## 3 Results and Discussion
### Electrochemical Characterization of the PLA Electrodes Electrodeposited with Graphene Oxide  
With the intention of improving the overall electrochemical performance of the electrodes, their surface was modified using graphene oxide as described in Section 2.1. The electrochemical performance of the electrodes thus modified was characterized using identical techniques. Three different electrodes were tested with differing results in the bare-electrode tests, ranging from poor results to the one that showed the best result (depth of the pattern of 0.2, 0.6 and 1.4 mm).  
The graphene electrodeposition led to a significant improvement in the performance of the electrodes for the three selected depths (Figure 6), which corroborates previous studies with graphene oxide that showed an improvement in the electrochemical performance of the electrode [24,30]. The CVs of the ED electrodes showed larger oxidation-reduction peaks compared to that of the bare electrodes, which reveals an improvement of the kinetics of the electrochemical reaction. This improvement is most clearly visible in the 1.4 mm depth. As for the EIS, the result showed a significant improvement for both the 0.2 mm and 0.6 mm electrodes (Figure 7). In fact, the latter is the one with the best results. For 0.2 mm, oxidation and reduction peaks became visible and showed a slight increase in current (thickening of the curve). In the case of 0.6 mm, we see a decrease in the distance  
Figure 4: Cyclic voltammetry for TPU.  
Figure 5: Electrochemical impedance results for the TPU electrode. (**A**) EIS and (**B**) electric equivalent circuit.  
between peaks and a considerable increase, almost double, of the oxidative part of the curve. Interestingly, in the case of the 1.4 mm pattern, there is a worsening of the results obtained without ED observing an increase in resistance and a decrease in capacitance (Table 3). This could be due to the arrangement of the GO on the electrode surface since there is a substantial worsening of the diffusion-associated resistance as well as charge transfer. Another explanation may be due to capacitance loss, i.e., graphene could increase the electron current through the electrode and decrease its accumulation.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & 0.2 & 0.6 & 1.4 \\ \hline Rs (\(\Omega\)) & 8.894 & 2.581 & 8.631 \\ C (P) & 1.56 10\({}^{-5}\) & 2.16 10\({}^{-5}\) & 3.41 0\({}^{-5}\) \\ Ret (\(\Omega\)) & 302.1 & 143.4 & 422.7 \\ RdI (\(\Omega\)) & 1.92 10\({}^{2}\) & 1.38 10\({}^{2}\) & 3.18 10\({}^{2}\) \\ td1 (s) & 7.023 & 7.734 & 7.056 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Electrical equivalent circuit data for PLA-ED electrodes.  
Figure 6: CV comparison between PLA and PLA-ED: (**A**) 0.2 mm electrode, (**B**) 0.6 mm electrode and (**C**) 1.4 mm electrode.  
Finally, it is important to note that the results here provided could not be directly extrapolated to a biological system. The positive influence of the depths of the pattern between 0.6 and 1.4 mm must be corroborated in biological tests, as the presence of a biofilm might affect not only charge transfer but also mass transfer resistance. In this regard, although a deeper pattern is expected to promote the formation of a more robust biofilm, resilient to the shearing stress caused by hydrodynamic forces, it might also give way to a thicker biofilm that limits mass transfer.