# Plasma-Sprayed Flexible Strain Sensor and Its Applications in Boxing Glove
## 4 Results and Discussion  
### Performance of the Flexible Strain Sensor  
#### 4.1.1 Sensitivity  
The conductive path of the flexible sensor was one or more linear paths formed by conductive nanoparticles. When subjected to an external force, the deformation in the length direction of the conductive path was very clear, and the change in the width direction was small. Therefore, when the original conductive path was subjected to an external force, the path between the conductive particles was reduced (Figure 10). Thus, the change in the external force was represented by the change in the resistance. In order to verify the stability and reliability of the strain sensor, a strain extensometer made in our laboratory was used to perform continuous strain-release cycle tests at different strains. The GF of the sensor was calculated based on the resistance change rate and the strain value (Equation (1)). \[\text{GF}=\frac{\text{R}-\text{R}_{0}}{\text{R}_{0}\cdot\delta} \tag{1}\]  
Figure 10: Microscopic mechanism of the developed flexible resistive sensor: (**a**) before applying the force; (**b**) after applying the force. Figure 9: The feedback principle of the smart boxing glove: (**a**) before receiving an external force; (**b**) after receiving an external force. (**c**) Desktop multimeter. (**d**) Change in the resistance over time. where R is the tensile resistance value, R\({}_{0}\) is the initial resistance value, and \(\delta\) is the tensile strain. Effect of the Plasma Treatment Process on Sensitivity  
Figure 11a,c indicate that the presence or absence of plasma treatment had a significant impact on the electrical properties of the sensing layer. Two resistance-strain sensors (a plasma-treated one and an untreated one) of the same size were stretched several times. The plasma-treated flexible resistance-strain sensor exhibited a relatively stable change in the tensile resistance. The tensile and recovery resistance of the untreated flexible resistive-strain sensor rapidly decreased within the first 300 reciprocating stretches, and the decrease ranged from large to small. After this, the resistance changes stabilized. This indicates that the low-temperature-plasma treatment enhanced the stability of the flexible resistive-strain sensor. After the reciprocating tensile resistance values of the two sensors stabilized, they were calculated using the resistance change curve. At a tensile strain of 10%, the GF of the plasma-treated sensor was 11.5, whereas the untreated sensor was approximately 3.9. This indicated that the plasma treatment increased the sensitivity of the sensor because the resistance of the conductive layer changed faster with the strain. After the low-temperature-plasma treatment, the original structure of the CNTs was destroyed because some carbon-carbon bonds were broken, resulting in a shortening in the length of CNTs. This length-shortening caused some CNTs to lose the ability to form end-to-end conductive paths. Compared with the conductive layers that have not undergone plasma treatment, the conductive paths in the treated conductive layers were reduced, which resulted in macroscopic resistance. During the stretching process of the treated sensor, with the increase in the stretching distance, the number of conductive branches rapidly decreased. This rapidly increased the resistance, which improved the sensitivity of the treated sensor. However, when stretched to a certain extent, no CNTs  
Figure 11: Responses of the untreated sensors and the sensors treated with low-temperature plasma to strain: (**a**) the change in the resistance of the untreated flexible sensor stretched 2500 times at 10% strain; (**b**) an enlarged view of the change in the resistance of the untreated flexible sensor; (**c**) the change in the resistance of the flexible sensor after plasma treatment under 10% strain after stretching 2500 times; (**d**) an enlarged view of the change in the resistance of the flexible sensor after plasma treatment.