Autonomous Spatiotemporal Regulation of Reversible Hydrogel Actuators by Chemical Reaction Networks.

Achieving autonomous spatiotemporal regulation of reversible hydrogel movement remains challenging, as existing approaches typically rely on external regulation or irreversible actuation mechanisms. This work presents the autonomous spatiotemporal regulation of reversible hydrogel actuators using thiol-based chemical reaction networks (CRNs). The core innovation is a bilayer actuator whose active layer is functionalized with phenylcyanoacrylate Michael acceptors. In their initial state, these groups are hydrophobic, keeping the active layer moderately collapsed. Binding to thiols, such as cysteamine or thiocholine, converts the acceptors into more hydrophilic adducts. This change drives water uptake, causing the active layer to swell and the entire structure to bend. The screening revealed that a methoxy-substituted acceptor provides the optimal balance of binding strength and reversibility for robust actuation. Autonomous control of reversible actuation is achieved by an autocatalytic, thiol-producing CRN coupled with negative feedback in the form of slow oxidation of thiols or their irreversible addition to acrylamides. In the hydrogel, this CRN generates a wave of thiols, which causes forward and reverse actuation. This principle is demonstrated with linear, flower-shaped, and hand-like actuators. This integrated system, which unites reversible chemistry with spatiotemporal control, takes a significant step toward emulating the autonomy of motion found in living systems.