Xinhu Gong

On the basis of the original hydrogen bonding interaction and physical entanglement, covalent cross-linking and ionic cross-linking were additionally introduced to construct a carboxymethyl chitosan/allyl glycidyl ether conductive hydrogel (CCH) through a one pot method by a graft reaction, an addition reaction, and simple immersion, successively. The multiple cross-linking networks significantly increased the strength of CCHs and endowed them with ionic conductivity and an antifreezing property at -40 °C, which showed stable, durable, and reversible sensitivity to finger bending activity at subzero temperature. The CCHs could even be assembled into a triboelectric nanogenerator (TENG) to provide electric energy, which demonstrated stability against temperature variation, multiple drawing, long-term storage, or large quantities of contact-separation motion cycles. CCH-TENG can also be used as a tactile sensor within the pressure range from 0.4 kPa to higher than 8000 kPa. This work provided a simple route to fabricate antifreezing conductive hydrogels based on carboxymethyl chitosan and to find potential applications in soft sensor devices under a low temperature environment.

Through a simple strategy of immersion in a mixed solution of water/ethylene glycol (EG)/lithium chloride (LiCl), self-healing carboxymethyl chitosan (CA) hydrogels, that is, CA/N-vinylpyrrolidone-EG-Li⁺ hydrogels (CEH) with an ultra-low-temperature freezing resistance below -70 °C were fabricated. The introduction of electrolyte ions and small-molecule polyol also made these hydrogels highly conductive (0.8 S m^-1) and imparted antidrying property to them, showing stable and reversible sensitivity to finger-wrist bending as well as 150 cycles of stretching. Such hydrogels also presented highly efficient self-healing ability, with a stress-strain healing efficiency of over 90%. Furthermore, the CEH-based sensors maintained a stable sensing performance over a wide range of temperatures below the freezing point (from -10 to -70 °C) and exhibited stable sensitivity to temperatures with fast response and no significant hysteresis. The present work is expected to provide a simple and sustainable route for the preparation of multifunctional antifreezing conductive hydrogels based on CA, leading to a wide range of potential applications in soft sensor devices.