.8 (2021), DOI:10.1063/5.0024177). Different 3D printing technologies have different requirements for the materials/inks to be used, and in most situations these requirements are highly demanding. The requirements for 3D printing materials/inks in tissue engineering include printability, biocompatibility, biodegradation properties, and mechanical properties of printed products. Biocompatibility is of paramount importance for a material in tissue engineering applications but it becomes highly important only when the material can be 3D printed where 3D printing is employed for structure formation. We have investigated / are investigating several 3D printing technologies, such as selective laser sintering (SLS), cryogenic extrusion 3D printing, and digital light projection (DLP), for fabricating advanced tissue engineering scaffolds and cell/scaffold constructs for the regeneration of bone, osteochondral tissue, blood vessel, etc. For example, for 3D printing of bone tissue engineering scaffolds via SLS, calcium phosphate (Ca-P)/ poly(hydroxybutyrate–co-hydroxyvalerate) (PHBV) nanocomposite was developed (B.Duan, et al., “Three-dimensional Nanocomposite Scaffolds Fabricated via Selective Laser Sintering for Bone Tissue Engineering”, Acta Biomaterialia, Vol.6 (2010), 4495–4505). For cryogenic extrusion 3D printing of scaffolds for osteochondral tissue regeneration, beta-tricalcium phosphate (beta-TCP)/poly(lactic-co-glycolic acid) (PLGA) nanocomposite was used (C.Wang, et al., “Cryogenic 3D Printing of Heterogeneous Scaffolds with Gradient Mechanical Strengths and Spatial Delivery of Osteogenic Peptide/TGF-beta1 for Osteochondral Tissue Regeneration”, Biofabrication, Vol.12 (2020), 025030). For shape-morphing scaffolds, beta-TCP/poly(D,L-lactide-co-trimethylene carbonate) (PDLLA-co-TMC) nanocomposite was developed (C.Wang, et al., “Advanced Reconfigurable Scaffolds Fabricated by 4D Printing for Treating Critical-size Bone Defects of Irregular Shapes”, Biofabrication, Vol.12 (2020), 045025). For obtaining complex shape-morphing structures, alginate (Alg) and methylcellulose (MC) blends were investigated (J.Lai, et al., “4D Printing of Highly Printable and Shape-morphing Hydrogels Composed of Alginate and Methylcellulose”, Materials & Design, Vol.205 (2021), 109699). This talk will give an overview of our work in developing different materials for 3D/4D printing in tissue engineering. It will provide design guidelines and practical approaches in developing composites and hybrids for biomedical 3D/4D printing.

Tissue engineering provides new vital means for human body tissue/organ repair. Tissue engineering scaffolds give temporary structural support to cells for their attachment, growth, and proliferation until the body has restored the mechanical and biological properties of the host tissue. However, the fabrication of tissue engineering scaffolds with controlled porous structures (pore shape, pore size, porosity, etc.) and desired external morphologies (shape, dimensions, etc.) is still a challenge. Digital light processing (DLP) is now recognized as a powerful 3D printing technology which can process ceramics into complex and interconnecting porous structures with high precision and resolution. Biphasic calcium phosphate (BCP), a physical mixture of hydroxyapatite (HAp) and beta-tricalcium phosphate (beta-TCP), is a promising bioceramic for bone tissue regeneration owing to its proven biocompatibility, osteoconductivity, possible osteoinductivity, and controllable biodegradation rates. But fabrication of BCP bioceramic scaffolds via DLP is rarely reported. In this study, a comprehensive investigation was conducted to assess and optimize DLP fabrication to produce high-performance porous BCP scaffolds. First, equal amounts of nano-sized HAp and beta-TCP powders were mixed using a planetary mill. The BCP powders were then formulated with photosensitive resins to prepare BCP slurries. In this step, different dispersants were used and compared for powder surface modification, aiming to decease bioceramic nanoparticle agglomeration and to achieve stable BCP slurry with high solid loading and low viscosity. The relationships between solid loading and slurry viscosity, DLP energy dose and cure depth and the optimal concentration of photo-initiator were systematically investigated so as to obtain the optimal processing parameters. Second, thermogravimetric analysis (TGA) was performed on DLP-formed green bodies to determine the debinding and sintering strategy for obtaining crack-free BCP porous scaffolds with desired biological and mechanical properties. Furthermore, the effects of sintering temperature (1100 - 1300 degree Celsius) on the shrinkage, phase stability, mechanical properties and biological properties of BCP scaffolds were investigated. Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), mechanical testing (compression tests and hardness testing) and surface roughness measurement were conducted for scaffold samples. Finally, BCP scaffolds with different designed pore morphologies, pore sizes and porosities were made via DLP and sintering using optimized processing parameters. The results showed that scaffolds sintered at 1300 degree Celsius had the best mechanical properties and the lowest surface roughness. Cell attachment and proliferation experiments indicated that DLP-formed BCP scaffolds had good cell viability. Also, the optimized DLP process demonstrated the DLP capability to fabricate complex tissue engineering scaffolds with great freedom and high accuracy. This study has therefore shown great potential of DLP for constructing complex bioceramic scaffolds and possibly other devices for biomedical applications.

Due to high stiffness and strength per unit mass, carbon fiber composite became more popular during the last few decades. The low conductivity of carbon fiber composite made aeronautical structures more vulnerable to withstand lighting strikes and unable to prevent electrostatic accumulation. To overcome this, typically different nanofiller are used in conjunction with conventional carbon fiber composites. Due to the high aspect ratio, extraordinary mechanical and electrical properties of nanomaterials, nanoengineered composites are becoming popular. In the present research work Graphene sheet (GS) of 120 µm thick, fabricated from 2D Graphene nanoplatelets, was incorporated into carbon fiber composites to enhance the electrical and mechanical properties of the composite laminate. Preliminary studies indicated that GS embedded composite laminates degraded the mechanical properties instead of enrichment due to weak graphene epoxy bonding. To improve the bonding in addition to vertical, horizontal, and square grids spiked roller was used to make holes in the GS. The utilization of the graphene grid helped to boost the graphene-epoxy bonding as well as mechanical properties. GS added at the mid-plane was expected to send the strong electric signal but in reality, the strength of the signal was not strong. To overcome the issue of weak bonding and electrical conductivity, a chemical deposition method was used to deposit copper nanoparticles on a Graphene sheet. The electrical conductivity of copper deposited GS was evaluated using Jandel four-point tester. Dynamic Mechanical Analysis (DMA) was carried out to evaluate and compare thermomechanical properties of copper deposited Graphene, plain graphene sheet reinforced composite and bare composite. Fragile interface observed due to weak plain GS- epoxy bonding resulted in degraded mechanical properties. The effect of copper nanoparticles deposited on the GS helped to enhance the bonding. Glass transition temperature (T_g), storage, and loss modulus, in addition to Electrical conductivity, were evaluated and compared. Scanning electron microscopy was used to evaluate and compare bonding at the mid-plane of the laminate. Overall study indicates that the copper deposited GS into carbon fiber composites would help to enhance the strength, stiffness, and conductivity of the carbon fiber-reinforced composite laminate.

The weight reduction of materials is crucial for building a resource- and energy-conscious society, realizing comfortable living spaces, and space exploration. Porous materials, such as aerogels, facilitate the fabrication of materials with density less than that of air. Owing to their high electrical, thermal, and mechanical properties, carbon aerogels have been studied for use in many applications. The lightest carbon aerogel, discovered to date, has a density of 0.16 mg cm^-3, as reported by Sun et al. in 2013 [1]. The density of this aerogel is approximately 1/6 of the density of air at room temperature; however, it does not levitate in air. This is because there are many pores in the ultralight aerogel that are filled with air, and therefore the apparent density is the sum of those of the material and inner air, thereby making it heavier than air. Many studies have been conducted on ultra-lightweight materials; however, the attempts to make them levitate in air have been very few and limited. If ultralight materials can levitate in air, they will behave as if they are defying gravity, providing new values to our lives. In recent years, research on carbon aerogels with a three-dimensional geometric structure designed using 3D printing technology has also been conducted to expand the applications of carbon aerogels further. This technology makes it possible to create aerogels that retain specific mechanical properties in a density range approaching air density by realizing complex geometric structures.
In this study, we show ultralight carbon aerogels that can intermittently levitate in air for a long time according to Archimedes' principle. Ultralight carbon aerogels fabricated with CNTs can be easily heated by light, and the temperature can be controlled. A carbon aerogel, with density that is less than that of air, is heated using a halogen lamp. As a result, the air inside expands to reduce the air density inside the aerogel, thereby creating a state in which the sum of the densities of the aerogel and inner air is less than that of the surrounding air. This allows the material to levitate due to buoyancy.
Ultralight aerogels, composed of CNTs and cellulose nanofibers (CNFs), were produced with a prepared density of 0.25, 0.50, 0.75, and 1.0 mg cm^-3. UV-visible/near-infrared spectroscopy (UV-vis/NIR) measurements using an integration sphere show that the diffuse reflectance of all samples is between 0 % and 4 % in the wavelength range of 350–2500 nm. The ultralight aerogel fabricated showed a heat capacity of approximately 9.3 × 10^-4 to 2.7 × 10^-3 J K^-1 cm^-3 per unit volume in the temperature range of 20–200 °C. The small heat capacity across the entire temperature range suggests that the aerogel could be heated instantaneously using a small amount of energy.
The levitation experiment was conducted by heating the ultralight aerogel with a 90 W halogen lamp in an acrylic cylinder whose bottom was covered with an aluminum lid placed in a container filled with liquid nitrogen. When the lamp was turned on, the aerogel was heated and began to levitate. A cylinder surrounded the aerogel, and the bottom was covered with an aluminum lid such that it was not affected by the updraft caused by the liquid nitrogen. The aerogel levitated as long as it remained exposed to light from the lamp. To confirm that the ultralight aerogel was heated by the halogen lamp and levitated via buoyancy according to Archimedes' principle, the densities of the aerogel (ρ_a) and ambient air (ρ_m) during levitation were calculated and compared. All the data obtained from the calculation results are near the line where ρ_a = ρ_m, indicating that air buoyancy levitates the ultralight aerogel, according to Archimedes' principle, when heated by the halogen lamp.
[1]. Sun, H., Xu, Z. & Gao, C. Multifunctional, ultra-flyweight, synergistically assembled carbon aerogels. Adv. Mater. 25, 2554–2560 (2013).

Achieving desired performance from self-assembly of nanoparticles (NPs) is very challenging due to the stochastic nature of interactions among the constituent building blocks. Self-assembly of Nano-colloids through evaporation of particle-laden droplets can be exploited to fabricate tailored nanostructures that add functionality and engineer the properties of the manufactured components. The particle-particle and particle-solvent interactions, as well as the forces arising from the evaporation of the solvent are the main driving factors that define the formation of the final nanostructure. In this study, we introduce a nanoparticle-agnostic approach that allows the fabrication of multi-material nanostructures with precisely engineered patterns. Evaporative droplets of aqueous suspensions of Carbon Nanotubes (CNTs), Graphene Nanoplatelets (GNPs), and Boron Nitride Nanotubes (BNNTs) representing NPs of different elemental composition (i.e., carbonaceous and ceramic) with different sizes and shapes (i.e., Nanotube, Nanoplatelets) are investigated. We utilize cellulose nanocrystals (CNCs) as a platform to make hybrid systems of CNC and the secondary NP (i.e., CNC-CNT, CNC-GNP, and CNC-BNNT). We then capitalize on fundamentally understanding the repulsive-attractive interactions in this hybrid system and their effects on the final pattern that is created. It is shown that the formation and thickness of deposited patterns of CNC-bonded CNTs, GNPs, and BNNTs after evaporation of nano-colloidal droplets depends only on the concentration and mass ratio of the NPs and not their shape and size or material that is utilized. This system involves non-toxic, abundant, and biocompatible agents such as water and CNC that promote its scalability. The proposed method enables new capabilities in the precisely controlled fabrication of engineered nanostructures, and programming smart self-assembly systems.

A thermoreversible thermoset formed by Diels-Alder (DA) chem. was used in Fused Filament Fabrication (FFF) 3D printing to obtain a printable, elastic thermoset with isotropic mech. properties. FFF additive manufg. typically uses thermoplastics that are limited in the amt. of filler that can be incorporated in composite prints due to their high melt viscosities and often exhibit anisotropic strengths according to the print geometry. Thermoreversible thermosets with weak links based on cycloadducts between maleimide and furan groups can revert from crosslinked network to liq. oligomeric resins with heating above 100 °C. In this study, filaments with up to 60wt% tricalcium phosphate and silica particles were prepd. and used in FFF printing. Thermoreversible resins were prepd. by reacting 1,1'-(methylenedi-4,4'-phenylene)bismaleimide (MPB) with furfuryl group functionalized oligomeric Jeffamines. The resulting resins were then heated to mix with particle filler and extruded to afford 1.75 mm filament. Tensile test specimens were printed between 130-150 °C with varying printing geometries to establish that the thermoreversible thermosets displayed isotropic strength relative to print orientation. The resulting printed structures were readily recyclable by chopping into fragments and re-extruding as filament for addnl. printing. The resins can be recycled back into filament and then printed up to five times. Irreversible crosslinking, through reaction of free maleimides, occurs after 5 h at 140 °C but could be used to prevent thermoreversion. Properties of the resins could be tuned with filler content and with the mol. wt. of the jeffamine.

Biomass natural fibers have been commonly used for reinforcing polymers (e.g., polylactic acid [PLA]) because of their low cost, low density, abundance, and biodegradability. In this study, PLA/wood flour composites were produced with wood flour loading between 30 and 60 wt% to investigate the effect of wood flour loading on the composite performance. The composites obtained were characterized using dynamic mechanical analysis (DMA), tensile testing, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and rheology. The rheological properties of these highly loaded composites were then modified using various coupling agents and viscosity modifiers to optimize the composites for large-scale additive manufacturing applications. The mechanism of the viscosity modification in the PLA/wood flour composites was investigated, and the chemical treatments were found to be effective in tuning the properties of the biocomposites for specific applications.

1. A layered Cu/Gr/Cu ultrathin foil via a one-step electrochemical route
A layered structure has a better effect on improving performance of the graphene (Gr)-reinforced composites due to its unique two-dimensional structure and excellent properties. In this work, a novel “one-step” electrochemical route was proposed for synthesizing the Gr-reinforced ultrathin copper (Cu) foil with high performance. The process includes: 1) A loose graphene oxide (GO) membrane, was prepared by electrophoresis deposition (EPD), that allows Cu ions passing through; 2) According to the difference of Cu deposition potential on different substrates, a potential step was designed for electrodepositing Cu successively on both sides of the GO membrane, i.e., the bottom Cu layer forms under low over-potential, while the top Cu layer forms under high over-potential. The experimental results show that the foil thickness reaches to as thin as 4-5 μm, and the tensile strength is almost twice as large as that of pure Cu foil. This process is simple, controllable and possible mass production, and expected to further practical applications in fields of Cu clad plate, printed circuit board and lithium-ion battery cathode collector system for saving raw material and also the space. In addition, this work proposes a new idea for preparing the layered composites via electrochemical route.

2. A Cu-(CNTs+Gr) composite ultrathin foil via electrodeposition
The incorporation of nano-reinforcement, such as carbon nanotubes (CNTs) and graphene (Gr), into copper (Cu) matrix via electrodeposition is a promising method to improve its comprehensive property. However, the challenge is to achieve a uniformly suspended electrolyte with these reinforcements or surfactant due to their inferior dispersibility. In this work, we report a process to produce a kind of high performance Cu-(CNTs+Gr) composite foil by using hybrid self-dispersing reinforcement CNTs+Gr via a direct current (DC) electrodeposition, which further results in an ultrathin foil reach to the thickness as low as 4 μm. In this process, firstly, the precursor graphene oxide (GO) with amphipathicity acts as a surfactant-like substance and disperses CNTs uniformly through the π-π bonding in-between GO and CNTs in the copper sulfate electrolyte. Then, GO is reduced into Gr during electrodeposition, and the synergistic effects of CNTs and Gr play a key rule to improve the ultrathin Cu-(CNTs+Gr) composite foil’s properties, including tensile strength up to 436 MPa, inoxidizability and superior electrical conductivity. This work provides a possibility to have mass-production of Cu foil as thin as 4 μm in industry and broad applications in the future.

Image-guided radiotherapy, particularly MR-Linac (magnetic resonance linear accelerator), has the potential to transform treatment outcomes in several challenging cancers, including lung cancer, by adjusting x-ray beams in real time to accurately target even moving tumours. Phantoms, manufactured objects which mimic the response of tissues to imaging and radiotherapy, are a critical technology for future development of these techniques. They provide a well-defined and stable "ground truth" which can be imaged during irradiation for long durations in a way that is impossible with human volunteers. Phantoms can also provide calibration and quality assurance, accelerating knowledge transfer in multi-institution partnerships.

3D Printing of phantoms has a long history for X-Ray imaging but is more difficult to achieve with MR imaging modalities because of the need to mimic the T₁ and T₂ relaxations of tissue that provide the imaging contrast. Current state of the art limits MR phantoms to simple geometries, containing aqueous solutions or gels with poor stability and are either static or only permit simple motion. The ability to replicate the unavoidable motion of internal organs (e.g. heart and lungs), while also mimicking complex tissue geometries, MR imaging properties and radiation response of tissues, is critical for advancing MR-Linac treatment. We are developing a palette of 3D-printable silicone materials with a clinically relevant range of MR imaging properties (T₁/T₂) for MRI and orthogonally tunable pre- and post-cure viscoelasticity to allow an appropriate mimicking of normal tissue, organs and tumours. It is also possible to incorporate dosimetric functionality to study the effect of image-guidance on delivered dose distribution in a realistic setting – not possible with any existing technology.

3D printing is very attractive for fabricating tissue engineering scaffolds because it can produce either simple or complex scaffolds precisely and rapidly according to the scaffold design and custom-make scaffolds to meet the requirements for individual patients. However, 3D printed structures have static shapes and properties during their application and hence do not accurately imitate the dynamic nature of body tissues. Shapes, properties or functions of tissue engineering products should evolve along with the dynamic tissue regeneration process. Therefore, 4D printing, an emerging manufacturing platform through the integration of time with 3D printing, is now hotly pursued for fabricating dynamic structures which can change the shape, properties or functions over time under appropriate stimulus/stimuli such as temperature, pH, humidity, light, electricity, magnetic field, sound, or a combination of these stimuli. Currently, shape memory polymers are the most popular responsive materials for 4D printing in tissue engineering. However, most of them are synthetic polymers whose biocompatibility is not as desired. Combining a synthetic shape memory polymer with a biocompatible natural hydrogel is an effective way for obtaining a polymer blend with both shape memory ability and enhanced biocompatibility for 4D printing in tissue engineering. Poly(D,L-lactide-co-trimethylene carbonate) (PDLLA-co-TMC) is a popular synthetic shape memory polymer. Structures made of PDLLA-co-TMC with a DLLA-to-TMC ratio of 80:20 respond at 37 degrees Celsius for shape memory, suggesting its potential for shape-morphing scaffolds for human bodies. Gelatin methacryloyl (GelMA), a photopolymerizable hydrogel composed of natural gelatin functionalized with methacrylic anhydride, retains the functional amino acid sequence of gelatin and hence exhibits good biological activity. It is shown to promote cell adhesion and proliferation. In this study, 4D printing of shape-morphing scaffolds with improved biocompatibility was investigated using polymer blends of PDLLA-co-TMC (80:20) and GelMA. The 4D printing process involved four steps: (1) Pure PDLLA-co-TMC and PDLLA-co-TMC/GelMA blends at 1:1, 2:1 and 3:1 blend ratios were printed into planar structures; (2) the planar structures were shaped into tubes by using a glass rod at 80 degree Celsius; (3) the tubular structures were flattened at 25 degree Celsius; (4) the flattened scaffolds could self-fold into tubular structures at 37 degree Celsius within a minute in an aqueous environment. The influence of blend ratio on the shape memory ability and printability was studied and compared with pure PDLLA-co-TMC. Surface morphologies of the 4D printed pure PDLLA-co-TMC and PDLLA-co-TMC/GelMA scaffolds were observed under SEM. Also, the mechanical properties of pure PDLLA-co-TMC and PDLLA-co-TMC/GelMA blends were assessed via tensile tests. Finally, the viability, attachment and proliferation of cells seeded on 4D printed pure PDLLA-co-TMC scaffolds and PDLLA-co-TMC/GelMA scaffolds were studied. It was found that the addition of GelMA into PDLLA-co-TMC improved printability, stretchability, and biocompatibility while maintaining the shape memory property. 4D printed PDLLA-co-TMC/GelMA scaffolds with desired functionalities such as biocompatibility and shape memory property have great potential for tissue engineering applications, especially for tubular tissues such as vasculature and gastrointestinal tract.

In recent several decades, the global warming has been accelerating due to increasing carbon dioxide emissions, and accordingly various environmental problems are newly raised. Among the solutions to reduce CO₂ emissions, the adsorptive approaches have been actively progressing. Research on CO₂ adsorbent using 3D printing technology which is one of the most focused issues in the scientific community has been conducted recently, however, the approaches using the existing traditional concept using clay as a binder can cause limitation of adsorption capacity and preparation of feed for 3D printing. Herein, we present a new unconventional processable adsorbent fabrication method using 3D printing technology which starts with a widely used printable polymer. CO₂ adsorption capacity of 3D-printed polymers is imposed by post-treatment processes such as hypercrosslinking and amine-grafting. 3D-printed monolithic adsorbents showed significantly improved CO₂ adsorption capability and CO₂/N₂ selectivity due to the presence of microporosity and amine-functionality, compared with their original polymer form. Moreover, tests under dynamic flow in dry and humid condition demonstrated that the 3D monolithic adsorbents effectively captured CO₂ at an extremely low flow rate, while possessing good stability and regenerability. Based on the promising results presented in this proof-of-concept study, we are aiming to further improve the CO₂ capture capacity by increasing both porosity and amine loading in future work.

Hydrothermal processing (HTP) is a green method to convert biomass (such as algae) and solid waste into valuable materials. In this study, HTP is conducted to treat and valorize sewage sludge and food wastes into biocrude oil and biochar. The effects of process parameters including feedstock, solids load, HTP temperature, and reaction time, are investigated. Statistical model is created to predict the yields of the HTP products. A techno-economic analysis (TEA) of a continuous HTP process is developed, assuming the biocrude oil is used as asphalt binder additive. Biochar is characterized and assembled with fiber materials into composites, which might be used as electrodes for energy storage. Activated carbon is also prepared from the biochar, then used for 3D printing.

Fused Deposition Modeling (FDM) is the cheapest 3D printing technique centered around thermoplastic printing. However, the current usage of FDM 3D printing has been limited, especially in the applications where mechanical strength becomes crucial. The main reason for this limitation is the inherent anisotropic nature of the layered prints. Epoxy-benzoxazine based filaments can reduce this anisotropy effect by forming covalent crosslinking during the print. Polybenzoxazine and epoxies are major thermoset classes with excellent thermomechanical properties and rich molecular design flexibility. Blends of benzoxazine and epoxy resins exhibit a long shelf life at room temperature but will react at elevated temperatures to afford copolymers with superior properties. Here, thermoplastic filaments were prepared from epoxy-benzoxazine blends for FDM 3D printing that cure during printing to afford robust thermoset prints. The benzoxazine can polymerize by a base-catalyzed condensation reaction with itself and with the bisphenol A-based epoxide resin. Benzoxazine's tertiary amine group can also catalyze the reaction of alcohol hydroxy groups with epoxides in the epoxide resin. The thermoplastic blend was formulated from poly(bisphenol A glycidyl ether end-capped), a bisphenol A-based benzoxazine, and fumed silica. DSC analysis of the blend revealed that its cure temperatures are higher than the softening points of the epoxy polymer and the benzoxazine monomer, permitting the filament to be extruded below the cure temperature with no crosslinking. Printing of the blend-based filament at higher temperatures initiates the curing reactions that ultimately lead to 3D filament printed thermosets.

Nanofillers in polymer composites will induce interface regions in the polymer with non-uniform nanostructures which can be approximated as multilayer shells surrounding nanofillers. This work studies topological nanostructures in polymers that interface with one-dimensional (1-D) and zero-dimensional (0-D) nanofillers. This new class of dielectric polymer nanocomposites involves nanofillers at ultra-low volume loading (< 1 volume percent) that generate large dielectric enhancement. The results show that the dipolar response of polyetherimide, a high temperature dielectric polymer, is increased by more than ten times with ultra-low (0.75 vol%) loading of 1-D nanofillers. The results reveal that 1-D nanofillers induce cylindrical shell-topology in the polymer matrix which is more effective in enhancing the high dielectric constant interfacial region than the spherical shell-topology generated by 0-D fillers. The cylindrical shells generated by 1-D fillers provide much higher dielectric enhancement over a broader interfacial region in the polymer matrix, as compared to the spherical shells induced by 0-D nanofillers.

Additive manufacturing which is also known as Three-dimensional printing (3D printing) has become the most promising material fabrication approach for academic, R&D and industrial use. Among various types of 3D printing technology, Fused Deposition Modeling (FDM) is currently the most popular category because of its cost-effectiveness, high scalability and flexibility as well as minimal post-processing required. Unfortunately, it is still challenging to print continuous fiber reinforced structures due to various issues such as complex machine design, fiber flexibility and durability, as well as fiber-matrix incompatibility. Some companies have developed alternative 3D printers capable of printing carbon fiber composite, but the machine cost is still a hindrance for small scale use. Moreover, 3D printing with other types of continuous fibers has not been explored.

Our research aims to develop and modify a commercial desktop 3D printer to be capable of printing continuous carbon nanotubes (CNT) assemblies. In our lab we grow vertically aligned multiwalled carbon nanotubes (CNTs) using Chemical Vapor Deposition process (CVD). The resulting CNTs are called “Spinnable CNTs” because they can be spun continuously as yarns, sheets, and fibers which can be incorporated in other matrices. With their inclusion in polymers, CNT reinforcement not only leads to mechanical strength improvements, but also results in unique multi-functionalities such as electrical conductivity. We successfully impregnated CNT yarns onto the surface of the polylactic acid (PLA) filament. The binary solvent system with 2 %w/v of PLA has been used as a binder during filament feeding without applying additional pull forces. Moreover, different types of commodity and industrial yarns such as cotton, polyester as well as Zylon yarns have been printed and studied. The 3D printed composites are fabricated by ROBO 3D R1 plus FDM printer. The structure-property relationship of the printed parts were investigated by varying layer thickness, fill density, infill type, printing temperature, printing speed, and delay time.

Results revealed that a 210°C extruder temperature leads to the best printability in comparison against 190-205°C. Minimizing printing speed also results in better printability as it prevents the dragging and the peeling off of the reinforcement yarns. Mechanical performance is investigated through measuring the tensile strength. At the same fiber volume fraction, the 1 mm thickness 3D printed part containing Zylon yarn shows the highest tensile strength of 45MPa and modulus of 1.40 GPa, followed by the CNT yarn as reinforcement which presents tensile strength of 39 MPa and modulus of 1.27 GPa. Results showed that 3D printed PLA reinforced Zylon yarn has 7.1% higher in tensile strength and 0.72% higher in tensile modulus than 3D printed neat PLA. This demonstrated that different yarn strengths lead to varying degree of the printed composite strength.

Morphology, fiber orientation, and void formation of the printed parts were characterized by SEM, electrical properties, and thermal properties by DSC and TGA. For future work, we aim to modify commercial desktop 3D printer for more seamless integration of the continuous fiber with high fiber volume fraction. Enhancing printing productivity and broadening various types of continuous fiber also need to be developed. Successful development of this hybrid 3D printing technology will be useful in a broad range of applications such as electronics, automotive, and aerospace.