A bioinspired layered hydrogel actuator vial-ascorbic acid-triggered interfacial self-growth from a stiff hydrogel†

Stimuli-responsive layered hydrogel actuators are highly attractive for broad applications in soft robots, intelligent devices, etc., owing to their softness, asymmetric responsiveness and deformability. However, current layered hydrogel actuators suffer from serious challenges such as tedious preparation, uncontrollable layer thickness and weak interfacial bonding force. Herein, we put forward a facile and highly efficient self-growing method to prepare a layered hydrogel actuator from a stiff hydrogel substrate by crafting l-ascorbic acid (Vc)-triggered interfacial radical polymerization. The redox reaction between Vc and confined Fe³⁺ in the stiff hydrogel substrate could produce Fe²⁺ catalysts, resulting in surface catalytically initiated radical polymerization (SCIRP) at room temperature. Various layered hydrogels were prepared rapidly and the thickness of the grown hydrogel layer can be accurately controlled. The obtained layered hydrogel exhibits asymmetric structural layers consisting of a dense layer and a porous layer, as well as a strong interfacial bonding force of about 250 N m⁻¹ between the porous layer and the stiff substrate. Furthermore, a thermo-responsive layered hydrogel actuator was developed, which showed reversible underwater bending ability in response to temperature changes and can also be designed as a smart manipulator to capture objects underwater. This work provides a novel and feasible approach for the highly efficient and controllable preparation of layered hydrogel actuators, which will find promising applications in the fields of soft robots, intelligent devices, sensors and so on.