Mussel adhesion – essential footwork

Abstract

Robust adhesion to wet, salt-encrusted, corroded and slimy surfaces has been an essential adaptation in the life histories of sessile marine organisms for hundreds of millions of years, but it remains a major impasse for technology. Mussel adhesion has served as one of many model systems providing a fundamental understanding of what is required for attachment to wet surfaces. Most polymer engineers have focused on the use of 3,4-dihydroxyphenyl-L-alanine (Dopa), a peculiar but abundant catecholic amino acid in mussel adhesive proteins. The premise of this Review is that although Dopa does have the potential for diverse cohesive and adhesive interactions, these will be difficult to achieve in synthetic homologs without a deeper knowledge of mussel biology; that is, how, at different length and time scales, mussels regulate the reactivity of their adhesive proteins. To deposit adhesive proteins onto target surfaces, the mussel foot creates an insulated reaction chamber with extreme reaction conditions such as low pH, low ionic strength and high reducing poise. These conditions enable adhesive proteins to undergo controlled fluid-fluid phase separation, surface adsorption and spreading, microstructure formation and, finally, solidification.

Mussel Byssus Structure‐Function and Fabrication as Inspiration for Biotechnological Production of Advanced Materials

“…Each thread is ≈2–5 cm long and further subdivided into three structurally and functionally distinct parts known as the plaque, core and cuticle (Figure A,B). The plaque is a versatile underwater adhesive glue that serves as the interface between the byssal thread and the surface . The core of the thread is a fibrous bio‐polymer anchored into the plaque that provides the energy‐damping function .…”

“…The byssus plaque has received the most attention from researchers wishing to replicate its remarkable ability to adhere to almost any surface chemistry in salty seawater environments, which has obvious biomedical and industrial relevance . Bound water, ionic double layers and biofilms present numerous challenges for wet adhesion; yet, mussels have evolved a protein‐based mechanism to circumvent this . Spearheaded by the Waite group, nearly forty years of research have gone into unraveling the complex adhesive mechanism in Mytilus spp .…”

“…Spearheaded by the Waite group, nearly forty years of research have gone into unraveling the complex adhesive mechanism in Mytilus spp . mussels, resulting in a very detailed understanding of the biochemistry behind their stickiness . The plaque consists of at least five proteins, although several more putative candidates were recently identified via transcriptomics .…”

“…The interaction of mfp‐3 and mfp‐5 with various surfaces has been intensively studied using a technique called surface force apparatus (SFA), which measures work of adhesion for monolayers of biomolecules. Using SFA on different surfaces, it was determined that mfp‐3 and −5 create strong interactions primarily mediated with DOPA, which interact via hydrogen bonds, hydrophobic interactions, metal coordination and covalent cross‐linking depending on the surface chemistry presented and the local conditions (Figure E) . DOPA's known tendency to oxidize at seawater pH to the quinone form, which tends to form covalent cross‐links and is less efficient at adhesion, is counteracted by a cysteine‐rich protein, mfp‐6, which functions as an effective reducing agent .…”

Simple peptide coacervates adapted for rapid pressure-sensitive wet adhesion

“…Since 1995, adhesion research has emphasized primarily interfacial chemistry, such as the role of DOPA and DOPA-inspired chemical moieties 9 . The relevance of this emphasis was brought into question by the recent discovery that interfacial chemistry provides less than 0.01% of the adhesion energy contributed by plaque geometry and architecture in whole plaques 10,11 .…”