Viscoelasticity-driven dynamical shape shifting of soft bilayer strips

Elastic bilayers with different strain recoveries can be used for dynamical shape shifting through ambient stimuli, such as temperature, mass diffusion, and light. As a fundamentally different approach to designing temporal shape change, we leverage constituent polymer molecular features (rather than external fields), specifically the viscoelasticity of gelatin bilayers, to achieve dynamical three-dimensional (3D) curls and helical twists. After stretching and releasing, the acquired 3D shape recovers its original flat shape on a timescale set by polymer viscoelasticity. The time-dependent bilayer curvature can be predicted from hyperelastic and viscoelastic functions: In one incarnation, the time-dependent strain obtained from modified creep recovery experiments is used in the Timoshenko bilayer model to predict curvature. In another incarnation, viscoelastic parameters found by fitting the stress-strain and the stress relaxation data are used in finite-element analysis to predict the nonlinear shape dynamics in space and time in quantitative agreement with experiments. Our framework leverages material properties as opposed to external field variations for biomimetic shape change, bringing us one step closer to acquiring autonomous shape-shifting capabilities of living systems.