Tough, antifreezing, and conductive double network zwitterionic-based hydrogel for flexible sensors

Hydrogels are considered to be one of the most promising flexible sensing materials, it is expected to be used in wearable devices, health monitoring, and electronic skin. However, the water in hydrogels can affect the performance at low temperatures due to icing. Therefore, it remains a challenge to prepare hydrogels that can maintain comprehensive performance at low temperatures. In this study, nanocellulose was added to gelatin and polyacrylamide to construct a brittle-tough combined double network. Then a mixture of zwitterionic betaine and ammonium sulfate replaced the water molecules in the hydrogel by a facile solution substitution method. A tough, high conductivity and antifreezing hydrogel was developed. The prepared hydrogel had excellent mechanical properties (compressive strength of 5.72 MPa) due to the introduction of nanocellulose and the “salting-out” effect of gelatin and ammonium sulfate. The zwitterionic betaine and ammonium sulfate endowed the hydrogel antifreezing properties (-40 °C) and high electrical conductivity (1.5 S/m). Based on the above advantages, the multifunctional zwitterionic-based hydrogel was applied in flexible sensors to monitor human motions, such as limb bending and facial expression. Therefore, both in mild or extreme environments zwitterionic-based hydrogel showed great potential in the field of flexible sensors.