Wet Adhesion: Learning How from Mussels

There are few adhesives with a greater spatial range of function than the underwater glue of mussels (Mytilus). At the nanoscale, mussel adhesive proteins interact energetically with diverse surface chemistries thereby securing the ends of byssal threads (scale: microns-millimeters) onto rocks, for example. Cohorts of threads collectively provide each mussel with a holdfast or byssus (scale: centimeters). Mussels rely on their byssus to network socially with one another and to build massive reef-like aggregates that stabilize shorelines and establish new ecosystems (scale: meters to kilometers). Unlike the tendons that they resemble, byssal threads are produced in minutes from complex fluids by injection molding and curing. The proteins in these fluids are well-adapted for their roles as tensile filaments, wear resistant coatings or bonding of wet surfaces. Regardless of function, all mussel adhesive proteins contain the modified catecholic amino acid DOPA (3, 4-dihydroxyphenylalanine), which is notoriously prone to oxidation. Despite this, mussels control DOPA reactivity with remarkable precision. At or near the interface between the adhesive proteins and the substratum, mussels impose a highly reducing local environment (low pH and high thiol content) in order to exploit the superior chemisorption of DOPA to a variety of surfaces. Farther from the interface, mussel proteins are maintained in either Fe³⁺-rich or oxidizing local environments (high pH and catecholoxidase). The first leads to protein gelation stabilized by multifunctional DOPA- Fe³⁺-complexation, whereas the latter, to the covalent cross-links between DOPAquinone and reactive amino acids. By adjusting the redox of the local environment, mussels “tune” the optimal level of adhesion (DOPA) or cohesion (quinone and cross-linking) needed in each part of the holdfast. A deeper understanding of mussel adhesive chemistry and its regulation is likely to inspire improvements in adhesive technology especially in wet applications.