Bilayer-type fluorescence hydrogels with intelligent response serve as temperature/pH driven soft actuators

Abstract In this study, we propose a fluorescent bilayer hydrogel soft actuator that responds quickly to multiple environmental stimuli. The two layers of hydrogel contain poly(N-isopropylacrylamide) and poly(2-(dimethylamino) ethyl methacrylate), respectively, which enable their mechanical response to temperature and pH. Additionally, both hydrogel layers contain poly(ethylene oxide) diacrylate, which serves as the crosslinker. Thanks to the anisotropic swelling of two layers, the bilayer hydrogel structure exhibits rapid, reversible, and repeatable bending motion, under varying temperature or pH. In addition, a photo-sensitive coumarin-based unit is introduced into the hydrogel network to add fluorescence. A four-arm gripper made of hydrogel bilayers demonstrates rapid actuation in response to both temperature and pH. Furthermore, we design a circuit switch using the hydrogel bilayer to serve as one “bidirectional alarm” and selectively turn on light emitting diodes in different media. This work demonstrates the applications of the hydrogel bilayer as a soft actuator and a circuit switch in ionic circuitry, enabling potential applications in a wide range of fields, including biomimetic devices, biocompatible/medical devices, and environmental sensors.