## 3 Results  
### The Electroporator  
In order to describe the operation principle in detail, the DRV0 signals were measured and compared with the resulting waveform on the load (R\({}_{\text{LOAD}}\) = 1 k\(\Omega\)). The waveforms are shown in Figure 4.  
As can be seen in Figure 4A, firstly, the DRV1 and DRV4 are triggered to form a 200 ns pulse by opening the respective switches (T\({}_{1}\)-T\({}_{4}\) refer to Figure 1). Then, a crowbar (C1) is used to rapidly cut the fall time (DRV3 is triggered, which opens T\({}_{7}\) and T\({}_{8}\)), followed by a 500 ns pulse of reverse polarity (DRV2 and DRV3 are active to control T\({}_{7}\) and T\({}_{8}\)). Similarly, in order to cut the fall time, the second crowbar is used (C2) and, thus, DRV4 is triggered (T\({}_{3}\) and T\({}_{4}\) are open). The effects of such a driving sequence can be observed in Figure 4B. It allows rapid switching of kV range pulses even in k\(\Omega\) loads. A programmable delay D1 can be introduced between repetitive sequences.  
Figure 4: The output waveforms of a 4-channel driver (**A**) and the output pulse on the load (**B**), where P1 and P2 are 200 and 500 ns pulses, respectively; C1 and C2 are the crowbar pulses (sub-100 ns) and D1 is the preprogrammed delay between sequences, which is in this case equal to 190 ns; R\({}_{\text{LOAD}}\) = 1 k\(\Omega\).  
The importance of the crowbar circuit can be highlighted when the prototype is tested with various loads. The effects of the crowbar circuits and pulse waveform dependence on the R\({}_{\text{LOAD}}\) are presented in Figure 5.  
It can be seen in Figure 5A that the crowbar circuit enables the pulse fall time to be in the range of 70 ns. If the crowbar is not involved, the fall time increases dramatically, which is highly undesirable. Further, the generator was tested with various loads to imitate various electroporation conditions. The respective waveforms are shown in Figure 5B.  
It can be seen that independently from the load, the pulse features the same waveform with almost identical dynamic parameters. In the case of high power pulses (R\({}_{\text{LOAD}}\) = 56 and 100 \(\Omega\)), the distributed parasitic inductance of the circuit starts to play a role, therefore, a short reverse voltage spike can be observed. Considering the short duration of the transient process, we believe that compensating the spike with additional snubber circuits is not necessary. However, it can be implemented if required. As was mentioned above, the implementation of snubber circuits will worsen the dynamic characteristics of the generator.  
The shortest possible pulses with the highest possible repetitive frequencies were further investigated. The waveforms are presented in Figure 6.  
The first 200 ns pulse was generated for comparison purposes and scaling of the following unipolar and bipolar sequences. In Figure 6A, it can be seen that the fastest possible sequence exceeds 5 MHz repetition frequency with the shortest possible pulse in the range of 65 ns. It is a significant improvement compared to available electroporators. The bipolar sequence (Figure 6B) can also feature a 65 ns waveform with the shortest period of the pulses in the range of 250 ns. A 33 ns step can be introduced to increase the pulse duration, which is a limitation of the microcontroller, which operates at a 32 MHz frequency.  
Figure 5: The effects of the crowbar circuit on the pulse waveform (**A**) and the dependence of the output pulse waveform on the load (**B**).  
Figure 6: Exemplary high frequency unipolar (**A**) and bipolar (**B**) sequences featuring shortest possible pulses, R\({}_{\text{LOAD}}\) = 1 k\(\Omega\).  
Finally, the capability to form arbitrary waveform sequences was confirmed, and an exemplary waveform is shown in Figure 7.  
It can be seen that the pulse shape and polarity, including the delays between separate pulses, can be easily pre-programmed, which offers high flexibility in parametric protocol design. The pulses can be symmetric and asymmetric both in terms of amplitude and duration.