## 3 Results  
### Electrical and Electrothermal Characterization  
First, the electrical conductivity tests (Figure 2a) showed that for both the 3D-printed circuits and coatings there was an increase in the electrical conductivity when increasing the nanoparticle content, which was expected, due to a higher volume fraction of nanoparticles. In addition, when comparing the 3D-printed circuits with the coatings, the latter showed a slightly higher electrical conductivity. This can be explained by the relatively low cross-sectional area of the printed ribbons (see Table 1), which makes them very sensitive to printing defects as voids or ink flow disruptions, which can hinder the formation of the electrically conductive network. On the other hand, the average temperature reached by the nanocomposite specimens during the self-heating tests, shown in Figure 2b, present a similar trend when they are compared to the electrical conductivity ones. In this regard, the \(\Delta\text{T}_{\text{av}}\) increases with the nanoparticle content, which can be explained by the previously mentioned expression for the electrical conductivity (1), the [PERSON]'s law (2), and the [PERSON]'s heating law (3):  
\[\text{V}=\text{I}\cdot\text{R} \tag{2}\]  
\[\text{Q}=\text{I}^{2}\cdot\text{R}\cdot\text{t} \tag{3}\]  
where V is the applied voltage, I is the current intensity, Q is the heat generated by [PERSON]'s heating, and t is the time during the experiment. Here, an increase in the electrical conductivity due to the higher volume fraction of the conductive filler induces a decrease in the electrical resistance. Moreover, this decrease in the electrical resistance is directly related to an increase in current intensity. Finally, the heat that is generated during [PERSON]'s heating is directly related to the square of the current intensity, leading thus, to a higher temperature. In addition to that fact, the coatings reached a significantly higher \(\Delta\)T\({}_{\text{av}}\) with regard to the 3D-printed circuits because of their higher cross-sectional area (almost three times higher, see Table 1), which reduces the electrical resistance and then increases the heat that is generated by [PERSON]'s heating, as explained before. When analyzing in detail the specimens with a global content of nanoparticles of 1.0 wt. %, the specimen doped with 0.5% CNT + 0.5% GNP reached a higher \(\Delta\)T\({}_{\text{av}}\) with regard to the specimen doped with only 1.0%CNT. This can be explained by the higher influence of GNP in the temperature reached by [PERSON]'s heating over CNT when added in \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen** & **Width (mm)** & **Thickness (\(\upmu\)m)** & **Cross-Sectional Area (mm2)** & **Volume (mm3)** \\ \hline
3D-printed ribbons & \(1.14\pm 0.07\) & \(439\pm 41\) & \(0.32\pm 0.04\) & \(16.43\pm 2.04\) \\
3D-printed circuit & \(13.69\pm 0.07\) & \(439\pm 41\) & \(3.88\pm 0.04\) & \(197.13\pm 24.54\) \\ Coating & \(52.79\pm 1.06\) & \(188\pm 22\) & \(9.93\pm 1.24\) & \(496.56\pm 61.82\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Morphological characterization. Figure 2: Electrical and electrothermal characterization as a function of nanoparticle content and specimen morphology. (**a**) Electrical conductivity, (**b**) average temperature increment with respect to room temperature, (**c**) examples of [PERSON]’s heating test for the 3D-printed circuit and coating with 1.0% CNT and 0.5% GNP, and (**d**) IR images taken during [PERSON]’s heating tests. low contents [6]. In this regard, the GNPs can act as a bridge between CNTs, enhancing the electrical conductivity and, thus, the temperature reached by the [PERSON]'s effect. Besides that, Figure 2c shows the \(\Delta\)T\({}_{\text{av}}\) and \(\Delta\)T\({}_{\text{max}}\) as a function of time during the self-heating tests that were reached by the 3D-printed circuit and coating with the best results in terms of [PERSON]'s heating (1.0% CNT + 0.5% GNP).