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1. A New 3D Printing Strategy by Harnessing Deformation, Instability, and Fracture of Viscoelastic Inks

   https://doi.org/10.1002/adma.201704028  

   Yuk, Hyunwoo; Zhao, Xuanhe (December 2017, Advanced Materials)

   Abstract Direct ink writing (DIW) has demonstrated great potential as a multimaterial multifunctional fabrication method in areas as diverse as electronics, structural materials, tissue engineering, and soft robotics. During DIW, viscoelastic inks are extruded out of a 3D printer's nozzle as printed fibers, which are deposited into patterns when the nozzle moves. Hence, the resolution of printed fibers is commonly limited by the nozzle's diameter, and the printed pattern is limited by the motion paths. These limits have severely hampered innovations and applications of DIW 3D printing. Here, a new strategy to exceed the limits of DIW 3D printing by harnessing deformation, instability, and fracture of viscoelastic inks is reported. It is shown that a single nozzle can print fibers with resolution much finer than the nozzle diameter by stretching the extruded ink, and print various thickened or curved patterns with straight nozzle motions by accumulating the ink. A quantitative phase diagram is constructed to rationally select parameters for the new strategy. Further, applications including structures with tunable stiffening, 3D structures with gradient and programmable swelling properties, all printed with a single nozzle are demonstrated. The current work demonstrates that the mechanics of inks plays a critical role in developing 3D printing technology. 

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2. Digital Light Processing Based Bioprinting with Composable Gradients

   https://doi.org/10.1002/adma.202107038  

   Wang, Mian; Li, Wanlu; Mille, Luis_S; Ching, Terry; Luo, Zeyu; Tang, Guosheng; Garciamendez, Carlos_Ezio; Lesha, Ami; Hashimoto, Michinao; Zhang, Yu_Shrike (October 2021, Advanced Materials)

   Abstract Recapitulation of complex tissues signifies a remarkable challenge and, to date, only a few approaches have emerged that can efficiently reconstruct necessary gradients in 3D constructs. This is true even though mimicry of these gradients is of great importance to establish the functionality of engineered tissues and devices. Here, a composable‐gradient Digital Light Processing (DLP)‐based (bio)printing system is developed, utilizing the unprecedented integration of a microfluidic mixer for the generation of either continual or discrete gradients of desired (bio)inks in real time. Notably, the precisely controlled gradients are composable on‐the‐fly by facilely by adjusting the (bio)ink flow ratios. In addition, this setup is designed in such a way that (bio)ink waste is minimized when exchanging the gradient (bio)inks, further enhancing this time‐ and (bio)ink‐saving strategy. Various planar and 3D structures exhibiting continual gradients of materials, of cell densities, of growth factor concentrations, of hydrogel stiffness, and of porosities in horizontal and/or vertical direction, are exemplified. The composable fabrication of multifunctional gradients strongly supports the potential of the unique bioprinting system in numerous biomedical applications. 

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3. 4D Printing of Freestanding Liquid Crystal Elastomers via Hybrid Additive Manufacturing

   https://doi.org/10.1002/adma.202204890  

   Peng, Xirui; Wu, Shuai; Sun, Xiaohao; Yue, Liang; Montgomery, S_Macrae; Demoly, Frédéric; Zhou, Kun; Zhao, Ruike_Renee; Qi, H_Jerry (August 2022, Advanced Materials)

   Abstract Liquid crystal elastomers (LCE) are appealing candidates among active materials for 4D printing, due to their reversible, programmable and rapid actuation capabilities. Recent progress has been made on direct ink writing (DIW) or Digital Light Processing (DLP) to print LCEs with certain actuation. However, it remains a challenge to achieve complicated structures, such as spatial lattices with large actuation, due to the limitation of printing LCEs on the build platform or the previous layer. Herein, a novel method to 4D print freestanding LCEs on‐the‐fly by using laser‐assisted DIW with an actuation strain up to −40% is proposed. This process is further hybridized with the DLP method for optional structural or removable supports to create active 3D architectures in a one‐step additive process. Various objects, including hybrid active lattices, active tensegrity, an actuator with tunable stability, and 3D spatial LCE lattices, can be additively fabricated. The combination of DIW‐printed functionally freestanding LCEs with the DLP‐printed supporting structures thus provides new design freedom and fabrication capability for applications including soft robotics, smart structures, active metamaterials, and smart wearable devices. 

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4. 3D Printing of Monolithic Proteinaceous Cantilevers Using Regenerated Silk Fibroin

   https://doi.org/10.3390/molecules27072148  

   Mu, Xuan; Gonzalez-Obeso, Constancio; Xia, Zhiyu; Sahoo, Jugal Kishore; Li, Gang; Cebe, Peggy; Zhang, Yu Shrike; Kaplan, David L. (April 2022, Molecules)

   Silk fibroin, regenerated from Bombyx mori, has shown considerable promise as a printable, aqueous-based ink using a bioinspired salt-bath system in our previous work. Here, we further developed and characterized silk fibroin inks that exhibit concentration-dependent fluorescence spectra at the molecular level. These insights supported extrusion-based 3D printing using concentrated silk fibroin solutions as printing inks. 3D monolithic proteinaceous structures with high aspect ratios were successfully printed using these approaches, including cantilevers only supported at one end. This work provides further insight and broadens the utility of 3D printing with silk fibroin inks for the microfabrication of proteinaceous structures. 

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5. Digital Light Process 3D Printing of Magnetically Aligned Liquid Crystalline Elastomer Free–forms

   https://doi.org/10.1002/adma.202414209  

   Herman, Jeremy_A; Telles, Rodrigo; Cook, Caitlyn_C; Leguizamon, Samuel_C; Lewis, Jennifer_A; Kaehr, Bryan; White, Timothy_J; Roach, Devin_J (October 2024, Advanced Materials)

   Abstract Liquid crystalline elastomers (LCEs) are anisotropic soft materials capable of large dimensional changes when subjected to a stimulus. The magnitude and directionality of the stimuli‐induced thermomechanical response is associated with the alignment of the LCE. Recent reports detail the preparation of LCEs by additive manufacturing (AM) techniques, predominately using direct ink write printing. Another AM technique, digital light process (DLP) 3D printing, has generated significant interest as it affords LCE free‐forms with high fidelity and resolution. However, one challenge of printing LCEs using vat polymerization methods such as DLP is enforcing alignment. Here, we document the preparation of aligned, main‐chain LCEs via DLP 3D printing using a 100 mT magnetic field. Systematic examination isolates the contribution of magnetic field strength, alignment time, and build layer thickness on the degree of orientation in 3D printed LCEs. Informed by this fundamental understanding, DLP is used to print complex LCE free‐forms with through‐thickness variation in both spatial orientations. The hierarchical variation in spatial orientation within LCE free‐forms is used to produce objects that exhibit mechanical instabilities upon heating. DLP printing of aligned LCEs opens new opportunities to fabricate stimuli‐responsive materials in form factors optimized for functional use in soft robotics and energy absorption. 

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