# Recent Progress in Pressure Sensors for Wearable Electronics: From Design to Applications
## 3 Fabrication Technologies
### Porous Structures  
A porous structure is used in almost all types of pressure sensors because it can broaden the surface area and improve the deformability by lowering the the density and Young's modulus of the layer. In this section, electrospinning, particle template, and emulsion template will be covered. Electrospinning is a promising fabrication technique to produce nano- and micro-sized nanofibers (Figure 2). The NFs promise sufficient deformability even if the material is relatively brittle. In the electrospinningning, a polymer solution is ejected from a nozzle by applying a high voltage to the nozzle, and the collector is grounded. After ejection, the jet experiences a whipping instability and extension before deposition due to interactions between charges existing in the jet, resulting in randomly deposited NFs. The electrospun fibers have a rough surface and remarkably large surface area per unit volume. Moreover, the electrospun NF layer has excellent deformability compared to the bulk PDMS layer. Our group has introduced an electrospun NF-based capacitive pressure sensor [111]. An electrospun PVDF-TrFE NF layer exhibited a stiffness 6 times lower than that of a spin-coated PDMS layer. [PERSON] et al. fabricated MXene composite nanofibrous scaffolds via electrospinningning [112]. The fabricated sensor demonstrated a high sensitivity of 0.51 kPa\({}^{-1}\) and a minimum detection limit of 1.5 Pa. In addition, the sensor showed stable output signals for 10,000 cyclic tests under pressure of 167 kPa. [PERSON] et al. produced PVDF NFs to fabricate a dielectric layer of a TENG [107]. In this work, polarized PVDF NFs showed higher electrical performances compared to flat-surface PVDF layers and unpolarized PVDF NFs. Core-shell NFs of a PDMS ion gel/PVDF-HFP was first introduced by [PERSON] et al., and the sensitivity with this NF mat was 0.43 kPa\({}^{-1}\)[108]. Electrospinning is rarely used for resistive pressure sensors because a high voltage difference of a few kV is induced between the nozzle and ground collector. In addition, electrospinning can only use materials with a high molecular weight. Figure 2: Schematic of electrospinningning technique and SEM images of electrospun NFs. (**a**) Electrospump PVDF-TrFE NFs at different solution concentrations of 14, 18, and 22 _avg_%/% from the top to bottom of images. Reproduced with permission from [90], Copyright (2016), American Chemical Society. (**b**) Electrospun NFs of PVDF and PVP. Reproduced with permission from [113], Copyright (2018), Elsevier. (**c**) Schematic diagram of core-shell electrospinningning technique and SEM images of NFs. Reproduced with permission from [108], Copyright (2018), Elsevier. The particle template method is one of the simple ways to produce porous structures [39; 67; 68; 69; 70; 75; 76]. The easily dissolvable particles, such as sugar and NaCl, are first mixed with a matrix material, which is usually silicone rubber (Figure 3). After the matrix material is cured, the particles are dissolved and removed to complete all fabrication processes. The particle dissolving process requires a long duration of several hours to more than 24 h, but as a relatively simple method, even a person without proper training can sufficiently secure reproducibility with the particle template method. [PERSON] et al. used this method to produce a PDMS mold with a PS bead as a dissolvable particle [114]. Another PDMS layer was produced via the PDMS mold, and Au was sputtered on the new PDMS layer. The spin-coated PVDF layer was then sandwiched between two PDMS layers. The sensor showed a sensitivity of 30.2 kPa\({}^{-1}\) which is sufficient to measure pressure of 0.7 Pa.  
To implement the emulsion template method, two or more aqueous solutions should be homogeneously mixed. For example, a fluorocarbon (FC-43) dispersed PDMS emulsion was used to fabricate a porous structured PDMS layer for TENG and capacitive pressure sensor applications by our group (Figure 4) [72; 115].