## 3 Results  
### PCL Polymer Solution, Microfibers and GNPs Impresation Optimization  
For the development of the PCL membranes, different variables were tested and optimized. First, different PCL concentrations of 15%, 20% and 25% (_w/v_) were tested in Chloroform (CHF)/Dimethylformamide (DMF) (4:1), taking into account several studies in the literature that have reported the use of CHF to ensure complete dissolution of the PCL and DMF in order to improve the properties of the solution (viscosity and conductivity) [32; 33; 34]. To assess the effect of the concentration of the polymeric solution on the morphology of the produced fibers, the conditions of the electrospinning process were kept constant using 240 mm of needle-collector distance, 1 mL/h of flow rate, and 25 kV of applied voltage [35]. The morphology, as well as the dimensions of the developed fibers, were evaluated by FESEM (Figure 3). Figure 2: Schematic representation of the experimental configuration for the piezoresistive performance evaluation. Figure 3 shows the FESEM images of the fibers produced by electrospinning using PCL, as well as the corresponding frequency distribution of fiber diameter, with a deposition of 30 min. As it can be seen in Figure 3, PCL fibers were successfully produced with all the evaluated PCL concentrations. However, an increase of the fibers' diameters was observed according to PCL concentration. In fact, using 15%, 20%, and 25% PCL an average diameter of 2.2 \(\upmu\)m, 3.1 \(\upmu\)m, and 4.5 \(\upmu\)m was recorded, respectively. Moreover, it can also be concluded that fibers obtained using 15% (_w/v_) of PCL were distributed more homogeneously than the other ones. Figure 3a shows very regular and uniform fiber diameters when compared to the rest of the formulations in study. On the other hand, the higher the PCL concentration, the more heterogeneous and irregular the fiber distribution and diameters. For the samples with 20% (_w/v_) of PCL (Figure 3b), a higher variation in diameters was observed, while it was considerably more noticeable for the 25% (Figure 3c). In this case, the fibers exhibited highly variable diameter values and, therefore, a heterogeneous distribution. Despite this, for this formulation, the fibers did not change their surface texture compared to the 15% (_w/v_) sample, maintaining a smooth appearance. Thus, the preliminary analysis of the FESEM images and diameter distribution histograms allowed us to determine that 15% was the best percentage of PCL to produce fibers with the most uniform structures and more regular and lower diameters. In order to produce defect free electrospun microfibers, not only the solution parameters, but also the electrospinning conditions need to be optimized. In this sense, parameters such as needle-collector distance, the applied voltage, and the flow rate were evaluated as well. FESEM images of the fibers obtained for each process conditions, as well as the diameters of the obtained fibers and the respective average values, are represented in Figure 4.  
Figure 3: Field Emission Scanning Electron Microscopy (FESEM) images of the electrospun fibers produced using different PCL concentrations and their diameter distribution histograms: (**a**) 15%; (**b**) 20%; (**c**) 25% (_w/v_) in Chloroform (CHF)/Dimethylformamide (DMF) (4:1). From left to right: 2000\(\times\) (50 \(\upmu\)m); 10,000\(\times\) (10 \(\upmu\)m). Figure 4: _Cont._  
Figure 4: FESEM images of the electrospun PCL fibers and their diameter distribution histograms produced with different process conditions (applied voltage, flow rate, and needle-collector distance: (**a**) 25 kV, 240 mm, 1 mL/h; (**b**) 25 kV, 240 mm, 2 mL/h; (**c**) 28 kV, 240 mm, 1 mL/h; (**d**) 28 kV, 240 mm, 2 mL/h; (**e**) 25 kV, 260 mm, 1 mL/h; (**f**) 25 kV, 260 mm, 2 mL/h; (**g**) 28 kV, 260 mm, 1 mL/h; (**h**) 28 kV, 260 mm, 2 mL/h, to a PCL concentration of 15% (_w_/_e_) in CHF/DMF (4:1). Figure 4 shows that all the electrospinning conditions led to the successful production of microfibers.