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Highly stretchable and tough hydrogels

Zhigang Suo Harvard University

Hydrogels are broadly used in bioengineering, but the scope of their applications is often severely limited by the mechanical behavior of hydrogels. Here we report exceptionally stretchable and tough hydrogels made of polymers forming networks via ionic and covalent crosslinks. Although such a gel contains 90% water, it can be stretched beyond 20 times its initial length, and has fracture energy of 9000 J/m2. Even for samples containing notches, a stretch of 17 is demonstrated. The high fracture energy is attributed to the synergy of two toughening mechanisms : crack bringing by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks over a large region of the gel. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed when the load is removed. The unzipped ionic crosslinks cause internal damage, which heals as ionic crosslinks re-zip. We envision that these hydrogels will serve as model systems to explore mechanisms of deformation and energy dissipation, and that hydrogels with enhanced mechanical properties will considerably expand the scope of their applications.