Highly water retention, flexible and self-extinguished temperature sensors based on double network hydrogel for early fire warning

Early fire warning represents an active and effective way to avoid potential fire hazards. The hydrogel-based temperature sensor shows high temperature sensitivity, flexibility, and flame retardancy, etc., which makes it an ideal candidate for early fire warning. Herein, we prepared double network (DN) SA-PAM-Fe hydrogels with sodium alginate (SA), acrylamide (AM) and FeCl3, and studied the influence of different water content and Fe3+ concentration on their sensing properties. The ion-conductive DN hydrogels are highly sensitive to temperatures, and the temperature sensitivity of SA-PAM-Fe hydrogels can reach a maximum of 1.28%/°C in the temperature range of 25–90 °C. This means that the SA-PAM-Fe hydrogel temperature sensor can be applied to monitor temperature changes with high resolution. The hydrogel sensor showed enhanced water retention performance with glycerol in SA-PAM-Fe hydrogel via solution replacement, because evaporation and crystallization of free water molecules in the hydrogel system could be suppressed by forming strong hydrogen bonds between glycerol and water molecules. Its maximum fluctuation of temperature sensitivity does not exceed 5.4% and 10% after 72 h preservation at room temperature and 24 h at 90 °C, respectively. The hydrogel sensor also exhibits excellent flame retardancy and flexibility due to the high water content. It is self-extinguished immediately as well as can be twisted, bent and lifted 500 g weight after being subjected to the flame. This work sheds light on the fabrication of self-extinguished and flexible temperature sensors for early fire warning.