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Recently, hydrogels have attracted great attention because of their unique properties, including stretchability, self-adhesion, transparency, and biocompatibility. They can transmit electrical signals for potential applications in flexible electronics, human-machine interfaces, sensors, actuators, et al. MXene, a newly emerged two-dimensional (2D) nanomaterial, is an ideal candidate for wearable sensors, benefitting from its surface's negatively charged hydrophilic nature, biocompatibility, high specific surface area, facile functionalization, and high metallic conductivity. However, stability has been a limiting factor for MXene-based applications, and fabricating MXene into hydrogels has been proven to significantly improve their stability. The unique and complex gel structure and gelation mechanism of MXene hydrogels require intensive research and engineering at nanoscale. Although the application of MXene-based composites in sensors has been widely studied, the preparation methods and applications of MXene-based hydrogels in wearable electronics is relatively rare. Thus, in order to facilitate the effective evolution of MXene hydrogel sensors, the design strategies, preparation methods, and applications of MXene hydrogels for flexible and wearable electronics are comprehensively discussed and summarized in this work.

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Robotics capable of human-like operations need to have electronic skin (e-skin) with not only tactile sensing functions but also proximity perception abilities. Especially, under the current widespread of COVID-19 pandemic, touchless interfaces are highly desirable. Magnetoreception, with inherent specificity for magnetic objects, is an effective approach to construct a non-contact sensing e-skin. In this work, we propose a new touchless sensing mechanism based on the magneto-piezoresistive effect. The substrate of the sensor is made of hierarchically microstructured ferromagnetic polydimethylsiloxane, coated with a three-dimensional (3D) piezoresistive network. The 3D network is constructed by stacked layers of reduced graphene oxide and carbon nanotubes through layer-by-layer deposition. With this integrated design, a magnetic force induced on the ferromagnetic substrate can seamlessly be applied to the piezoresistive layer of the sensor. Because the magnetic force relates strongly to the approaching distance, the position information can be transduced into the resistance change of the piezoresistive network. The flexible proximity sensor exhibits an ultrahigh spatial resolution of 60 µm, a sensitivity of 50.47 cm-1, a wide working range of 6 cm, and a fast response of 10 ms. The repeatable performance of the sensor is shown by over 5000 cycles of approaching-separation test. We also demonstrate successful application of the sensor in 3D positioning and motion tracking settings, which is critical for touchless tactile perception-based human-machine interactions.

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Wearable electronics, electronic skins, and human-machine interfaces demand flexible sensors with not only high sensitivity but also a wide linear working range. The latter remains a great challenge and has become a big hurdle for some of the key advancements imperative to these fields. Here, we present a flexible capacitive pressure sensor with ultrabroad linear working range and high sensitivity. The dielectric layer of the sensor is composed of multiple layers of double-sided microstructured ionic gel films. The multilayered structure and the gaps between adjacent films with random topography and size enhance the compressibility of the sensor and distribute the stress evenly to each layer, enabling a linear working range from 0.013 to 2063 kPa. Also, the densely distributed protrusive microstructures in the electric double layer contribute to a sensitivity of 9.17 kPa-1 for the entire linear working range. For the first time, a highly sensitive pressure sensor that can measure loading conditions across 6 orders of magnitude is demonstrated. With the consistent and stable performance from a low- to high-measurement range, the proposed pressure sensor can be used in many applications without the need for recalibration to suit different loading scenarios.

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Recent years have seen a rapid development of electronic skin for wearable devices, autonomous robotics, and human-machine interaction. As a result, the demand for flexible pressure sensors as the critical sensing element in electronic skin is also increasing. These sensors need to feature high sensitivity, short response time, low detection limit, and so on. In this paper, inspired from the cobweb in nature, we propose a piezoresistive pressure sensor by forming a cobweb-like network made of a zinc octaethylphorphyrin (ZnOEP)/carbon nanotube (CNT) hybrid on an array of polydimethylsiloxane (PDMS) microposts. The hybrid material exhibits excellent adhesion to PDMS, benefitting from ZnOEP's low Young's modulus and the nonpolar bonding between ZnOEP and PDMS such that no delamination and resistance variation are found after thousands of cycles of bending and twisting. With the overhanging morphology of the ZnOEP/CNT network on the micropost array, we realized a pressure sensor with an ultrahigh sensitivity of 39.4 kPa-1, a super-fast response time of 3 ms, a low detection limit of 10 Pa, and a reproducible response without degradation after 5000 cycles of pressure loading/unloading. The sensor can be employed for a variety of applications, including wrist pulse measurement, sound level detection, mechanical vibration monitoring, etc., proving its great potential for use in electronic skin systems.

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In the present study, 3D biomimetic platforms were fabricated with guiding grating to mimic extracellular matrix topography, porous membrane to resemble the epithelial porous interface and trenches below to represent blood vessels as an in vitro tissue microenvironment. Fabrication technologies were developed to integrate the transparent biocompatible polydimethylsiloxane platforms with preciously controlled dimensions. Cell migration behaviors of an immortalized nasopharyngeal epithelial cell line (NP460) and a nasopharyngeal carcinoma cell line (NPC43) were studied on the 2D and 3D platforms. The NP460 and NPC43 cells traversing through the porous membrane and migrating in the trenches below were studied by time-lapse imaging. Before traversing through the pores, NP460 and NPC43 cells migrated around the pores but NPC43 cells had a lower migration speed with less lamellipodia spreading. After traversing to trenches below, NPC43 cells moved faster with an alternated elongated morphology (mesenchymal migration mode) and round morphology (amoeboid migration mode) compared with only mesenchymal migration mode for NP460 cells. The cell traversing probability through porous membrane on platforms with 30 µm wide trenches below was found to be the highest when the guiding grating was perpendicular to the trenches below and the lowest when the guiding grating was parallel to the trenches below. The present study shows important information on cell migration in complex 3D microenvironment with various dimensions and could provide insight for pathology and treatment of nasopharyngeal carcinoma.

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