Sundew-Inspired Simultaneous Actuation and Adhesion/Friction Control for Reversibly Capturing Objects Underwater

The development of intelligent underwater soft actuators by using stimuli-responsive polymer materials is of immense scientific interest because of its wide application potential. Inspired by the hunting mechanism of sundews in nature, having rosettes of leaves that are sticky and capable of actuating, a novel structural hydrogel actuator made by high mechanical strength hydrogel poly(N-isopropyl acrylamide-acrylic acid)/Fe (PNIPAAm/PAAc-Fe) with asymmetric structures in its two sides is fabricated, which is defined as structural hydrogel film. The as-prepared hydrogel actuator can demonstrate reversible bending in response to temperature change (below/above the lower critical solution temperature of PNIPAAm) or encountering fast solvent exchange between ethanol and water, along with the switching of surface friction/adhesion force. During responsive bending, the actuator can generate large actuation force of ≈21 mN. Meanwhile, the combination of fast-responsive bending, switchable surface friction, and strong actuation force endows the actuator excellent capacity to reversibly catch and release objects underwater. Especially, the existing fibrillar hydrogel pillars on actuator makes it possible to be used as a manipulator for realizing precise transportation of small objects underwater. The current actuator shows fantastic actuation capacity similar to natural sundews, and provides inspiration for developing novel soft robotics underwater.