Designing anti-dehydration and ion-conductive tough hydrogels as environment-adaptable strain sensors for e-skin

In order to adapt environmental changes, a hydrogel that possesses high mechanical strength, long-lasting moisture, good temperature tolerance, and exceptional electrical conductivity simultaneously is extremely desirable but remains challenging. Inspired by muscles becoming more ion-rich and stronger after swelling with blood, herein we propose a simple secondary soaking method to achieve ultralong anti-dehydration lifespan and high conductivity in a swollen yet strengthened polyampholytes (PA) hydrogel via the synergy of ionic and two types of metal–ligand bonds. An as-prepared PA gel is first dialyzed in a ZnCl2 solution to reach a swelling equilibrium, and then moved to a FeCl3 solution to achieve a new equilibrium. Through this approach, the as-prepared PA gel network could be optimized and eventually constructed by ionic and two kinds of metal–ligand bonds, enabling a synergistic reinforcement. To understand the reinforcing mechanisms, we also discuss the viscoelastic and elastic contributions to the mechanical properties of the hydrogels by a viscoelastic model. Additionally, the hydrogels possess ultralong anti-dehydration lifespan (>300 days), high ion conductivity (≈50 S/m) and temperature- and humidity-responsiveness, exhibiting high potential as e-skin to monitor human activity. The proposed strategy is simple but effective for achieving anti-dehydration and highly ion-conductive tough hydrogels.