Symposium F.SF05—Advanced Materials for Additive Manufacturing

Additive manufacturing (AM) spread widely not only for prototyping but also for the production of end-use products. Quality control is a vital importance in the production of parts. One of the challenges of quality control in AM technology - such as fused deposition modeling (FDM) - is that the mechanical strength of the 3d-printed part could be anisotropic (not uniform). The part’s printing orientation could affect the mechanical strength of 3d-printed parts. Among the AM technology spectrum, the stereolithography (SLA) process is known to be able to produce parts with relatively uniform mechanical strength (isotropic).
Many studies were done using FFF 3D printing method. However, to our knowledge, only very few studies had done to investigate the effect of printing orientation on the elongation-at-break of 3d-printed parts using SLA technology. Moreover, previous studies only experimented with a limited amount of samples and orientations. Therefore, strength and elongation-at-break need to be examined in detail and with more variations of print orientation. For this purpose, this study investigated the influence of part’s printing orientation to the tensile mechanical properties (tensile-stress-at-max-load, tensile-strength-at-break, tensile-stress-at-yield, young-modulus, elongation-at-break) of SLA technology. This work covered a large number of orientations (27 printing orientations) testing five different mechanical properties.
We used Form 2 3D printer to 3D print with White photopolymer resin of FLGPWH04 material. We used Preform 3.03.1 software to set the sample positions. The samples were oriented with respect to x, y, and z axes according to the software axes. Permutation-with-repetition of 3 different angles (0°, 45°, and 90°) with respect to 3 different axes resulted in 27 part’s printing orientations. Furthermore, we printed 3 samples for each orientation with for a total of 81 samples for all orientations. The printed samples were washed in isopropyl alcohol (IPA) for 20 minutes. Afterward, we carefully removed the support structures from the samples using a tweezer; followed by drying and curing process at 60 °C for a total of 20 minutes (samples were flipped every 10 minutes).
We followed the ASTM D638 V standard tensile test design. Each printed sample was kept at the same time (13 days) before the tensile testing to avoid time impact on the properties. Then we performed the tensile testing using Instron 3367. In Bluehill LE 3.77 software, the sizes were entered for each sample. It took the measurements while the test was running and calculated the strain, stress, modulus, and yield. The raw data from the samples compiled in Microsoft Excel.
The results showed relatively uniform isotropy of the tensile-stress-at-max-load and it has 3.8% relative standard deviation. In addition, tensile-strength-at-break, tensile-stress-at-yield, and young-modulus has 4.15%, 5.67% and 7.94% relative standard deviation, respectively. In contrast, a relatively large anisotropy of elongation-at-break has been observed with 21% relative standard deviation. The sample printed vertically (90 degrees normal to the bed) elongated to an average of 10% whereas the sample printed horizontally (0 degrees normal to the bed) elongated to an average of 15%. One hypothesis that explains this behavior is that printing vertically created many layers comparing to printing horizontally. Thus, the high number of interfaces layers make the part elongate less. The results show that the elongation-at-break could vary and could be tuned in SLA technology; a new finding in this research field.
We recommend that the designer of SLA parts to consider the part’s printing orientation depending on the possible application (whether the application requires large elongation-at-break or short elongation-at-break). By knowing the effect of printing orientation, high-quality parts could be produced consistently according to the application’s requirement.

A common design rule in synthetic particle-filled composites is to align all reinforcing particles parallel with the primary stress and thereby perpendicular to a likely fracture. Doing so can often result in crack deflection, which is commonly touted as evidence that a material has enhanced fracture toughness. In reality, as soon as the crack deflection occurs, the material is fracturing along an “easy axis”. Much better is for the crack not to deflect and be forced to propagate forward along a “hard axis” spending much more energy to grow. Geometric conditions alone can create T-stress that confine the crack to propagate straight along a hard-direction. However, real-world applications of filled composites often dictate geometric design and put toughness at risk. Instead, designed texturing of reinforcing particles has the potential to steer cracks along a “hard axis” without reliance on geometric loading conditions. Recent advances in the field of composite manufacturing techniques, make such architectures possible, allowing the creation of microstructures which can locally tune the fracture toughness anisotropy.
In the studies presented, we explore a unique variant of this principle through the creation of an alignment architecture reminiscent of the spine of a herringbone fish, designed to trap a fracture along the boundary between two regions of distinct particle alignment. This robs the fracture of a preferred direction of initial fracture deflection, delaying the initiation of fracture formation, and forces it along a “hard” direction of fracture initiation, effectively raising both its initial and overall fracture resistance significantly beyond monolithically aligned composite polymer systems. However, while this custom microstructure proved successful in demonstrating this method of boosting mechanical performance over traditional monolithic systems in principle, it remains limited in application by the necessity of a fracture initiating at or near the boundary between two alignment regions. Without filling this pre-condition, the fracture would initiate in a manner similar to a monolithic alignment system.
To overcome this limitation, we applied these principles to generate a new class of beautiful herringbone mosaics that provide isotropic toughness in 2D. This structure was composed of a series of hexagonal unit cells, further divided into a series of triangular sub-cells, each with herringbone patterns. Mechanical testing of these samples determined that this structure not only demonstrated superior initial fracture toughness to monolithic samples, but that this effect was approximately isotropic, showing superior mechanical performance over the highest performing monolithic samples regardless of the direction of the tensile force relative to the microstructure placement. In applications in which the geometric design of a particle-filled composite is constrained, texturing materials with herringbone mosaics provide a meaningful route toward boosting isotropic toughness through microstructural design alone.

In aerospace applications, variations of temperature caused by changing environmental conditions or generation of heat is a common problem. The resulting thermal strains cause separation and alignment errors in optical systems, and stresses due to mismatches of thermal expansion at material interfaces cause failure of components and structures. Because minimizing weight, cost, and lead time are also critical requirements, lightweight, easily machined metals are attractive for aerospace applications, but these materials all have substantial coefficients of thermal expansion. Previous work has shown that the superposition of two metals with dissimilar coefficients of thermal expansion in periodic cellular configurations can result in a composite material with zero effective thermal expansion. However, fabrication of these material architectures typically is either impossible or requires manual assembly. Here, we propose methods to design and fabricate cellular metal composites with tunable thermal expansion and optimized specific stiffness that can be manufactured at useful scales entirely with automated methods. Samples were manufactured with ultrasonic additive manufacturing and computer-controlled machining, and thermal expansion was measured optically. The results demonstrate the practical fabrication of cellular metal composites with zero or negative thermal expansion in one direction, tunable thermal expansion in a second direction, and structural performance indices competitive with the indices of conventional aerospace alloys.

In additive manufacturing, a titanium alloy Ti6Al4V is the most widely used and was reported to form columnar grains during process. However, columnar grains are generally regarded to be unfavorable since their presence can impart solidification defects and mechanical property anisotropy. Therefore, fine equiaxed microstructures are desired. Recently more or less 0.3vol% addition of TiC particles resulted in grain refinement of Ti6Al4V during Laser-based powder bed fusion of metals[1]. Furthermore it was clarified that TiC particles partially melted in molten Ti6Al4V and the residual TiC particles generated fine and equiaxed grains from the heterogeneous nuclei during directional solidification of Ti6Al4V[2]. Although equiaxed grains can be obtained at high solidification rates in general, the effect has not been clear in an alloy with TiC. In this study, as a fundamental research for additive manufacturing, we aimed to reveal the effects of solidification rate, which varies the amount of melted TiC and the degree of constitutional undercooling.

Ti6Al4V powder (with 2.0vol% TiC and without TiC) in a Ti6Al4V sheath was melted and directionally solidified with a floating zone melting method at various solidification rates V(=1,3,5,10,15mm/min). During the process, the temperature around the melt pool was measured using a two-color pyrometer. Microscopic images on the cross section through the center of directionally solidified samples were obtained to evaluate the numbers and equivalent diameter of the prior β grains. The area fractions of the precipitate and matrix were analyzed by an image processing software, and the carbon contents in them were analyzed by Electron Probe Micro Analyzer(EPMA) to estimate the amount of the melted TiC. Also using phase field method, microstructure evolution of Ti6Al4V during solidification was simulated and constitutional undercooling around solidification front was estimated.

From microstructural observations, the number of fine equiaxed prior β grains increased by TiC addition except at 1mm/min and the total number of grains increased as solidification rate increased. Also with TiC added microstructure, precipitation appeared in the prior β grains regardless of solidification rates. EPMA analysis results showed that the precipitation was Ti₂C and the area percentage of Ti₂C decreased with increasing solidification rate. Estimating carbon amount from Ti₂C fraction, carbon amount derived from melted TiC is 0.262 mass% at 1mm/min and 0.178 mass% at 15mm/min. Considering total carbon amount 0.270 mass% from 2.0vol% added TiC, carbon amount derived from melted TiC is smaller when solidification rate is higher. From phase field simulation, constitutional undercooling around solidification front was larger as solidification rates increased. From above, it is considered that at a higher solidification rate, the amount of melted TiC particles is smaller, and the amount of residual potent heterogeneous nucleus TiC particles is larger at a large constitutional undercooling .

References
[1] Y.Watanabe et al., Metall. Mater. Trans. A, 51(2020)1345.
[2] S.Yamamoto et al., Metall. Mater. Trans. A, 50(2019)3174.

Recently, many researchers have studied 4D printing by fused deposition modeling (FDM)-3D printer using thermoplastic material including shape memory polymer (SMP) [1]. 4D printing is 3D printing with shape-shifting-function triggered by material shrinkage and/or expansion. A conventional 4D-printed device by FDM-3D printer is, however, difficult to deform after shape-shifting, because thermoplastic material is rigid below the glass transition temperature. Therefore, a conventional 4D-printed device is not possible to use as an origami stretchable device which is deformed by origami-deformation of elastic hinges [2].

In this study, we proposed a 4D printing device with a hybrid hinge. The hybrid hinge has a structure in which a self-folding hinge made of SMP and soft hinges made of elastomer are arranged in series. By applying heat to the 3D printed device with a hybrid hinge, the self-folding hinge works to form a folded structure. The elastic deformation after self-folded can be realized not by a self-folded hinge but by soft hinges made of an elastomer with wide elastic range. In order to confirm the elasticity of hybrid hinge, we evaluated a change of folding angle when the devices were repeatedly bent. As a comparison to our proposed device with a hybrid hinge, we prepared a device without a hybrid hinge, i.e. without soft hinges. In the device without a hybrid hinge, we confirmed that the folding angle changed at one cycle bending due to plastic deformation of SMP. On the contrary, the bending angle of the device with a hybrid hinge did not change by a repetition of bending deformation. Finally, we demonstrated an origami stretchable device with multiple hybrid hinges. We fabricated miura-ori as origami stretchable device. Unlike a previous demonstration of miura-ori using 4D printing via FDM-3D printer, our fabricated device can be deformed from folded shape to deployed shape by external force even under room temperature at 25°C.

In conclusions, our proposed hybrid hinge was confirmed as an effective way to realize the 4D printing with elastic hinge. We believe that our proposed way will accelerate the development of stretchable origami device.

[1] Teunis van Manen et al., Materials Horizons, vol. 4, pp. 1064-1069, 2017.
[2] Shuguang Li et al., Proceedings of the National Academy of Sciences, vol. 114, pp. 13132-13137, 2017.

3D printing or additive manufacturing has great attention in recent years as it allows the manufacturing of complex shapes and various geometries. Specially, these techniques have various application fields, which are making prototypes for electronics, aerospace, automotive, and so on. For 3D printing technology to be applied to electric vehicles, it needs to be light, high strength and high thermal stability. Many researchers are working on high-temperature, high-strength materials such as PEEK and composites to increase strength.
Therefore, we have designed a hybrid organic-inorganic composite material, which are glass fiber, carbon fiber, and modified PEEK, for high-temperature, high-strength materials stability. We successfully made organic-inorganic 3D printing filaments using a mini-extruder machine for fused deposition modelling (FDM) printer. Also, we studied thermal and mechanical properties of 3D printed samples with or without post heat treatment at 250oC for 1 hr. These printed samples were measured by thermal mechanical analyzer (TMA), bending strength and tensile strength with micro-universal testing machine. Surface morphology and the crystallinity of PEEK of those are measured by scanning electron microscope (SEM) and the Differential Scanning Calorimetry (DSC) method.
We will present the detailed experiment results and other analysis results for improved 3D printed samples.
This work was partly supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CRC-19-02-ETRI) and the Technology Innovation Program (No. 20005139) funded by the Ministry of Trade, Industry & Energy(MOTIE, Korea)

3D printing, commonly known as additive manufacturing, is a manufacturing process of producing three dimensional objects having complex features through layer by layer printing. The 3D printing technique has been developed rapidly and applied to various industrial fields since it was proposed in 1980s. Even though there are many 3D printing techniques introduced so far, the photocuring 3D printing which is based on photopolymerization of photo-sensitive materials is becoming more important due to its high printing speed, high precision and smooth surface of products. However, the materials which are mainly utilized in the photocuring 3D printing often encounter limited mechanical properties which are originated from their own nature. Thus, it is necessary to design photo-sensitive materials satisfying the requirements. Recently, poly(ethylene glycol) diacrylate(PEGMA) which is commonly a highly biocompatible photocurable material is widely used in the photocuring 3D printing for biomaterials, dental materials, biomimetics, and tissue engineering and so on despite its poor mechanical and thermal properties. Thus, there are many demand to overcome the disadvantages of PEGDA and to find suitable materials for the fields. Here, we report the polyetherimide-based photocurable resin for biocompatible 3D printing. Polyetherimide(PEI) is one of engineering plastics, which has superior mechanical strength, long-term thermal stability, excellent chemical resistance and good biocompatibility. By integrating those properties of polyetherimide to advanced biocompatible 3D printing, we synthesized a polyetherimide-based photocurable resin for DLP stereolithography 3D printing. To synthesize the polyetherimide-based resin condensation reaction between dianhydrides of BPADA and diamines of 3,5-diaminobenzoic aicd under the nitrogen flow are conducted to produce amic acid groups in chain backbones and chain ends. As thermal imidization of the reactants is then proceeded in sequential 3 steps of 60°C for 2 hours, 110°C for 2hours and 205°C for 4 hours, the amic acid groups are successfully converted to imide linkages. It is noted that the carboxylic acid of diamono acid are still remained after the imidization. After cooling down to the 30°C, glycidil methacrylate (GMA) is additionally grafted to the PEI molecules by the reaction with the carboxylic acids in presence of tetraethylammonium bromide (TEAB), triethylamine and hydroquinone under the nitrogen flow of 100°C. The GMA groups impart the photochemical reactivity to in the PEI molecules. After the precipitation in distilled water and filtration, PEI-g-GMA powder is obtained and dried in vacuum oven at 40°C for 24hours. To prepare the photocurable resin for 3D printing, the synthesized PEI-g-GMA is mixed with crosslinkers(PEGDA), reactive diluents(1-vinyl-2-pyrrolidinone(NVP) and photo initiators(Igarure819^®). To test the biocompatible property of our resins, three dimensional specimens are printed with the photocurable resins prepared here, by precisely adjusting light dosage and compositions of the mixtures. It is found that the specimens are printed in 0.4 seconds of exposure time and 30 μm of layer thickness. The printed specimens are rinsed with isopropanol and post-cured in a UV light chamber. The photocurable resin and the printed specimens are characterized by using universal test machine(UTM), differential scanning calorimeter(DSC), thermal gravimetric analysis(TGA), FT-IR and H¹-NMR spectroscopy. In addition, the molecular weight of PEI-g-GMA is estimated by end group analysis based on H¹-NMR data.

The fused deposition modeling (FDM) is an advanced 3D printing technique in which the thermoplastic polymer is melted and extruded by the shear force coming from the nozzle to create layer by layer structure according to the user input file. Compared to the conventional molding, a major challenge of the FDM process is the highly nonequilibrium thermal gradients that impact the filament fusion and the thermo-mechanical and electrical integrity of the materials produced. Furthermore, since these gradients are poorly understood, tructure/property models predicting the outcome of printing have been difficult to establish. In order to visualize the gradients, while probing the internal structure of the printed samples, we designed an apparatus which can simultaneously profile both macrostructure and microstructure during printing, as a function of time, using synchrotron X-ray scattering, and together with infrared thermal imaging. This information is then combined with Raman spectroscopy, rheology, and DMA studies, in order to understand the evolution of the internal structure and its corresponding influence on the properties of the materials. We had previously applied these techniques to crystalline polymers, PLA and iPP [1]. Here, we extend these measurements to the study of the internal structure of FDM printed Polypropylene (PP)/Graphene Nanoplatelet (GNPs) nanocomposites. Micro-beam wide-angle scattering analysis showed that the GNPs cluster towards the center of the filament during extrusion from the nozzle when the GNP concentration is higher than 10%. Previous studies had demonstrated that iPP interacts strongly with graphene [2], and hence the segregation of the GNP away from the interface facilitates chain diffusion across the interfaces, consistent with the high impact toughness observed. Thermal imaging confirms the non-uniform distribution, with the highest conductivity, 267% increase, occurring along the center of the fibers. Hence incorporation of GNP and the subsequent alignment enables the printing or oriented thermal or conductors. In operando time-dependent small angle scattering analysis of the filament during extrusion shows the interesting correlated motion of the GNPs, which stops abruptly once the temperature is below the melting point. This confirms the gel-like state which occurs with the GNP in the iPP melt state and drives the particles along the velocity gradient in the moving nanocomposite stream.
1. Shmueli, Y., et al., Compos. Sci. Technol, 2020. 196, 108227
2. Kai, Y., et al., Nanocomposites, 2015. 1(3) p. 126-137.

Polylactic acid (PLA) is commonly used for 3D-printing, nevertheless it is characterized by poor self-adhesion. The quick cooling rate of PLA does not allow enough time for the fibers in the filament to properly penetrate across an interface and form strong polymer chains. As a result, adhesion is inhibited and the overall strength of prints is reduced, making 3D-printed PLA a nonviable material for load bearing applications. This work aims at tackling this problem with a binary polymer blend of PLA combined with a polyolefin, more specifically high-density polyethylene (HDPE) or isotactic polypropylene (iPP). HDPE has superior self-adhesive properties to PLA and a lower surface energy than PLA. This coupled with its low glass transition temperature causes the HDPE to segregate towards the surface of the filament, forming an outer shell during extrusion. We hypothesized that a shell thickness of three radii of gyration will provide a polymer chain long enough to enhance the interpenetration of fibers between layers while keeping phase separation to a minimum. Three radii of gyration are theoretically found at a mass composition of 0.36 wt% HDPE/99.64 wt% PLA. The binary composite showing the highest improvement in mechanical properties, as compared to neat PLA, was 0.25 wt% HDPE/99.75 wt% PLA. This supports the hypothesis that the addition of a small loading of HDPE can improve the mechanical properties of a 3D-print by manipulating the polymer structure via migration of HDPE to the surface of PLA, as also verified by water contact angle measurements and secondary-ion mass spectrometry. Improved interlayer diffusion was also observed when adding low concentrations of iPP to PLA. The inherent immiscibility of PLA and iPP, as a result of the lower surface energy of the latter, allows for an enhanced filament structure: when present at low concentrations, iPP migrates to the surface of the extruded PLA matrix as well, creating a filament characterized by a PLA core and an iPP shell, as confirmed by water contact angle goniometry. Mechanical testing showed that in comparison to the pure PLA control, an optimized filament of 99 wt.% PLA/1 wt.% iPP displayed a 66% increase in toughness. Optical microscopy validated this data, revealing indistinguishable filament-to-filament interfaces in the optimized composite as opposed to pure PLA

The thermoelectric(TE) generator is a solid-state device that directly converts the flow rate (temperature difference) into electrical energy. This TE energy transformation offers a unique solution for producing electricity from waste heat. For enhancing the efficiency of the TE generator, 3d printing method to produce TE materials are needed with geometries suitable for heat sources. Herein, in order to attain high-quality 3D printing of TE inorganic materials, a comprehensive evaluation of appropriate rheological properties and methodologies for formulating inorganic inks is required. For 3D printing, the most important feature is the thixotropy that implies time-dependent shear thinning property.
In this study, we analyzed the rheological behavior of the Bi₂Te₃-based and Cu₂Se-based TE inorganic inks to assess printability and 3D structural retention concerning the content of ion binder which has a strong electrostatic interaction among their particles. Moreover, for evaluating the thixotropy, 3D printing was imitated using a rheometer by three-interval thixotropy test (3ITT) under the nonlinear viscoelastic region.
In static state, the inks showed Bingham plot and it becomes solid like property with increasing ion binder content. Especially, the 3ITT indicated that in Bi₂Te₃ based ink without ion binder, storage modulus(G’) increased dramatically when subjected to stress over the 10 Pa. It implied that the particles are aggregated and turned into unstable states. Using quantitative functions such as degree of structural deformation(De), Degree of structural recovery(Rec), we numerically examined the degree of ink recovery after high shear stress. Both inks have generally shown similar rheological characteristics. Phase stability is improved when binder content increases, but above a certain binder concentration the particle aggregation occurs, and recovery rate and printability are reduced. Thus, it is essential to find the optimum ion binder concentration of inorganic ink suitable for 3D printing. These results were able to demonstrate that ion binder of 25wt% in Bi₂Te₃-based ink and 50wt% in Cu₂Se-based ink were the optimized ion binder conditions for high-quality 3d printing TE materials.

Poly(vinylidene fluoride) (PVDF) has been used extensively as a membrane material for water treatment. It can be easily processed into porous films with high mechanical strength directly from the solution. However, the chemical instability of PVDF remains an issue, particularly in the caustic environments found during the membrane cleaning process, which consequently decreases the lifetime of PVDF membranes. Despite possessing high mechanical strength, PVDF membranes exhibit decreased applicability for low-fouling piezoelectric separation applications, owing to a low electroactive phase following deposition. For the use as piezoelectric membranes, it is important to maximize the electroactive phase, in order to produce vibrations through the application of AC voltage. Moreover, the techniques currently used to enhance the electroactive phase in PVDF, such as poling, mechanical stretching, and quenching, result in the instability and hinder high-temperature separation applications in piezoelectric PVDF membranes.
To overcome these issues, PVDF can be chemically modified to make dehydrofluorinated PVDF (dPVDF). This modification has multiple benefits. The primary advantage of this modified material is the presence of pendant alkene moieties, which can be used for further post modification strategies to fabricate smart and reactive membranes that respond to external stimuli, such as pH, temperature or electric potential, in order to mitigate fouling. An additional advantage of this material is an increased electroactive phase relative to PVDF, which makes it a promising candidate for low-fouling piezoelectric membrane applications.
Here, for the first time, we present the fabrication of porous dPVDF films by direct ink writing (DIW) and subsequent phase inversion. The dPVDF solutions in N,N-dimethylacetamide (DMAc) exhibited Bingham behavior at polymer concentrations between 7 wt% and 15 wt%, and thus could be DIW to produce continuous films. The dPVDF inks (dPVDF (15 wt% in DMAc), and 5, 15, 30 wt% PVP), were subsequently DIW into continuous films, and phase inverted in deionized water (non-solvent) to produce porous films. The porosity of the dPVDF films was confirmed using helium beam ion microscopy. The presence of alkene moieties in the dPVDF and dPVDF porous films was confirmed through attenuated total reflection – Fourier transform infrared (ATR-FTIR) spectroscopy and Raman spectroscopy. An increase in electroactive phase fraction was observed in the DIW films, confirming the enhancement in piezoelectric characteristics. The DIW porous dPVDF membranes presented herein show exceptional promise for the production of piezoelectric membranes and the subsequent utilization as separation membranes in harsh environments.

When 3D printing, the quick cooling rate of PLA does not allow enough time for the filament to properly diffuse across an interface¹, which inhibits adhesion and reduces the overall strength of prints. We aimed to tackle this problem by adding low concentrations of polypropylene (PP) to PLA to create a 3D-printing filament with improved interlayer diffusion.² Our hypothesis is when a PLA/PP blend is formed through 3D printing, PP migrates to the surface of the filaments due to its lower surface energy and lower glass transition temperature, forming a core-shell structure that produces pseudo-"self-healing"² by fusing fibers. Due to PP's lower surface tension compared to PLA (30.7 mN/m vs 44 mN/m)⁵, PP will travel to the surface of the fibers when extruded, displacing air pockets formed when the fibers crack.^3,4 Previous testing shows that 1% PP blends are the most resistant to tensile testing, and this study seeks to determine if the PP/PLA blend produced by molding or 3D printing is more effective, and which concentration of PP is most resistant to tensile testing.
To find how the core-shell structure impacted the physical properties of the blends, the ratio of the Volume of the Shell vs. Volume of the Fiber and Shell thickness were both plotted against the Volume fraction of PP. Radius of gyration (R_g) of PLA and PP were also calculated. By using the thickness of the shell and the diameter of the fiber, the number of R_gs of PLA and PP in each core-shell sample was found.
5 groups differing in PP % by weight were used to compare which concentration of PP in a PLA/PP blend best resisted tensile testing: 0%, 0.5%, 1%, 2.5%, and 5%. After preparation, the Young’s Modulus, UTS (Ultimate Tensile Strength), and Elongation at Break were analyzed. The Young’s Modulus of the molded samples showed that the Pure PLA had the highest Young’s Modulus, meaning that the molded samples did not exhibit “self-healing”. The results for the 0.5% PP, 1% PP, 2.5 % PP, 5% PP, and Pure PLA 3D printed samples indicate that 0.5% PP has a higher Young’s Modulus compared to 1% and Pure PLA, making it more reliable for 3D printing. The contact angle measurements indicate that even a small % of PP in a blend produces a contact angle similar to pure PP, meaning that adding PP lowers the surface tension of the blend, therefore in the 3D printed blends PP travels to the fibers' surface and produces "self-healing".
In conclusion, this study shows that the optimal concentrations for making a PLA/PP blend for 3D printing is 99.5% PLA/0.5% PP. It also shows that compared to the 3D printed blends, the molded blends do not strengthen the blends to the same extent during tensile testing.

¹ Liu, Zengguang, et al. “A Critical Review of Fused Deposition Modeling 3D Printing Technology in Manufacturing Polylactic Acid Parts.” International Journal of Advanced Manufacturing Technology, vol. 102, no. 9-12, Springer Science and Business Media LLC, 2019, pp. 2877–89, doi:10.1007/s00170-019-03332-x.
² Peng, Fang, et al. “3D Printing with Core–Shell Filaments Containing High or Low Density Polyethylene Shells.” ACS Applied Polymer Materials, vol. 1, no. 2, American Chemical Society, Feb. 2019, pp. 275–85, doi:10.1021/acsapm.8b00186.
³ Mehrabi Mazidi, Majid, et al. “Highly-Toughened Polylactide- (PLA) Based Ternary Blends with Significantly Enhanced Glass Transition and Melt Strength: Tailoring the Interfacial Interactions, Phase Morphology, and Performance.” Macromolecules, vol. 51, no. 11, American Chemical Society, June 2018, pp. 4298–314, doi:10.1021/acs.macromol.8b00557.
⁴ “Polymer Properties Database.” Surface Tension, polymerdatabase.com/polymer%20physics/sigma.html.
⁵ Guo, Yichen, et al. “Enhancing Impact Resistance of Polymer Blends via Self-Assembled Nanoscale Interfacial Structures.” Macromolecules, vol. 51, no. 11, American Chemical Society, June 2018, pp. 3897–910, doi:10.1021/acs.macromol.8b00297.

We gratefully acknowledge support from the Louis Morin Charitable Trust.

Here we designed a non-toxic and environmental sustainable polymer nanocomposite PLA/PBAT/APP/RDP-Clay which achieved good flame resistance and balanced mechanical properties with the addition of 3wt% ammonium phosphate (APP) and 2wt% resorcinol diphenyl phosphate (RDP)-coated montmorillonite clays (C-Na⁺). Polylactic acid (PLA) received attention due to its biodegradability and mechanical properties. However, its poor impact strength and flammability largely limits its application in wide range of areas. In our previous study¹, APP was found to be rigid and made the impact strength of PLA even worse. We were able to increase the ductility without affecting flammability of the PLA/APP composite by decreasing the APP concentration to only 2 wt% with the addition of only 0.12wt%RDP. Hence, poly(butylene adipate-co-butylene terephthalate)(PBAT), a ductile and biodegradable polymer, is introduced to balanced impact strength. But PBAT was found to be a poor efficient carbon source in APP intumescent system. For PLA/PBAT(75/25) over 10wt% APP/RDP is required to make PLA/PBAT(75/25) resisted to combustion, while pure PLA only need 2wt%APP/RDP. The huge increase for the use of APP not only increase the cost but also sacrificing the desired impact strength. According to Contact Angle Measurement and work of adhesion (W_a) between related materials, APP is more affinity to PBAT than PLA which possibly draw part of APP into PBAT phase and decrease the overall flame retardancy. In order to minimize this effect, 2wt%RDP coated sodium clay(C-Na+) is added to reduce the interfacial energy between PLA and PBAT which decrease PBAT domain size. Compatibilization was further confirmed by observation of a 32% decrease (from 2.53μm to 1.73μm) in PBAT domain size via Scanning Electron Microscopy. With 2wt%RDP-clay into the composites, we achieved a V-0 designation with a low loading of APP(3wt%). This was attributed to the decreased PBAT domain size, increasing the efficiency of APP to decrease the heat release in the combustion at a reduced overall concentration. From Thermogravimetric Analysis (TGA) result, the reaction mechanism was confirmed by the presence of intumescent char. Which is a byproduct of the reaction in the solid state. The increased ductility below Tg and viscosity in the melt enables fiber drawing and comparison of the combustibility upon FDM printing, which will be discussed.

We gratefully acknowledge support from the Louis Morin Charitable Trust.

1. Y. Xue, X. Zuo, L. Wang, Y. Zhou, Y. Pan, J. Li, Y. Yin, D. Li, R. Yang, M.H. Rafailovich and Y. Guo: Enhanced flame retardancy of poly(lactic acid) with ultra-low loading of ammonium polyphosphate. Composites Part B: Engineering 196, 108124 (2020).

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

Material properties are governed by the chemical composition and spatial arrangement of constituent elements at multiple length scales. This fundamentally limits material properties with respect to each other creating trade-offs when selecting materials for a specific application. For example, strength and density are inherently linked so that, in general, the denser the material, the stronger it is in bulk form. We are combining advanced micro-architected design with additive manufacturing techniques to create material systems and devices with previously unachievable property combinations. We have demonstrated designer properties resulting from architected materials in polymers, metals, ceramics, and combinations thereof for unprecedented control of structural, mechanical, thermal, chemical, and functional properties. Our manufacturing techniques can generate designed structures which are highly three-dimensional and composed of micro- and nano-scale features with multiple constituent materials in the same structure. In this talk, I will discuss our advanced modeling, materials, and manufacturing methods with focus on applying these approaches to cellular fluidics—a novel 3D fluidic control and delivery platform—and printed electrochemical reactors for CO2 reduction reactions.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Water electrolysis has been gaining century-long attention as a hydrogen production technique due to its simplicity and effectiveness. Coupling with renewable energy, water electrolysis paves the way for clean and sustainable hydrogen generation without worsening the depletion of fossil fuels and degradation of ecosystems. Particularly, alkaline water electrolysis (AWE) is one mature industrial strategy, and megawatt-level stacks have been well established in the market recently owing to many of the following advantages. The less corrosive nature of alkaline electrolyte compared to acid media allows for the use of non-platinum catalysts which significantly lowers the cost of AWE. In addition, the low vapor pressure of alkaline media is helpful in realizing high-purity hydrogen gas production. AWE generally exhibits a decent energy efficiency of about 80-85% at mid-high current densities of 100-200 mA/cm², but the efficiency drops significantly when increasing the current to 400 mA/cm² or higher. The considerable energy efficiency loss is related with the both limited intrinsic and extrinsic activity of electrodes. In spite of recent efforts in developing intrinsically active electrodes for AWS with facile proton-coupled electron transfer (PCET) and fast reaction kinetics, it is equally urgent to promote the extrinsic activity of electrodes at high current densities. Three-dimensional (3D) porous electrodes, such as commercial Ni foam, have been routinely used for as AWS electrode substrate because of their high surface area and ability in enhancing the mass diffusion of electrolyte. Nonetheless, the large amount of gas bubbles generated at high current densities are unfavorable to be released out of the 3D Ni foam due to the disordered pore arrangement. The sluggish bubble transport and release behavior of gas bubbles in 3D Ni foam block the efficient reactant diffusion and cause high ohmic loss, leading to substantial high overpotentials and low energy efficiency. Through an additive manufacturing strategy, we prepared a 3D printed Ni electrode with periodical pore structures as the substrate of AWS electrodes. The pore channels aligning with the buoyancy direction is believed to facilitate the upward movement of gas bubbles, and therefore enhance their diffusion and release out of the 3D electrodes. By growing intrinsically active AWS catalyst of carbon doped NiO on the 3D printed Ni, the electrodes exhibit much higher extrinsic activity than the counterparts supported on Ni foam. A high current density of 1000 mA/cm² can be obtained in 1.0 M KOH at low overpotentials of 245 and 425 mV for HER and OER, respectively. The work provides a new strategy of enhancing extrinsic activity of AWS electrodes at practically high current densities through pore structure engineering.

Due to the advantages of small sample volumes and well-controlled microenvironments, microfluidic devices have enabled exciting applications such as lab-on-a-chip diagnostics, point-of-care systems, organ-on-a-chip, nanomaterial synthesis, and bioassays. Soft lithography is a widely employed device fabrication method that can achieve sub-micron resolution using the elastomer polydimethylsiloxane (PDMS). Yet, the requirement of microfabrication facilities, stamp distortions, as well as time-consuming manual steps such as PDMS molding, layer alignment and bonding imposes constraints on the use of soft lithography to produce ubiquitous and widely deployable microfluidic devices. Additive manufacturing, or 3D printing, techniques have recently emerged as promising complements or alternatives to augment the manufacturability of microfluidic devices. 3D printing methods such as stereolithography (SL) and multi-jet modeling (MJM) utilize photo-curable polymers to create well-defined micro channels with feature sizes below 100 μm and a range of components for fluid manipulation. However, due to the potential for contamination from either the residual precursor resins or sacrificial supporting materials, directly printing microfluidic structures onto substrates that contain pre-deposited 3D structures or electronic sensing elements is challenging for SL and MJM. The degree of automation of SL and MJM is also compromised due to necessary post-processing steps to remove residual precursor and supporting materials from the channels. In addition, microfluidic devices printed by SL and MJM with photo-curable polymers have low elasticity and cannot be aligned to existing structures. Microfluidic device fabrication methods can be further enhanced by directly integrating microfluidic structures with electronic sensors and curvilinear surfaces as well as improving automation.
Here, we introduce an automatable extrusion-based printing methodology that can directly align and print elastomeric microfluidic structures onto planar and curvilinear substrates with minimal post-processing. By selecting a silicone-based ink of proper viscosity and controlling the profiles of printed overhung structures, self-supporting walls were realized and further enclosed to form hollow structures such as channels and chambers. Since the microfluidic spanning distance is in the sub-millimeter regime, a sufficiently small bending moment results that the as-printed walls can withstand, rendering this method highly suitable for printing microfluidic structures. Printing toolpaths can then be specifically designed to create leakage-free transitions between channels and chambers, T-shaped intersections and overlapping channels. The mechanical equilibrium states of the 3D printed silicone structures eliminate the need for supporting materials and rigid exterior covers that are conventionally required to build hollow structures with viscoelastic materials. Multi-material mixers were fabricated by integrating printed herringbone ridges to achieve high mixing efficiency in an enclosed microfluidic channel. Further, seamless integration of microfluidics and sensing microelectrodes was realized via synchronous alignment and printing onto the micro-chip. High-throughput and real-time impedance measurements of NaCl solutions were demonstrated with the 3D printed salinity sensor array. Most compellingly, we conformably printed 3D microfluidic networks that were integrated with valves on a spherical surface, demonstrating a novel device form-factor that can be potentially extended to freeform surfaces such as the human body as microfluidics-based wearable health sensors. The devices demonstrated in this work serve as a proof of concept for the automatable capabilities of extrusion-based 3D printing to fabricate self-supporting and aligned multifunctional microfluidic structures.
[This document has been authorized for unlimited public release. PAO number U20-2128.]

3D printing with light-based vat photopolymerization (VP-3DP) can make challenging geometries (e.g. overhangs, cavities, channels) with soft elastic materials, resulting in great strides in soft robotics. As printing speeds and resolution improve, soft robotics still face challenges to produce large-scale features via VP-3DP due to a limited build area and the need for structural supports for complex features. In an effort to bridge this gap, we have developed tunable photopolymerizable resin systems amenable to VP-3DP to produce self-healing elastomers. Materials consist of dynamically exchangeable moieties that can be printed with wide-ranging moduli (e.g. elastic to rigid). Printed parts have high elongations (up to 2,000% strain); the capacity to self-heal to elastic or rigid parts; and reset permanent shape via bond exchange with applied heat. By printing these materials into small modular structures and utilizing their capacity for dynamic bond exchange, we demonstrate the facile construction of highly complex, reconfigurable, and macro-sized soft robotic actuators. We then combine these soft robotic actuators with self-healing conductors in the form of bulk liquid metal or Polymerized Liquid Metal Networks for embedded strain sensing, joule heating, and power delivery in a self-healable soft robot.

Carbon nanomaterials, including 0D carbon nanopowders, 1D carbon nanotubes, and 2D graphene, exhibit scaling effect and exceptional properties with great interest in structural and functional applications. The integration of carbon nanomaterials and architectural features at multiple length scales into structural mechanics and functional materials has significantly shifted the paradigm of materials engineering toward materials by design with structural and functional capabilities. Three-dimensional carbon multiscale architectures featuring the characteristics of both the constituent CNT material, which brings the effects of its feature nanoscale resolution and properties, as well as the structures across multiscale lengths offer breakthrough advances in many applications including ultra-light structural materials, energy storage, and biomedical devices. However, there is no effect approach to assemble those carbon nanomaterials into macro-scale structures with desired architecture while ensuring scaling properties. Therefore, it is highly anticipated to develop new manufacturing technology to handle nanomaterials and assemble them into large-scale structure with complex architectural features in a fast and efficient way.
In my talk, I will introduce our recent work on additive manufacturing of three-dimensional carbon-based multiscale architectures, and demonstrate its applications in electrochemical energy storage. Our method will retain the properties of carbon nanomaterials and there no additional binder or polymer in the final 3D components. We have demonstrated that carbon nanomaterials, including 0D carbon nanopowders, 1D carbon nanotubes, and 2D graphene, could be all processed into complex three-dimensional multiscale carbon architectures.

High mass loading and efficient charge transport are two crucial but often mutually exclusive characteristics of supercapacitors. 3D printing has emerged as a new tool to fabricate high-performance electrodes for energy storage devices. The interconnected conductive pathways in the printed 3D structures allow efficient electron transport. The periodic large pores in the 3D printed structures provide a short cut for electrolyte ions to the electrode surface. This efficient electron transport and ion diffusion in 3D printed electrodes enables fast charging and discharging even for thick electrodes at high mass loadings without sacrificing their gravimetric and volumetric capacitive performance, which successfully addresses the long-term challenge in this area. In this talk, I will present a millimeter-thick 3D-printed graphene aerogel electrode that could support pseudocapacitive manganese oxide (MnO₂) up to 180 mg cm^-2. The ultrahigh mass loading and efficient kinetic process enable them a record-high areal capacitance of 44.13 F cm^-2 for supercapacitors. More importantly, the 3D-printed graphene/MnO₂ electrode can simultaneously achieve excellent capacitance normalized to area, gravimetry, and volume, which is the trade-off for most electrodes. Furthermore, by pairing the 3D-printed graphene aerogel/MnO₂ as a cathode and surface-functionalized graphene aerogel as an anode, the assembled asymmetric supercapacitor (ASC) could further be extended to 2V. This high mass loading-ASC could achieve a remarkable energy density of 0.65 mWh cm^-2 at an ultrahigh power density of 164.5 mW cm^-2, outperforming all the previously reported supercapacitor devices operated at the same power density. These works have successfully proved the feasibility of fabricating pseudocapacitive electrodes via a 3D printing method, which might revolutionize the conventional fabrication process of commercial supercapacitors.

Every year in the United States, hundreds of billions dollars are spent on farm production. The two largest farming costs are related to crop input and labor. The cost on labor can be reduced by mechanization. However, because of the heterogeneity of farmlands, farmers face a challenge of reduced resource efficiency after switching to large-scale farming. To further reduce the cost by improving the resource efficiency, precision agriculture (PA) technologies has been widely adopted since the early 1990s. PA technologies use various sensors to measure temporal and site-specific environment, soil, and crop data. The currently available commercial sensors fabricated using conventional components are cost-prohibitive, therefore the deployment density and data resolution are limited.
Recently, additive manufacturing (AM), owing to its reduced material wastage, fast prototyping, and lower equipment cost features, has been used to fabricated sensors for PA applications. Various AM techniques such as inkjet-printing, screen-printing, stencil-printing, and extrusion-printing have been used to fabricate soil and environment sensors. Of the two, soil sensors probably benefit the most from low fabrication costs because accommodating the spatial and temporal heterogeneity of soil requires a high sensor density. For instance, soil moisture level could be different between two sampling spots at 2.5 m apart. Deploying sensors at this high density is obviously infeasible. In reality, only 1 sensor is deployed per hectare which is insufficient for high-resolution PA applications.
Researches have tried several printing approaches to reduce the fabrication cost of soil moisture sensors in order to improve the measuring resolution. Most printed soil moisture sensors use interdigitated electrodes to measure the dielectric constant of soil at different moisture levels. The electrodes are typically printed using silver or copper inks on polymer substrates. These printed sensors have demonstrated reliable performances in laboratory environments; however, still pose two challenges: (1) the sensors are made of non-degradable materials therefore require labor-intensive retrievals to avoid accumulation in the field, (2) the conductive components (i.e. silver or copper) are toxic to soil microbes and crops.
In this work, we report on a printed soil moisture sensor made of biodegradable and ecofriendly materials. The sensor is comprised of a slowly degradable hydrophobic encapsulant and rapidly degradable functional components (i.e. the electrodes and substrates). This combination of materials allows the sensor to function in soil for an extended period then rapidly fail after the encapsulant is decomposed by soil microbes. The rapid breakdown facilitates easy assessment of each sensor’s service life so that the maintenance effort can be reduced. The sensor will eventually decompose completely hence eliminates the needs of labor-intensive retrievals. Ecotoxicity tests show that the sensor materials do not hinder the crop growth. Sensors with encapsulants of different thickness were fabricated and their sensitivities as a function of volumetric water content (VWC) were measured. The long-term stability of the sensor was assessed by accelerating the degradation under controlled temperature. Finally, the breakdown of the sensor was characterized with an extended accelerated test.

Power electronic devices have a driving demand to both decrease in size and weight while increasing in performance, which requires next generation thermal management methods. GE Research and the US Army Research Lab have teamed up to explore the concept of utilizing a 3D printed ceramic cooling channel with an internal swirler to enable efficient use of two-phase flow in a Package Integrated Cyclone COoler (PICCO). This work will describe the challenges and learnings associated with 3D printing the ceramic swirler in alumina by stereolithography. Processing effects were evaluated including printing orientation, substrate effects, and slurry grade. Design effects were evaluated including support structures, addition of fillets and geometric non-uniformities.
Research was sponsored by the U.S. Army Research Laboratory’s Sensors and Electron Devices Directorate (SEDD) and was accomplished under Cooperative Agreement Number W911NF1920276. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Army Research Laboratory’s Sensors and Electron Devices Directorate (SEDD) or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.

Medical devices such as artificial implants and prostheses are commonly made of metallic materials, for instance, Cobalt-Chromium alloy, Titanium alloy and stainless steel, for they demonstrate satisfactory biomechanical properties and biocompatibility. The introduction of additive manufacturing has brought the design and fabrication of medical device into a new era, customizing not only shape but also porosity, internal design and surface finishing. Here we present the use of novel Integrated Machining Direct Metal Laser Sintering (IM-DMLS) 3D printing technology to enable hybrid fabrication of prosthetic implant customizing to specific bone anatomy to a dimensional accuracy of ±O(10) microns. Subsequent fine polishing results in a surface roughness (Ra) in micron to submicron range. Tailoring the internal structure of prosthetic implant reduces the weight of implant while preserving good biomechanical properties. Furthermore, we demonstrate the feasibility to form Cobalt-Chromium alloy scaffolds with tunable lattice size and porosity in the biological-relevant range which allows osseointegration. Our work has prominent potential in medical applications of customized medical devices that benefits patients in terms of better bone-implant integration in long term and improved patient experience of size fitting and weight reduction.

Aerogels are highly porous structures produced by replacing the liquid solvent of a gel by air without causing a collapse in the solid network. Unlike conventional fabrication methods, additive manufacturing has been proposed to fabricate aerogels having complex geometries with designed pore morphologies, which can be advantageous for various applications. However, the fabrication of Aeroges with tailored macrostructures by scalable and controllable methods remains a big challenge. The vision of tailoring the architecture of aerogels for broad applications, including separation, all-solid-state batteries, micro pressure sensors, electrodes, and electrochemical catalyst templates, has stimulated the research on 3D printing of aerogels. Herein, we propose a printing methodology that combines multi-nozzle drop-on-demand inkjet printing of nanomaterials suspension with freeze casting for rapid printing of 3D aerogels architectures to achieve several key qualities, namely, pure, continuous, boundary-free, controlled microstructure, and truly 3D architectures (e.g., 3D truss with overhang structures). To date, we have successfully printed world’s lightest material via 3D printing through the proposed technique.

Architected materials are a new class of engineered materials with carefully controlled internal structures that give rise to properties that differ from or surpass those of their constituent materials. Additive manufacturing provides an extraordinary opportunity to rationally design the structure and the chemical composition of architected materials across multiple length scales to optimize properties and functionalities for a variety of applications. These functional architected materials are capable of decoupling critical trade-offs to reach new regions of the material property space and enabling exotic properties that rarely exist in classical materials. Electrochemistry describes the mutual interaction between electricity and chemical reactions, which enables electrically controlled and oftentimes reversible release/storage of energy through electrochemical reactions in, for example, batteries, fuel cells and electrodeposition processes. Designing an electrochemical system is a multi-faceted engineering challenge with trade-offs and constraints such as ion transport through electrolyte, electron transport inside both electrodes, active electrode surface area, chemical reversibility, and mechanical/chemical stability of electrodes and products during reaction. Recent development in high resolution additive manufacturing has enabled an unprecedented degree of structural control at length scales relevant to electrochemical transport kinetics and thus open up possibilities to rationally design material architectures that improve the performance of these electrochemical systems.

Recently, we created additively manufactured architected materials that can response to and facilitate electrochemical reactions in the local environment. We developed novel fabrication methods based on a suite of 3D printing methods such as two-photon lithography and stereolithography and various physical and chemical post-processing techniques to create architected materials with multi-level design freedom including feature sizes, structural geometries, and material compositions, which resonates with the entangled challenges in electrochemical systems. We demonstrated that additive manufacturing provides a new platform to design 3D-architected electrodes for batteries and electrochemical reactors for CO₂ reduction with enhanced transport kinetics and mechanical stability. Furthermore, we presented a new class of electrochemically reconfigurable architected materials that could transform their structures in a programmable, reversible and non-volatile fashion, which provide new vistas for designing mechanical metamaterials with tunable phononic bandgaps and deployable micro-devices for biomedical applications. The multi-scale and multi-physics nature of these electrochemically active architected materials prompted us to develop a toolset of in situ microscopy techniques and various simulation frameworks to capture and analyze their dynamic behaviors undergoing electrochemical reactions in order to provide an in-depth understanding of the underlying mechanisms as well as design principles generalizable for other functional architected material systems.

While considerable progress has made in additive manufacturing of structural materials, including polymer, metal and ceramics, adding multi-functionality into the material pallet is compelling. However, such efforts has been challenged by the trade-off between active properties and manufacturablility. In this talk, I will present the additive manufacturing of a suite of electro-mechanical coupling materials (e.g. piezoelectric materials and electronic integration) for energy transduction, where electrical and mechanical behaviors and their couplings can be inversely designed and tailored by an end-user at will. This unleashes new design freedoms for rapid material property discovery and functional product realizations. I will discuss a suite of all-in-one transducers and their new applications, including self-sensing autonomous materials, microsystems for robotics, as well as air and maritime self-sensing system.

As we move towards more sustainable manufacturing systems, cellulosic materials are extremely promising due to their biocompatible and sustainable characteristics. Hydroxypropyl cellulose (HPC) is a biocompatible cellulose derivative capable of self–assembling into a lyotropic chiral nematic phase in aqueous solution, which can then reflect vibrant colourations.

We report here a functionalised photocrosslinkable HPC ink compatible with 3D printing. We demonstrate that we can 3D print solid materials with a large variety of shapes, and that the HPC retains its liquid crystalline state and therefore, its colouration after the printing. The final colour of the material can be tuned on–line during printing either by changing the amount of water in the ink formulation or by altering the temperature at which the ink is photo–crosslinked. Our developed system, therefore, allows us to extend beyond the limits of traditional multi-color printing systems based on dyes, as it allows to create materials with colours across the visible range using a single feedstock.

Over the past decade, additive manufacturing has emerged as one of the most powerful manufacturing tools available today. Vat photopolymerization techniques, in particular, are especially promising as they are capable of achieving high resolutions and throughputs. However, the fabrication of non-polymeric materials with this technique still remains a challenge in the field — ceramics can be fabricated using vat photopolymerization but are often either based on photosensitive slurries that can be challenging to use and have poor resolutions or on inorganic-organic photoresins that are limited in composition achievable and require some complex synthesis. Metals, in general, have not been explored with vat photopolymerization techniques.

To address these, we introduce the concept of architected “chemical reactors” and demonstrate how we can use this general idea to adapt the field of materials chemistry to additive manufacturing. We present a novel technique called photopolymer complex synthesis that combines solution combustion synthesis with vat photopolymerization to address the issues mentioned above and enable the additive manufacturing of both complex metal oxides and metals. In brief, aqueous metal-ion containing homogenous photoresins are prepared and used with photolithography, in conjunction with post-processing techniques to fabricate architected metal oxide or metal structures. As examples of this technique, we fabricated and characterized zinc oxide (ZnO) and lithium cobalt oxide (LCO) architected structures with sub-micron and sub-millimetre features respectively. Compression of the ZnO structures resulted in an electromechanical response, and electrochemical cycling of the LCO structures showed efficient performance as a lithium ion battery cathode. We also demonstrate the fabrication and characterization of metal structures made from copper and cupronickel alloys. Characterization of these structures using X-ray diffraction, energy-dispersive spectroscopy and transmission electron microscopy indicate that the structures are indeed comprised of the appropriate material, highlighting the utility of photopolymer complex synthesis as a facile and accessible method for the fabrication of these metal oxide and metal materials.

Additive manufacturing of batteries offers great potential for the future of power integration into electronics and devices, such as in thin film and flexible devices where battery packaging physically prevents intimate incorporation. Additionally, printing technologies open up new design spaces for batteries such as in distributed power supplies and utilization of unused negative space around other features within a device. To achieve this, additive manufacturing approaches must be developed that permit the deposition of battery component materials with fine feature sizes and with control over complex 3D topologies and thin film morphologies.
We demonstrate the use of aerosol jet printing approaches for the deposition and control of 3D printed battery component materials. The printed ink is atomized into micro-droplets and entrained into a flowing carrier gas. This flow is then focused at the printing nozzle where it exits in a high speed collimated stream. This process couples high resolution feature sizes (10-100 µm) with a large working distance, making it ideal for incorporating batteries into minute spaces and on non-planar structures. However, the complexity of the aerosol jet process puts specific constraints on the inks in order to maintain proper atomization.
Here we focus on controlling the separator/electrolyte membrane morphology, which is known to be one of the most critical challenges in 3D printed battery systems. A relatively high level of separator porosity is required to maintain a high ionic conductivity, however these features must be small enough in size to prevent dendritic growth and shorting. We achieve this porosity control through a drying induced phase separation process. A mixed solvent system with carefully controlled relative evaporation rates is used together with a variety of polymer/nanoparticle host materials. This leads to instability in the ink upon drying thereby causing phase separation into polymer poor and polymer rich phases, creating porosity upon solidification. This approach enables the deposition of extremely uniform separator films with fine porosity control. In this study, we examine printing the separator material both on flat substrates and directly onto rough graphite anodes as a means of printing Li-ion batteries. We explore how the printing processes directly impacts battery performance and demonstrate how intimate contact between the electrode and separator at this interface and can impact dendrite growth, a key safety concern in Li-ion batteries.

Mycelium-bound composites are promising materials for sustainable packaging, insulation, fashion, architecture. They consist in exploiting a living organism, a fungus, to develop its intricate mycelium network to form a composite material. To date, moulding is the main fabrication process explored for mycelium-bound composites that strongly limits the ability to design complex shapes that could widen the range of applications. As an alternative, extrusion printing is a facile and low energy-cost process that has not been explored yet but that could offer with design freedom and structural properties. In this study, we combine bamboo microfibres, chitosan, and mycelium from Ganoderma Lucidum to establish a composite mixture that is workable, extrudable and buildable. All those components are issued from agricultural waste, which is key in obtaining interesting composites that are also sustainable. We studied the impact of bamboo fibre size, chitosan concentration, pH and ratio to determine the optimum growth condition for the mycelium as well as highest mechanical stiffness and building capability. The resulting materials that we fabricated have thus low energy costs, are sustainable, and present mechanical properties that makes the process promising for structural applications. The printing method now allows the investigation of complex structures for more optimal mycelium growth thanks to the design of 3D porous structures allowing spreading of the loads and high surface contact with oxygen.

Three-dimensional (3D) bioprinting is a promising technology to produce living, tissue-like structures, but a current lack of diversity in bioink materials is a major limitation. A cell’s phenotype is directly influenced by its surrounding microenvironment; thus, ideally each cell type would be printed in its own customizable bioink. To fulfill this need for a universally applicable bioink strategy, we developed a versatile, bioorthogonal crosslinking mechanism that is completely cell compatible and works with any type of biopolymer. We term this family of materials UNIversal, Orthogonal Network (UNION) bioinks. To prepare these materials, the biopolymer backbone is modified with either an azide or a bicyclononyne (BCN) group. The functionalized polymer is mixed with cells to create a bioink, and then extruded into a support bath containing soluble, multifunctional crosslinkers modified with the partner bioorthogonal reactive group. As demonstration of UNION bioink versatility, gelatin, hyaluronic acid (HA), recombinant elastin-like protein (ELP), and polyethylene glycol (PEG) were each used as backbone polymers to create inks with storage moduli spanning 200 to 10,000 Pa. Because UNION bioinks are crosslinked by a common chemistry, multiple materials can be printed together to form a unified, cohesive structure. This approach is compatible with any support bath that enables diffusion of the UNION crosslinker. Both matrix-adherent human corneal mesenchymal stromal cells and non-matrix-adherent human induced pluripotent stem cell-derived neural progenitor spheroids were printed with UNION bioinks. The cells retained high viability and expressed characteristic phenotypic markers after printing. Thus, UNION bioinks are a versatile strategy to expand the toolkit of customizable materials available for 3D bioprinting.

In order to create crisp, defined patterns of biologically-created materials, new technologies need to be developed and implemented. The Meyer lab is developing first-of-their-kind DIY bacterial 3D printers that can deposit engineered bacteria in specific three-dimensional patterns using simple devices and chemistries. Our affordable, easy-to-build bacterial 3D printers have fully automated, coordinated control of the pumps and printhead, allowing for high spatial resolution (<mm-scale) printing of bacteria onto wet or dry surfaces. Our printers mix an alginate-containing bacterial culture with a calcium chloride solution upon printing, triggering cross-linking of the alginate molecules to form a stable, biocompatible scaffold to support the bacteria. After printing, the bacteria survive for several weeks and can be induced to produce a variety of biomaterials. We are applying our printer to the fabrication of engineered biofilms, groups of bacteria that live within a spatially structured polymer matrix. We use our 3D printer to deposit engineered E. coli that are able to produce CsgA fibrils, or curli fibrils, the major protein component of biofilm polymer matrices. These engineered bacteria can stick to the printing surface even upon attempting to dissolve the printing scaffold, creating free-standing, stably patterned biofilms. Our patterned biofilms develop emergent properties analogous to those seen in natural biofilms, including resistance to anti-bacterial treatment and oxygen depletion within the biofilm interior. These model biofilms will be crucial for future development of anti-biofilm strategies, for which no reproducible model biofilm test system is currently available, as well as for the reliable production of beneficial living materials, which could be applied for water and soil purification or mineral extraction.

Bio-active materials integrated with living cells are capable of sensing and responding to a wide range of environmental stimuli and show great promise in biological and chemical sensing applications. The field of bioprinting has typically been geared towards mimicking biology for the purpose of creating human ex-vivo tissues and little work aimed at engineering materials to perform a synthetic function. In this work we engineer hybrid materials composed of living and non-living materials to structure and support the viability and function of living microbes through the fabrication of artificial biofilms

We have previously demonstrated the ability to additively manufacture three-dimensional living tissues using optimized scaffolding materials that sustain the long-term viability of living cells and are amenable to creating diverse living constructs with high degrees of cellular organization. Using the 3D structuring capabilities of LLNL’s BioPuSL system (a custom-built biological projection micro-stereolithography), we demonstrate the viable patterning and subsequent growth of E. coli bacteria within 3D bioprinted, bacterial-laden engineered biofilms. We show how altering biofilm 3D architecture influences the mass-transport to govern the target application of metal ion absorption. This approach of incorporating sensing cells within novel substrates and architecture designs can enable the development of a new generation of biological generated functional materials with many applications from biosensors to the production of biofuels and bioproducts.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNLCONF-760584.

AM technology is a rapidly growing biomaterial processing technique that has an enormous potential for (bio)medical applications such as tissue engineering and regenerative medicine, however greater resolution is required for the next generation of biomedical materials. Melt electro (spinning) writing (MEW) is one such unique AM process that has gained the spotlight in regenerative medicine due to its potential to engineer highly-ordered fibrous scaffolds, capable of replicating extracellular microenvironment functions. Highly resolved porous constructs fabricated in this manner have great utility for both in vitro and in vivo applications and represents the cutting-edge of 3D printing in biomedical applications. This talk will highlight some of latest developments in such high-resolution 3D MEW processing. This includes developments in processing parameters, functional designs, modelling biomechanics of MEW fibre constructs, and bioactive and hydrogel-based materials for MEW. A particular focus will be placed on the application of MEW constructs for cartilage and cardiac muscle regeneration. The second part of the presentation discusses an even more enticing feature in advancing MEW technology: the convergency of MEW with other 3D printing technologies, e.g. extrusion-based printing, in a single printing platform. The simultaneous use of MEW with other complementary 3D printing processes allows the strategic arrangement of multiple material compositions and operation at different length scales, from sub-micron to centimetre scale, not possible with any single component deposition method. Applications of this novel multitechnology fabrication in soft tissue engineering will be discussed.

3D printing helps to provide further control on electrode design. By using 3D printing to tune the geometry, porosity and dimensions of the structure, mass transport to functional microbes or sites in the structure can be improved. By embedding living microbes directly into the ink we can create 3D printed structures to help harness the activity of microbes and use them in functional devices. Here we demonstrate the incorporation of the living bacteria Shewanella Oneidensis MR-1 (S. Oneidensis MR-1) directly into an ink used for creating 3D printed structures. By incorporating S. Oneidensis MR-1 into a sodium alginate-cellulose ink we showed that S. Oneidensis MR-1 survives the 3D printing process through the prominent degradation of methyl orange azo dye from the surrounding solution in the presence of the living 3D printed structure. The viability was further confirmed using confocal microscopy. By incorporating carbon black into this ink we further demonstrate the direct printing of a living microbial fuel cell (MFC) anode. Electrochemical measurements showed there was good charge transfer between the S. Oneidensis MR-1 and the electrode surface. To our knowledge, this is the first report on implementing 3D printed bacteria structure as a living electrode for an MFC system. The capability of printing living and functional 3D bacterial structure could open up new possibilities in design and fabrication of microbial devices as well as fundamental research on the interaction between different bacterial strains, electrode materials, and surrounding environments.

In order to advance the state-of-the-art of tissue engineering, there is a need for degradable materials with adequate mechanical properties, tunable degradation, and the ability to form biologically relevant complex geometries. The overarching goal of this work is to 3D print personalized stiff structural scaffold which can be infilled with a soft cellular niche hydrogel, satisfying the conflicting design requirement of both a soft matrix to promote neo-tissue growth and a stiff scaffold to withstand physiological loads. To achieve this goal, mechanical integration of these two materials is needed to enable transfer of biomechanical cues and prevent delamination at the interface and engineer the scaffold to degrade at pre-engineered rates.

In our previous study, a relationship between the exposure conditions during 3D printing and the local mesh size of the network formed was established (Muralidharan et. al. Adv. Mater Technol. 2019). Control over mesh size of the 3D printed polymer network ranging from 1.3nm to 11.5nm was achieved. Mesh size ultimately controls the transport of second material (soft hydrogel) into the printed network, and integration distances ranging from 10s of microns to millimeters in 3D printed parts were demonstrated.

This study aimed to develop 3D printable degradable materials with high fidelity and predictable properties through two specific objectives: 1) Design and develop 3D printable materials with controlled degradation. The synthesis of traditional multifunctional macromers that form degradable networks commonly involves multiple functionalization and purification steps, which makes the development of large numbers of polymers with diverse properties difficult. Poly (β-amino esters) (PBAEs) are an attractive biomaterial for degradable materials that has one-step synthesis with no further purification steps and tunable degradation. Linear macromers based on diacrylate-terminated PBAEs were synthesized by reacting 1^o amines (RNH₂) with diacrylates. 3D printable poly (β-amino ester) based acrylate terminated macromers were subsequently photopolymerized and characterized for degradation. Through rational selection of acrylates and amines used to synthesize the PBAEs, the hydrophobicity of the network can be controlled. This directly influences the degradation mechanism (i.e. bulk vs surface degradation). We demonstrate degradation profiles with an exponential decay in mechanical properties over time, evident of bulk erosion, to networks with minimal change in mechanical properties, evident of surface erosion. Mass loss was measured over time in an accelerated degradation solution of 1M NaOH. The degradation profiles ranged from linear mass loss to systems where mass loss is slow at early times and accelerates as the network degrades. The polymers with similar modulus of 8-10MPa, exhibited a wide range of degradation rate from 100% mass loss within 90 min to 100% mass loss in 10 days to minimal mass loss over 45 days. These variations, achieved through simple chemical modifications, illustrate the versatility of these PBAE based polymers. 2) High resolution 3D printing using PBAE macromers. Through high resolution projection microstereolithography based 3D printing, customized medical image derived geometries with precisely prescribed microarchitectures can be produced. The high molecular weight macromers limit the diffusion of propagating radical into dark during 3D printing, thus allowing high fidelity features as low as 20μm. Moreover, the spatio-temporal degradation control was demonstrated with three orders of magnitude variation in degradation rates. In summary, with an advanced toolbox of synthetic and manufacturing methodologies in hand, we can rationally design and manufacture polymer materials with specific properties.

3D printing has been widely investigated for biomedical applications and attracted considerable attention as a cost-effective manufacturing technique to produce bespoke devices for patients. However, it is still facing many challenges for realizing this target due to a lack of suitable materials. At the University of Nottingham, we have developed a methodology that allows us to rapidly identify printable formulations for use with inkjet 3D printing, selected from libraries of materials with cell instructive behaviour. We show how we can exploit this high throughput approach to quickly identify useful, printable materials with potential for scale up and for the production of multimaterial, multifunctional devices. We will show how we used this method to produce devices composed of formulations that can inhibit biofilm formation (Pseudomonas Aeruginosa and Staphylococcus aureus) by more than 90% in comparison with commonly used devices made from silicone rubber. We go on to show how the design and manufacture of these multimaterial devices can be coupled to design optimisation approaches that generate compositional maps of materials that tell us where to put which materials in order to get a precise mechanical response. As a result, we are able to show that we can select for material, shape and the distribution of material composition to create bespoke implants or other devices that offer significantly reduced infection mortality rate during application, as well as offering all the personalisation benefits that come with 3D printing. We have carried out a range of characterizations that show our devices are capable of safe use and retain their efficacious properties both in vitro and in vivo.

In developing 3D printed implants, we face considerable challenges in defining which materials to utilize and what combinations to adopt to personalize their performance. A pertinent example is the long-term subdermal contraceptive implant. This is usually cylindrical with a size (maximum diameter 2 mm, length range 1 to 4 cm) defined by a combination of implantation method and anatomical positioning. Currently, such devices are manufactured by a process which heats a blend of polymer and active pharmaceutical ingredient (API) to around 100°C, However, current systems are not personalizable, nor is it possible to combine multiple drugs into a single treatment. One route to achieving personalization is through 3D printing. Ink jet based 3D printing, offers multiple benefits including its scalability, high resolution and importantly, drop by drop deposition that can provide both control over material properties at the microscale as well as the ability to co-deposit multiple materials (drugs).

In this study, we show that ink jet 3D printing of polymer blends gives rise to controllable phase separation that can be used to tailor the release of drugs. We predicted phase separation using high throughput screening combined with a model based on the Flory-Huggins interaction parameter, and were able to show that drug release from 3D printed structures can be predicted from observations based on single drops of mixtures.

Here, we report the creation of a library of multicomponent biodegradable inks, whose diversity of physicochemical properties allows for the range of phase separation behaviour required for tailoring drug release. We show that, by understanding the mechanisms that drive formation of this microstructure, we can predict microstructure that arises out of the ink jet printing process and reliably design and manufacture implants for tailored release.The screen considers the behavior of formulations designed for the release of drugs from 3D printed constructs and uses single drops screening (SDS) to characterize behavior in large, printed constructs.

We observed microstructure in larger-scale printed material that is similar to the drops but not evident in cast materials. We show that, by understanding both this microstructuring and the material composition characteristics, we can design and manufacture implants exhibiting tailored release that does not require varying the implant geometry. This is an important advance for implants that need to be delivered by cannula, where the shape is highly constrained and thus the usual geometrical freedoms associated with 3D printing cannot be exploited, bringing a hitherto unseen level of understanding to emergent material properties of 3D printing.

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

The three-dimensional localization of bacterial cultures provides a valuable tool for studying the behavior of microbes and for generating engineered living materials, but remains a challenging feat to achieve. Recent efforts have been made to confine bacteria into printable hydrogel matrices, mainly focusing on sustaining the growth of microbes while ensuring adequate material properties for 3D printing. At times, though, the rheological features required for conventional direct ink writing do not match the needs of the bacterial strain of choice and hinder the growth of the microorganisms. To circumvent this obstacle, we developed a 3D printing approach that allows us to directly deposit bacteria suspensions into a matrix that both supports free-form printing and allows oxygen diffusion. In our approach, we choose bacteria that have a potential for functional applications or for the synthesis of materials. Amongst these, bacteria capable of synthesizing bacterial cellulose such as G. xylinus are of particular interest as they are able to produce personalized biomaterials with outstanding properties such as biocompatibility, tensile strength, and high chemical purity. The rheological behavior of the matrix selected to host such bacteria must prevent the shape-distorting effects of surface tension and gravity. Moreover, when a needle ploughs through the support matrix, it is necessary to ensure a rapid closure of the groove behind the tip to maintain the resolution of the print. Using this approach, we are able to print freeform bacteria suspensions and, when using G. xylinus, directly form complex 3D cellulose structures. The use of bacteria to directly grow functional materials in accurate 3D shapes opens the way to a new generation of tailorable living materials, which do not only perform the designed function but also retain the ability to regenerate and respond to environmental stimuli. We envision this new paradigm to be of interest for a wide variety of applications as, for instance, wound dressing and drug delivery.

Additive manufacturing (AM) offers new opportunities to shape functional materials into complex three-dimensional geometries that are not accessible via conventional fabrication technologies. The layer-by-layer approach used in AM processes also allows us to build artificial structures in a similar way to the methods used by living organisms to grow complex hierarchical materials with unusual sets of properties. Living organisms strongly rely on self-assembly processes to create such functional hierarchical structures. In this talk, I will show how additive manufacturing can be combined with self-assembly processes to fabricate macroscopic functional objects with structural features spanning over multiple length scales. The underlying concept is to exploit the shaping capabilities of AM to define the structure of the object at the macroscale (> 1mm), while implementing self-assembly strategies within the feedstock ink to achieve structural control at the sub-millimeter scale. Alike biological materials, the 3D objects manufactured through this approach exhibit properties that emerge primarily from the hierarchical structure rather than the specific chemistry of its constituents. This provides an attractive route towards the design and fabrication of functional materials with enhanced performance using more sustainable chemical resources.

The ability to pattern multiple materials, on micrometer length scales, in three dimensions is critical for several technological applications including composites, microfluidics, photonics, and tissue engineering. The aim of the recently established NSF CREST Center at Florida A&M University is to promote additive manufacturing of novel device structures with an effort towards ab-initio fundamental understanding of material-property relationships that govern the working forces behind high-rate processing applications, and thus low-cost energy products. It is our goal to develop a materials-to-manufacturing framework for research in 3D printing of structures and devices for a wide variety of applications. We aim to (1) gain a fundamental understanding of the materials assembly and additive processing through a combination of experiments and theory; (2) develop and use novel processing/characterization tools for the proposed materials/devices; and (3) use AM to layer-by-layer process these structures/devices. The three main subprojects in the center include - (1) Nanostructured lightweight magnetic materials for shielding/sensing applications. (2) Nanostructured materials for energy applications and (3) Nanostructured Materials for Biological Applications. In this talk, I will detail recent developments in the CREST on these particular sub projects – flow and field assisted processing of multifunctional nanocomposites – where we use combined flow and magnetic fields to orient nickel nanorods, direct printability and performance of 3D conductive graphite structures – where we use extrusion printing to fabricate graphitic structures with enhanced electrical and thermal conductivities, microscopic origin of elasticity and yielding in colloidal gels – where we use a combination of rheology and x-ray scattering to get a microscopic view of macroscopic properties and characterization and printability of Sodium alginate-Gelatin hydrogel for bioprinting non small cell lung cancer cells (NSCLC) co-culture. Research is tightly integrated with education of minority students at FAMU which is the only HBCU to offer a PhD in chemical and industrial engineering.

Current state-of-the-art graphene inks for extrusion based additive manufacturing (AM) typically contain large quantities of polymers and other additives that require removal through post processing heat treatment to allow access to many functionalities of the 2D materials. Alternative routes, e.g. freeze casting, require complex multiple steps under thermal control to allow printability. Here, an alternative route to additive manufacturing with 2D materials is proposed, based on the concept of the capillary suspension. Capillary suspensions can form when a solid phase is suspended in a mixture of two immiscible solvents, one of which (the minor phase of the mixture) strongly wets the particles in suspension. The microstructure of a capillary suspension leads to a Bingham bevavior, with low resistance to flow above a critical stress when the capillary bridges within the suspension are ruptured and is thus ideally suited to extrusion based AM. A capillary suspension containing 20 wt.% graphene in aqueous suspension, with carboxymethyl cellulose as dispersant, is transformed to a capillary suspension through the addition of 5% octanol. The resulting ink has fluid properties compatible with 3D printing and simple lattice structures are produced with strength sufficient strength for handling after air drying. Similar inks and demonstrator structures with 2D BN in suspension have also been produced. The graphene structures have open porosity of 69%, a compression strength of 1.3 MPa and electrical conductivity of 250 Sm^-1. The stability of the ink is shown to be controlled by the relative wetting angle with the two fluids and particle size and aspect ratio.

There are increasing research interests in natural materials and composites for sustainable additive manufacturing. Natural materials, such as cellulose fibers, have been used as reinforcement fillers in petroleum-based matrix polymers. However, the hydrophilic cellulose fibers are incompatible with many synthetic hydrophobic polymers, leading to weak interfacial bonding and poor mechanical properties. More importantly, the majority of these composites remains not recyclable or degradable. Manufacturing pure natural materials into mechanically-robust engineering materials can benefit in both environmental and economic perspectives associated with the replacement of petroleum-based materials. The natural resources are renewable, biodegradable, and inexpensive. For example, corn is the largest agricultural product in the US with an annual output over 300 million tons. Processing this large amount of corn results in more than 200 million tons of waste, such as husks and cobs. Although there are emerging researches using natural materials, such as rice and protein, to develop 3D printable materials, limitations on printable materials and their mechanical properties exist. Here, we develop 3D printable biocomposites entirely from corn products and waste, namely starch and cellulose fibers from husks. We extract cellulose microfibrils from corn husks via an alkalization process to achieve controlled dimension and structures. The cellulose fibers are distributed into compatible matrices derived from corn starch, to formulate 3D printable biocomposites using an extrusion-based direct ink writing (DIW) method. We perform a systematic design of experiments to investigate the ink formula, processing steps, and resulted mechanical properties to establish process-structure-property relationships and obtain the optimal mechanical properties. The results indicate comparable stiffness and strength compared with other 3D printed thermoplastics. Since our biocomposites consist of purely natural components, they have potential applications in medical and food area.

Fused deposition modeling (FDM), as one of the most dominant 3D printing techniques, has been increasingly applied in various fields, yet it suffers from the anisotropic performance in the printed materials and the difficulty to print thermoset polymer. Here, we present an unprecedented 4D printing process that achieves, for the first time, FDM printability of highly isotropic thermoset polymer with excellent shape memory behavior and superb thermo-mechanical performance. A specially modified Epoxy-benzoxazine/carbon nanotube hybrid resin was formulated that behaves as a low-temperature thermoplastic allowing FDM printing, and a high-temperature thermoset achieving covalently crosslinked structure. The cross-layer reaction fuses individual layers into an integrity, thus eliminating layer delamination induced by FDM to offer isotropic mechanical properties regardless of the printing orientations. The highly crosslinked epoxy-benzoxazine network offers excellent shape memory behavior with fast recovery rate (full recovery in 80 s) and large recovery degree (98%), as well as outstanding mechanical strength and thermal stability that are superior over most FDM printed polymer.

Nowadays, there are only a limited number of commercial alloy systems for Laser Powder Bed Fusion (LPBF) , i.e. pure titanium and Ti6Al4V alloys, stainless steels, nickel-based superalloys, Al-Si alloys, cobalt chromium and recently high-entropy alloys. At the same time, developing new alloy compositions is now becoming a key challenge to address in the additive manufacturing field. However, the development of new alloys for LPBF generally requires high economical investments. In fact, the starting powders have a high cost because of they must present highly sphericity and specific particle size distribution to guarantee good flowability and to prevent the formation of porosities in the final parts. In addition, finding the right window for the main process parameters is a core procedure for adopting novel alloy compositions for LPBF. There are many process parameters that can affect the final density of the parts:
considering a fixed value for the layer thickness, selected on the basis of the powder mean size and distribution, part quality depends from laser power, scan speed, hatching distance and scanning strategy. The traditional Design Of Experiment (DOE) approaches for defining the operating process windows are time consuming and expensive requiring the manufacturing of several matrices of massive samples.
Recently in literature it has been demonstrated that the proper P and v values for can be determined via the Single Scan Tracks (SSTs) approach, which requires small quantity of powders with respect to the manufacturing of massive samples. In fact, SSTs are useful for the evaluation of the melting and consolidation behaviour of the material by changing laser power and scan speed, but without considering the effect of the hatching distance. For this reason, according to the traditional approaches SST analyses must be followed by matrices of massive samples with different hatching distances with a consequent increase of the employed powders.
In this work, a novel method to define LPBF process parameter of new alloys through SSTs is proposed. At first, a power - speed operating window is easily defined. Then, the proper hatching distance is designed considering to have an optimum overlap among adjacent scan tracks. In this way, massive samples can be directly produced by using the best P-v- combinations, avoiding many experiments. In particular, in the first jobs a range of P-v values for which stable and regular SSTs are identified. For the SST characterization an automated and fast method is set up by developing a code for analysing SST images, also avoiding the subjectivity of each researcher in measuring SST features. After that, the measurement of SST width allows to directly select the hatching distance to ensure a good overlapping. In this way the manufacturing of dense samples can be easily carried out by employing a small quantity of powder and time.

Stereolithography (SLA) 3D printing is often used to fabricate complex 3D structures requiring precise and clean surface finish due to its geometrical accuracy compared to other additive manufacturing methods. However, photopolymer resin used in the SLA has limited mechanical, electrical, and thermal properties. Nanofibers or nanoparticles were used as fillers to improve these properties, and studies showed that adding Carbon Nanotubes (CNT) as reinforcement enhanced the electrical conductivity of the resin, but these lacked in the analysis of the geometrical accuracy performance.
In this study, we evaluated the effect of CNT concentration on electrical conductivity, curing conditions, and dimensional accuracy. Polyurethane acrylate resin was mixed with varying MWCNT concentration from 0 weight percentage (wt%) to 0.8 wt%, and structures were printed using a desktop masked SLA 3D printer. Electrical conductivity was measured using a four-point probe method and dimensional accuracy was evaluated with the geometrical benchmark model.
From the measured data, 0.6 wt% was the maximum concentration of MWCNT that performed a high precision level of SLA method, which also showed 0.033 S/m of electrical conductivity. We applied these findings to fabricate complex multi-material 3D circuit and capacitive sensor.

Many emerging applications in microscale engineering require the fabrication of three-dimensional architectures in inorganic materials. Small-scale additive manufacturing (AM) aspires to provide access to these geometries with feature sizes in the micro- and submicrometer range. Yet, the synthesis of device-grade inorganic materials is still a challenge for AM [1] – a major handicap for its incorporation in advanced micro- and nanofabrication processes.
Here, we present our latest advances in electrohydrodynamic redox printing (EHD-RP), focusing on the broadening of the materials range and the optimization of the deposited microstructure. EHD-RP has previously been shown to enable the direct, ink-free fabrication of polycrystalline multi-metal 3D structures with a resolution of 250 nm and a feature size of ≈100 nm [2]. The electrochemical concept can result in excellent as-printed materials quality printed at speeds that outperform alternative electrochemical techniques by one order of magnitude. Additionally, as a most unique feature, EHD-RP has demonstrated nanoscale multi-metal printing with unprecedented detail. Yet, until now, the materials available to EHD-RP have been limited to copper, silver, and gold.
In this talk, we will discuss our latest findings on microstructure optimization in copper. Further, we will present our efforts to expand the printable materials palette to different metals, including alloys and magnetic metals, as well as semiconductors and insulators. We will show that the method is compatible with different sources of ions (sacrificial anodes and salt solutions) and will discuss the role of the solvent. The results present a further step towards the additive deposition of inorganic materials of high quality by microscale AM.

[1] Reiser, A., Koch, L., Dunn, K. A., Matsuura, T., Iwata, F., Fogel, O., Kotler, Z., Zhou, N., Charipar, K., Piqué, A., Rohner, P., Poulikakos, D., Lee, S., Seol, S. K., Utke, I., Nisselroy, C., Zambelli, T., Wheeler, J. M., Spolenak, R., Metals by Micro-Scale Additive Manufacturing: Comparison of Microstructure and Mechanical Properties. Adv. Funct. Mater. 1910491, 1910491 (2020).

[2] Reiser, A., Lindén, M., Rohner, P., Marchand, A., Galinski, H., Sologubenko, A. S., Wheeler, J. M., Zenobi, R., Poulikakos, D., Spolenak, R., Multi-metal electrohydrodynamic redox 3D printing at the submicron scale. Nat. Commun. 10, 1853 (2019).

[3] Reiser, A., Hirt, L., Spolenak, R. & Zambelli, T. Additive Manufacturing of Metal Structures at the Micrometer Scale. Adv. Mater. 29, 1604211 (2017).

The consistent growth of electronic devices is one poses one of the most serious ecological challenges of the decade. The rapid pace of innovation correlates with rapid obsolescence of existing devices. Disposal of these devices often poses problems since the electronic circuitry is often printed using toxic heavy metals and other hazardous chemicals which are harmful to both global environment and human health. To address these issues we have engineered a new degradable, environmentally sustainable, and electronically and thermally conductive nanocomposite, which could be extruded into filaments and used for 3-D printing of circuits and antennas. We blended poly (butylene adipate-co-butylene terephthalate) (PBAT) with a semi-crystalline harder polymer, poly (lactic acid) (PLA), and added graphene nano-platelets (GNPs) as filler. PLA and PBAT are incompatible and phase segregate, but the interfacial tension between GNPs and PLA is even higher than that of the two polymers, and hence the GNPs are sequestered only within the PBAT phase, and do not segregate at the polymer interfaces. The blend is also ductile and easily extruded into filaments for 3-D printing. TEM imaging of the filaments showed that the additional confinement during extrusion further aligned the GNPs allowing them to form percolated rope-like structures within the matrix. Electrcial and thermal conductivity measurements indicated that samples formed via 3D printing had better thermal and electrical properties than those formed via molding, without sacrificing the mechanical performance. Compared with molded PBAT/PLA/GNPs at 20% graphene loading, printed electrical conductivity significantly increased from 2.49S/m to 597 S/m, thermal conductivity increased from 0.664W/mK to 1.794W/mK, while the Young’s Modulus increased from 867MPa to 1258Mpa.

3D printing is gaining importance for the fabrication of energy devices with superior performance, controllable dimension and customized shape. The complex architectures enabled by printing processes are able to enhance electron and mass transport and provide high surface area, which constitute fundamental features for energy systems. Among extrusion-based techniques, Direct Ink Writing offers the possibility to print a wide range of different materials, and it is suitable for the multi-material deposition of full energy devices, such as supercapacitors and rechargeable batteries.
Despite the great potential held by 3D printing processes, the fabrication of full devices via Direct Ink Writing is bound to the formulation of printable inks from energy materials. Inks must meet specific rheological requirements to be printable, which include high shear-thinning behaviour during flow, solid-like response at rest and fast recovery of the elastic properties after extrusion. The ink design strategy must be carefully optimized to simultaneously obtain high printability and achieve favourable electrochemical properties.
Here the most common ink formulation strategies for energy devices are discussed, highlighting the role played by the different ink components to achieve high printability and their impact on the electrochemical performance of the final device. Viable ink formulations for the fabrication of sodium-ion batteries are presented, giving insight into the connection between rheological behaviour and printability.

The carbon nanotube (CNT)/polydimethylsiloxane (PDMS) composite has been studied and fabricated as soft electronics, such as strain sensors, supercapacitors, and electrodes, due to its excellent flexibility and electrical conductivity. An engineered foam structure provides the capability of large-scale deformation as stretchable devices and also affects the percolation threshold of CNT concentration in the CNT/PDMS foam composites. It has been investigated that PDMS-rich domains form at the surface of the composites, where limited reinforcements distribute in this region. However, little research has been focused on the influence of this domain on the electrical conductivity of composites. In this work, we present the effect of the PDMS-rich domain on the CNT/PDMS foam nanocomposites. The material extrusion 3D printing method is used to fabricate the composites due to its advantage of free design to form the scaffold model. The foam structure is built by introducing a foaming agent into CNT/PDMS composite ink as feedstock for material extrusion 3D printing. Electrical pathways formed by interconnected CNTs increase due to PDMS-rich regions at the surface of micropores in the foam structure. Therefore, we hypothesize that the percolation threshold of CNT concentration in the foam composites decreases compared to that in the solid composites. The percolation threshold is inversely proportional to the density of micropores in the foam structure. It is also noted that the electrical conductivity of CNT/PDMS foam composites is higher than the solid composites at identical CNT concentrations. We will compare this hypothesis to our achieved foam structure fabricated using a sacrificial material (sodium chloride). This study provides a broader insight into the mechanism of electrical conductivity in polymer-based foam nanocomposites.

In this work, laser powder bed fusion (LPBF) is used for the additive manufacturing of composite samples consisting of tungsten carbide particles with a nickel binder. Such process can become a viable low-energy alternative to the conventional production of ceramic-metal composites for applications including tooling, electronics, and wear components. Single track experiments are used to evaluate the melting behavior of the composite material. Samples are then printed with of process parameters that adequately melted the material. The density, microstructure, and functional properties of these samples are measured. Highly dense (>98%) samples are successfully manufactured and analyzed as a function of LPBF parameters. Macro- and microdefects resulting from the laser processing are also discussed. A methodical approach to evaluate an acceptable processing region is presented and used to investigate the feasibility of additively manufacturing tungsten carbide-nickel composites via LPBF.

Aluminum metal matrix composites (MMCs) are highly desirable materials for precision aerospace applications, where stiffness and low mass are critical, but they are rarely used because of their cost and machining difficulty. In these applications, components typically have complex geometries and are produced in small numbers. Therefore, processing metal matrix composites additively would be the ideal solution, but MMCs with high ceramic content are not commercially available for AM. Here, using purely mechanical methods and readily available constituent powders, we fabricate composite feedstock powders optimized for laser powder bed fusion in a cost-effective, scalable manner. We demonstrate that these powders can be laser consolidated to nearly 100% relative density, providing a potential pathway for the broader adoption of these high-performance yet underused materials.

Ultrasonic additive manufacturing combines layer-by-layer ultrasonic welding of foils with CNC machining to produce net-shape components without melting the feedstock. The solid-phase nature of ultrasonic additive manufacturing makes it attractive for building bulk nanocrystalline metals; however, the high hardness of nanostructured materials makes it difficult to achieve the plastic flow necessary for welding without excessive frictional heating, which could compromise their grain structure. Here, we circumvent this processing challenge by creating laminar composites comprising nanocrystalline NiCo feedstock alternated with soft Al interlayers which accommodate the deformation required for bonding. During the layer deposition process, the NiCo grain structure was preserved and the Al asperities completely conformed to the NiCo surface. TEM chemical analysis of the NiCo/Al interfaces combined with thermodynamic calculations reveals a 100 nm interdiffusion zone containing several intermetallic phases. Tensile testing of the composites and the monolithic NiCo shows that an improved energy absorption in the laminates is realized in part through delocalized damage mechanisms. Ultimately, we couple this experimental work with process models that quantitatively illustrate the tradeoff between frictional heating induced- grain growth and interfacial plastic flow, theoretically validating the addition of softer interlayers as a processing strategy for nanostructure preservation.

In stereolithography (SLA), diffusion of reactive species between light and dark regions of the printing plane limits the achievable resolution. Partially crosslinked macromolecules diffuse away from initiating regions, leading to overpolymerization in dark zones and loss of print fidelity. Additionally, gradients within polymerized regions cause mechanical heterogeneity in the final part. These processes occur on the individual voxel scale and below, so bulk measurement techniques such as photorheology and infrared spectroscopy are unable to adequately measure them, and new measurement techniques are needed. As resolution requirements for new applications push the limits of scalable single-photon processes, the ability to meet this demand hinges on an increased understanding of the fundamental photopolymerization processes involved.

In this talk, I will describe an atomic force microscopy (AFM) technique to directly map the generation and diffusion of polymeric species during stereolithography. We use an AFM cantilever with a nanocylinder extending from the tip, and when the cylindrical probe is inserted into a resin, the drag on the cylinder can be used to measure to the local viscosity of the liquid, and in turn, the local extent of reaction. Vibrating the cantilever at one of its resonance frequencies (10⁴ Hz - 10⁷ Hz) affords high temporal resolution. Computational fluid dynamics modeling indicates that the fluid motion induced by the oscillating cantilever is localized within 1 μm of the probe. Employing a custom-built instrument, we can direct arbitrary light patterns from a spatial light modulator to the imaging plane of the AFM to achieve photopolymerization. By translating the tip and light pattern relative to one another, the full spatial evolution of the reaction profile can be measured.

We employed this technique to measure the reaction dynamics during SLA photopolymerization of a thiol-ene resin. The thiol-ene click reaction has some advantages over the photopolymerization of acrylate resins, including reduced shrinkage stress and insensitivity to oxygen inhibition. We find a non-reciprocal dependence of the reaction on light intensity; increasing the light intensity decreases the efficiency of the reaction, likely by increasing the bi-radical termination rate. Thus, at constant light dose, lower light intensities lead to increased conversion in the reaction volume, even accounting for increased time for diffusion. This disproves a common assumption in stereolithography, namely that gelation depends solely on the light dose, often referred to as the “critical dose.”

Mapping the spatial evolution of the polymer conversion profile also allows us to easily identify the diffusion length scales during the reaction, and we measure polymer accumulation 20 μm away from the illuminated region within one second of light exposure, identifying the relevant length scale of our process. We then created an open ring test structure (inner diameter = 15 μm, outer diameter = 45 μm) based on this finding to determine the effect that diffusion plays on the resolution of a printed part. Consistent with our measurements of the reaction profile, we observe significant overpolymerization in the ring center, leading to gelation and loss of fidelity to the projected mask. This new technique provides adequate spatial (1 μm) and temporal (<1 ms) resolution for modern stereolithography applications, and it can be used to predict the ultimate printing resolution and rapidly determine the optimal exposure conditions for a given resin.

Dynamic covalent chemistries have attracted much attention to create reversibly crosslinked covalent polymer networks that show improved processability compared to irreversibly crosslinked networks. Two types of dynamic covalent mechanisms exist: (i) dissociative reversible reactions where the covalent bonds dissociate upon application of the corresponding stimulus, such as heat or light and (ii) associative exchange reactions where a certain functional group can exchange bonds with existing covalent bonds. These dynamic covalent chemistries have also become very popular to create self-healing materials that are able to reform broken bonds to recover the functional properties of materials and hence increase the service lifetime of structures and systems by recovering their functional performance.
The thermally reversible Diels-Alder cycloaddition reaction between furan and maleimide functional groups is a widely studied reversible covalent chemistry for creating thermally reversible covalent polymer networks. While scientific articles reporting synthesis of new reversible polymer networks are numerous, accurate knowledge of the reaction kinetics and thermodynamics of the dynamically reversible equilibrium reaction and the structure and property development of derived stimuli-responsive materials are much less widespread. Such knowledge allows to design and optimize the functional properties of the materials in view of intended applications and to tailor the processability of the reversible polymer networks. Heating the thermally reversible polymer networks based on the Diels-Alder reaction shifts the reaction equilibrium towards the gradual breaking of the cycloadduct crosslinks in the network. At a certain point so many bonds are broken that the material undergoes a solid-liquid transition, referred to as degelation, as the network structure is broken down. This reversible gel transition was employed to create filaments for the 3D printing of 3D structures for flexible, compliant components for a self-healing soft robotic gripper.
The dynamically reversible chemistry, choice of monomers and resulting polymer network architecture determine the (thermo)mechanical properties, thermoresponsive behaviour and healing capabilities of the created reversible covalent polymer networks. Optima need to be sought between (i) reasonably fast reaction kinetics for fast and efficient damage healing at moderate temperatures, (ii) reasonable processing and manufacturing conditions and (iii) adequate mechanical properties and structural stability required for the intended application. Stress relaxation is desirable property to make materials tougher, relieving stress before microscopic defects can grow into macroscopic cracks and ultimately lead to failure, while creep behaviour during application can’t be allowed. Recycling and reprocessing of materials are desirable from an ecological viewpoint, while the materials should also be able to withstand static and dynamic loading in a considerable range of environmental conditions.

Additive manufacturing of functional polymers remains limited by the challenge of precisely controlling multiscale hierarchical assembly during processing. In this work, we address this challenge by integrating nonequilibrium self-assembly of densely grafted bottle brush block copolymers with a highly versatile direct-write 3D printing approach. We demonstrate that programmatic variation of printing conditions enables control of peak reflected wavelength for bottlebrush photonic crystals across a range of 403 to 626 nm (blue to red) with a single ink material. We further demonstrate spatial and functional patterning of photonic crystals to achieve colorful prints of chameleon patterns through on-the-fly tuning of assembly kinetics during printing. Via scanning electron microscopy and synchrotron small-angle X-ray scattering analysis of printed films, we then clarify the microstructural underpinning of this phenomena as tuning of lamellar domain d-spacing (total thickness of A+B block) over a range of greater than 30 nm, which we attribute to modulation of polymer conformation during printing. Finally, we establish the role of kinetic trapping of metastable microstructures as the mechanism for domain size control based on in situ optical microscopy and solvent-vapor annealing experiments. Thus, we demonstrate a new method for tunable block copolymer photonics without synthetic variation. Furthermore, we present a hardware and software integrated 3D printing scheme that can serve as a test bed for functional property modulation for a variety of functional polymeric and material systems.

Additive manufacturing (AM), or 3D printing, represents a set of processes that enable layer-by-layer fabrication of complex structures using a wide range of materials that include ceramics, polymers, and metals. AM has allowed exploiting novel material properties, especially those that arise at the nano-scale, that do not occur in conventional materials, revolutionizing the production of complex parts for aerospace, military, automotive, optial, and medical applications. Facilitating these technologies requires a fabrication process to create a variety of functional materials in 3D, however the material choice for AM at the nano- and micro-scale is limited. A conspicuous example is a lack of AM processes for high refractive index (n), low absorption materials with nano-sized dimensions, which are typically required for microoptics and device applications. To harness the beneficial properties of 3D nano-architected meta-materials, it is critical to assess their properties at each relevant scale while capturing over­all structural complexity. We present the fabrication and synthesis of nano- and micro-architected materials using 3D (two-photon and interference) lithography, as well as characterization of their properties as a function of architecture, constituent materials, and microstructural detail. We focus on additive manufacturing of complex, multi-component and multi-material architectures comprised of metal oxides, metals, and hydrogels that are derived from (metal) salt-based, aqueous photoresin synthesis. The discussion will focus on each representative materials system (metals, oxides, and hydrogels), with a detailed overview of titanium dioxide, with critical feature dimensions between 150 and 600 nm and <1% porosity. Using the described methodology, we created 3D dielectric photonic crystals (PhCs) out of rutile TiO2 patterned into woodpile face-centered tetragonal (FCT) architectures with beam dimensions of 300-600nm and lateral periods of 0.8-1.5 μm. We use Plane Wave Expansion (PWE) simulations and Fourier Transform Infrared Spectroscopy (FTIR) to demonstrate the full photonic bandgaps centered at 1.8-2.9 μm. This fundamental research provides insights into the range of possible, often transient attainable chemical compositions, microstructures and architectures of chemically-derived, 3D-archtiected ceramics and metals, enabling advances in multiple technologies, i.e. 3D MEMS, micro-optics, and prototyping of 3D PhCs.

Dual-phase titanium alloys, incorporating α and β phases with contrasting mechanical properties, exhibit balanced and comprehensive mechanical performance, which is suitable for many structural applications, especially in aerospace and biomedical sectors. Laser melting deposition (LMD, one of the primary additive manufacturing methods) provides an ideal method to efficiently fabricate metallic components with complex geometries, as well as controlling microstructure. The complex phases, microstructure, and related phase/grain boundaries significantly influence the deformation and fracture behavior of AM-produced titanium parts. So far, the correlation between the microscale mechanics of individual phases/grains and global mechanics of AM-produced parts has been poorly understood. This thesis will focus on analyzing the micromechanical properties of α+β titanium alloys, including hardness/modulus in different phases, micro-compression/tension/bending tests, towards a better understanding the role of micromechanical properties of each phase and interfaces on plastic deformation and damage behavior of bulk materials. Thus, the goal of this work is to bridge the microscale and macroscale mechanical properties in additive manufactured titanium alloys.

Sub-micron resolution oxide materials have immense potential in the fabrication of various nanotechnologies. Zinc oxide (ZnO) is of particular interest due to its piezoelectric properties and its application as sensors in microelectromechanical systems (MEMS). We have recently developed a technique called Photopolymer Complex Synthesis that enables the creation of nano-architected polycrystalline ZnO with feature sizes of 250nm. We achieve this by first patterning zinc ion-containing aqueous photoresins using two-photon lithography and subsequently calcining them at a variety of temperatures to give ZnO. In-situ microcompression experiments with concurrent electrical measurements on these architected ZnO structures demonstrate an electromechanical response, i.e. a voltage drop is observed during compression of the structure. Since ZnO exhibits an intrinsic piezoelectricity effect primarily along the c-axis of the crystal, some net crystallographic alignment is necessary for any piezoelectric effect to be observed in polycrystalline ZnO. However, TEM analysis of these polycrystalline ZnO architectures show that they have a homogeneous random crystal orientation. Thus, a more in-depth study of this material and its microstructure-property relationship is necessary to understand the emergence of the observed electromechanical properties.

By adopting a statistical approach that accounts for the random nature of this microstructure, we show through analytical models and experimental results the role of relative length scales, i.e. the microstructure grain size compared to the architecture feature size, in this observed phenomenon. Our findings also demonstrate that microstructure can be used to tune the piezoelectric response of these structures and highlights the possibility of exploiting localized strain in complex architectures. Investigation of this unique processing of ZnO demonstrates not only the specific properties of this architected material, but highlights the importance of microstructural investigation in novel micro-scale 3D printing materials. Additionally, this fundamental exploration provides an insight into the underlying mechanisms of electromechanical behavior in intrinsic polycrystalline peizoelectrics and lays the foundation for future design and engineering of these materials at the nanoscale.

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

Fused deposition modeling (FDM) is one of the most widely used additive manufacturing techniques for the production of plastic parts. Until recently, prototyping activities had been the main application of FDM, where mechanical properties of printed structures are of minor importance. However, with the decreasing costs of FDM-based manufacturing, and improvements in filament technologies, the manufacturing of functional load-bearing components has become feasible for many applications. This new application route requires a better understanding of the process-mechanical property relationships in FDM-produced parts. Parameters such as nozzle temperature, raster orientation, print orientation, and infill ratio directly affect the strength and ductility of FDM parts. There have been many studies on this topic in the last decade, mostly focusing on the widely used filament materials such as ABS and PLA. In the meantime, new advances in filament technology have made elastomers available for FDM. Low stiffness and extreme elongation capabilities of elastomers provide new 3D printing opportunities for a wide range of structural applications. Further advancement in the field has recently enabled the printing of elastomeric foams by using the readily available FDM printers. Mechanical properties conventional elastomeric foams are well-characterized in the literature; however, there is no study in the literature about the mechanical behavior of 3D-printed elastomeric foams to our knowledge.
In this work, we investigate the process-property relationships of an elastomeric filament designed to foam during extrusion, providing a low-density and porous structure. We focused on the effect of infill ratio, layer thickness, feed rate, and nozzle temperature on the mechanical properties. An Ultimaker 2+ FDM printer produced tensile dogbone specimens by using Varioshore TPU natural filament manufactured by Colorfabb (Netherlands). Scanning electron microscopy characterized the microstructure of the specimens, and conventional tensile mechanical testing quantified the mechanical properties. Increasing nozzle temperature, increasing layer thickness, and decreasing feed rate promote foaming and provides a lower density structure. As the extent of foaming increases, the elastic modulus, strength, and elongation at failure decrease. After the characterization step, we combined the excellent capability of FDM in producing custom cellular structures with the intrinsically low stiffness of the elastomeric foam and demonstrated the fabrication of extremely compliant cellular structures. Compression testing results of these model structures demonstrate the enormous potential of the foaming filament technology for the generation of novel compliant structures.

This article addresses modeling of the solidifying material structure during 3D welding/printing of fully dense Mg alloy products by fused deposition of molten droplets from a uniform droplet spray source on a motorized X-Y table substrate. The resulting crystallite size distribution is simulated by a solidification model consisting of nucleation/fragmentation and constrained growth description, calibrated via structural data from a single droplet splat. This is enabled by a semi-analytical thermal modeling framework, based on superposition of moving Green’s and Rosenthal functions for the temperature field from a Gaussian source distribution, in which the deposit solid geometry and heat transfer boundary conditions are accounted for by mirror source images of modulated efficiency. The simulation model is implemented for layered ellipsoidal deposit sections on planar substrates by multi-pass spraying, and its predictions are validated against measured crystal size by image analysis of experimental micrographs of a Mg₉₇ZnY₂ alloy, to an error margin of +15%. The computationally efficient simulation provides insight to the deposit microstructure, and is intended as a process observer in a closed-loop, adaptive control scheme based on infrared temperature measurements.

Additive manufacturing processes produce materials with hierarchical microstructures containing fusion boundaries at the macroscale, irregular grains and grain boundaries at the microscale, and subgrain dislocation structures at the nanoscale. In 316L stainless steels, corrosion performance has been discussed in the context of chemical heterogeneities formed in the presence of these hierarchical microstructures. However, the large variability in reported measurements underscores the need for statistically significant microstructural data, which is often difficult to access via electron microscopy alone. In this presentation, we explore multi-modal synchrotron x-ray techniques for quantifying hierarchical microstructures and their connection to the underlying chemical distribution in 316L stainless steel. Our results show that the dislocation density depends on the printing conditions and ultimately influences chemical distribution at the nanoscale with implications for the formation of deleterious second phases from a corrosion perspective. On this basis, corrosion rates and pit densities are discussed to build a connection between printing conditions and corrosion performance vis-à-vis microstructural and chemical information from correlative electron microscopy and synchrotron characterization experiments. A decreased pitting potential and loss of passive behavior accompanied by elongated regions of pitting attack were attributed to Cr depletion degrading passive film formation with Mn carbides serving as initiation sites for localized attack.

Additive Manufacturing (AM) has been developed expeditiously in the past years, and the application domain and material range are broadening steadily. The 3D printing era has started in 1987 when the first commercial 3D printer was released utilizing stereolithography technique. Ever since, new AM processes are introduced and implemented to face current challenges and accommodate the new requirements in the manufacturing field along the way of 3D printing development.
In opposition to subtractive manufacturing techniques that include material removal to obtain the desired shape, additive manufacturing (AM) is a process where joining of materials takes a place to create three-dimensional (3D) objects from 3D model data. The printed designs are made by adding subsequent layers generated from computer-aided design. The ultimate advantage of AM is that it gives the user the ability to create bespoke designs of both different geometric and different materials. However, AM is a very diversified term that comprises different methods such as direct metal laser sintering, fused filament fabrication (FFF), stereolithography, powder bed fusion, selective laser melting (SLM), and binder jetting.
The 3D printing of metallic materials is based on the layered consolidation of feedstock materials in the form of: i) powder, ii) wire, or iii) sheet while using various energy sources to form complex structures and designs. Metal printing methods are categorized as electron- beam-based, arc-based, laser-based, welding-based, and ultrasonic. However, the most common energy sources for metal 3D printing are electron beams and lasers. Laser-based metal 3D printing processes are classified into laser sintering (LS), laser melting (LM), and laser metal deposition (LMD). LS is a powder bed fusion technique where a scanning laser is used to consolidate the layers of metal powder sequentially. The LM is another powder bed fusion technique involving the consolidation of metal powders using high-intensity lasers. All equipment setup, configuration, and processing methodology are similar in LS and LM, but in LM, the powder is almost or completely melted to produce a dense structure. LM enables the production of metal parts with a considerably high level of microstructural homogeneity when compared with LS. In LMD, the feedstock material is either a wire or powder. In powder-based LMD, a pressurized gas transfers the powder through a nozzle into a melt pool created by a laser beam on a substrate. The laser and powder integrated system deposits the material while following a specified path. In wire-based LMD, a wire is continuously fed into the path of the laser, while the tip of the wire is continuously melted to make deposits on a substrate.
The FFF technique for printing metallic parts is a particular interest in the investigation of this work due to acceptable printing resolution and relatively low-cost process compared to LS,LM, and LMD costs. To our knowledge, no one has reported the effect of printing angles to the tensile property for metal FFF process. We will use the Markforged Metal X 3D printer using 17-PH stainless steel material in this work. The 3D printing consists of 3 steps: FFF 3D printing of metal-binder filament, washing to remove the binder, and sintering to achieve metal mechanical property. We will use Eiger software to import and slice the STL file of the sample design. We will use standard tensile test sample ASTM D638 V.
This work will investigate the effect of 3D printing angles on the mechanical properties of the printed samples. The default orientation in the printer is zero for X, Y, and Z, which corresponds to sample X0Y0Z0. We will do experiments of five different print angles of 0, 30, 45, 60, and 90 degrees normal to the surface. Finally, we will measure and discuss the tensile properties (stress, strain, Young’s modulus and yield) of the 17-4 PH stainless steel using Instron 3367 apparatus.

The emergence of 3D printing has allowed for the rapid prototyping of complex geometries that were previously inaccessible using traditional manufacturing techniques. Amongst these additive manufacturing techniques, stereolithography stands out in terms of the surface smoothness, isotropy in properties and fabrication speed it can provide. The materials that can be 3D printed with stereolithography, however, are limited and fairly weak in mechanical properties. Here, we aim to improve the mechanical performance of stereolithographically printed polymers through the addition of graphene fillers. The effects of filler concentration, post-fabrication baking and loading direction on the quasistatic and dynamic mechanical properties of the composite were investigated. It was found that the graphene platelets were selectively aligned along the print axis, the graphene platelets were well-dispersed up to a concentration of ~ 0.05 wt% and post-print bake has the effect of increasing the degree of cure in the PMMA polymer resin up to 120°C, beyond which it would begin to degrade. As a result of such optimization, polymers reinforced with as little as 0.02 wt. % graphene were found to increase their stiffness by 88.3 % and strength by 115.2 %. Split-Hopkinson pressure bar tests also showed that these composites had higher specific strengths and specific energy absorption compared to other lightweight materials such as syntactic ceramic foams and Al alloys (in both bulk and foam form factors).

Additive manufacturing (AM), or 3D printing, represents a set of processes that enable layer-by-layer fabrication of complex structures using a wide range of materials that include ceramics, polymers, and metals. AM has allowed exploiting novel material properties, especially those that arise at the nano-scale, that do not occur in conventional materials, revolutionizing the production of complex parts for aerospace, military, automotive, optial, and medical applications. Facilitating these technologies requires a fabrication process to create a variety of functional materials in 3D, however the material choice for AM at the nano- and micro-scale is limited. A conspicuous example is a lack of AM processes for high refractive index (n), low absorption materials with nano-sized dimensions, which are typically required for microoptics and device applications. To harness the beneficial properties of 3D nano-architected meta-materials, it is critical to assess their properties at each relevant scale while capturing over­all structural complexity. We present the fabrication and synthesis of nano- and micro-architected materials using 3D (two-photon and interference) lithography, as well as characterization of their properties as a function of architecture, constituent materials, and microstructural detail. We focus on additive manufacturing of complex, multi-component and multi-material architectures comprised of metal oxides, metals, and hydrogels that are derived from (metal) salt-based, aqueous photoresin synthesis. The discussion will focus on each representative materials system (metals, oxides, and hydrogels), with a detailed overview of titanium dioxide, with critical feature dimensions between 150 and 600 nm and <1% porosity. Using the described methodology, we created 3D dielectric photonic crystals (PhCs) out of rutile TiO2 patterned into woodpile face-centered tetragonal (FCT) architectures with beam dimensions of 300-600nm and lateral periods of 0.8-1.5 μm. We use Plane Wave Expansion (PWE) simulations and Fourier Transform Infrared Spectroscopy (FTIR) to demonstrate the full photonic bandgaps centered at 1.8-2.9 μm. This fundamental research provides insights into the range of possible, often transient attainable chemical compositions, microstructures and architectures of chemically-derived, 3D-archtiected ceramics and metals, enabling advances in multiple technologies, i.e. 3D MEMS, micro-optics, and prototyping of 3D PhCs.