Mechanically adaptive crack-resistant hydrogels based on strain-induced macroscopic phase separation and hierarchical...

It remains a significant challenge to simultaneously achieve high stiffness, large extensibility, and strong resistance to crack propagation in hydrogels. Here, we report a hydrogel that undergoes a fully reversible strain-induced whitening transition with a high modulus ( ~ 30 MPa), high tensile strength (~15 MPa), large stretchability ( > 1000%). Importantly, the hydrogel achieves an exceptional fracture toughness around 100 kJ m^-2 and exhibits the ability to autonomously suppress crack growth under load. The hydrogel is constructed from a strong polycation and a weak polyanion at an asymmetric molar ratio, producing a transparent and initially isotropic network containing ionic-bond-rich and hydrogen-bond-rich nanodomains. In situ X-ray scattering measurements, combined with molecular dynamics simulations, reveal a multiscale structural transition under tensile deformation. Specifically, yielding of the hydrogen-bond-rich soft domains enhances interdomain contrast, inducing whitening, together with orientation and adaptive dissociation of ionic-bond-rich domains. This process forms an extended anisotropic dissipative zone, thus effectively blunting and arresting cracks. This work demonstrates how compositional asymmetry enables a macroscopically uniform and transparent hydrogel to autonomously evolve anisotropic energy-dissipation pathways under strain, providing an efficient strategy for creating damage-tolerant soft materials for extreme mechanical environments.