Strong Tough Polyampholyte Hydrogels via the Synergistic Effect of Ionic and Metal-Ligand Bonds

Despite existing in biological systems, developing synthetic polyampholyte (PA) hydrogels constructed by both ionic and metal-ligand bonds remains challenging. Herein, a simple secondary equilibrium approach is proposed to fabricate strong and tough PA hydrogels via the synergy of ionic and metal-ligand bonds. The original PA gels (constructed by ionic bonds) are first dialyzed in multivalent metal-ion solutions to reach a swelling equilibrium and then moved to deionized water to dialyze excess free ions to achieve a new equilibrium. Through this approach, the original PA gel network can be optimized and eventually constructed by ionic and metal-ligand bonds, enabling a synergistic reinforcement. By selecting different original PA gel systems and diverse multivalent metal-ions, the proposed approach is proved to be generalizable to fabricate strong and tough PA gels. Additionally, the hydrogels have stable ion-conductivity even at the water-equilibrium state, making them promising as strain sensors. The viscoelastic and elastic contributions to the mechanical properties of the hydrogels by a viscoelastic model are also discussed to further understand the strengthening and toughening mechanisms. The proposed strategy is simple but effective for achieving strong and tough PA-based hydrogels. This study also provides new insights for PA hydrogels in electrolyte environments.