Elasticity and energy dissipation in the double network hydrogel adhesive of the slug Arion subfuscus.

The slug Arion subfuscus produces a mucus-based defensive secretion that is remarkably tough. This glue appears to be a double network hydrogel, gaining its toughness through the synergistic actions of two networks of polymers, a relatively stiff network and a relatively deformable network. The double network mechanism has great potential to guide the development of synthetic adhesives. Mechanical tests were performed to analyse key predictions of the mechanism. Stress relaxation tests and tensile tests support the presence of stable cross-links. Cyclic stress-strain tests demonstrate that the glue dissipates a great deal of energy through the failure of these cross-links as sacrificial bonds. Energy dissipation by failure of sacrificial bonds rather than viscous processes is supported by the minimal effect of the time course of the experiments on the measured properties. These sacrificial bonds appear able to reform within minutes after failure. Finally, the glue's stiffness decreases at pH values below 5.5, whereas magnesium and calcium rapidly dissociate from the glue at all pH values tested. Thus, these ions may not be the primary cross-linkers generating the glue's stiffness. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.