Printed shape-morphing materials

For applications from electronics to tissue engineering, researchers have devised materials that distort into complex shapes in response to heat, light, or other stimuli. Sometimes the goal is to create a device that undergoes a reversible, controllable change; other times it’s simply to manufacture three-dimensional shapes that are difficult or impossible to make in any other way. Typical approaches require exerting exquisite control over the spatially varying material composition or the local orientation of an embedded liquid crystal (see the article by Carl Modes and Mark Warner, Physics Today, January 2016, page 32 ). Now a team of Harvard researchers has developed a simpler method using a 3D printer. Doctoral student Sydney Gladman and her adviser Jennifer Lewis designed a viscoelastic ink composed of cellulose fibrils embedded in a hydrogel. Shear forces in the printer nozzle cause the fibrils to align parallel to the extruded hydrogel filament. Immersing the material in water makes the hydrogel expand; thanks to the fibrils’ stiffness, each filament swells just 10% in length but 40% in diameter. When the extruded filament is laid out on a surface in a spiral or crisscross pattern, the combination of forces causes the hydrated structure to buckle out of the plane. Collaborators Elisabetta Matsumoto and L. Mahadevan developed a mathematical model to predict what 3D structure would form from a particular printed pattern—and, more importantly, what print path should be used to create a desired 3D structure. Together, the researchers demonstrated the power of their technique by creating a variety of biologically inspired shapes with spatially varying curvature, such as the calla lily flower shown here. (A. S. Gladman et al., Nat. Mater., in press, doi:10.1038/nmat4544 .)