Invention of the double network (DN) hydrogel in 2003 significantly modified the knowledge that hydrogels are weak materials. The extra-ordinary toughness of DN gels is a consequence of the internal fracture of the brittle network to dissipate significant amounts of energy under large deformation and the elasticity of the second network to maintain their original configurations after the deformation. Thus, the covalent bond of the brittle first network serves as true “sacrificial bond”, for the toughness of the DN gels. Many recent works have replaced the covalent bonds with non-covalent bonds to allow the fractured bond to be re-formed in hydrogels. Studies along these lines have successfully produced tough hydrogels with self-recovery after internal rupture.<br/>The double network concept naturally suggests a more general strategy for designing tough soft materials: incorporating, on purpose, a mechanically fragile structure to toughen the material as a whole. This strategy gives more freedom in the molecular design, not limited to double or multiple network systems but also applies to single network system as long as it has sacrificial bond to dissipate energy and can retain original configurations of the material after large deformation. This strategy has been proved effective using ionic bonds in hydrogels. One further goal is to develop tough soft materials with anisotropic mechanical performance resembling to skins, cartilages, muscle, and tendons. Using self-assembly molecules, such as lipid molecules, rod-like macromolecules, block copolymers, to build anisotropic energy dissipation structure would be a strategy. This possibility is demonstrated by a fully anisotropic hydrogels consisting of layered lipid membranes entrapped in the matrix of neutral network. The integrations and interactions of different mechanisms for dissipating energy and maintaining elasticity are essential to the design of tough soft materials.

Toughening soft materials with sacrificial bonds

253rd National Meeting (2017)

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