4D printing of reversible shape morphing hydrogel structures

A series of hydrogel-based inks are developed to print 3D structures capable of reversible shape deformation in response to hydration and temperature. The inks are made of large polymer chains and UV curable monomers which form interpenetrating polymer networks after polymerization. By taking advantage of the long polymer chains in the ink formulations, it is possible to adjust the rheological properties of the inks to enable 3D printing. Hydrogels produced from the inks exhibit robust mechanical performance with their mechanical properties controlled by the nature of the long polymer chains within their networks. In this paper, hydrogel hinges are made from various ink formulations and a simple model is developed to predict their bending characteristics, including the bending curvature and bending angle. This model can be used as a guide to determine optimal parameters for a wide range of materials combination to create all-hydrogel structures that undergo desired shape transitions.