## 3 Results
### Properties of the Strain Sensor  
We first studied the sensitivity and linearity of the strain sensor; the resistance was measured by a digital source meter. After fixing two ends of the sensor to two tensile grips of an universal machine, the sensor was quasi-statically stretched to a maximum strain of 21% with a 3% increment per step. The measured data relating the relative change in resistance to strain is shown in Figure 3, where \(\Delta R\) and \(R_{0}\) are the resistance change and the initial resistance, respectively. In order to demonstrate the effect played by the microfolded rGO structures, the response of a MWCNTs/rGO sensor which was fabricated without the third dip-coating step is also presented. It can be seen that although the sensitivity of the MWCNTs/rGO sensor is better at small strains, the sensitivity of the MWCNTs/rGO/rGO sensor stays good over entire strains. This can be explained by the sensing  
Figure 1: A schematic illustration about fabricating conductive fabric through three dip-coating steps.  
mechanism in Figure 4 where the cyan rectangle and the pink curve respectively represent the rGO and MWCNTs. When the sensor is stretched, the decrease of the overlapping area of the upper and lower rGO sheets due to their relative sliding causes the contact resistance to increase. It is worth noting that the contact resistance is the horizontal-direction resistance between two rGO sheets, which is schematically shown in Figure 4. Compared with the breakdown of the conductive paths of rGO sheets, the change of the conductive path caused by the slip of the overlapping area was not dramatic, thereby lowering the sensitivity at small strains. On the other hand, since the contact resistance between two rGO layers is linearly proportional to the strain [37], the obtained sensor has good linearity.  
As mentioned above, another aspect attracting our attention was the stability of the strain sensor. It is well known that the irreversible breakage of the conductive paths is the main reason for the poor stability. Due to the large aspect ratio of MWCNTs, it was expected to achieve good  
Figure 4: Schematics of the mechanism due to MWCNTs and two rGO conductive networks.  
Figure 3: Linearity of the fabric-based strain sensors with and without microfolded rGO structures.  
Figure 2: SEM characterization of the conductive fabric.  
stability in combination with rGO. Ten stretching-releasing cycles were exerted on the fabric-based MWCNTs/rGO/rGO strain sensor, and the measured relative change in resistance is presented in Figure 5. For comparison, the corresponding response of a rGO/rGO strain sensor without the first dip-coating step is also shown. It can be seen that the stability of the MWCNTs/rGO/rGO strain sensor was better compared to the rGO/rGO strain sensor in two aspects, i.e., the overall relative change in resistance between each cycle, and the recovery of its value at the zero strain.  
Furthermore, the effect of rGO concentration on the sensor sensitivity was also studied. Besides the strain sensor coated by 5 mg/mL rGO solution, another one was developed by using 10 mg/mL rGO solution. The two sensors experienced the same stretch; the measured data are shown in Figure 6. It can be seen that, irrespective of the rGO concentration, both the two sensors have good linearities over the strain range 0\(\sim\)21%. However, the higher the rGO concentration, the higher the sensitivity of the sensor. The reason is that more rGO sheets can be deposited on the fabric surface when using the higher rGO concentration; thus, more microfolded structures are formed.  
Figure 5: Stability of the fabric-based strain sensors with and without MWCNTs.  
Figure 6: The effect of rGO concentration on the sensor’s response.