A Water State Manipulation Strategy for Ultra-Stiff Yet Highly Sensitive Hydrogels

Abstract

Hydrogels in water or aqueous solutions swell or shrink unidirectionally until reaching an equilibrium state, which has been extensively studied through both experimental analysis and theoretical prediction. In this work, an unexpected concurrent swelling and deswelling behavior of hydrogels is reported, which manipulates the water state in the polymer networks. The osmotic pressure-driven rapid deswelling and the ionic electrostatic repulsion-driven slow swelling of hydrogels occur simultaneously, affording double-layered polyacrylamide (PAM) hydrogels, in which the rigid inner layer does not contains free water, while the soft outer layer comprises both bound and free water. The PAM hydrogels show uniaxial tensile fracture strengths and Young's moduli up to 104.2 MPa and 2.4 GPa, respectively, outperforming various polymeric and biological structural materials. This strategy is applicable to the general molding and three-dimensional (3D) printing techniques, providing wide possibilities for engineering applications. Such strong and rigid hydrogels can simultaneously serve as structural materials for load-bearing and act as functional materials for super-stress (25 to 6000 N) and highly adaptable tactile sensing. These findings enrich the hydrogel swell/deswelling theory and broaden the engineering applications of hydrogels as structural materials.