Robust Hydrogel Actuators Functioning in Multi‐Environments Enabled by Thermo‐Responsive Polymer Nanoparticle Coatings on Hydrogel Surfaces

Hydrogel actuators with anisotropic structures exhibit reversible responsiveness upon the trigger of various external stimuli, rendering them promising for applications in many fields including artificial muscles and soft robotics. However, their effective operation across multiple environments remains a persistent challenge, even for widely studied thermo-responsive polymers like poly(N-isopropyl acrylamide) (PNIPAm). Current attempts to address this issue are hindered by complex synthetic procedures or specific substrates. This study introduces a straightforward methodology to grow a thin, dense PNIPAm nanoparticle layer on diverse hydrogel surfaces, creating a highly temperature-sensitive hydrogel actuator. This actuator demonstrates adaptability across various environments, including water, oil, and open air, owing to its distinct structure facilitating self-water circulation during actuation. The thin PNIPAm layer consists of interconnected PNIPAm nanoparticles synthesized via in situ interfacial precipitation polymerization, seamlessly bonded to the hydrogel substrate through an interfacial layer containing hybrid hydrogel/PNIPAm nanoparticles. This unique anisotropic structure ensures exceptional structural stability without interfacial delamination, even enduring harsh treatments such as freezing, ultrasonic irradiation, and prolonged water immersion. Remarkably, PNIPAm films on hydrogel surfaces which enable programmable 3D actuation can also be precisely patterned. This synthetic approach opens a novel pathway for fabricating advanced hydrogel actuators with broad-ranging applications.