Temperature and pressure sensitive ionic conductive triple-network hydrogel for high-durability dual signal sensors

In this work, the fabrication of strengthened triple network hydrogels was successfully achieved based on in-situ polymerization of polyacrylamide by combining both chemical and physical cross-linking methods. The ion conductive phase of lithium chloride (LiCl) and solvent in the hydrogel were regulated through soaking solution. The pressure and temperature sensing behavior and durability of the hydrogel were investigated. The hydrogel containing 1 mol/L LiCl and 30 %v/v glycerol displayed a pressure sensitivity of 4.16 kPa  1 and a temperature sensitivity of 2.04 %/oC ranging from 20 to 50 degrees C. The durability results reveal that the hydrogel could maintain water retention rate of 69 % after 20 days of ageing. The presence of LiCl disrupted the interactions among water molecules and made it possible for the hydrogel to respond to changes in environment humidity. The dual signal testing revealed that the delay of temperature response over time (about 100 s) is much different from the rapidity of pressure response (in 0.5 s). This leads to the obvious separation of the temperature-pressure dual signal output. The assembled hydrogel sensor was further applied to monitor human motion and skin temperature. The signals can be distinguished by different resistance variation values and curve shapes in the typical temperature-pressure dual signal performance of human breathing. This demonstrates that this ion conductive hydrogel has the potential for application in flexible sensors and human-machine interfaces.