The Molecular Origin and Number of Interfacial Interactions Dictate the Adhesion of Bioinspired Adhesives

Inspired by the strong adhesion of mussel byssal threads to surfaces, the incorporation of 3,4-dihydroxyphenylalanine (DOPA) in polymer architecture is a popular strategy to improve adhesion. Even though there have been numerous literature reports of this bioinspired method to improve the adhesion performance of polymers, the mechanism behind the success of DOPA-based adhesion continues to be a puzzle. Herein, we designed mussel-inspired elastomers (MIE) with four different functionalities to test the importance of aromatic and hydroxyl groups in determining the adhesion performance. With a combination of adhesion measurements, surface-sensitive spectroscopy, and molecular dynamics simulations we show that the aromatic groups form multivalent weaker acid-base interactions with the surface hydroxyl groups on a sapphire substrate (-OH_sap). Also, we found that the combination of the interaction between phenyl and two -OH groups of DOPA with -OH_sap groups increases the adhesion of DOPA-based polymers compared to polymer analogs functionalized with either phenylalanine (only aromatic) or serine (only hydroxyl) or tyrosine (aromatic and one hydroxyl). Thus, this study illustrates the importance of both strong and weak acid-base interactions in enhancing adhesion.