Andrew Smith. Society for Integrative and Comparative Biology, San Francisco, California, January 2013: “Double network gels and biological glues: a powerful new toughening mechanism”.
Limpets, marsh periwinkles and some terrestrial slugs produce remarkable glues that are gels. A key question has been how they can achieve tenacities on the order of several hundred kilopascals using only a dilute gel that is a modified lubricating mucus. Previous work has shown that the essential change is the addition of relatively small, cross-linked proteins. Nevertheless, highly cross-linked gels are typically brittle and fail easily. Molluscs may avoid this through the use of a “double network”. Recent work in materials science has found that combining two highly dissimilar, interpenetrating gel networks can increase gel strength by a factor of 100 to 1000 over the strength of the two gels separately. A prototypical double network gel combines a deformable network of very large polymers and a highly cross-linked network of much smaller polymers. Initial fracture occurs in the stiffer, highly cross-linked network. Fracturing the soft network as well, though, requires extensive deformation. This deformation damages the rigid network in a large volume surrounding the crack. This can increase the energy required to propagate the crack by several orders of magnitude. Such a mechanism is likely at play in molluscan adhesive gels given their structure. In fact, any biological gel containing proteoglycans or similarly large polymers in combination with smaller cross-linked proteins has the potential to operate this way. This talk will outline the structural and mechanical criteria for double network gels and consider the applicability of this mechanism to different biological materials.