## 3 Results and Discussion
### Electrical Conductivity  
Once the device for housing the UV light guide was selected due to the better distribution of the printed ribbons shown by the circuits made with the 2-stages printing device, electrical conductivity tests were performed (Figure 6).  
First, Figure 6a shows the electrical conductivity of the circuits as a function of the number of printed layers. Here, the electrical conductivity values obtained for the specimens with different number of printed layers were in the same order of magnitude, in the range of 0.03-0.05 S/m, demonstrating the homogeneity of the printing ink and the reproducibility of the manufacturing process. However, when analyzing in detail, a slight increase in the electrical conductivity was observed when increasing the number of printed layers. Electrical conductivity is an intrinsic property of the material, so the value should not change when increasing the number of printed layers. However, the relatively small cross-sectional area of the printed ribbons makes them very sensitive to changes in the electrically conductive network formed by CNTs, which can be caused by presence of voids, changes in the ink flow through the printing nozzle, CNT aggregates or local heterogeneities, among others (Figure 6b). In this regard, the circuits which presented the highest cross-sectional area also presented the highest electrical conductivity due to the lower probability of containing critical defects that hindered the formation of the electrically conductive network. An ohmic behavior of the 3D printed nanocomposites was confirmed as there was a linear V-I correlation in the electrical test performed (see Figure S1).  
Figure 6c shows the electrical conductivity of the ribbons as a function of the electrode configuration used for measuring (Figure 3), allowing us to study how the interface between layers affects the electrical conductivity. Here, a slight decrease in the electrical conductivity was observed for the tests carried out with the electrodes positioned in the first and last layers, which can be caused by some heterogeneities of trapped gases from the photopolymerization process or air trapped when depositing a new layer, as it is schematically shown in Figure 6d. Moreover, the decrease in the electrical conductivity observed in the specimens with the electrodes positioned in the first and last layers, with regard to the specimens with the electrodes positioned within the same layer, was higher when increasing the number of printed layers. These results support the statement above mentioned, regarding the negative effect of the printing defects when increasing the number of printing layers on the electrical conductivity. Furthermore, it has been proven that, despite the volumetric conductive network, the electrical current is able to flow through different pathways depending on the placement of the electrodes in the 3D printed circuits. Nevertheless, the electrical conductivity results evince that the interface between layers do not significantly affect the electrical conductivity of the circuits since the decrease in the electrical conductivity values was under 10% for the four-layer circuit.  
Therefore, the electrical conductivity increased with the number of printed layers in spite of the evidence that there were structural defects at the interface between them.  
Figure 6: Electrical conductivity tests. (**a**) electrical conductivity of the circuits as a function of number of printed layers, (**b**) local heterogeneities that can hinder the formation of the electrically conductive network, (**c**) electrical conductivity of the ribbons as a function of electrode disposition and the number of printed layers and (**d**) scheme of air trapped between printed layers and trapped gases from the photocuring process.  
Moreover, the electrical measurements have proven to be efficient and easy indirect tests to determine the quality of the printed circuits.