Soft-tough polyacrylamide and polyacrylamide/polyvinyl alcohol/sodium alginate bilayer hydrogels for adaptive...

Human skin achieves high sensitivity and a wide range of tactile perception through its gradient structure and the synergistic function of various receptors. However, significant challenges remain in achieving multifunctional synergy enhancement for artificial skin. This study develops a soft-tough bilayer gradient hydrogel based on a salting-out and ion coordination toughening strategy, achieving high sensitivity (7.35 kPa^-1, 0-3 kPa) and a broad stress sensing range (0-75.44 kPa). The soft layer is designed with a low-density polyacrylamide network, where weak chemical crosslinking improves both elasticity and sensitivity. Meanwhile, the tough layer is formed by a high-density dual crosslinked network of polyacrylamide /polyvinyl alcohol/sodium alginate, followed by post-treatment in ZnSO₄ solution. The combined effects of salting-out and ion coordination greatly enhance toughness, extend the stress detection range, and strengthen interlayer adhesion. Furthermore, this study integrates the hydrogel into a robotic arm and develops a gel-controller-computer tactile feedback system that simulates the conditioned reflex mechanism, enabling non-destructive recognition and precise grasping of objects with varying hardness. Moreover, the bilayer gradient hydrogel-based sensor facilitates comprehensive recognition of an object's weight and shape. This research offers a novel approach to advancing artificial skin, bioinspired robotics, and intelligent sensing technologies.