Bioinspired hydraulic anisotropic hydrogel actuators for artificial joints

Spiders exhibit remarkable agility due to their unique hydraulic actuation system in the leg joints, enabling them to capture prey and construct intricate webs. Despite the folding structure of spider articular membranes can be replicated, accurately simulating both structural features (e.g., water content, anisotropy) and functional properties (e.g., actuation strain, stress generation rate) of spider joints remains a challenge. Herein, inspired by the articular membranes of spiders, we present a miniaturized hydraulic actuator featuring a bellows-like structure, achieved through a periodic arrangement of anisotropic hydrogel coils. Benefiting from this unique structure, tubular hydrogel actuators can achieve an impressive actuation strain of 447% in the axial direction under hydraulic pressure. When integrated into a notched Teflon tube serving as the exoskeleton, the assembled artificial joints enable reversible extension-bending deformation under hydraulic pressure, with a rapid, repeatable stress generation speed of 0.61 MPa s⁻1. We further demonstrate that our miniaturized hydrogel-based carrier robots can transport loads weighing up to 1,200 times their weight in various non-aqueous environments. Our strategy not only offers a novel approach for designing bionic anisotropic hydrogels for artificial joints but also advances the development of hydraulic actuators as microscale soft robots.