Dynamic, Viscoelasticity-Driven Shape Change of Elastomer Bilayers

Thin bilayers made of elastic sheets with different strain recoveries can be used for dynamic shape morphing through ambient stimuli such as temperature, mass diffusion, and light. As a fundamentally different approach to designing temporal shape change, constituent polymer molecular features (rather than external fields) are leveraged, specifically the viscoelasticity of gelatin bilayers, to achieve dynamic three-dimensional (3D) curls and helical twists with curvatures as high as 1.25 cm^–1 when the strain difference between the layers is 0.45 cm/cm. After stretching and releasing, the acquired 3D shape recovers its original flat shape on a time scale originating from the polymer viscoelasticity. The recovery time is found to be dependent on formulation and applied strain such that the recovery times at an applied strain of 1 cm/cm are about 2 and 10 s when there is more and less water plasticizer, respectively. The bilayer-time-dependent curvature can be accurately predicted from hyperelastic and viscoelastic functions using finite element analysis (FEA). FEA reveals the nonlinear shape dynamics in space and time, in quantitative agreement with experiments. The bilayers exploit intrinsic material properties in contrast with state-of-the-art methods relying on external field variations, moving one step closer to acquiring the autonomous shape-shifting capabilities of biological systems for building engineered devices.