# Recent Progress in Pressure Sensors for Wearable Electronics: From Design to Applications
## 3 Fabrication Technologies
### Surface Modification  
Surface modification aims to produce a pattern of special shape confined to the active layer surface. These patterns have a high aspect ratio and use a mold produced through photolithography or soft-lithography. Pyramid or rod patterns are widely used to enhance deformability and surface area of the active layer surfaces (Figure 5). For resistive pressure sensors, the surface of the soft active layer, including patterns, is coated with conductive materials, such as Pt [116], poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) [117], and rGO [118]. Since the contact surface of the two electrodes becomes larger under external pressure, the deformability of the pattern should be improved for larger variation in resistance [116; 117; 118]. [PERSON] et al. introduced a sandblasted silicon mold to fabricate vertically-aligned CNTs on a PDMS substrate. As a resistive pressure sensor, it showed a sensitivity of 0.3 kPa\({}^{-1}\) at pressure \(<\) 0.7 kPa. [PERSON] et al. used micropatterned PDMS as a dielectric layer of a triboelectric tactile sensor [101]. The micropatterned PDMS was developed on various polymer foams, such as PU, ethylene vinyl acetate (EVA), and polyvinyl chloride (PVC). Among these polymer foams, PVC showed the highest sensitivity of 14.3 mV/kPa, but it had the narrowest measurement range. [PERSON] et al. introduced a novel fabrication method for porous pyramidal patterns [73]. Porous structures could be obtained with a particle template method with PS beads, and the pyramidal patterns were fabricated with a silicon mold. The PS beads were stacked in and on the pyramid-patterned mold, and then the PDMS solution was spin-coated. Under sufficient pressure during the curing process, a porous pyramidal PDMS dielectric layer could be obtained. The capacitive pressure sensor with this dielectric layer has a sensitivity of 44.5 kPa\({}^{-1}\) which is 10-20 times higher sensitivity compared to other capacitive pressure sensors. Figure 4: (a) Typical fabrication process of the emulsion-template method and (b) the cross-sectional SEM images of porous PDMS. Reproduced from [115]. Readily market-available molds, rather than specially prepared molds, or even the leaves of trees, have also been used to modify the surface of the active layer. [PERSON] et al. used a mimosa leaf as a mold to fabricate a leaf replica for a resistive pressure sensor [47]. Two gold-deposited replicas were assembled to construct a resistive pressure sensor, and it showed a sensitivity 50.17 kPa\({}^{-1}\). This work is significant in that it has produced a highly sensitive pressure sensor using an uncomplicated and low-cost process. [PERSON] et al. produced sandpaper-based PDMS microhump patterns [49]. The patterns were then coated with PEDOT:PSS to fabricate a contact-resistance-based resistive pressure sensor. This sensor showed a remarkably high sensitivity of 851 kPa\({}^{-1}\) at \(<\)3 kPa. The fabric is another patterned structure that is commercially available around the world. Using the fabric as a mold to improve the surface roughness of the sensing layer can be simplified at a low cost. [PERSON] et al. used a silk mold to obtain a patterned PDMS surface and investigated the effect of the pattern density on the sensitivity of the resistive pressure sensor [119]. The results indicate that a higher pattern density is more advantageous due to its more effective contact sites, which leads to larger charge transfer when pressure is applied. To optimize the geometry of the pyramidal pattern for high sensitivity, [PERSON] et al. presented an analytical model that investigated the structural effects of the patterns [120]. From the results of this work, the compressive deformation of the pyramidal patterns increases as the height of the pyramid increases, and the bottom length of the pyramid decreases. That is, the high aspect ratio of the patterns leads to more deformation of the patterns under the same applied pressure, resulting in higher sensitivity for any type of pressure sensor.