A spider silk-inspired, transparent, anti-freezing ionic conductive hydrogel as a flexible sensor device†

Abstract

As soft material ionic conductors, ionically conductive hydrogels are of great significance for the development of flexible electronics. However, it is still a great challenge to effectively design functional hydrogel structures to address various practical application scenarios (such as low temperature environments) and expand their application range (such as transparent display devices). In this paper, an anti-bacterial and ionically conductive TEMPO-oxidized cellulose nanofiber/polyvinyl alcohol/quaternary ammonium chitosan/Al³⁺ (CPQA–EH) hydrogel (conductivity of 7.50 ms cm⁻¹) with high transparency (93.7%) is constructed by a simple method of solution mixing and immersion. An organic solvent is used to induce in situ phase separation and multiple interactions between molecular chains to promote crystallization. The hydrogel network structure is regulated step by step, and nanofibrils are induced in situ to form a nano-fishnet structure. The CPQA–EH ionically conductive hydrogel with a nanofibrous network exhibits excellent tensile strength (1341.86 kPa) and toughness (6992.53 kJ m⁻³). Meanwhile, it shows low-temperature sensing ability even at −80 °C (freezing point of −122.08 °C). The flexible sensor based on the CPQA–EH conductive hydrogel can sensitively recognize external stimuli (strain/pressure). It shows stable detection of the movement of human joints and vocalization, and the hydrogel with high transparency can also be used as a display device to recognize writing signals.