Mineral-Enhanced Polyacrylic Acid Hydrogel as an Oyster-Inspired Organic–Inorganic Hybrid Adhesive

Underwater adhesion is crucial to many marine life forms living a sedentary lifestyle. Amongst them, mussel adhesion has been mostly studied, which inspires numerous investigations of 3,4-dihydroxyphenylalanine (DOPA)-based organic adhesives. In contrast, reef-building oysters represent another important "inorganic" strategy of marine molluscs for adhesion by generating biomineralized organic-inorganic adhesives, which is still rarely studied and no synthetic analogues have ever been reported so far. Here, a novel type of oyster-inspired organic-inorganic adhesive based on a biomineralized polyelectrolyte hydrogel is reported, which consists of polyacrylic acid physically cross-linked by very small amorphous calcium carbonate nanoparticles (<3 nm). The mineral-enhanced polyelectrolyte hydrogel adhesive is shown to be injectable, reusable, and optically clear upon curing in air. Moreover, comparable adhesion performance to DOPA-based adhesives is found for the hydrogel adhesive in both dry and wet conditions, which can even be further enhanced by introducing a small amount of second large cross-linker such as negatively charged nanoparticles. The present mineral hydrogel represents a new type of bio-inspired organic-inorganic adhesive that may find a variety of potential applications in adhesive chemistry.