  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.
After stretching and releasing, the acquired 3D shape recovers its original
flat shape on a timescale originating from the polymer viscoelasticity. 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 findings present a new frontier in dynamic biomimetic
shape-morphing by exploiting intrinsic material properties in contrast with
state-of-the-art methods relying on external field variations, moving one step
closer to acquiring autonomous shape-shifting capabilities of biological
systems.
