Wet adhesives inspired by mussel foot proteins in nature provide great alternatives to existing medical adhesives, many of which are either cytotoxic or ineffective. These materials have catecholic (3,4-dihydroxyphenylalanine), or DOPA, and cationic lysine residues which synergistically drive the adhesion process. However, DOPA is oxidized at basic pH, thus limiting its application where adhesion at a large range of pH is desirable. To address this molecular limitation, we studied hydroxypyridinone (HOPO) compounds hypothesized to be more resistant to oxidation than catechols, as a candidate for multifunctional wet adhesives. We compared the pH dependence on adhesion of 1,2-HOPO compound against previously reported catechol-based molecules that mediate strong wet adhesion at acidic pHs, namely- Tren-Lys-Cam with 2,3-dihydroxybenzoyl group (TLC or tren-Lys-2,3-DHB) and its derivative 3,4-dihydroxybenzoyl group (3,4-DHB), using a Surface Force Apparatus. The results suggest that the adhesion force of 1,2-HOPO between two atomically smooth mica surfaces at pH 10 falls to  ~30% of the peak adhesion force of ~ -40 mN/m or adhesion energy of –8.5 mJ/m<sup>2</sup>, similar to catecholic compounds with dihydroxybenzoyl groups. But more interestingly, after switching back to acidic pH of 3, the 1,2-HOPO compound recovers ~75% of the peak adhesion force, unlike the catecholic TLCs that do not recover adhesion. The results of this work should guide the design of a new generation of smart adhesives for binding to charged surfaces in saline environments.

pH dependent interfacial adhesion of hydroxypyridinone based wet adhesives

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