Tough and Rapidly Relaxing Hydrogels Via Programmable Crosslink Kinetics.

Replicating the synergy of high toughness and rapid stress relaxation found in native tissues remains a central challenge for synthetic hydrogels on account of their intrinsic mechanical-temporal trade-off. Here we introduce a supramolecular hydrogel platform that leverages kinetic programming to precisely regulate crosslink dynamics through molecular dissociation kinetics. This molecular design allows independent tuning of relaxation dynamics and fracture toughness, decoupling properties that are typically correlated. The resulting hydrogels exhibit stress relaxation ( t 1 / 2 ${t}_{1/2}$ = 0.1-100 s) two orders of magnitude faster than conventional networks while achieving exceptional fracture energy ( G c = 14 , 500 J m - 2 $G_c = 14{,}500\,\mathrm{J\,m^{-2}}$ ), well above natural rubber. Slowing crosslink dissociation significantly enhances energy dissipation under load, revealing a kinetic principle for toughening viscoelastic networks. This work establishes a molecular blueprint for designing soft materials with programmable, time-dependent mechanics.