PH-based regulation of hydrogel mechanical properties through mussel-inspired chemistry and processing

The mechanical holdfast of the mussel, the byssus, is processed at acidic pH yet functions at alkaline pH. Byssi are enriched in Fe³⁺ and catechol-containing proteins, species with chemical interactions that vary widely over the pH range of byssal processing. Currently, the link between pH, Fe³⁺-catechol reactions, and mechanical function is poorly understood. Herein, it is described how pH influences the mechanical performance of materials formed by reacting synthetic catechol polymers with Fe ³⁺. Processing Fe³⁺-catechol polymer materials through a mussel-mimetic acidic-to-alkaline pH change leads to mechanically tough materials based on a covalent network fortified by sacrificial Fe ³⁺-catechol coordination bonds. These findings offer the first direct evidence of Fe³⁺-induced covalent cross-linking of catechol polymers, reveal additional insight into the pH dependence and mechanical role of Fe³⁺-catechol interactions in mussel byssi, and illustrate the wide range of physical properties accessible in synthetic materials through mimicry of mussel-protein chemistry and processing. Novel bioinspired and tough hydrogels are designed by regulating the pH of the reaction between catechol-terminated poly(ethylene glycol) (PEG) and Fe³⁺. A covalently cross-linked network is fortified with a series of coordination bonds, which act as sacrificial and reversible interactions to dissipate energy during deformation. These hydrogels represent a novel class of mussel-mimetic biomaterials inspired in both content and processing.