A Dual-Responsive, Freezing-Tolerant Hydrogel Sensor and Related Thermal- and Strain-Sensitive Mechanisms

Freezing-tolerant and conductive hydrogels have attracted tremendous interest as promising materials for flexible sensors under the gelid condition. In this work, a freezing-tolerant, dual-responsive hydrogel sensor was developed by using an ionic–glycerol hybrid hydrogel. The fabricated hydrogel sensor was demonstrated to detect efficiently different temperatures ranging from −30 to 100 °C or to detect both small strains (e.g., 1%, 2%, 4%) and large strains (e.g., 10%, 20% and 30%) as a freezing-tolerant strain sensor. A systematic investigation was conducted to explore the thermal- and strain-sensitive mechanisms of the obtained hydrogel. It was found that the resistance of the hydrogel rapidly increased as the temperature (T) decreased. In addition, the strain sensitivity increased as T decreased from 0 °C to −30 °C. Surprisingly, an abrupt resistance increase was observed when the tensile strain of the gel reached a critical value (e.g., 2620% at 0 °C) or upon moving the hydrogel from 25 to −10 to −30 °C. Such a sharp increase in resistance was found to be mainly caused by the abrupt fracture of a vast amount of the inner hydrogel network at large strains or the appearance of microcrystals in the gel network at low temperatures. This work provides fundamental and practical insights into fabricating functional freezing-tolerant hydrogel sensors for engineering applications.