The biochemistry and mechanics of gastropod adhesive gels

Several different types of gastropods produce tough adhesive gels. These gels consist of more than 95% water, yet they are far stiffer and tougher than the mucus used for other activities. This chapter reviews the structure and properties of adhesive gels from limpets, marsh periwinkles, terrestrial snails, and terrestrial slugs. While some of these gels contain large, entangled, carbohydrate-rich polymers such as those found in the mucus that is secreted for locomotion and lubrication, what sets the adhesive gels apart is the additional presence of substantial quantities of gel-stiffening proteins. Adhesive gels have been studied in depth in the slug Arion subfuscus. In this glue, the gel-stiffening proteins bind iron, and metals such as iron and calcium stiffen the gel. Metals bind directly to polymers in the glue and can cross-link them through coordinate covalent bonds. Additionally, key proteins in the glue are rich in carbonyls, presumably due to metal-catalyzed oxidation. These carbonyls appear to serve as sites for reversible cross-links between proteins. Furthermore, the action of the proteins does not merely stiffen the gel; it creates a second network that interpenetrates the tangled carbohydrate network creating a double network. In a double network gel, the two networks act together to achieve toughness values that are orders of magnitude greater than what they can achieve separately