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

Abstract

low density of polymer cross-links. Typical fracture energy of a synthetic hydrogel is about 10 J m −2 , [11] which is orders of magnitude lower than that of cartilage (≈1000 J m −2 ), [12] and natural rubbers (10000 J m −2 ). [13] More often than not, a compromise between high strength and toughness has to be made [14] as they are typically mutually exclusive properties. In recent years, extensive efforts have been directed toward addressing this problem by introducing energy dissipative mechanisms into the gel network. [15] Several strategies have been reported in the literature to design hydrogels with improved mechanical properties. Examples include organic-inorganic nanocomposite hydrogels, [16] slide-ring hydrogels, [17] nonswellable hydrogels, [18] and double network (DN) hydrogels. [19] Among these approaches, the DN strategy combines two polymer networks, one of which contains sacrificial bonds that dissociate during application of stress to provide dissipation of energy, and the other network is flexible and lightly cross-linked, and maintains hydrogel structural integrity during the process of deformation. Unlike chemically cross-linked DN hydrogels which do not exhibit fatigue resistance due to irreversible breakage of the cross-links during loading, [19] utilizing physical cross-links as sacrificial bonds enables gel recovery after a loading cycle. [20] However, in most instances, the hydrogel requires long time and elevated temperature to repair the damage of loading. Henderson et al. designed a triblock amphiphilic copolymer where the polyelectrolyte midblock poly(methacrylic acid) was flanked by glassy end blocks of poly(methyl methacrylate). The midblock was ionically cross-linked using divalent cations to obtain a high strength (1 MPa) hydrogel. [21] However, 12 h of waiting time at room temperature was required for the hydrogel to recover 60% of its dissipated energy. Zheng and coworkers reported a physically cross-linked Agar/Polyacrylamide (PAM) DN hydrogel with tensile strength of 1 MPa, but after first loading even after resting for 4 days at room temperature the mechanical properties of the recovered hydrogel did not improve significantly. [22] Heating the gel at 100 °C was required for the gel to recover back 65% of dissipated energy and 90% of elastic modulus compared to the first loading cycle. When the PAM network was modified by introducing hydrophobic Dual Cross-Linked Hydrogels 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 Ni 2+ ions with low density of chemical cross-linking. The IMZ-Ni 2+ 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/ Ni 2+ r...

Engineering hydrophobically associated hydrogels with rapid self‐recovery and tunable mechanical properties using metal‐ligand interactions

“…This result can be attributed to reorganization to a stronger network during the reassociation of Zn 2+ ‐terpy bonds. We [ 17 ] and others [ 30 ] have previously observed similar strengthening of hydrogel networks. Considering that this recovery took place from a residual strain considerably higher than the Fe 2+ ‐based gel after the same resting period (10 min) for both the gels, the self‐recovery for the Zn 2+ gel was more efficient.…”

“…In order to enable recovery after the loading–unloading cycle, physical crosslinks acting as sacrificial bonds have been introduced into hydrogels. Along with other physical interactions such as H‐bonding [ 10–12 ] and ionic bonds [ 13 ] , hydrophobic association [ 14 ] and metal‐ligand co‐ordination [ 15–17 ] are promising reversible interactions that can be employed as sacrificial motifs to provide energy dissipation pathways to the hydrogels. Chen et al reported on agar/hydrophobically associated polyacrylamide (PAM) network, which showed tensile strength of 0.267 MPa.…”

Intrinsically Freezing-Tolerant, Conductive, and Adhesive Proton Donor–Acceptor Hydrogel for Multifunctional Applications

“…Interestingly, the hydrogel recovered the lost mechanical energy and the hysteresis area was slightly higher, as the first loading–unloading cycle of the rested and original hydrogel was compared (Figure F). A similar increase had been noticed previously by our group , and others and suggested the possibility of stronger network formation with better energy-dissipating ability. Similar to tensile force, the extent of recovery was evaluated under the compressive force by subjecting the hydrogel to successive compressive–relaxation cycles.…”

Integration of Macro-Cross-Linker and Metal Coordination: A Super Stretchable Hydrogel with High Toughness

“…Accordingly, in recent decades, enormous research efforts have been focused on realizing hydrogels suitable for real-world applications that require superior mechanical integrity. Typical strategies explored to achieve this include the formation of double-networks (DNs) ,− and dynamic cross-linking as well as the incorporation of nanofillers, , metal coordination, − macromolecular cross-linking, − and dual physical cross-linking. ,− Several attempts were also made by combining the abovementioned methods. − …”