Special Issue Editorial: Advanced Materials for Additive Manufacturing

Abstract

Additive manufacturing (AM or 3D printing) has advanced significantly over the past decade and has seen a proliferation of new materials, new methods, and new applications. As AM is fundamentally a material processing technology, its advance closely relates to materials. This special issue consists of 32 articles from renowned groups worldwide, presenting their perspective and recent advancements in AM. The articles cover a wide range of topics, which reflect the diversity and breadth of the AM field. The collections in this special issue have attracted seven review articles that cover different aspects of the AM field. Zhu et al. (adma.202314204) discussed the recent progress in multi-material and multi-scale AM. Chen et al. (adma.202307686) also explored multimaterial 3D/4D printing for tissue engineering applications. Wan et al. (adma.202312263) reviewed the latest achievements in 4D printing across diverse fields, including biomedical engineering, electronics, robotics, and photonics. Machine learning has immersed into different fields, including AM. Ng et al. (adma.202310006) discussed the challenges and opportunities of integrating machine learning with AM, including quality control, process optimization, design optimization, microstructure analysis, and material formulation, etc. Laser is one of the most commonly used energy sources for AM. Park et al. (adma.202307586) discussed the principle, material selection, and applications of laser-based AM. The progress of AM also significantly advances other fields, such as soft robotics. Xin et al. (adma.202307963) provided an overview of how AM promotes the fabrication of soft grippers, which are an important component of soft robots. Tissue engineering applications are another sub-field where AM finds tremendous potential. Yuan et al. (adma.202403641) presented an in-depth review of 3D printing of smart scaffolds with stimulus-responsive properties, which can generate tailored and controllable therapeutic effects for bone tissue engineering and regeneration. This special issue has a strong focus on materials in 3D printing. Liquid crystal elastomers (LCEs) have attracted significant attention in recent years due to their capability of large reversible actuation upon external stimuli. LCEs are typically actuated by heat. However, typical heating methods, such as water baths, are relatively slow. Maurin et al. (adma.202302765) proposed an LCE-liquid metal (LM) composite where the LM was used to heat the 3D-printed LCEs ultrarapidly through eddy current under a high-frequency magnetic field. In their demonstrations, the LCE structures could be activated in less than a second. Photothermal heating by incorporating nanoparticles into resin is another common method for LCE actuators. Gold nanorods (AuNRs) are typically used due to their high photothermal efficiency. Skillin et al. (adma.202313745) grafted AuNR with poly(ethylene glycol) (PEG) that greatly improved the dispersion of AuNR in the LCE resin. Because of this, the LCE-AuNR nanocomposites with very low PEG-AuNR content (0.01 wt.%) were shown to be highly efficient photothermal actuators with rapid response (within a second). Chen et al. (adma.202303969) incorporated photochromic titanium-based nanocrystals (TiNC) into an LCE ink. Upon UV irradiation, TiNC could change color from white to black and absorb infrared (IR) light to generate heat. The 3D-printed structures could thus be globally or locally programmed, erased, and reprogrammed for color and shape change. Kotikian et al. (adma.202310743) used a multi-nozzle 3D printer to fabricate LCE lattice structures with spatially programmed nematic director order. These structures demonstrated different interesting shape morphing upon actuation. Escobar and White (adma.202401140) developed actuators by twisting LCE fibers. These actuators showed dramatically increased deformation rate, specific work, and achievable force output. Liquid metals , because of their high conductivity and large deformation capability almost without resistance, have attracted significant research interest in recent years. For example, Maurin et al. (adma.202302765) utilized this feature and spray-printed large patches of the LM so that eddy current could be generated for rapid heating. Wu et al. (adma.202307632) utilized digital light processing (DLP) method to print conductive patterns with high resolution (≈20 µm) by using LM particles. By a simple 5–10s UV irradiation, highly conductive and stretchable patterns could be printed using a photo-cross-linkable LM particle ink. Wu et al. (adma.202307546) used selective laser sintering (SLS) to fabricate a lattice structure, which was then coated by LM. After the lattice was magnetized by a magnetic field, any deformation of the lattice could generate a voltage change for sensing. LMs also have many other interesting merits. Krisnadi et al. (adma.202308862) found that mixing a small amount of water (as low as 1%) with LM foams could lead to significant foaming where the volume could increase by 4–5 times yet still retain good conductivity. They further used this feature to create a 4D printing where the conductors could grow, fill cavities, and change shape and density over time. This special issue also introduces some interesting new materials for 3D printing. Materials or structures capable of growth have attracted much research interest in recent years. In addition to the work by Krisnadi et al. (adma.202308862), Wang et al. (adma.202309818) developed a hydrogel ink that contained yeast and magnetic particles. After a structure was fabricated, it could be deformed by a magnetic field, and then the growth of yeast could then fix the deformation. The growth could be reversed by removing the yeast cell walls. The directed growth and recovery process could be repeated several times. Sustainability has also drawn significant attention in the AM community. Yue et al. (adma.202310040) developed a biobased δ-valerolactone as a platform photoprecursor for DLP printing. Both thermoplastic and thermoset structures could be printed. These printed structures could be recovered to monomers at more than 90% yield by a simple heating. Volumetric printing has also seen rapid development in recent years, but the resins (or inks) usable with this emerging method remain limited. In this special issue, Lian et al. (adma.202304846) developed a bioink from porcine and human renal cortex to fabricate renal decellularized extracellular matrix (dECM) hydrogels using volumetric printing. One requirement for the resin in volumetric printing is high viscosity, which limits the choice of resins. Riffe et al. (adma.202309026) addressed this challenge by using sacrificial gelatin to modulate resin viscosity to support the cell-compatible volumetric printing of macromers based on poly(ethylene glycol), hyaluronic acid, and polyacrylamide. The gelatin could be removed by washing at an elevated temperature. They further expanded this approach to multimaterial volumetric printing. The AM field has also seen many new methods that have led to new capabilities. Godshall et al. (adma.202307881) used a custom heated material extrusion device to print aerogels of engineering thermoplastics through in-situ thermally induced phase separation. The printed aerogels had tailorable porosities (50.0–74.8%) and densities (0.345–0.684 g cm−3), with moduli ranging from 26.3 to 135.0 MPa. Using AM in space has always been a fascinating research area. There are tremendous challenges that one needs to overcome due to the difference in the environments between on-ground and space. Mo et al. (adma.202309618) developed a 3D bioprinting device that could be operated on a satellite. They also created corresponding bioink and suspension medium that supported the on-satellite printing and in situ culture of complex tumor models. Additionally, they developed a control algorithm based on machine learning to enable the automatic control of 3D printing, autofocusing, fluorescence imaging, and data transfer back to the ground. Metal AM has seen wide applications in automobile and aerospace industries. However, one important aspect of metal AM is how to improve the properties of printed parts. Hu et al. (adma.202307825) significantly improved the mechanical performance of Al6xxx alloy by introducing a nucleation agent during the AM process, followed by a heat treatment. This led to a substantial enhancement in plastic stability. Niu et al. (adma.202310160) presented an approach to inhibit cracks by manipulating stacking fault energy (SFE) in a high-entropy alloy from the laser powder bed fusion (LPBF) AM process. They introduced a small amount of Al doping, which could effectively lower SFE, efficiently dissipate thermal stress during LPBF processing, as well as enhance the resistance to crack propagation. Noronha et al. (adma.202308715) used a design approach to improve the mechanical properties while maintaining the lightweight. Specifically, they integrated thin-plate lattice with hollow-strut lattice (HSL) metamaterials, which enhance the resistance of the irregular HSL nodes to deformation and uniformly distribute the applied stresses in the new topology for significantly improved strength. One important aspect of improving the properties is to understand those properties. However, AM field faces the challenges of no or few standard testing methods to gain insights into properties. Koch et al. (adma.202308497) took advantage of the recent development of two-photon polymerization (2PP) 3D printing technique to create macro-sized specimens (centimeter range) and tested three different photopolymers using a high-throughput 2PP system. They characterized the mechanical, thermo-mechanical, and fracture properties of 2PP processed materials, which laid the foundation for future expansion of the 2PP technique to broader industrial applications. As shown in the work by Noronha et al. (adma.202308715), design plays an important role in AM for improving mechanical properties. Deng et al. (adma.202308149) further demonstrated the importance of design in obtaining direction-dependent elastic properties in nonperiodic 3D architectures. This is a challenging problem as it is computationally expensive and experimentally nontrivial. They combined artificial intelligence (AI) and 2PP to design and fabricate 3D porous scaffolds with prescribed elastic properties. Multimaterial printing is highly desirable in AM but is challenging to achieve. Smith and MacCurdy (adma.202308491) took a design approach to achieve continuous change in multiple mechanical properties in composite materials AM. By using multi-inkjet printing, they obtained materials that span four orders of magnitude in modulus and two orders of magnitude in toughness. 4D printing has emerged in recent years as one of the most active subfields of AM. It combines AM with active materials so that the printed structures can change shapes or properties as a function of time (the 4th dimension). Wan et al. (adma.202312263) conducted a comprehensive review of the development 4D printing in the past five years and provided perspectives on future development of this subfield. The work by Maurin et al (adma.202302765), Chen et al. (adma.202303969), Kotikian et al. (adma.202310743), Krisnadi et al. (adma.202308862), and Wang et al. (adma.202309818), also demonstrated features of 4D printing. Shi et al. (adma.202307601) developed a gelatin/sodium-alginate/magnetic (GSM) bioink for 4D printing with the application of sutureless internal tissue sealing. The ink could be precisely placed by the gastroscope with the assistance of an external magnetic field. In addition, the magnetic field could bring the solidified material together, resulting in a sutureless sealing. Bioprinting has seen tremendous growth in the past few years. This is also reflected in this special issue. Chen et al. (adma.202307686) reviewed multimaterial 3D/4D printing for tissue engineering applications. Yuan et al. (adma.202403641) provided an in-depth review of 3D printing of smart scaffolds with stimulus-responsive properties, which can generate tailored and controllable therapeutic effects for bone tissue engineering and regeneration. The works by Lian et al. (adma.202304846), Riffe et al.(adma.202309026), Mo et al. (adma.202309618), and Shi et al. (adma.202307601) were all aiming at bioprinting. In addition, Li et al. (adma.202308875) developed a nanocomposite bioink that consisted of magnesium peroxide and poly (lactide-co-glycolide) for low-temperature printing. The printed structure could release magnesium ions in a time-sequential manner to prevent tumor recurrence, inhibit bacterial infection, and promote bone defect repair. Camarero-Espinosa et al. (adma.202310258) 3D-printed scaffolds inspired by the bone marrow niche that could recapitulate the natural healing process after injury. They conducted in vivo tests of these niche-inspired scaffolds in different animal models. Garreta et al. (adma.202400306) fabricated dECM from porcine and human renal cortex to enrich cell-to-ECM crosstalk during the onset of kidney organoid differentiation from human pluripotent stem cells (hPSCs). They found that the printed renal dECM together with hPSC-derived renal progenitor cells presented new approaches for 2D and 3D kidney organoid differentiation, exhibiting renal differentiation features and the formation of an endogenous vascular component. Hwang et al. (adma.202400364) presented a bioprinting-assisted tissue assembly (BATA) approach to fabricating biological tissues with complex structures. As a demonstration, they fabricated a model for the left ventricular twist that exhibits synchronized contraction between layers and mimics the native cardiac architecture. As mentioned earlier, AM is a highly active field. The collections in this special issue may represent only a small portion of the excellent work being done, but we hope that they can give other researchers a glimpse into this dynamic and rapidly developing field. The authors declare no conflict of interest. Kun Zhou is a Professor of Mechanical Engineering in the School of Mechanical and Aerospace Engineering at Nanyang Technological University and is a member of European Academy of Science. He currently serves as Programme Director (Marine & Offshore) in Singapore Centre for 3D Printing. He received both his B.Eng. and M.Eng. degrees from Tsinghua University, China and his Ph.D. from Nanyang Technological University. He has been conducting multidisciplinary research at the crossroads of mechanics, additive manufacturing, materials science, and molecular physics. Ruike Renee Zhao is an Assistant Professor of Mechanical Engineering at Stanford University where she directs the Soft Intelligent Materials Laboratory. Renee received her Ph.D. and postdoc training from Brown University and MIT, respectively. Her research focuses on the development of stimuli-responsive soft composites for applications in soft robotics, miniaturized biomedical devices, flexible electronics, and deployable and morphing structures. H. Jerry Qi is the Woodruff Endowed Professor of Mechanical Engineering at Georgia Institute of Technology and is the site director of NSF IUCRC Center SHAP3D on 3D printing. He received his doctoral degree from MIT. He joined University of Colorado Boulder in 2004 and moved to Georgia Tech in 2014. His research is nonlinear mechanics of active polymers and their integration with 3D printing for active structures and sustainability.

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