Mechanically robust and adaptive, elastic sensing microelectronics [i.e., electronic skin (e-skin)] with portability, foldability, and biocompatibility are highly desirable for emerging technologies and bio-integrated devices, including next-generation prosthetics, soft robotics, human-machine interfaces, and personalized healthcare systems. Hydrogel, a viscoelastic material composed of 3D networks for hydrophilic polymers that are chemically or physically cross-linked, shows promising potential for e-skin due to its excellent stretchability, designable conductivity, and conformability. However, current hydrogel-based sensors usually have low sensitivity and responsibility and are limited by toughness-hysteresis trade-off, which prevents their successful implementation in e-skin. MXene, a booming class of 2D early-transition metal carbides/carbonitrides with hydrophilic surfaces, high electrical conductivity, and rich functional groups, has recently raised broad interest in next-generation e-skin sensors by creating MXene-hydrogel systems. <b>Herein, we present multifunctional e-skin systems with MXene-hydrogel composites published in Sci. Adv. 2020 and ACS Nano 2022.</b> The hydrogel was first engineered to achieve both high toughness and low hysteresis. Two types of composites, i.e., MXene embedded in the bulk hydrogel material to form MXene@hydrogel and MXene on the surface of hydrogel material to form MXene/hydrogel. They showed different sensing mechanisms and thus can be used for wearable strain sensors with different working ranges. The optimized e-skin sensors were capable of detecting, quantifying, and remotely monitoring stretching motions in multi-dimensions, pressure exertions (40 Pa-30 kPa), tactile and proximity sensing (from a distance of ~20 cm), temperature, and light variations, providing a promising platform for next-generation smart flexible electronics with an ultrabroad working range.

MXene-hydrogel composites for flexible electronic skin sensors

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