Bilayer hydrogel mixed composites that respond to multiple stimuli for environmental sensing and underwater actuation

Bilayer-type hydrogel composites that respond to multiple stimuli provide a useful approach for manufacturing soft actuators and intelligent sensors. In this study, we propose two types of bilayer hydrogel composites (referred to as N type and D type). The N type and the D type separately consist of poly(N-isopropylacrylamide) (PNIPAM) and poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) bilayer hydrogels, with acrylamide (AM) as the co-monomer. The N type hydrogel composite exhibits oriented bending as temperature increases or as environmental salt concentration increases. The D type also presents oriented bending as temperature increases. Additionally, it demonstrates bending and changes color based on the pH of the surrounding solution, since it is doped with an acid-based pH-indicator. In a demonstration, we present several straight rods composed of a mixture of N and D type hydrogel composites that are able to morph into the letters SOS under environmental changes (temperature or pH). This can be used as a visual environmental indicator, or as an alarm. Furthermore, we show structures composed of a mixture of N and D composites imitating the flexible motions of a ballet dancer or a Tai Chi master. These mixed bilayer polymer composites may be used for applications in soft robotics, biomimetic devices, and environmental sensors.