MXenes, a kind of two-dimensional material of early transition metal carbides and carbonitrides, have emerged as a unique class of layered-structured metallic materials with attractive features, as good conductivity comparable to metals, enhanced ionic conductivity, hydrophilic property derived from their hydroxyl or oxygen-terminated surfaces, and mechanical flexibility. With tunable etching methods, the morphology of MXenes can be effectively controlled to form nanoparticles, single layer, or multi-layer nanosheets, which exhibit large specific surface areas and is favorable for enhancing the sensing performance of MXenes based sensors. Moreover, MXenes are available to form composites with other materials facilely. With structure design, MXenes or its composite show enhanced mechanical flexibility and stretchability, which enabled its wide application in the fields of wearable sensors, energy storage, and electromagnetic shielding. In this review, recent progress in MXenes is summarized, focusing on its application in wearable sensors including pressure/strain sensing, biochemical sensing, temperature, and gas sensing. Furthermore, the main challenges and future research are also discussed.

Solar steam generation, utilizing abundant solar energy and floating photothermal materials, has been considered as one of the most sustainable, efficient ways to solve the problem of water shortage. Here, a new system for solar steam generation is fabricated based on a PEGylated MoS-cotton cloth (PMoS-CC). 80.5-90 ± 3.5% of high-efficiency solar steam generation is achieved under a light density of 1-5 kW m because of the good gas permeability of CC and the hydrophilic property of PMoS-CC. The self-growth PMoS-CC provides good photothermal performances in pure water and saline water. The water evaporation rate with PMoS-CC keeps a stable value after a long-time illumination (4 h) and 32 times cycle tests. Our result provides a way to prepare pure water in the applications for alleviating a scarcity of drinking water.

Solar vapor generation is a promising and whole new branch of photothermal conversion for harvesting solar energy. Various materials and devices for solar thermal conversion were successively produced and reported for higher solar energy utilization in the past few years. Herein, a compact device of reduced graphene oxides (rGO) and paper fibers was designed and assembled for efficient solar steam generation under light illumination, and it consists of water supply pipelines (WSP), a thermal insulator (TI) and a double-sided absorbing film (DSF). Heat localization is enabled by the black DSF due to its broad absorption of sunlight. More importantly, the heat transfer, from the hot DSF to the cold base fluid (water), was suppressed by TI with a low thermal conductivity. Meanwhile, bulk water was continuously transported to the DSF by WSP through TI, which was driven by the surface energy and surface tension based on the capillary effect. The effects of reduction degrees of rGO on the photothermal conversion were explored, and the evaporation efficiency reached 89.2% under one sun with 60 mg rGO. This new microdevice provided a basic technical support for distillation, desalination, sewage treatment, and related technologies.

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On-skin devices that show both high performance and imperceptibility are desired for physiological information detection, individual protection, and bioenergy conversion with minimal sensory interference. Herein, versatile electrospun micropyramid arrays (EMPAs) combined with ultrathin, ultralight, gas-permeable structures are developed through a self-assembly technology based on wet heterostructured electrified jets to endow various on-skin devices with both superior performance and imperceptibility. The designable self-assembly allows structural and material optimization of EMPAs for on-skin devices applied in daytime radiative cooling, pressure sensing, and bioenergy harvesting. A temperature drop of ~4 °C is obtained via an EMPA-based radiative cooling fabric under a solar intensity of 1 kW m–2. Moreover, detection of an ultraweak fingertip pulse for health diagnosis during monitoring of natural finger manipulation over a wide frequency range is realized by an EMPA piezocapacitive-triboelectric hybrid sensor, which has high sensitivity (19 kPa−1), ultralow detection limit (0.05 Pa), and ultrafast response (≤0.8 ms). Additionally, EMPA nanogenerators with high triboelectric and piezoelectric outputs achieve reliable biomechanical energy harvesting. The flexible self-assembly of EMPAs exhibits immense potential in superb individual healthcare and excellent human-machine interaction in an interference-free and comfortable manner.

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