Mussels Put Their Best “pH”oot Forward: Importance of pH in Formation of Biological and Bio‐Inspired Materials

Mussel byssus offers bio‐inspired designs for advanced adhesives, coatings, and supramolecular hydrogels. Solidification of secreted proteins into mechanically robust fibers is triggered by a pH jump from acidic fluid condensates to basic seawater conditions driving protein crosslinking by metal coordination with 3,4‐dihydroxyphenylalanine (DOPA). Knowledge of the dynamic and localized pH changes during secretion is currently lacking. Yet, this information is crucial for controlling the gelation and properties of mussel‐inspired DOPA‐modified materials. Here, an iridium oxide‐based pH microsensor combining high mechanical stability and spatial resolution is used to measure the acidic pH during byssus secretion and to map mussel‐inspired hydrogels revealing compositional heterogeneity resulting in mechanically distinct regions. Within the hydrogel, high pH values >12 are measured which are detrimental to DOPA due to its strong propensity for oxidation, which adversely alters gel properties. This demonstrates the need to improve preparation methods of mussel‐inspired adhesive materials to more closely mimic native processes. An iridium oxide‐based pH microsensor with a large linear range, fast response time, and low interference is fabricated to track localized pH changes in mussel and mussel‐inspired adhesive materials. An acidification of ≈4 pH units is recorded inside the mussel foot distal depression. Mapping of pH variations within the mussel‐inspired hydrogels indicates current manual mixing methods for preparing mussel‐inspired hydrogels tend to cause irreversible structural changes and local differences in mechanical properties.