Yongfa Cheng

Recently, wearable and flexible pressure sensors have sparked tremendous research interest, and considerable applications including human activity monitoring, biomedical research, and artificial intelligence interaction are reported. However, the large-scale preparation of low-cost, high-sensitivity piezoresistive sensors still face huge challenges. Inspired by the specific structures and excellent metal conductivity of a family of two-dimensional (2D) transition-metal carbides and nitrides (MXene) and the high-performance sensing effect of human skin including randomly distributed microstructural receptors, we fabricate a highly sensitive MXene-based piezoresistive sensor with bioinspired microspinous microstructures formed by a simple abrasive paper stencil printing process. The obtained piezoresistive sensor shows high sensitivity (151.4 kPa–1), relatively short response time (<130 ms), subtle pressure detection limit of 4.4 Pa, and excellent cycle stability over 10,000 cycles. The mechanism of the high sensitivity of the sensor is dynamically revealed from the structural perspective by means of in situ electron microscopy experiment and finite element simulation. Bioinspired microspinous microstructures can effectively improve the sensitivity of the pressure sensor and the limit of the detectable subtle pressure. In practice, the sensor shows great performance in monitoring human physiological signals, detecting quantitatively pressure distributions, and remote monitoring of intelligent robot motion in real time.

MXenes have received increasing attention due to their two-dimensional layered structure, high conductivity, hydrophilicity, and large specific surface area. Because of these distinctive advantages, MXenes are considered as very competitive pressure-sensitive materials in applications of flexible piezoresistive sensors. This work reviews the preparation methods, basic properties, and assembly methods of MXenes and their recent developments in piezoresistive sensor applications. The recent developments of MXene-based flexible piezoresistive sensors can be categorized into one-dimensional fibrous, two-dimensional planar, and three-dimensional sensors according to their various structures. The trends of multifunctional integration of MXene-based pressure sensors are also summarized. Finally, we end this review by describing the opportunities and challenges for MXene-based pressure sensors and the great prospects of MXenes in the field of pressure sensor applications.

Abstract High‐performance flexible pressure sensors have attracted a great deal of attention, owing to its potential applications such as human activity monitoring, man–machine interaction, and robotics. However, most high‐performance flexible pressure sensors are complex and costly to manufacture. These sensors cannot be repaired after external mechanical damage and lack of tactile feedback applications. Herein, a high‐performance flexible pressure sensor based on MXene/polyurethane (PU)/interdigital electrodes is fabricated by using a low‐cost and universal spray method. The sprayed MXene on the spinosum structure PU and other arbitrary flexible substrates (represented by polyimide and membrane filter) act as the sensitive layer and the interdigital electrodes, respectively. The sensor shows an ultrahigh sensitivity (up to 509.8 kPa –1 ), extremely fast response speed (67.3 ms), recovery speed (44.8 ms), and good stability (10 000 cycles) due to the interaction between the sensitive layer and the interdigital electrodes. In addition, the hydrogen bond of PU endows the device with the self‐healing function. The sensor can also be integrated with a circuit, which can realize tactile feedback function. This MXene‐based high‐performance pressure sensor, along with its designing/fabrication, is expected to be widely used in human activity detection, electronic skin, intelligent robots, and many other aspects.

Abstract Pressure sensing is key to smart wearable electronics and human–machine interaction interfaces. To achieve a high‐performance pressure sensor that has broad linear range and is capable of detecting subtle changes of pressure, the good choice of sensing materials and rational design of structures are both needed. A novel piezoresistive sensor based on hollow MXene spheres/reduced graphene composite aerogel and flexible interdigital electrodes is presented. Benefiting from the unique microstructure of the composite aerogel, the prepared pressure sensor exhibits high sensitivity (609 kPa −1 in the range of 6.4–10 kPa), broad linear range (0–10 kPa), low detection limit (6 Pa), short response time (232 ms), and good durability (6000 cycles). Moreover, the device is able to monitor various human activities in real time, as well as distinguish tiny differences of grain. The potential application of mapping the location and intensity of the pressures is also explored.

Abstract Conductive Ti 3 C 2 T x MXenes have been widely investigated for the construction of flexible and highly‐sensitive pressure sensors. Although the inevitable oxidation of solution‐processed MXene has been recognized, the effect of the irreversible oxidation of MXene on its electrical conductivity and sensing properties is yet to be understood. Herein, we construct a highly‐sensitive and degradable piezoresistive pressure sensor by coating Ti 3 C 2 T x MXene flakes with different degrees of in situ oxidation onto paper substrates using the dipping‐drying method. In situ oxidation can tune the intrinsic resistance and expand the interlayer distance of MXene nanosheets. The partially oxidized MXene‐based piezoresistive pressure sensor exhibits a high sensitivity of 28.43 kPa −1 , which is greater than those of pristine MXene, over‐oxidized MXene, and state‐of‐the‐art paper‐based pressure sensors. Additionally, these sensors exhibit a short response time of 98.3 ms, good durability over 5000 measurement cycles, and a low force detection limit of 0.8 Pa. Moreover, MXene‐based sensing elements are easily degraded and environmentally friendly. The MXene‐based pressure sensor shows promise for practical applications in tracking body movements, sports coaching, remote health monitoring, and human–computer interactions. image