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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 | M A Y / J U N E 2 0 1 9 5 6 3D PRINTSHOP MORPHING METAMATERIALS Engineers at Rutgers University– New Brunswick are using 4D printing to create flexible, lightweight materials that could lead to better shock absorp- tion, morphing airplane wings, soft ro- bots, and implantable medical devices. In addition to changing shape, these smart materials can go from as stiff as wood to as soft as a sponge. 4D printing is based on the same layering technol- ogy as 3D printing, with one big differ- ence: It uses special materials and de- signs to print objects that change shape with environmental conditions such as temperature acting as a trigger. Time is the fourth dimension that allows these materials to take on a new shape. The Rutgers team can tune their plastic-like materials with heat, so they stay rigid when struck or become soft as a sponge to absorb shock. Stiffness can be adjusted more than 100-fold in temperatures from 73° to 194°F, allowing precise control of shock absorption. The mate- rials could be used in airplane or drone wings that change shape to improve performance, and in lightweight structures that are collapsed for space launches and reformed in space for a larger structure, such as a solar panel. Soft robots made of flexible and rubbery materials inspired by the octopus could have variable flex- ibility or stiffness tailored to the environment and application. Tiny devices implanted in people for diagnosis or treatment could be temporarily made soft and flexible for minimally invasive and less painful insertion into the body. rutgers.edu. USING LIGHT TO BUILD MULTI-MATERIAL DESIGNS 3D-printing techniques typically produce parts made of one material at a time. However, more complex appli- cations could be developed if printers could use different feedstocks to build multi-material parts. Scientists at the University of Wisconsin (UW)–Madison developed a novel 3D printer that uses patterns of visible and ultraviolet light to dictate which of two monomers are polymerized to form a solid material. Different patterns of light provide the spatial control necessary to yield multi-material parts. Chemistry professor A.J. Boyd- ston and graduate student Johanna Schwartz knew that improved printing materials required a chemical approach to complement engineering advances. Most multi-material 3D printing meth- ods use separate reservoirs to get dif- ferent materials in the right positions. Boydston realized that a one-vat, multi- ple-component approach—similar to a chemist’s one-pot approach when syn- thesizing molecules—would be more practical than multiple reservoirs with different materials. This approach is based on the ability of different wave- lengths of light to control which starting materials polymerize into different sec- tions of the solid product. Researchers createmultiple digital images that produce a 3D design when stacked. Then they simultaneously di- rect light from two projectors toward a vat of liquid starting materials, where layers are built one-by-one on a plat- form. After each layer is constructed, the build platform moves up and light helps create the next layer. The team’s unique approach to multi-material 3D printing could enable designers, art- ists, and engineers to build significantly more complex systems. wisc.edu . Graduate student Johanna Schwartz built this multi-material printing setup. Courtesy of UW. 4D-printedmetamaterials can be temporarily transformed into any shape and returned to their original formwhen heat is applied. Scale bar represents 2 mm. Courtesy of Rutgers/C. Yang.