## 3 Results and Discussion  
### Effects on APPJ Electrical Parameters  
The behavior of \(P_{dis}\) as a function of different parameters varied in this work are shown in Figure 2, which also depicts the behavior of \(i_{RMS}\) as a function of the O\({}_{2}\) added to the plasma. In Figure 2a we can see that the \(P_{dis}\) values increase monotonically with the increment in He flow rate. In that case, the distance between plasma outlet and the target (**d**) was 7 mm. Regarding the influence of **d** in the \(P_{dis}\) values, it can be seen in Figure 2b that the higher the **d** the higher the \(P_{dis}\) until **d** is too large for a stable discharge to be formed. The data in Figure 2a,b were obtained using pure He as the working gas. Concerning the influence of the additional O\({}_{2}\) content, the results obtained for \(P_{dis}\) and \(i_{RMS}\) shown in Figure 2c,d present the first attempt to understand how these quantities evolve as a function of the O\({}_{2}\) percentage added to the working gas. The measurements of \(P_{dis}\) and \(i_{RMS}\) for He flow rates of 1.5 and 2.0 L/min were performed changing the additional O\({}_{2}\) content from 0 to 5%, in steps of 1%. From the curves shown in Figure 2c it can be seen that in the range of 0-2% of additional O\({}_{2}\) only small variations in the \(P_{dis}\) values were detected. Above 2% of O\({}_{2}\) more pronounced decrease of mean discharge power was observed. Additionally, the values obtained for \(P_{dis}\) using a He flow rate of 2.0 L/min change considerably less than when the He flow rate is 1.5 L/min. On the other hand, it can be seen in Figure 2d that the \(i_{RMS}\) values decrease monotonically as more O\({}_{2}\) is added to the system, which is a behavior that can be considered an advantage for in vivo applications of the plasma jet. To understand the reasons behind the behavior of the \(P_{dis}\) and \(i_{RMS}\) curves shown in Figure 2c,d, one can inspect the waveforms of the \(V\), \(q\) and \(i\) signals that were acquired for different quantities of O\({}_{2}\) in the range of 0-5%. Those waveforms are presented in Figure 3, which shows an interesting time behavior of \(V\), \(q\) and \(i\) depending on the additional concentration of O\({}_{2}\) present in the working gas. Notice that for better comparison of the evolution of these quantities as a function of O\({}_{2}\) content, all plots shown in Figure 3 are presented in the same scale (see the caption of Figure 3). From the waveforms presented in Figure 3 one can see that for an O\({}_{2}\) concentration higher than 1%, at least three main effects take place as more O\({}_{2}\) is added to the plasma: the peak values of the applied voltage (\(V_{peak}\)) slightly increase, the discharge ignition (indicated by sudden rises in the charge and current waveforms) occurs later and consequently the discharge duration tends to be shorter. It is very likely that the shorter duration of the discharge due to the delayed ignition is responsible for the decrease in the \(P_{dis}\) and \(i_{RMS}\) values presented in Figure 2c,d. The increase in the voltage values can explain the increment in the peak current measurements, which is another effect of the addition of O\({}_{2}\) observed in Figure 3. All waveforms in Figure 3 were obtained with a He flow rate of 1.5 L/min. However, very similar behaviors for the \(V\), \(q\) and \(i\) waveforms were observed when different He flow rates, as well as different distances between the plasma outlet and the target were used. The increase in the \(V_{peak}\) values with more O\({}_{2}\) present in the working gas, and also the reduction of \(P_{dis}\) and \(i_{RMS}\) values, are trends already observed in other works studying the addition of O\({}_{2}\) to the plasma [25,26]. However, since most of these experiments employed sinusoidal voltage waveforms to produce plasma jets, in none of those cases it was possible to observe a delay in the ignition of the discharge, even when a pulsed power source was used.