Dual-Responsive Hydrogel Actuators with Anisotropic Metal Ion Cross-Linking and Polydopamine-Coated Cellulose Nanocrystals as Photothermal Fillers

Nature-inspired stimuli-responsive hydrogels hold great potential for advanced functional applications in soft robotics, tissue engineering, and bionic systems. Here, we report anisotropic poly(N-isopropylacrylamide)-based hydrogel actuators incorporating polydopamine-coated cellulose nanocrystals (PDACNCs) for simultaneous photothermal conversion and mechanical reinforcement combined with a two-step metal ion cross-linking strategy targeting carboxylate groups in the network. Uniform Al3+ cross-linking enhanced the stiffness and toughness of the hydrogels, while controlled Fe3+ diffusion produced cross-linking density gradients that induced anisotropic swelling and deswelling. Optimizing the Al3+ and PDACNC contents yielded the best balance between mechanical robustness and actuation speed. Under thermal stimulation, the actuators achieved bending angles of up to 280° within 0.3 min, whereas higher Al3+ or PDACNC levels slowed the response and reduced the deformation. Patterned Fe3+ diffusion enabled diverse programmable deformation modes including bending, twisting, and folding. Moreover, the photothermal effect of PDACNCs facilitated rapid, remote actuation under near-infrared irradiation. This work presents a versatile design platform for dual-responsive hydrogel actuators with tunable performance and complex, programmable shape transformations.