Development of a two-mode hydrogel sensor with a thermal diffusion effect for intelligent sensing and temperature warning

Abstract

With the rapid development of flexible sensor technology, dual-responsive polymer hydrogel sensors have attracted considerable attention owing to their high sensitivity and multifunctional capabilities. However, conventional poly(N-isopropylacrylamide) (PNIPAM) hydrogels, while demonstrating excellent thermal responsiveness, are limited by inadequate mechanical strength, poor conductivity, and restricted functionality, making the development of high-performance multifunctional PNIPAM hydrogels a significant challenge. In this study, a smart dual-response hydrogel based on PNIPAM is proposed, incorporating methyl methacrylate (MMA) and sodium chloride (NaCl), which, when combined with hydroxypropyl methylcellulose (HPMC), forms a double-network structure, thereby achieving synergistic optimization of the mechanical properties and temperature response. The hydrogel precisely tunes its lower critical solution temperature (LCST) to 80 °C through the hydrophobic groups of MMA while significantly enhancing conductivity via Na⁺/Cl⁻ ionic shielding effects. Additionally, it provides exceptional mechanical properties, including a stretching strain of 1077 % and a compressive strength of 128.6 kPa. Moreover, it has excellent strain sensing sensitivity over a wide threshold range of 1–400 % (GF = 6.6). Importantly, the incorporation of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF₄) substantially enhanced the thermoelectric properties, inducing a distinctive P-to-N-type transition in the Seebeck coefficient across the phase change (from +2.37 mV K⁻¹ to −6.84 mV K⁻¹). Herein, successful object shape recognition and graded temperature warning functions were achieved by integrating a deep learning algorithm (with an accuracy of 99.40 %) with a manipulator system. The experimental results demonstrate that the hydrogel shows great potential for human motion monitoring, high-temperature human-machine interactions, and smart robotics, offering new ideas for multifunctional e-skin design.