Toughening Hydrogels by Forming Robust Hydrazide-Transition Metal Coordination Complexes

Graphical Tough hydrogels were developed by forming robust hydrazide-Zn 2+ coordination complexes. The mechanical properties of the resultant gels exhibited a notable temperature dependence and thus a good shape-memory ability due to the dynamic nature of coordination bonds. The reactive amine group of the hydrazide enables facile surface modification and functionalization based on Schiff's base reactions. Energy dissipation based on dynamic fracture of metal ligands is an effective way to toughen hydrogels for specific applications in biomedical and engineering fields. Exploration of new kinds of metal-ligand coordinates with robust bonding strength is crucial for the facile synthesis of tough gels. Here a hydrogel toughening strategy based on the formation of robust coordination complexes between the hydrazide ligands and zinc ions is reported. The resultant hydrogels exhibit high strength and toughness at room temperature. Their mechanical properties show temperature dependence due to the dynamic nature of coordination bonds. In addition, the amine group of hydrazides in the gel matrix provides a reactive site for Schiff's base reaction, enabling surface modification without influence on overall mechanical performances of the gel. The hydrazide ligands are easy to synthesize and can coordinate very well with several transition metals. Such a metal-ligand coordination should be suitable to develop tough soft materials with versatile applications.