Thermoplastic charge-transfer hydrogels for highly sensitive strain and temperature sensors

Abstract

Recently, flexible wearable electronics have drawn increasing interest in health monitoring, human motion detection, artificial skin, and so on, owing to their comfort of wear and their ability to transmit real-time information. However, they generally show poor stretchability, low sensitivity, and single sensory function, which limit their practical applications. Herein, a series of conductive hydrogels (CT-K hydrogels) are developed using the CT complex of a viologen derivative and pyranine derivative (MS-CT complex) as the crosslinker. Their mechanical properties were highly tunable by adding different concentrations of KCl, reaching a maximum fracture strain of 1044%. Their ionic structure endowed them with an excellent conductivity of up to 6.75 S m⁻¹. These hydrogels were fabricated as multifunctional sensors capable of detecting both strain and temperature with high sensitivity (gauge factor up to 8.71 and temperature coefficients of resistance up to −3.20% °C⁻¹), a wide sensing range (900% strain and 5–70 °C) and reliable stability. Benefiting from the thermosensitivity and reversible interaction of the MS-CT complex, CT-K hydrogels had thermoplastic properties, and their sensing performance remained good after thermal treatment, demonstrating that the CT-K hydrogel-based sensors were recyclable and adaptable. This work paves the way for the development of high-performance hydrogel sensors using CT complexes, and promotes the versatile electronic applications of hydrogel sensors.