## 3 Results and Discussions
### Scanning Electron Microscopy (SEM)  
The SEM images (Figure 6) describe, at \(\times\)100 and \(\times\)10,000 magnifications, the morphology and topography of the surface of the biofilms formed inside of the microfluidic devices at voltages ranging from 10 V to 60 V. Figure 6(a1,a3) illustrate the structure of the biofilm formed at 10 V voltage on sides A and B, respectively, at a magnification of \(\times\)100. As can be observed, the filament deposition lines, resulting during the printing process, are more defined on the A side of the channel, suggesting a thinner layer of the deposited biofilm compared to that on side B. Another aspect to be taken into consideration is the more grainy structure of the biofilm on side B, with more systematically organized bacterial colonies. At a closer look at the extracellular matrix, at a magnification of \(\times\)10,000, Figure 6(a2,a4) display a denser structure of the biofilm on side B. A similar situation occurs in the case of the samples prepared at 20 V, and 30 V. As shown at \(\times\)100 magnification in Figure 6(b1,b3,c1,c3), a thicker biofilm can be observed on side B, compared to side A. Figure 6(b4,c4) allow the visualization of the cell clusters within the ECM, suggesting the existence of organized structures of the cells under and in between the pores and layers of the protective ECM. Figure 6(c2) indicates the existence of only the ECM inside the channel, with no visible individual cells. This could be explained by both the flow rate acting upon the cell attachment inside the channel as well as by choice of area for microscopic visualization (images taken closer to the central area of the microchannel). A common behaviour is observed on side B of the samples prepared at 40 V, 50 V, and 60 V, where the bacterial biofilm layer appears to cover the whole width of the channel, with the deposition lines completely disappearing under the biofilm (Figure 6(d3,e3,f3)). In Figure 6(d1), it is noticeable that for the 40 V sample, on side A, the deposition lines are still visible, with noticeable gaps between the filaments, whereas for the 50 V sample's side A (Figure 6(e1)), the deposition of the biofilm seems more \"hill\" or \"dome\" like. For the 60 V sample, the filament deposition lines disappear completely on side A (Figure 6(f1)) as well as on side B, suggesting a change in behaviour due to the more uniform electric field. Figure 6(e2,e4) illustrate whole-structured cells, whereas Figure 6(f2,f4) present a more fractured or damaged structure, with cells less round and organized. This phenomenon could be a result of the high electric gradient interacting and destroying the cell walls. According to the SEM images, the dielectrophoretic effect of the non-uniform electric field upon the bacterial cells allows for the formation of a more robust, thicker, and overall more compact biofilm on the B side of the channel that accommodates the perpendicular electrodes, therefore conferring the cells a positive dielectrophoretic behaviour. Differences in the biofilms' overall structures are more noticeable after the 40 V mark when the dielectrophoretic effect seems to intensify, the biofilms becoming more dense, all together with a more comprehensive structure. The effects of the dielectrophoretic forces seem to decrease once the 60 V mark is reached, as suggested both by the simulations and the SEM images. Cell structures seem to lose definition, while the biofilm takes the shape of a comprehensive mass rather than intercommunicating colonies (Figure 6(f1,f3)). Another noticeable aspect emphasized by the SEM images is the close cell-cell contact occurring in the biofilm and the fact that the biofilm matrix may act to stabilize contacts between neighboring bacteria [48; 49]. The biofilm's architecture appears as an extended, tightly packed biomass in which cells cluster in microcolonies with relatively uniform adhesion [50].