# Recent Progress in Pressure Sensors for Wearable Electronics: From Design to Applications
## 2 Pressure Transduction Mechanism and Design Principles
### Capacitive Type  
A dielectric layer of a capacitive type pressure sensor plays a critically important role in achieving high sensitivity (Figure 1b). The capacitive pressure sensor is usually based on a parallel plate capacitor, and the capacitance is defined by the equation: \(C=\varepsilon_{0}\varepsilon_{r}A/d\), where \(\varepsilon_{0}\) and \(\varepsilon_{r}\) are the dielectric constants of the vacuum and the dielectric layer, \(A\) is the overlapped area of the two electrodes, and \(d\) is the distance between the electrodes. When pressure is applied on the sensor, the dielectric layer deforms, i.e., \(d\) changes, resulting in variation in the capacitance value. PDMS and Ecoflex are often used as the dielectric materials of the capacitive pressure sensor due to their highly deformable nature [57; 58; 59; 60; 61]. Moreover, by modifying the surface of the dielectric layer with microstructures [62], micropillar arrays [63], wrinkled patterns [64], and pyramidal shapes [65; 66], the sensitivity of the sensor can be dramatically enhanced. These patterns of various shapes not only improve the deformability of the dielectric layer, but also vary the effective dielectric constant (\(\varepsilon_{r}\)), resulting in more variation in the capacitance. The pores between patterns are occupied by air, and their volumes are reduced by external pressure due to the deformation of the patterns. In general, since the permittivity \(\varepsilon_{r}\) is greater than that of the air, as the portion of the air decreases, the effective dielectric constant increases, and consequently, the capacitance increases. With the synergetic effect of enhanced deformability of the dielectric layer and the varied effective dielectric constant, the sensitivity of the sensor can be dramatically enhanced. However, the surface modification method generally causes the pattern to collapse even at small pressure because the size of the pattern is typically only a few tens to hundreds of micrometers. It has high sensitivity until the pattern collapses, but the range is relatively short. In addition, since mold manufacture through photolithography is usually used, it is disadvantageous in terms of fabrication cost. However, once the mold is manufactured, it can be used semi-permanently and shows excellent reproducibility as long as the mold is not damaged. Another way to improve the deformability of the dielectric layer is to form porous structures [67; 68]. [PERSON] et al. introduced the fabrication strategy of using porous Ecoflex with a particle template method [69]. In this work, a sugar cube containing Ecoflex was cured, and then the cube was dissolved with hot water to obtain porous structures. [PERSON] et al. described a particle template method with polystyrene (PS) beads as a sacrificial material and PDMS as a dielectric material [70]. PS beads were dissolved in dimethylformamide (DMF) for 12 h, and the fabricated pressure sensor with porous structures showed a sensitivity of 0.63 kPa\({}^{-1}\), which is 7.85 times higher than a bare PDMS dielectric layer. To fabricate a porous structured elastomer, [PERSON] et al. first prepared an emulsion of water and PDMS [71]. The phase of water changed from liquid to gas during the curing process to form micro-sized pores, and the PDMS was simultaneously cured, while the pores were trapped in the layer. Our group introduced using fluorocarbon (FC-43) with the emulsion template method, and the highly sensitive capacitive pressure sensor with a sensitivity of 1.43 kPa\({}^{-1}\) could be obtained [72]. [PERSON] et al. achieved a great enhancement in sensitivity by using porous pyramidal patterned PDMS [73]. The deformability of the dielectric layer was further modified with both surface structures and porous structures, resulting in an extremely high sensitivity of 44.5 kPa\({}^{-1}\). Furthermore, the sensitivity of the porous structure-based capacitive pressure sensor can be further improved with a composite dielectric layer combined with Ag NPs [74], Cu/Ni NFs [39], carbon paste [75], and CNTs [76].