PNA-Bi ₂ S ₃ @PDA Hydrogel with Dual High Mechanical Strength and Rapid Photothermal Actuation: Preparation and Performance

Inspired by stimulus-responsive motions in natural organisms, numerous hydrogel actuators with diverse functionalities have been fabricated for soft robotics applications. However, integrating high mechanical strength with rapid shape-memory response remains a fundamental challenge in hydrogel-based actuation systems. This study addresses this gap by developing a photothermal-responsive poly(<i>N</i>-isopropylacrylamide)-acrylic acid (PNA) hydrogel incorporated with polydopamine-functionalized Bi₂S₃ nanoparticles (denoted as PNA-Bi₂S₃@PDA), which exhibits synergistic enhancement in mechanical robustness and actuation kinetics. The system leverages the narrow-bandgap semiconductor properties of Bi₂S₃ for efficient near-infrared photothermal conversion, while the surface-grafted PDA layer enables multifunctional hydrogen bonding with the PNA network. This dual-interaction strategy ensures homogeneous nanoparticle dispersion and achieves remarkable mechanical reinforcement─exhibiting 618% elongation at break and 183 kPa tensile strength, representing a 4.5-fold enhancement compared to unmodified hydrogels. The hydrogel demonstrates rapid programmable deformation under thermal and photonic stimuli, including 384° bending within 15 s in hot water (70 °C) and 73° bending within 30 s under NIR irradiation (808 nm, 0.6 W cm^-2). Furthermore, it shows significant potential for applications in programmable actuation, temperature-regulated fluidic valves, thermal-responsive circuit switches, and soft robotic manipulators. Notably, its response speed surpasses conventional PNIPAM-based hydrogel actuators, providing a new paradigm for developing high-performance underwater photoresponsive soft actuators and advancing the field of intelligent flexible materials.