Ionic coordination strengthening of temperature-driven gradient hydrogel actuators with rapid responsiveness

Emerging applications of stimuli-responsive hydrogels as soft actuators and robotics have attracted great attention, but they face challenges in practical applications due to their poor mechanical properties, low driving force, and low actuation speed. Herein, we presented tough hydrogel actuators composed of a copolymer of N-isopropylacrylamide (NIPAM) and 3-sulfopropyl methacrylate potassium salt (SPA) monomers, and tunicate cellulose nanocrystals (TCNCs). The negatively charged TCNCs as nanofillers were gradient arranged by a direct current electric field (DC-EF) via electrophoresis. After in situ polymerization, TCNCs gradient distributed across the thickness of hydrogel, resulting in the formation of gradient crosslinking density of networks. Hydrogel actuators with high strength were obtained via Zr4+/−SO3- ionic coordination, whose tensile strength was about 200 times of hydrogel without ionic coordination. These hydrogel actuators exhibited rapid bending velocity, excellent stability, and good cycling performance in response to temperature. Importantly, these hydrogel actuators could be designed as soft robots to lift and transport objects by changing environment temperature. This work provided a facile yet efficient strategy to fabricate soft actuators with high strength and rapid responsiveness.