Zwitterion-Incorporated Antifreeze Hydrogel with Highly Stretchable, Transparent, and Self-Adhesive Properties for Flexible Strain Sensors

Hydrogels are considered to be one of the most promising flexible sensing materials for a variety of practical applications. However, hydrogels usually freeze at temperatures below freezing and lose their inherent properties. Thus, it is highly desirable to prepare hydrogels that can simultaneously have excellent mechanical properties and antifreezing properties at low temperatures. Herein, a facile photoinitiated polymerization strategy is employed to introduce zwitterionic proline (ZP) into the flexible poly(2-hydroxyethyl acrylate) (PHEA) matrix to prepare a stretchable and adhesive PHEA/ZP hydrogel with good freezing-tolerant properties. Due to the presence of abundant hydrogen bonds and electrostatic interaction forces, the obtained PHEA/ZP hydrogel exhibits excellent mechanical properties (stretch of 810%), a self-adhesive nature, high transparency (>90%), and rapid electrical self-healing capability. More importantly, the presence of ZP endows the hydrogel with considerable antifreeze properties, making it completely transparent and stretchable even at −40 °C. In addition, the use of PHEA/ZP hydrogel as a flexible strain sensor can accurately monitor human motions, such as the flexion of human limbs and joint microstress. Therefore, antifreeze hydrogel assisted by zwitterions enables wearable flexible sensors for motion detection at room temperature and low temperature, which has great potential in the field of flexible sensors.