## 3 Results
### Performances of Multielectrode Unipolar (MEU) and Multielectrode Bipolar (MEB) Modes  
All temperature distributions obtained from the comprehensive simulation study were bound by the 50 \({}^{\circ}\)C isothermal surface; thus, only the effective thermal lesion is considered hereafter. Supplementary Figure S2 illustrates the process of lesion growth in the myocardium in the MEU and MEB modes at four time points during RFCA, until a preliminary effective continuous lesion was created. The overall thermal damage to the insides of the myocardium was achieved by spreading and merging the respective lesion under each electrode. In the MEU mode, the lesion began to form from the outer edge of each electrode. In the MEB mode, the lesion first appeared at the two sides that connected the electrode and catheter body. The time required for an effective continuous lesion in the MEB mode was 4.5 s, while that for the MEU mode was 25.6 s. Electrodes 1 and 10 in the MEU mode were the first two electrodes that merged their lesions with neighboring electrodes, but the last two to achieve the same in the MEB mode. Figure 4a,b present the surface and internal temperature distributions of the myocardium in the MEU and MEB modes at 60 s of RFCA. Only the MEB mode could produce continuous lesions both on the tissue surface and inside the myocardium. The surface lesion was discontinuous and shaped like circles in the MEU mode. In both modes, the surface temperature was always lower than the temperature inside the myocardium, resulting from the thermal convection brought by blood flow. From Figure 4c, the temperature in the myocardium increased sharply in the MEB mode. The control algorithm worked properly that maintain the maximum temperature around 95 \({}^{\circ}\)C. We identified a concave region between the adjacent active electrodes in the MEU mode, as shown in Figure 5a. We noted the minimum distance \(d\) between every two peaks of the lesion on the tissue surface and the depth of concave \(D\) at 30 s and 60 s of RFCA, respectively (Figure 5b). The gap and the concave area increased from the two ends to the middle of the catheter. At 60 s of RFCA, the effect of \(D\) is small, but discontinuous lesions still exist between the adjacent electrodes on the tissue surface. For the MEB mode, we found no such evidence of a concave. Figure 4: **(a)** Temperature distribution on the tissue surface and **(b)** inside of the myocardium in the MEU and MEB modes at 60 s of RFCA. **(c)** The maximum temperature detected in the myocardium during 60 s of RFCA. The highest surface temperatures in the MEU and MEB modes were generated at the outer edge of electrode 10 and the inner edge of electrode 9, respectively. A higher temperature field always occurs at the outer area of an electrode in the MEU mode and the inner area of the electrode in the MEB mode. This finding matched the current flow and the subsequent distribution of current density in both modes, as shown in Figure 6, in which the voltage distribution is also displayed. The current in both modes flowed spatially throughout the entire model. As indicated in Figure 6a, most of the current in the MEU mode spread out from the active electrodes and pointed outward to the dispersive electrode along the surrounding edges of the model bottom. A portion of the current gathered near a particular section of the catheter body and headed either to the above or below the blood chamber but eventually flowed toward the dispersive electrode. In addition, the areas between the adjacent electrodes were barely involved in current flows. As shown in Figure 6b, the current in the MEB mode splits out from the middle of each active electrode and flows toward the adjacent grounding electrodes. Current flows were more concentrated along the inner edge of the CMC, except for the transmission between electrodes 1 and 10. According to the routes of current flow, the distribution of current density was generally discontinuous in the MEU mode between adjacent electrodes.