Harnessing Chain Mobility via Protonation for Tough and Isotropic Hydrogel

Fabricating hydrogels with isotropically high tensile strength, stretchability, and toughness is crucial for applications in tissue engineering, stretchable bioelectronics and soft robots. However, many toughening strategies, including mechanical training, directional freezing, and solvent exchange, often induce anisotropy or fail to enhance all these metrics simultaneously. Herein, we report a strategy to fabricate ultra-tough, isotropic poly(vinyl alcohol) (PVA) hydrogels by synergistically modulating polymer chain mobility and physical crosslinking through sequential acidification, freeze-thawing, and salting-out. Acidification protonates the hydroxyl groups, suppressing premature interchain hydrogen bonding and promoting network homogenization. Subsequent salting-out deprotonates the hydroxyl groups to strengthen the interpolymer hydrogen bonds, forming crystalline domains that act as strong, reversible physical crosslinks. The resulting hydrogel achieves a high tensile strength of 29.5 MPa, stretchability of 2683%, and record-high toughness of 424 MJ m^-3 among isotropic hydrogels, even surpassing most anisotropic hydrogels in their reinforced direction. This strategy offers a generalizable platform for engineering tough, isotropic hydrogels with broad potential across bioengineering, additive manufacturing, and soft robotics.