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 are poorly understood. Herein, we describe how pH influences the mechanical performance of materials formed by reacting synthetic catechol polymers with Fe³⁺. 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 Fe³⁺-catechol coordination bonds. Our 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.