# A High Compressibility Pressure--Sensitive Structure Based on CB@PU Yarn Network
## 3 Results and Discussion
### Pressure-Sensing Mechanism  
When increasing pressure is applied upon the CB@PU yarn network, the bending of the conductive PU yarn will cause the deposited CB conductive layers to deform and mutually contact, leading to a change in electrical resistance and the electrical signal. Most piezoresistive sensors, such as polymer sensors and flexible sensors, deform as a whole under pressure. But the CB@PU network sensor is not directionally compressed. That is, the sensor can be divided into two parts: compressed part and uncompressed part. During the press-down process, the upper layer is subjected to a direct force and is compressed. The yarns in the upper layer closely contact with each other and then form a dense compressed layer. The uncompressed part below will not be affected by the external force. The yarns in this part are still separated from each other, so the density of the yarns in network will not change. This isillustrated in Figure 4.  
For the compressed layer, there are two types of deformation in the conductive network structure: deformation of the macro PU network and of the microscopic CB conductive layer. From the microscopic point of view, when the PU network is compressed, the bending of yarn causes tension on the CB layer. The CB nanoparticles' distribution on the conductive layer becomes sparse, and the electrical resistance of conductive yarn will increase. After releasing compression strain, the PU yarn can still retain its original length. The CB layer merges and returns to a tight particle distribution, which causes the decrease of resistance. This reversible stretching deformation of CB conductive layer generates a corresponding change in yarn's conductivity. It can be calculated that the maximum elongation of a single yarn in the network is 20.6%. The specific calculation process is explained in Figure 5 and the following equations. Figure 4: Schematic evolutions of the CB@PU network during continuous compressive deformation. Figure 5: Schematic diagram of the length change of single yarn in CB@PU network under a compressive deformation. When the yarn is compressed from the uppermost layer to the lowermost layer, the strain of the yarn is greatest. The height of the PU yarn network was 11.2 mm before compression. The height of the PU yarn network was 0.29 mm after a full compression, so the maximum height difference was 10.9 mm:  
\[\sqrt{16^{2}+10.9^{2}}=19.3\]  
\[(19.3-16)/16\times 100\%=20.6\%\]  
therefore the maximum length change of the single yarn in network was 20.6% after a full compression. The Fluke 124 oscilloscope was connected with the both ends of yarn to measure the resistances of single CB@PU yarn at different strains. Figure 6 reveals that the relative resistance of a single CB@PU yarn corresponded to a gradual increase of the strain. In the process of making network structure, each yarn in the network was stretched to 200% of the original length it was, and when the sensor was unstressed, the yarn in the network already had a 100% strain, so when the sensor was under pressure, the calculations of amount of deformation and of the corresponding change in resistance were both based on the 100% strain which had been generated. The corresponding resistance change rate could be obtained through the following equation:  
\[200\%\times(1+20.6\%)=241.2\%,\text{ i.e., }141.2\%\text{ strain}\]  
So the corresponding resistance change rate was:  
\[(6.75-5.34)/5.34\times 100\%=26.4\%\]  
The data is displayed in Figure 6.  
However, the sensing mechanism is mainly based on the macro change in the network. Macroscopically, due to the high density of the yarns in network (up to 40.71 cm per cubic centimeter, which could be calculated based on the sensor volume and total length of yarns), just under a slight pressure, the yarns would mutually contact and generate dense point contacts and line contacts on CB layers. Yarns in the compressed layer overlap with each other, CB particles contact with each other and form chain-shaped conductive channels which can be seen in Figure 7a.