# A High Compressibility Pressure--Sensitive Structure Based on CB@PU Yarn Network
## 3 Results and Discussion
### Piezoresistive Characteristics of CB@PU Network Pressure Sensors
The combination of the two factors caused the sensitivity to decrease gradually from the micro-pressure zone to the high-pressure zone. As the pressure increased, the resistivity curves showed a similar trend. However, the thinner PU yarn in the micro-pressure zone showed higher sensitivity of 2.33 kPa\({}^{-1}\). This was because the yarns in different sensors had the same density per unit area. The thicker the yarn, the larger the area superimposed between the yarns, and the larger the effective contact area of the conductive layer. From no-pressure to micro-pressure, the network with thicker yarn had a more obvious current change. In the remaining regions, the sensor made of finer PU yarn had higher sensitivity. The yarn with thinner network had smaller elastic modulus and softer structure that could be easily compressed, so the sensitivity was higher. What's more, the experimental results showed that the thinner yarn had higher GF but lower linearity. The resistivity curve of the 73 tex yarn was more stable than that of the 12 tex yarn. The stiffness of the thick yarn network was higher and it could afford higher force, thus the stability of the network was higher. The CB@PU network fabricated with 12 tex PU yarn was preferred. The resistance response upon cyclic loading was studied to evaluate the potential of the CB@PU network as a strain sensor. Figure 13 illustrates the hysteresis curve for the CB@PU network sensor. As revealed in Figure 13, for small stress such as \(\sigma=500\) Pa, the relative change of the resistance overlapped with that in the stress-release cycle for the sample, indicating a small hysteresis in the response. For a larger stress such as \(\sigma=10\) kPa, there existed a relatively large hysteresis in the response. However, even in this case, the original resistance of the sensor was fully recovered after releasing it from strain. It should be mentioned that in the process of making network structure, each yarn in the network was stretched to 200% of the original length, that was, when the sensor was unstressed, the yarn in the network had a 100% strain, so the yarn had an elastic potential energy in the rest state. Therefore, when subjected to a pressure, the reaction time of the sensor and the rebound time of the yarn were thus reduced. The hysteresis properties caused by PU yarn materials were alleviated. In the subsequent research process, we will continue to explore methods to improve the hysteresis. The elastic performance was tested and the capability of the device as a strain sensor was evaluated. The responsive behaviors of CB@PU networks in different regions were recorded. The normalized current response of repeated pressure loading and unloading cycles was recorded and plotted in Figure 14a,b. As the pressure increased from 25 Pa to 1250 Pa, the responsive current signals became more and more obvious. As seen in Figure 14a, stable and continuous current responses were clearly visible when the detected pressure was as low as 25 Pa. This minimum detectable limit was lower than some recently reported sensors [8; 35]. The high sensitivity of pressure sensors to small pressure change could be attributed to the soft structure of the elastic network. Light touching could cause a significant contact between the yarns and produce sensitive responses. These responses of sensors to small pressures indicated their ability to detect small movements. Figure 13: Hysteresis curves for the CB@PU network structured sensor. In the repeated high-pressure region, the current curve of the sensor was obtained and plotted in Figure 14b. The intensity as well as the shape of these signal peaks varied for different pressure values. The higher the pressure, the sharper the peaks would be. The current signal was still relatively stable within the pressure range of up to 20 kPa. This maximum detection limit was higher than most of the known small piezoresistive sensors.