## 3 Results
### Functional Properties Evaluation
#### 3.4.2 Piezoresistive Behavior  
In line with the main objective of this work, a flexible sensor based on microfibers with a piezoresistive behavior was developed, the electrical response under mechanical stimuli of the samples was studied. For this, mechanical cyclic compressions were performed according to the methodology described before. The strain over a GNPs system results in a change of its global electronic properties [17]. When a material that has certain electrical characteristics is subjected to mechanical compression in a specific area, the difference between the internal and external pressure outside the studied area causes a bending in one part and, therefore, a bulge in another section of the membrane. This bulge will generate a pseudo-magnetic field, consequently decreasing the electrical resistance, which means that the electrons will be scattered by the compressed region. This behavior is reflected and quantified by the GF values for piezoresistive materials [6]  
All the piezoresistive behavior tests were performed after the successive washing and drying treatments, with the objective to ensure the highest stability as possible in the results. In addition, 50 consecutive compression cycles were carried out, considering only the last 10 measurements in order to ensure the correct material relaxation and thus obtain the most reliable results possible. The pressure applied on the samples induced their damage due to their very low thickness. The electrodes came into contact, making a short-circuit and resulting in unrealistic electrical resistance values. For this reason, a superposition of layers of the same membrane was carried out in order to obtain a suitable thickness for the test. The piezoresistive response obtained for the samples developed in this work are presented on Figure 10.  
The neat PCL sample did not show any change in electrical resistance under mechanical compression, which confirms that this sample did not present piezoresistive behavior (Figure 10a). Since the development of piezoresistive sensors requires a conductive surface, this result was expected, considering that PCL does not have any electrical conductivity property per se. The same behavior occurred for 0.05%GNPs-PCL (Figure 10b) sample as well, demonstrating that the GNPs present onto the sample were not enough to obtain electrical conductivity nor a piezo resistivity response. However, using 0.1%, 0.2%, 0.5%, 1%, and 2% of GNPs (Figure 10c-g, respectively), it is possible to observe a change in the electrical resistance under mechanical deformation, demonstrating the piezoresistive behavior of these samples. Moreover, a linearity in the variation of the electrical resistance was observed for all the strain/release cycles. In addition, the sample with 0.5% GNPs was the one to demonstrate higher variation in the electrical resistance with the applied strain. The sensitivity of the sensors can be expressed by the GF. The GF values were calculated using Equation (6). These results are presented on Table 3 and were calculated as the average of 10 ascending and descending cycles. The highest obtained GF value (3.20) was observed for the sample with 0.5% of GNPs, being that an increase of the GF values between 0.1% and 0.5% was verified. The finding shows that this concentration of GNPs (0.5%) resulted in the higher sensitivity to mechanical deformation, which is related with greater variations in the electrical resistance, as confirmed by the Figure 10e. Hence, the 0.5%GNPs-PCL sample is the most promising to be used as a piezoresistive sensor. Above the 0.5% concentration, the GF values decreased, reaching 1.21 for the 2%GNPS-PCL sample. Moreover, Figure 11 shows the relation between the measured electrical conductivity and the calculated GF for each GNPs-PCL sample as a function of the GNPs concentration. For GNPs percentages between 0.1% and 0.5%, there was an increase for both electrical conductivity and GF.