This review summarizes the progress in flexible pressure sensor arrays from their fundamental designs to device applications.

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Recently, piezoelectric characteristics have been a research focus for 2D materials because of their broad potential applications. Black phosphorus (BP) is a monoelemental 2D material predicted to be piezoelectric because of its highly directional properties and non‐centrosymmetric lattice structure. However, piezoelectricity is hardly reported in monoelemental materials owing to their lack of ionic polarization, but piezoelectric generation is consistent with the non‐centrosymmetric structure of BP. Theoretical calculations of phosphorene have explained the origin of piezoelectric polarization among P atoms. However, the disappearance of piezoelectricity in multilayer 2D material generally arises from the opposite orientations of adjacent atomic layers, whereas this effect is limited in BP lattices due to their spring‐shaped space structure. Here, the existence of in‐plane piezoelectricity is experimentally reported for multilayer BP along the armchair direction. Current–voltage measurements demonstrate a piezotronic effect in this orientation, and cyclic compression and release of BP flakes show an intrinsic current output as large as 4 pA under a compressive strain of −0.72%. The discovery of piezoelectricity in multilayer BP can lead to further understanding of this mechanism in monoelemental materials.

With the arrival of intelligent terminals, tactile sensors which are capable of sensing various external physical stimuli are considered among the most vital devices for the next generation of smart electronics. To create a self-powered tactile sensor system that can function sustainably and continuously without an external power source is of crucial significance. An overview of the development in self-powered tactile sensor array system based on the triboelectric effect is systematically presented. The combination of multi-functionalization and high performance of tactile sensors aimed at achieving highly comprehensive performance is presented. For the tactile sensor unit, a development is summarized based on the two primary modes which are vertical contact-separation and single-electrode. For the pressure mapping array, the resolution is significantly enhanced by the novel cross-type configuration based on the single-electrode mode. Integrated with other mechanisms, the performance will be further elevated by broadening of the detect range and realizing of visualization of pressure imaging. Then, two main applications of human-machine interaction (HMI) and trajectory monitoring are comprehensively summarized. Finally, the future perspectives of selfpowered tactile sensor system based on triboelectric effect are discussed.

Two-dimensional (2D) materials, possessing numerous remarkable properties including high electrical conductivity, optical transparency and mechanical strength, have supplied a fertile soil for theoretical research and practical application in electronics and optoelectronics. Due to a persistent need of strain sensors, microelectromechnical system (MEMS), nanorobots and active flexible electronics, piezotronic and piezo-phototronic effect of 2D materials have been attracting growing attentions in latest years. Therefore, a comprehensive and intensive understanding of piezotronic and piezo-phototronic effect of 2D materials is required. Here we review the recent progress in theoretical analysis and experimental observation of piezotronic and piezo-phototronic effect of 2D materials enabled by non-centrosymmetric crystal structure. After introducing the fundamental physics of piezotronic and piezo-phototronic effect concisely, the origination and analysis of the piezoelectricity in 2D materials are discussed in detail. Furthermore, we focus on the application in piezotronic and piezo-phototronic effect of transition-metal dichalcogenides (TMDCs) including transistor, nanogenerator, humidity sensor and photodetector. Moreover, some other 2D piezoelectric materials (PEMs) are also been summarized owing to their potential applications in piezotronics and piezo-phtotronics. Finally, some perspectives are put forward on the following opportunities and challenges for future research in this emerging field.

We report a dynamic tuning on coherent light emission wavelengths of single ZnO microwire by using the piezoelectric effect. Owing to the dominant role occupied by the piezoelectric polarization effect in the wurtzite-structure ZnO microwire, the effective dielectric constant (or refraction index) of the gain media was modulated toward an increasing trend by applying a tensile strain, resulting in a shift of the strain-mediated whispering-gallery mode (WGM) lasing at room temperature. Also, the strain required to resolve the spectra in the two operating types of PL and lasing were systematically analyzed and compared. Because of the narrow line width in the lasing mode, the strain-dependent spectral resolution was improved by an order of magnitude, making it feasible for achieving high-precision, ultrasensitive, and noncontact stress sensing. Our results have an important impact on laser modulation, optical communication, and optical sensing technology.

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The continuous development of strain sensors offers significant opportunities for improving human-machine interfaces and health monitoring. The dynamically modulated lasing mode is a novel approach to realize a flexible, noncontact, high color-resolvability, high-resolution, and ultrasensitive strain sensor. Here, a flexible strain sensor perceiving stress variations is reported via the dynamical regulation of a GaN whispering gallery lasing mode based on the piezoelectric effect. The refraction index of GaN shows a linear relationship with the applied external tensile strain, resulting in a redshift phenomenon of the lasing mode peak at room temperature due to the predominant function of the piezoelectric polarization in the GaN microwire. Compared with a strain sensor relying on the wavelength shift of a photoluminescence (PL) emission peak, the differences and advantages of a sensor based on the strain-induced lasing mode variation are also investigated and analyzed systematically. This strain sensor may serve as an essential step toward the color mapping of mechanical signals by optical methods, with potential applications in color-perceived touching sensing, noncontact stress measurement, laser modulation, and optical communication technologies.