The development of wearable electronics, point-of-care testing, and soft robotics requires strain sensors that are highly sensitive, stretchable, capable of adhering conformably to arbitrary and complex surfaces, and preferably self-healable. Conductive hydrogels hold great promise as sensing materials for these applications. However, their sensitivities are generally low, and they suffer from signal hysteresis and fluctuation due to their viscoelastic property, which can compromise their sensing performance. We demonstrate that hydrogel composites incorporating MXene (Ti₃C₂T_x) outperform all reported hydrogels for strain sensors. The obtained composite hydrogel [MXene-based hydrogel (M-hydrogel)] exhibits outstanding tensile strain sensitivity with a gauge factor (GF) of 25, which is 10 times higher than that of pristine hydrogel. Furthermore, the M-hydrogel exhibits remarkable stretchability of more than 3400%, an instantaneous self-healing ability, excellent conformability, and adhesiveness to various surfaces, including human skin. The M-hydrogel composite exhibits much higher sensitivity under compressive strains (GF of 80) than under tensile strains. We exploit this asymmetrical strain sensitivity coupled with viscous deformation (self-recoverable residual deformation) to add new dimensions to the sensing capability of hydrogels. Consequently, both the direction and speed of motions on the hydrogel surface can be detected conveniently. Based on this effect, M-hydrogel demonstrates superior sensing performance in advanced sensing applications. Thus, the traditionally disadvantageous viscoelastic property of hydrogels can be transformed into an advantage for sensing, which reveals prospects for hydrogel sensors.

MXenes, a rapidly growing family of 2D materials based on transition metal carbides and nitrides, have shown great potentials as multi-functional nanomaterials. Their unique combination of metallic conductivity, hydrophilicity, and highly charged surfaces endow them with excellent electrochemical performance. Yet, these same characteristics are equally import in electronic and optoelectronic applications of MXenes, henceforth termed MXetronics. MXenes are suitable for solution processing in various polar solvents, allowing the preparation of MXene thin films with controlled transparency, sheet resistance, interlayer spacing, and surface chemistry. The wide selection of various MXene structures, transition metal composition, and surface functional groups enables tunable work function, band gap, and hence electronic and optical properties. In this Review, we summarize the recent advances in MXetronics and discuss future possibilities for this rising class of 2D materials.