Flexible pressure sensor is a kind of sensor that can convert external force signals (such as bending, torsion, stretching and other stimuli) into resistance, current or capacitance signals. According to the different sensing mechanisms, it can be divided into four types: piezoresistive, capacitive, piezoelectric and triboelectric^{[15⇓⇓~18]}.

Piezoresistive pressure sensor is a kind of sensor that converts pressure change into resistance or current signal. Its resistance change formula is: R = ρL/A (ρ is the resistivity of the material, L and A are the length and surface area, respectively). The sensing mechanism is shown in Figure 1A. When external pressure is applied, the material will deform, which indirectly changes the distribution density and contact state of the conductive material inside the sensor, so that the electrical signal of the sensor changes regularly. Therefore, the piezoresistive pressure sensor has a simple structure and a wide detection range compared with other sensors, and is widely used in real-time health monitoring, intelligent robots, and artificial limbs^[19,20]. For example, Zhou et al. Used electrospinning technology to prepare an ultra-thin sensing layer, which is a composite nanofiber film composed of poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) and polyamide 6, and connected electrodes on the upper and lower sides of the sensing layer.A flexible piezoresistive pressure sensor with high sensitivity (6554.6 kPa^-1), fast response time (53 ms), wide detection range (0 ~ 60 kPa) and good cyclic stability (> 10 000 times) is obtained, which can detect human movements such as respiration, pulse, hand and wrist^[21].

Capacitive pressure sensor is a kind of sensor with parallel plate capacitor as the sensing element, which is usually composed of upper and lower electrode plates, insulator and substrate. The capacitance change formula is C=ε₀ε_rA/d(C is the capacitance, ε₀ is the vacuum permittivity, ε_r is the relative permittivity, and d is the plate spacing). The sensing mechanism is shown in Figure 1b. When subjected to pressure, the distance between the upper and lower electrode plates changes, resulting in a change in capacitance. Capacitive pressure sensor has the advantages of large detection range, high sensitivity, simple structure and low energy consumption. For example, Yu et al. Designed a multi-walled carbon nanotube/polydimethylsiloxane (MWCNTs/PDMS) electrode layer and a barium titanate/polydimethylsiloxane (BaTiO₃/PDMS) dielectric layer with micro-convex structure embedded in the surface, and obtained a flexible capacitive pressure sensor with high sensitivity (2.39 kPa^-1), wide pressure detection range (0 – 120 kPa), low detection limit (6.8 Pa) and fast response time (16 ms), which can identify postures such as sitting, standing, walking and running^[22].

Piezoelectric pressure sensor is a kind of sensor based on pressure-sensitive effect, which can realize the conversion between mechanical energy and electrical energy. The conduction mechanism is shown in Figure 1C. When the material is deformed under pressure, the positive and negative charges inside the functional material are separated, and opposite charges are generated on the two surfaces, and a potential difference is formed inside. The pressure is determined by detecting the change of the potential difference. Compared with other sensors, piezoelectric pressure sensors can directly convert mechanical energy into electrical energy, which is of great significance to promote the realization of self-powered sensing technology. Park et al. Synthesized a lead zirconate titanate piezoelectric film and bonded it on a polyethylene terephthalate substrate to prepare a self-supplied voltage electric pulse sensor with high sensitivity (0.018 kPa^-1), fast response time (60 ms) and good mechanical stability, which can be completely attached to the skin surface to respond to tiny pulse signals from the human body^[23].

The triboelectric pressure sensor is a pressure sensor based on the coupling effect of triboelectrification and electrostatic induction, which is composed of two kinds of friction materials with opposite polarities. The sensing mechanism is shown in Figure 1 d. When pressure is applied, two different materials come into contact with each other, and opposite electrostatic charges are generated on the surface. As the pressure decreases, the two materials separate from each other, and the electrodes on the back of the two triboelectrically charged materials generate charges by electrostatic induction. This creates a potential difference that causes electrons to transfer through an external circuit until an equilibrium state is reached when the two materials are completely separated. Compared with piezoresistive and capacitive pressure sensors, triboelectric pressure sensors can work normally without power supply. Compared with piezoelectric pressure sensors, triboelectric pressure sensors have high sensitivity and many kinds of sensing materials to choose from^[24]. Shlomy et al. Used PDMS as the negative electric layer, nylon and cellulose butyl acetate as the positive electric layer, and prepared a triboelectric pressure sensor by bonding metal electrodes. Implanted it under the skin, it can convert tactile pressure into potential, which can be transmitted to sensory nerves through electrodes to simulate tactile sensation and compensate for traumatic peripheral nerve injury^[25]. The triboelectric pressure sensor-based triboelectric nanogenerator exhibits the advantages of high energy conversion efficiency, excellent performance, and easy processing, which brings new opportunities and challenges for self-powered wearable electronic devices.

Although different flexible pressure sensors have different sensing mechanisms and have their own advantages and disadvantages, the key performance indicators of flexible pressure sensors are the same.It mainly includes sensitivity, pressure detection range, detection limit, response/recovery time, cycle stability, etc. See Table 1 for the specific definition of performance indicators, the meaning expressed, and the way to improve.

It can be seen from Table 1 that a flexible pressure sensor with excellent performance requires a device with both high sensitivity and wide pressure response range. At the same time, low detection limit, short response/recovery time, high cycling stability and linearity are also the key indicators to be pursued for flexible pressure sensors, which are crucial for the reliable operation of sensors in new health care, pulse detection and electronic skin^{[30⇓⇓~33]}. However, it is unrealistic to pursue all of the above properties to the extreme. Therefore, at present, the focus of attention in the development of flexible pressure sensors is first on the sensitivity, followed by the pressure response range, that is, to ensure the performance of the sensor before paying attention to its scope of application. According to the literature, the main factors affecting the sensitivity of the sensor are the substrate material and the microstructure of the sensing material.

Typical micro-convex structures mainly include micro-dome structure, micro-cone structure and micro-cylinder structure. Each micro-convex structure has its own characteristics, which can meet the needs of pressure sensors in different application directions. The micro-convex structure is easy to deform under small pressure, which can significantly increase the contact area of the active layer, thus producing a large electrical signal change and effectively improving the sensitivity of the pressure sensor. In addition, the deformation of the micro-convex structure can also broaden the pressure detection range.

Micro-dome structure is one of the most common surface micro-convex structures. Generally speaking, there are many smooth micro-bulges on the surface of the substrate material. Tang et al. Fabricated a graphene oxide (GO)/silicone rubber resistive pressure sensor based on a micro-dome structure using a one-step foaming strategy and a photolithography technique, as shown in Fig. 2^[37]. By changing the foaming time to change the structural characteristics of the micro-dome, the sensitivity and linear working range of the pressure sensor can be adjusted. When the foaming time is 3 min, the sensitivity of the sensor is 0.19 kPa^-1(0~4.2 kPa), which is much higher than that of the pressure sensor with flat surface (0.04 kPa^-1). In addition, the sensor has a wide sensing range (0 ~ 100 kPa), short response time (75 ms) and recovery time (80 ms). The above results show that the flexible pressure sensor based on micro-dome structure has a good balance between sensitivity and linear sensing range, and can be applied in human body pulse detection, finger click detection, grip force sensing, etc. In order to further improve the sensitivity of the pressure sensor, Park et al. coated a mixture of CNT and PDMS on a silicon mold with a micro-dome structure, peeled it off from the mold after curing, prepared a conductive film with a uniform micro-dome array, and assembled it into a pressure sensor, as shown in Figure 3^[38]. The sensor is based on the change of tunneling current between two microdome arrays. The external pressure causes stress concentration at small contact points and local deformation in the microdomain, which leads to the increase of contact area and tunneling current, and slows down the effect of hysteresis on the sensing performance. The sensor has a sensitivity of 15.1 kPa^-1, a response time of 40 ms, and a detection limit of 0. 2 Pa, and has application prospects in health monitoring, motion detection, and so on. Although the sensitivity of the pressure sensor based on the micro-dome structure can be improved, and the response time and the sensing range can be improved, the improvement of the sensitivity of the structure is limited due to the smooth surface bulge.

The stress distribution of the micro-cone structure is not uniform, usually concentrated at the tip, so it shows the advantages of improving the sensitivity that the micro-dome structure does not have^[39]. When pressure is applied, the tip of the micro-cone structure will be compressed more, resulting in higher mechanical deformation, greater electrical signal changes and improved sensitivity. Cao et al. First prepared a silicon mold with an inverted micro-cone structure, and sprayed single-walled carbon nanotubes (SWCNTs) on its surface. Finally, they spun the PDMS prepolymer and cured it to obtain a SWCNTs/PDMS composite film with a micro-cone structure, based on which they prepared a pressure sensor.The sensor has high sensitivity (3.26 kPa^-1), short response/recovery time (200 ms/100 ms) and good cycling stability. It can recognize various textures like human fingerprint epidermis, such as textures of different fabrics, Braille, etc. It has great potential applications in robot skin and tactile perception^[40]. As such,

Ma et al. Used the inverse template method to fabricate a flexible pressure sensor based on PDMS and CNT conductive micro-cone structure, as shown in Fig. 4^[28]. A further improvement is that the sensor systematically adjusts the pressure sensing characteristics of the device by adjusting the spacing of the micro-cones and the length of the substrate. When the ratio of microcone spacing to substrate length is 1:1, the sensor has high sensitivity (0.34 kPa^-1) in the range of 10 – 100 kPa, fast response time (48 ms), low hysteresis, low operating voltage (0.1 mV), and high mechanical robustness. The optimized sensor is integrated into a wearable pressure sensor system to realize the potential application in electronic skin. However, the tip of the micro-cone structure is easy to be worn during use, so its cyclic stability is not enough compared with the micro-dome structure.

The micro-cylinder structure has a larger contact area than the micro-dome structure, and has better structural stability than the micro-cone structure, so the advantages of the two structures can be integrated to better improve the sensing performance of the flexible pressure sensor. Lin et al. Prepared a PDMS film with a microcylinder array and a polyvinylidene fluoride (PVDF) fiber sensing electrode, and assembled them into a capacitive pressure sensor^[41]. The high dielectric coefficient of PVDF and the voids generated by electrospinning are beneficial to the formation of a special expansion layer, and a novel double dielectric layer structure is obtained by combining the array micropillars. When pressure is applied on the sensor, the fiber layer releases air in the gap, and a slight pressure also changes the distance between the electrodes, thus improving the sensitivity of the sensor. When the pressure on the sensor is released, the cylinder pressed into the fiber quickly expands the distance between the electrodes, and the fiber layer compressed by the pressure quickly returns to its original bulky state. This microcylinder structure can respond quickly to changes in the distance between the electrodes and slow down the effects of hysteresis, thereby increasing the capacitance change and durability of the sensor. The sensor has a sensitivity of 0.6 kPa^-1, a response time of 25 ms, a detection limit of 0. 06 Pa, and a good cyclic stability (> 10 000 cycles), and can be successfully applied to pulse, limb movement, respiration, and acoustic vibration. In addition, Lu et al. Prepared anodic aluminum oxide templates (t-AAO) with different heights by alternating oxidation and pre-broadening using phosphoric acid/oxalic acid as electrolyte, and used them as reaction vessels to polymerize pyrrole monomers into polypyrrole (PPy) on the pore walls of t-AAO templates through in-situ polymerization.Finally, a microcylinder structured film was prepared by coating a polymethylmethacrylate/N, N-dimethylformamide mixed solution, and the film was assembled together face to face to obtain a highly interlocked structure with interlocked nanoarrays (IOCA) pressure sensor, as shown in fig. 5^[42]. The sensor has high sensitivity (268.36 kPa^-1) in the pressure range of 0 to 200 Pa, fast response/recovery (48/56 ms) and good repeatability and continuity, and can accurately detect wind speed, wrist torsion and bending, which has a wide range of applications in wearable medical monitoring, electronic skin and so on.

The small initial current and the significant change of current with pressure are the key to improve the sensitivity and linearity of the sensor^[43,44]. The rough structure of the thorn-like conductive fibrous membrane can endow it with a large specific surface area and increase the contact between the sensitive layer and the electrode, thus effectively improving the sensing performance of the sensor. Hu et al. Prepared a carbon hybrid thorn-like fiber (CHF) with a fluffy structure by wet spinning. The skeleton of the fiber was composed of GO, and the entangled CNTs on the skeleton formed a thorn-like structure. Then the CHF and silver electrode were integrated on the PDMS substrate to obtain a flexible piezoresistive sensor, as shown in Figure 6^[45]. The sensor's fluffy structure expands the internal space, allowing the fiber to deform, resulting in satisfactory sensitivity. When subjected to bending strain, the GO nanosheets begin to slide, and the CHF fibers deform to form conductive paths. With the increase of stress, the edge of GO nanosheets is disconnected, and the charge is transmitted through the thorn-like CNTs to ensure the integrity of the overall structure under strain loading. When the stress is removed, the sensor recovers the layered structure. The sensor has a sensitivity of 1127 kPa^-1, a response time of 70 ms, and a cyclic stability of more than 2000 cycles, and can be used to record real-time sitting signals from the lumbar and cervical spines. Similarly, Sharma et al. Successfully prepared thorn-like P-CPCM fiber membranes by blending acrylonitrile, cellulose and MXene to prepare nanofiber membranes using electrospinning technology, followed by carbonization and in situ polymerization of polyaniline (PANI)^[46]. A flexible piezoresistive sensor was successfully constructed by stacking two Cu electrodes and three layers of P-CPCM fiber membranes on a PDMS flexible substrate in turn. When pressure is applied, the thickness of the film decreases, the current passes through a large number of spine tip contacts, the conductive path increases, and the resistance decreases; The greater the pressure, the larger the contact area and the better the conductivity; After the pressure is released, the film thickness and the electrode are restored to the original state. The introduction of thorn-like nanofibers,

High sensitivity (179.1 kPa^-1), low detection limit (1.2 Pa), wide pressure range (0 ~ 50 kPa) and high durability (> 10 000 cycles) of the piezoresistive sensor are achieved. It can be seen from the above that the bramble structure is mainly produced by attaching conductive materials to the fiber through spinning technology. Although the bramble structure can improve the sensitivity of the sensor, its preparation process is complex, and it is difficult to control the uniformity of the bristle structure, which may affect the sensing stability of the sensor.

Wrinkle structure is a special surface micropattern, which is usually formed under mechanical stress. Baek et al. Prepared silicone rubber with a wrinkled surface structure by mechanical stretching, and then placed gold-coated silicon electrodes on both sides of the silicone rubber to prepare a capacitive pressure sensor (Fig. 7)^[47]. The wrinkle structure effectively increases the contact point density of the sensor, thereby improving the sensitivity of the sensor. Compared with the wrinkle-free sensor, the response and recovery time of the single-sided wrinkle sensor are improved by 9.7% and 17%, respectively, and the response and recovery time of the double-sided wrinkle sensor are improved by 42% and 25%, respectively.The results show that the introduction of wrinkled structure into the elastic matrix is an effective way to improve the sensing performance, and the degree of wrinkling has a significant impact on the response and recovery time of the sensor. In order to simplify the process, Luo et al. Prepared a PPy film with a wrinkled structure by electroplating, and then filled polyvinyl alcohol nanowires (PVANW) between the polyethylene terephthalate film sprayed with indium tin oxide and the wrinkled PPy film to prepare a capacitive pressure sensor^[48]. When the sensor is subjected to external pressure, the wrinkled PPy film is deformed, the conductive path is increased, and the isolation function of PVANW is overcome, resulting in an ultra-high sensitivity (228.5 kPa^-1), a low detection limit (2.97 Pa), and a fast response (66.8 ms). Compared with the surface micro-convex structure and the thorn structure, the wrinkle structure has the advantages of simple formation process, uniform wrinkle size, easy control, and relatively good stability in the use process, so it is an effective strategy to prepare a high-sensitivity flexible pressure sensor.

To sum up, the substrate material with surface microstructure is used to prepare the pressure sensor. Because the substrate material has good flexibility, the stability of the microstructure is relatively good, and the cycle stability is basically not affected while the sensitivity of the pressure sensor is improved. However, the contribution of different surface microstructures to the performance of the sensor is still different, as shown in Table 2, and the sensitivity of the pressure sensor formed by this strategy is limited, and the response/recovery time is longer.