Multi-responsive hydrogel actuator with photo-switchable color changing behaviors

Hydrogel actuators, in addition to their programmable shape deformations triggered by external stimuli, are expected to integrate multi-functionality in response to multiple stimuli from the environment and to mimic the biology systems. In this work, inspired by animals (i.e. cephalopods or chameleon) that can change the color of the skin to mimic the environment, a hydrogel actuator was designed to possess both solvent and thermoresponsive bending properties as well as photo-switchable color changing behaviors. This hydrogel actuator consists of a poly (N-isopropylacrylamide) (PNIPAAm) layer and a PNIPAAm layer with spiropyran moiety (PNTPAAm-SP). Spiropyran, a hydrophobic and photochromic compound, endowed the bilayer gel with solvent and temperature triggered reversible bending behaviors as well as the reversible displaying of different patterns that can be drawn or erased by light. Moreover, accompanying with its shape deformation, the bilayer gel showed a change in fluorescent behavior, i.e. its fluorescent intensity increased upon switching the solvent from DI water to EtOH or increasing the temperature. The potential applications of this bilayer hydrogel in biomimetic devices, gripper, and information storage were demonstrated. With complicated 3D shape-morphing as well as color changing ability integrated in one bilayer gel system, this bilayer hydrogel may find its broad application including bio-mimetic robotics, wearable devices, biocompatible/medical devices, and environmental sensors.