Polymer Physics Prize Winner: Toughening hydrogels with sacrificial bonds

Hydrogels are a class of soft materials composed of cross-linked polymer networks and abundant water. Due to their rubbery nature, conventional hydrogels lack an energy dissipation mechanism during deformation and thus have weak mechanical properties. The invention of double-network hydrogels with two interpenetrating, contrasting network structure demonstrates a strategy to significantly toughen such rubbery materials (Gong et al., Adv. Mater. 2003). In double-network hydrogels, the hard and brittle short-strand network breaks upon deformation to dissipate large amounts of energy, while the soft and stretchable long-strand network disperses the stress and maintains the integrity of the materials. Therefore, the covalent bonds of the brittle network act as "sacrificial bonds" for toughening the materials [Gong, Soft Matter, 2010]. This sacrificial bond principle has been found to be general and also applicable to elastomers [Ducrot, Creton et al., Science, 2014]. This principle has been extended to non-covalent bonds to further endow materials with self-healing capabilities [Sun, Suo et al., Nature, 2012; Sun, Gong et al., Nature Mater. 2013]. The sacrificial bond principle further leads to a more general strategy for designing tough soft materials: deliberately combining a mechanically fragile structure or weak bonds to make the entire material tough. This strategy gives greater freedom in molecular design, not only limited to double or multiple network systems but also applicable to dually cross-linked single-network systems. Recently, it has been demonstrated that "self-renovative" materials that behave like muscles through repetitive mechanical training can be developed by using sacrificial bonds in double network system [Matsuda, Nakajima, Gong et al., Science 2019].