# Simultaneous Sensing of Touch and Pressure by Using Highly Elastic e-Fabrics
## 3 Experimental
### Structure
#### 3.1.2 The Capacitive Pressure Sensor Layer  
Figure 3a shows the structure of the capacitive pressure sensor layer, consisting of a vertical or capacitive column layer (silver paste/SWCNT) on the top side, a dielectric layer (spacer fabric), and a horizontal or capacitive row layer (silver paste/SWCNT) on the bottom side. The column structures have been selected for the capacitive pressure layer with a width of each sensing plate (column/row) of 12 mm, and a gap between two plates of 5 mm. These dimensions/resolutions of the two sensing layers were chosen in order to easily make the prototype samples. In the screen-printing technologies on the fabric, the smallest size of each electrode line should be about 0.1 mm to ensure the working performance, but it needs more experiments to obtain the final size. Figure 3c shows the electrical response of the capacitive pressure sensor layer under the pressure of a finger. When the spacer fabric is under pressure, the spacer layer (PET yarns) is bent from the stretching state. When the pressure is released, the spacer layer returns to the initial state. The pressing position could be detected by the capacitive change between the column layer on the top side and the row layer on the bottom side.  
At the pressing point, the capacitive pressure sensors are based on the principle of parallel-plate capacitance [40; 41; 42]. The capacitance is accordingly varying as a function of changes in the distance between parallel-plate electrodes (Figure 3d). In wearable applications, the design of the capacitive pressure sensors is difficult by the low elasticity of the materials. Otherwise, the outputs are easily influenced by parasitic capacitance. The spacer layer, by PET yarns of the spacer fabric, will improve the elasticity of the electrodes, thereby increasing the sensitivity of the sensor for flexible applications in wearable research [26; 43; 44].  
Figure 2: (**a**) Structure of the capacitive sensor layer, (**b**) real picture of the capacitive sensor layer, and (**c**) working principle of the capacitive sensor layer.  
The capacitance at the pressing point (C) can be calculated by Equation (1), where A represents electrode area, d represents dielectric thickness, \(\varepsilon_{0}\) represents a constant for the dielectric permittivity of vacuum, and \(\varepsilon_{r}\) represents the permittivity of a dielectric. Following that, the capacitance change relies on the change in dielectric thickness of the spacer fabric.  
\[C_{sensor}=\ \varepsilon_{o}\varepsilon_{r}\frac{A}{d_{0}}. \tag{1}\]  
The variations in the permittivity of dielectric layers also contribute to a change in capacitance, as shown in Equation (2), where \(\varepsilon_{air}=1\) and \(\varepsilon_{PET}=3.3\). The sensitivity increases when the volume of the air gaps decreases. The decrease in dielectric thickness (d) and increase in permittivity (\(\varepsilon_{e}\)) under pressure together contribute to the increase in the sensitivity of the textile capacitance sensor.  
\[\varepsilon_{e}=(\%V_{air}\cdot\varepsilon_{air}+\%V_{PET}\cdot\varepsilon_ {PET}). \tag{2}\]