To realize continuously and stably work in a “moist/hot environment”, flexible electronics with excellent humid resistance, anti-swelling, and detection sensitivity are demanding. Herein, a solvent-resistant and temperature-ultrasensitive hydrogel sensor was prepared by combining MXene and quaternized chitosan (QCS) with the binary polymer chain. The strong electrostatic interaction between the QCS chain and the poly(acrylic acid) (PAA) network endows the hydrogel stability against solvent erosion, high temperature, and high humidity. The strong dynamic interaction between MXene and polymer matrix significantly improves the mechanical properties and sensing (strain and temperature) sensitivity of the hydrogel. The hydrogel strain sensor exhibits a high gauge factor (5.53), temperature/humidity tolerance (equilibrium swelling ratio of 2.5% at 80 °C), and excellent cycle stability, which could achieve a remote and accurate perception of complex human motion and environment fluctuation under aquatic conditions. Moreover, the hydrogel sensor exhibits impressive thermal response sensitivity (−3.183%/°C), ultra-short response time (< 2.53 s), and a low detection limit (< 0.5 °C) in a wide temperature range, which is applied as an indicator of the body surface and ambient temperature. In short, this study broadens the application scenarios of hydrogels in persistent extreme thermal and wet environments.

With the widespread prevailing of flexible electronics in human–machine interfaces, health monitor, and human motion detection, ultrasoft flexible sensors are urgently desired with critical demands in conformality. Herein, a temperature-sensitive ionogel with near-infrared (NIR)-light controlled adhesion is prepared by electrostatic interaction of poly(diallyl dimethylammonium chloride) (PDDA) and acrylic acid, as well as the incorporation of the conductive polydopamine modified polypyrrole nanoparticles (PPy-PDA NPs). The PPy-PDA NPs could weaken the tough interaction between polymer chains and depress the Young’s modulus of the ionogel, thus promoting the ionogel ultrasoft (34 kPa) and highly stretchable (1,013%) performance to tensile deformations. In addition, the high photothermal conversion capacity of PPy-PDA NPs ensured the ionogel excellent NIR-light controlled adhesion and temperature sensitivity, which facilitated the ionogel on-demand removal and promised a reliable thermal sensor. Moreover, the resulted ultrasoft flexible sensor exhibited high sensitivity and stability to both strain and pressure in a broad range of deformations, enabling a precise monitoring on various human motions and physiological activities. The temperature-sensitive, ultrasoft, and controlled adhesive capabilities prompted great potential of the flexible ionogel in medical diagnosis and wearable electronics.

Hydrogel is a potential matrix material of electronic-skins (E-skins) because of its excellent ductility, tunability, and biocompatibility. However, hydrogel-based E-Skins will inevitably lose their sensing performance in practical applications for water loss, physical damage, and ambient interferences. It remains a challenge to manufacture highly durable gel-based E-skins. Herein, an E-Skin is fabricated by introducing ionic liquids (ILs) into MXene-composited binary polymer network. The obtained ionic gel shows excellent mechanical properties, strong adhesion, and superior tolerance to harsh environments. The E-skin exhibits high sensitivity to both strain and pressure in a wide range of deformations, which enables a monitoring function for various human motions and physiological activities. Importantly, the E-skin shows consistent electrical response after being stored in the open air for 30 days and can be quickly healed by irradiation with 808 nm near-infrared light, originating from the photo-thermal effect induced self-healing acceleration. It is noteworthy that the E-skin also reveals a highly sensitive perception of temperature and near-infrared light, displaying the promising potential applications in the multifunctional flexible sensor.