The Mussel Attachment Plaque: A Load Bearing Protein Scaffold

Abstract

The attachment of marine mussels to each other, to rocks, as well as to a variety of pristine and bio-fouled surfaces (glass, metals or plastics) while under seawater is impressive. The attachment is accomplished via a wide adhesive plaque interpenetrated by the collagen-rich fibers of a thin long thread that connects the plaque at the distal end to the mussel at the proximal end. While the robust and versatile adhesion of the plaque has been attributed to the molecular adhesive properties, we propose that microscopic geometry, interior structure and the penetration (or lack thereof) of the thread are critical in load distribution and enhancement of adhesive performance.We present novel results on the structure of plaques from a variety of genera (Modiolus, Mytilus and Septifer) and species (M. californianus, M. galloprovincialis) studied via electron microscopy and neutron scattering. Despite living in the same coastal area, these species live in micro-environments subjected to different wave motion, allowing us to investigate how ecology might influence plaque structure and load-bearing capacity. Their architecture is reminiscent of various structural foams, ranging from closed-cell foams with one relevant length scale, to open-cell foams characterized by two lengthscales. Similar foam architectures have been found in other marine secretions, such as the sandcastle worm's tube cement. For the mussel plaques, the most complex structure we observe exhibits a collection of pores having an inner network, further connected with an outer mesh network. We reveal a robust structure, largely unaffected by guanidine hydrochloride treatment, and study the effect of temperature on the plaque's mechanical properties. Finally, we develop models to demonstrate the load distribution in the plaque and explore the potential benefits of such structures in wet environments.