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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 0 8 0 3D PRINTSHOP 3D-printed liquid crystal elastomer spinal cage with porous lattice architecture. THE PATH TO 4D PRINTING Soft robots and biomedical im- plants that reconfigure themselves on demand are one step closer to reality due to a new way to print shapeshifting materials. Rafael Verduzco and gradu- ate student Morgan Barnes of Rice Uni- versity’s Brown School of Engineering, Houston, developed a method to print objects that can be manipulated to take on alternate forms when exposed to changes in temperature, electric cur- rent, or stress. The researchers consider this reactive 4D printing. They first re- ported their ability to make morphing structures in a mold in 2018. But using the same chemistry for 3D printing limited structures to shapes that stay in the same plane, meaning that no bumps or other complex curves could be programmed as an alternate shape. Overcoming the limitation to decouple the printing process from shaping is a significant step toward more useful materials, says Verduzco. The lab’s challenge was to create a liquid crystal polymer ink that incorporates mu- tually exclusive sets of chemical links be- tween molecules. One establishes the original printed shape while the other can be set by physically manipu- lating the printed-and- dried material. Curing the alternate form under ultraviolet light locks in those links. Once the two programmed forms are set, thematerial can thenmorph back and forthwhen it’s heated or cooled, for example. The team had to find a polymer mix that could be printed in a catalyst bath and still hold its original programmed shape. One re- maining limitation of the process is the ability to print unsupported structures, like columns. To do so would require a solution that gels just enough to sup- port itself during printing. Gaining this ability would allow researchers to print far more complex shape combinations, a future research goal. rice.edu . 3D-PRINTED MATERIAL MIMICS CARTILAGE Scientists at the University of Col- orado Denver, led by mechanical engi- neering professor Chris Yakacki, are the first to report 3D printing a complex, po- rous lattice structure using liquid crystal elastomers (LCEs), creating devices that can mimic cartilage and other biologi- cal tissues. Until now, most researchers working with LCEs could create either large objects with minimal detail or tiny objects with high detail. Yakacki’s team used a combination of chemicals and printing processes that significantly re- duced the difficulty. The scientists used a process called digital light processing (DLP) and developed a honey-like LC resin that cures when hit with ultraviolet light, forming new bonds in a succession of thin photopolymer layers. The final cured resin forms a soft, strong, and compliant elastomer. When printed in lattice structures, the resin begins to mimic cartilage. The combination of the LCE resin and printing process led to 12 times greater rate dependence and up to 27 times greater strain energy dissipation compared to those printed from a commercial photocurable elas- tomer resin. The new structures have many potential applications, such as shock-absorbing helmet foam, biomed- ical implants for toes, and numerous possibilities in the spine. ucdenver.edu . Shapeshifting materials made with a 3D printer morph from their original form to an alternate shape through changes in temperature, electric current, or stress. Courtesy of Verduzco Laboratory/Rice University. Researchers at the DOE’s Oak Ridge National Laboratory, Tenn., are refining their design of a 3D-printed nuclear reactor core, scaling up the additive manufacturing process necessary to build it and developing methods to confirm the reliability of its printed components. The lab aims to turn on the Transformational Challenge Reactor by 2023. ornl.gov. BRIEF