Engineering Elasticity and Relaxation Time in Metal-Coordinate Cross-Linked Hydrogels

Reversible cross-links between polymer chains are a promising avenue to engineer the mechanical properties of soft materials and in particular hydrogels. Such developments are however challenged by the complexity of these materials, which unlike conventional, permanently cross-linked gels involve multiple relaxation time scales. To address this challenge, we study a model system of tetra-arm poly(ethylene glycol) hydrogels transiently cross linked by reversible histidine:Ni2+ coordinate complexes and explore the separate influences of polymer structure and cross-link density on the time-dependent hydrogel rheology. We show that the characteristics of the polymer matrix primarily control the hydrogels' static elasticity, implying that its dynamics are largely governed by coordinate-bond rearrangement kinetics rather than polymer relaxation time scales. By contrast, the ion concentration has a strong influence on both the hydrogel's statics and dynamics, and we quantitatively account for the former using a simple model based on the known equilibrium bonding properties of histidine:Ni2+ complexes. Our findings establish specific engineering principles for the viscoelastic mechanics of metal-coordinate hydrogel materials, thus opening new perspectives for the optimization of their use in (bio)functional applications.