# Single-Layer Pressure Textile Sensors with Woven Conductive Yarm Circuit
## 3 Results and Discussion
### Electrical Properties  
To analyze the characteristics of sensors, we use a universal testing machine (UTM) from Dacell Co. Ltd., Seoul, Korea, and a source meter (Keysight B2902A). As shown in Figure 3a, sensor samples are placed above the sole and between the grips of the UTM machine. For the measurements of the  
Figure 2: (**a**) Single-layer pressure textile sensor, (**b**) nonconductive wool yarn at 100 \(\upmu\)m, (**c**) nonconductive wool yarn at 50 \(\upmu\)m, (**d**) conductive wool yarn at 100 \(\upmu\)m, and (**e**) conductive wool yarn at 50 \(\upmu\)m.  
electrical resistance, electrical wires connected directly to the two electrode threads of the samples by the grips. A constant source voltage applied to the samples during the test. Resistance and pressure are continuously measured when loading the force. At least five samples are tested, and the experiments are repeated 50 times in order to record average values, respectively. The working principle (Figure 3b,c) of one-layer pressure sensors is based on sensing the change of electrical resistance when the distance between conductive particles (CNTs) changes in the conductive fibers, as well as between the CNTs and electrode threads. Equation (1) shows the electrical conduction mechanism under the loading of pressure where \(R\), \(\rho\), \(l\), and A present the resistance, resistivity, fiber length, and cross-section area, respectively. As described in Figure 3c, d\({}_{0}\) is the initial distance between CNTs, d is the distance between CNTs after pressuring, de0 is the initial distance between CNTs and electrodes, and de is the distance between CNTs and electrodes after pressuring. When pressure is applied, these distances become closer (d \(<\) d\({}_{0}\) and d\({}_{e}\) \(<\) d\({}_{e0}\)); accordingly, the contact area becomes wider. Thus, total resistance will decrease. \[R=\rho\frac{l}{A} \tag{1}\]  
Figure 4a shows a relationship between the pressure and resistance of sensors. Resistance changes from a maximum of 283.2 \(\Omega\) to a minimum of 158.7 \(\Omega\), following different levels of load pressures. It is clear that the sensors have a large working range from 0 kPa to 1,000 kPa, and fit many applications. Figure 4b shows a set of the current-voltage (I-V) graphical curves, which are used to define the operation of the sensor under different static pressures at 250 kPa, 500 kPa, 750 kPa, and 1000 kPa within the system. The applied voltage from -1.8 V to 1.8 V demonstrated an ohmic behavior of the sensors. The slope of I-V curves reduces with a decrease of pressure, indicating that an increase in applied pressure led to a decrease in the sensor's resistance. Figure 3: (**a**) Universal tensile machine (UTM), (**b**) sensing layer, and (**c**) working principle. Figure 5a describes the resistance of sensors after 100 loading cycles. The sensitivity decreases initially but stabilizes after some cyclic loading/unloading periods. The reason for this behavior is the deformation (permanent) in the structure under pressure leads to a change in the electrical response of the sensors. However, after 85 cycles, that deformation of the fibers remains stable, and the electrical output of the sensor becomes stable (Figure 5b). Response time is an important parameter that defines the time lag between an electronic input and the output signal. The response delay in the sensor is caused by the viscoelastic nature of wool yarns and the connectivity between the CNT coatings under pressure. As shown in Figure 5b, the fast response time is about 50 ms. The fast connection time of conductive particles ensures the rapid electrical property of these sensors in a practical application. As described in Figure 6a, we also evaluated the hysteresis of the proposed sensor at different frequencies of 0.05 Hz, 0.1 Hz, 0.2 Hz, 0.5 Hz, and 1 Hz. With each frequency, the upper line presents the loading phase, and the lower line presents the releasing phase.