Intrinsic Anti-Freezing, Tough, and Transparent Hydrogels for Smart Optical and Multi-Modal Sensing Applications.

Hydrogels have received great attention due to their molecular designability and wide application range. However, they are prone to freeze at low temperatures due to the existence of mass water molecules, which can damage their flexibility and transparency, greatly limiting their use in cold environments. Although adding cryoprotectants can reduce the freezing point of hydrogels, it may also deteriorate the mechanical properties and face the risk of cryoprotectant leakage. Herein, the microphase-separated structures of hydrogels are regulated to confine water molecules in sub-6 nm nanochannels and increase the proportion of bound water, endowing the hydrogels with intrinsic anti-freezing properties, high mechanical strength, good stretchability, remarkable fracture energy, and puncture resistance. Even after being kept in liquid nitrogen for 1000 h, the hydrogel still maintains good transparency. The hydrogel can exhibit excellent low-temperature shape memory and intelligent optical waveguide properties. Additionally, the hydrogel can be assembled into strain and pressure sensors for flexible sensing at both room and low temperatures. The intrinsically anti-freezing microphase-separated hydrogel offers broad prospects in low-temperature electronic and optical applications.