Dual Cross-Linked Hydrogels with High Strength, Toughness, and Rapid Self-Recovery Using Dynamic Metal–Ligand Interactions

Abstract

A dual cross-linking design principle enables access to hydrogels with high strength, toughness, fast self-recovery, and robust fatigue resistant properties. Imidazole (IMZ) containing random poly(acrylamide-co-vinylimidazole) based hydrogels are synthesized in the presence of Ni2+ ions with low density of chemical cross-linking. The IMZ-Ni2+ metal–ligand cross-links act as sacrificial motifs to effectively dissipate energy during mechanical loading of the hydrogel. The hydrogel mechanical properties can be tuned by varying the mol% of vinylimidazole (VIMZ) in the copolymer and by changing the VIMZ/Ni2+ ratio. The resultant metallogels under optimal conditions (15 mol% VIMZ and VIMZ/Ni2+ = 2:1) show the best mechanical properties such as high tensile strength (750 kPa) and elastic modulus (190 kPa), combined with high fracture energy (1580 J m−2) and stretchability (800–900% strain). The hydrogels are pH responsive and the extent of energy dissipation can be drastically reduced by exposure to acidic pH. These hydrogels also exhibit excellent anti-fatigue properties (complete recovery of dissipated energy within 10 min after ten successive loading–unloading cycles at 400% strain), high compressive strength without fracture (17 MPa at 96% strain), and self-healing capability due to the reversible dissociation and re-association of the metal ion mediated cross-links.