Temperature-Responsive\nIonic Conductive Hydrogel for\nStrain and Temperature Sensors

Abstract

Flexible wearable devices have achieved\nremarkable applications\nin health monitoring because of the advantages of multisignal collecting\nand real-time wireless transmission of information. However, the integration\nof bulky sensing elements and rigid metal circuit components in traditional\nwearable devices may lead to a mechanical and signal-conducting mismatch\nbetween wearable devices and biological tissues, thus restricting\ntheir wide applications in the human body. The excellent mechanical\nproperties, conductivity, and high tissue resemblance of conductive\nhydrogel contribute to its application in flexible electronic sensors\nto monitor human health. In this work, a dual-network, temperature-responsive\nionic conductive hydrogel with excellent stretchability, fast temperature\nresponsiveness, and good conductivity was developed by introducing\na polyvinylpyrrolidone (PVP)/ tannic acid (TA)/ Fe³⁺ cross-linked\nnetwork into the <i>N</i>,<i>N</i>-methylene diacrylamide\n(MBAA) cross-linked poly­(<i>N</i>-isopropylacrylamide-<i>co</i>-acrylamide) (P­(NIPAAm-<i>co</i>-AM)) network.\nFurthermore, the introduction of the PVP/TA/Fe³⁺ cross-linked\nnetwork endowed the hydrogel with excellent stretchability and conductivity.\nBy adjusting the molar ratio of TA and Fe³⁺ to 3:5, a hydrogel\nwith a maximal stretching ratio of 720% and sensitive strain response\n(GF = 3.61) was achieved, showing a promising application in wearable\nstrain sensors to monitor both large and fine human motions. Moreover,\nby introducing PNIPAAm with a lower critical solution temperature\n(LCST), the hydrogel may be used to monitor the environmental temperature\nthrough the temperature–conductivity responsiveness, which\ncan be applied as a wearable temperature sensor to detect fever or\ntissue hyperthermia in the human body.