Wet adhesives inspired by mussel foot proteins provide excellent 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 neutral pH, thus limiting its application where adhesion at an extensive range of pH values is desirable. To address this limitation, we studied hydroxypyridinone (HOPO) compounds, namely Tren(Lys-1,2-HOPO)3 and Tren(Lys-3,4-HOPO)3 hypothesized to be more resistant to oxidation than catechols, as a candidate for multifunctional wet adhesives. We compared the pH dependence on adhesion of Tren(Lys-1,2-HOPO)3 compound against previously reported catechol-based molecules that mediate strong wet adhesion at acidic pHs, namely- Tren(Lys 2,3-dihydroxybenzoyl group)3, using a Surface Force Apparatus. The results suggest that the adhesion force of Tren(Lys-1,2-HOPO)3 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/m2), similar to catecholic compounds with dihydroxybenzoyl groups. More interestingly, after switching back to acidic pH of 3, the 1,2-HOPO compound retains ~75% of the peak adhesion force, unlike the catecholic TLCs that do not recover adhesion. Tren(Lys-3,4-HOPO)3 compound shows a similar trend. With the absence of lysine, the adhesion of Tren(1,2-HOPO)3 is completely abolished. Also, only the Tren-core or Tris(2-aminoethyl amine) does not participate in the adhesion mechanism. The results of this work should guide the design of a new generation of smart adhesives for binding to charged surfaces in saline environments.
