Poly(<i>N</i>-isopropylacrylamide)/MXene Nanosheet/Fe₃O₄ Nanoparticle Composite Hydrogel Sensors for Monitoring Human Movement and Magnetic/Temperature Switches

The rise of the Internet of Things highlights the need for wearable piezoresistive sensors capable of integrating sensing devices with broader applicability and more advanced sensing functionalities. However, the development of multifunctional hydrogel sensors that meet the diverse needs of practical applications continues to present a substantial challenge. In this study, N-isopropylacrylamide/MXene/Fe3O4 (PNMF) composite hydrogels were synthesized via a straightforward one-pot method. The PNMF-based sensor exhibits multimode sensing abilities for pressure, magnetic, and temperature, with MXene and Fe3O4 nanoparticle providing electrical and magnetic response properties. Importantly, the temperature response is controlled by adjusting the MXene nanosheet content, which modifies the lower critical solution temperature (LCST) of the PNMF composite hydrogels. The PNM2F2-based pressure/magnetic/temperature sensor shows sensitivities of 0.40 and 0.23 kPa–1 in the 0–390 and 390–1040 Pa ranges, respectively. In the magnetic field ranges of 0–1.61, 1.61–4.25, and 4.25–40.93 mT, the field sensitivities are 26.93, 4.54, and 0.36 T–1, respectively. The PNMF composite hydrogel, as a multimode sensor, not only detects real-time human joint movement through pressure sensing but also operates as both a magnetic and temperature control switch, enabling detection of magnetic field strength and body surface temperature. This demonstrates its multimode sensing capability in more complex scenarios.