Dynamic-Covalent Origins of Strain-Stiffening in Synthetic Hydrogels

The role of dynamic bonds in cross-link-driven strain-stiffening of synthetic hydrogels has remained unresolved. This study compares the rheological behavior of covalently cross-linked hydrogels formed via irreversible thiol–maleimide bonds to dynamic-covalent hydrogels cross-linked through thia-Michael chemistry, demonstrating that dynamic cross-links are essential for hydrogel strain-stiffening. Modulation of pH and temperature to tune the kinetics of the thia-Michael reaction reveals a direct relationship between cross-link dynamicity and the extent of strain-stiffening. Specifically, the stiffening index scales as a power law with the reverse rate constant of cross-link exchange. These findings establish a role for dynamic cross-linking in strain-stiffening of synthetic hydrogels and provide a framework for the design of adaptive, biomimetic materials with tunable mechanical properties.