# Cancer Treatment: An Overview of Pulsed Electric Field Utilization and Generation
## 8 Voltage Pulse Parameters and Selection  
There are many voltage pulse shapes to choose from, of which three waveforms are the most popular for many electroporation applications. These shapes, shown in Figure 7, are rectangular, multipulse, and exponentially decayed waveforms [29]. There are many parameters that can describe the shape of the voltage pulse. A typical rectangular wave with a sharp rise and fall transition can be parametrized by the peak, pulse duration, and frequency. However, in real applications, parasitic elements (i.e., stray inductance and capacitance from transmission lines) mean that in any system the voltage pulse takes time to reach the plateau peak and additional time to decay to zero. The rise and fall time are two parameters used to describe the transition of the pulse; these are of high importance, as they define the quality of the pulse. During the rising time of the pulse, overshooting may occur; conversely, undershooting may occur during the falling time. Voltage pulse generation techniques depend on energy storage elements, capacitors, or inductors to boost the input voltage and allow fast delivery of the pulse to the load. For this reason, the peak of the pulse decays slightly from the plateau value. This can be noticed based on the capacitance or inductance values and the time constant of the elements. As described in Section 4, the cell is modelled as capacitive and resistive; thus, the current can be used as an indication of electroporation. During the rise time, the capacitive component is dominant, as the capacitor is charging, after which it decays to a fixed value. When the cell is electroporated, the permeability of the cell increases; this means that the resistor value decreases, leading to an increase in the current, which oscillates and then stabilizes at a new value [63]. The most standardized and widely used waveform is the rectangular wave pulse, which can be characterized by the parameters listed in Table 5 [31] and is graphically depicted in Figure 8.  
Figure 6: Different electrode shapes: (**a**) plate, (**b**) needle with one positive and four negative electrodes, and (**c**) clamp. Figure 7: Most popular voltage pulse shapes for PEF: (**a**) rectangular, (**b**) exponential, (**c**) ramp, and (**d**) multipulse [29]. Electroporation voltage pulses can be unipolar or bipolar. For unipolar pulses, the polarity can only be positive or negative, while bipolar pulses can have both positive and negative polarities. The two pulses can be of the same parameters, with no delay inserted between them, or they can be of different parameters with or without delay time inserted in between. The peak of the pulse defines the efficacy of operation, number of pulses, and rise and fall time, as well as the shape of the pulse; delivering bipolar pulses has been shown in research to be more promising than unipolar [7; 35; 51]. Unipolar pulses can lead to electrode movement and muscle contraction, which leads to increased pain [65]. On the contrary, bipolar pulses mitigate muscle contraction and electrode movement and loss, and can expedite treatment time thanks to their inducing mechanical stresses on cells in addition to electrical ones. In addition, bipolar pulses have shown better tumour ablation ability than unipolar pulses [66; 67]. It is rare that any application needs only one voltage pulse to be delivered to the load. Rather, voltage pulses are delivered in series through a train of pulses with different numbers and repetition frequencies, which can be combined into different shapes to obtain a better effect. Figure 9 shows voltage pulses divided into many trains of pulses with the same pulse parameters and with delay time inserted in the first part; the second part shows trains of pulses that share the same parameters in the same group and have different parameters in the second group.