Freezing-Tolerant, Highly Sensitive Strain and Pressure Sensors Assembled from Ionic Conductive Hydrogels with Dynamic Cross-Links

Conductive hydrogels have attracted intensive attention for versatile functions in flexible electronics because of their unique combination of mechanical flexibility and conductivity. However, hydrogels containing plenty of water inevitably freeze at subzero temperature, leading to invalid electronics with failed mechanical advantages and negligible conductivity. Moreover, the inferior elasticity and fatigue resistance of hydrogels result in unstable sensing performance and poor reusability of hydrogel-based electronics. Herein, a freezing-tolerant, high-sensitive, durable strain and pressure sensor was constructed from an ionic conductive chitosan-poly(acrylamide-co-acrylic acid) double-network [CS-P(AM-co-AA) DN] hydrogel with dual-dynamic cross-links (chitosan physical network and ionic coordination [CO₂LFe^III]), which was feasibly fabricated by soaking the CS-P(AM-co-AA) composite hydrogel in FeCl₃ solution. The ions immobilized in dynamic cross-links exerted crucial effects on improving mechanics [prominent tensile performance, supercompressibility, extraordinary elasticity, fast self-recovery capacity, and remarkable fatigue resistance (1000 cycles)]; meanwhile, the free ions in the hydrogel rendered the hydrogel excellent conductivity and strong freezing tolerance concurrently. The sensor assembled from the DN hydrogel exhibited cycling stability and good durability in detecting pressure, various deformations (elongation, compression, and bend), and human motions even at a low temperature (-20 °C). Notably, the sensitivity on detecting strain and pressure at both room and subzero temperature was superior than most of the reported organohydrogel and hydrogel sensors. Thus, we believe that this work will provide a platform for construction and application of high-sensitive strain and pressure hydrogel sensors with cycling stability and good durability in a wide temperature range.