In Situ Stimuli Transfer in Multi-Environment Shape-Morphing Hydrogels Based on the Copolymer Between Spiropyran and Acrylic Acid.

Smart hydrogels are considered as close mimics to the functions of biological entities. However, the stimuli-responsive performance of hydrogels is often limited by the slow diffusion process during water exchange with the surrounding environment. Here, a homogenous hydrogel composed of a water-soluble spiropyran covalently attached to the polyacrylic acid network is reported. This hydrogel demonstrates rapid and reversible shape-morphing behavior in air, underwater, or in oil. The mechanism involves the reversible protonation of spiropyran triggered by light stimuli. The release/capture of protons regulates the local proton concentration near the carboxyl groups in the polyacrylic acid network, distinguishing it from existing stimuli-responsiveness based on bulk water diffusion. The environment-independent shape-morphing performance of the unique in-situ stimuli transfer process, resulting in local water transfer amongst parts of a single piece of hydrogel is attributed. Eventually, light-controlled reversible actuation of the hydrogel is demonstrated, offering exciting possibilities for applications in flexible electronics, and soft actuators/robots.