Understanding the Role of DOPA in Underwater Adhesion*

Removal of interfacial water is a major challenge in making adhesives that can stick underwater. However, there are many examples of biological adhesives that work in wet environments such as mussels, caddisfly and sandcastle worm. It is found that mussels stick to rocks underwater using byssal threads and the proteins that binds these threads to surfaces contain 20 % 3,4-dihydroxyphenylalanine (DOPA), indicating the importance of this molecule in mussel adhesion. Many new polymers have been synthesized that incorporates the DOPA unit to improve adhesion underwater. Although these materials show promising results, the role of catechol in interfacial adhesion is still under debate. In my research, we have focused on the role of hydrophobic groups along with DOPA in underwater adhesion. We have designed polymers that contain DOPA mimetic groups along with a significant fraction of hydrophobic side chains, and a UV cross-linkable molecule. We have used surface sensitive sum-frequency generation spectroscopy to probe the contact interface of this polymer with a solid substrate underwater to understand the role of DOPA. Also we related the spectroscopic observations with adhesion measured using JKR geometry.