Bioinspired hydrogel actuators with multiple and synergistic responses towards a single stimulus

Abstract

In nature, many living creatures coordinate multiple parts of the body like color, shape or posture in response to external stimulus. This inspires scientists to develop fluorescent hydrogels with stimuli-responsive behaviors for the creation of artificial actuators. However, most reported fluorescent hydrogels need more than two kinds of stimuli to achieve abundant macro changes, which is far from the intelligent and cooperative characteristics of living creatures. Here, we construct a smart fluorescent hydrogel by polymerizing sodium 4-styrenesulfonate (NaSS), (Z)-N-(4-(1-cyano-2-(4-(diethylamino)phenyl)vinyl)-phenyl)methacrylamide (NE) and acrylamide (AM). Under acidic conditions, the gel volume decreases due to the polyelectrolyte effect and acid/base responsiveness of benzenesulfonate groups from NaSS. Meanwhile, the protonation process of N, N-diethylaniline group from NE increases the solubility and breaks the intramolecular charge transfer (ICT) process. Therefore, a smart fluorescent hydrogel with synergistic shape, color, fluorescence and transparency response behaviors towards a single acid stimulus was successfully constructed. This hydrogel is further made into intelligent actuators with bilayer structures, which provides new insights in the design of biomimetic systems.