Differential diffusion driven far-from-equilibrium shape-shifting of hydrogels

Far-from-equilibrium (FFE) conditions give rise to many unusual phenomena in nature. In contrast, synthetic shape-shifting materials typically rely on monotonic evolution between equilibrium states, limiting inherently the richness of the shape-shifting behaviors. Here we report an unanticipated shape-shifting behavior for a hydrogel that can be programmed to operate FFE-like behavior. During its temperature triggered shape-shifting event, the programmed stress induces uneven water diffusion, which pushes the hydrogel off the equilibrium based natural pathway. The resulting geometric change enhances the diffusion contrast in return, creating a self-amplifying sequence that drives the system into an FFE condition. Consequently, the hydrogel exhibits counterintuitive two opposite shape-shifting events under one single stimulation, at a speed accelerated by more than one order magnitude. Our discovery points to a future direction in creating FFE conditions to access otherwise unattainable shape-shifting behaviors, with potential implications for many engineering applications including soft robotics and medical devices.

Yue Zhang, Kangkang Liu, Tao Liu, Chujun Ni, Di Chen, Jiamei Guo, Chang Liu, Jian Zhou, Zheng Jia, Qian Zhao, Pengju Pan, Tao Xie