# Biological Decontamination by Microplasma
## 4 Discussions
coli_ using a DBD system. The results were obtained at a power of 50 W after 30 s of treatment. These correspond to the calculated D values in Joule of \(1.76\) kJ/mL and \(0.87\) kJ/mL for _S. aureus_ and \(E. coli\), respectively. For the values obtained with _S. aureus_ at \(-1.7\) kV shown in Table 1, it was difficult to calculate the D values, since the average control values were higher than the treated values. This can be explained by the variations in the initial bacterial count due to the method of swabbing that we used to spread bacteria on the Petri dish. Nonetheless, the effect of inactivation is clear, as shown in Figures 5 and 6. Considering only a 45 mm diameter area below the electrode, as shown in Figure 5, where the inhibition was 100%, the D value for _E. coli_ at \(-1.7\) kV was \(5.6\) kJ/mL and \(0.28\) kJ for 50 \(\upmu\)L. These values are comparable with the results reported in [9; 17; 30], although our results are obtained at relatively low discharge voltage of \(-1.7\) kV compared with voltages of more than 10 kV. [PERSON] et al. [12] obtained a \(5.8\) log CFU/mL reduction in _E. coli_ \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline  & \multicolumn{6}{c}{**D Values**} \\ \cline{2-7}  & \multicolumn{2}{c}{\(-1.5\) kV} & \multicolumn{2}{c}{\(-1.7\) kV} & \multicolumn{2}{c}{\(-1.7\) kV} & \multicolumn{2}{c}{\(-1.7\) kV45 mm} \\ \cline{2-7} _Bacterial Liquid_ & **50 \(\upmu\)L** & **1 mL** & **50 \(\upmu\)L** & **1 mL** & **50 \(\upmu\)L** & **1 mL** \\ _Quantity_ & **(k/50 \(\upmu\)L)** & **(k/J/mL)** & **(k/J/50 \(\upmu\)L)** & **(k/J/50 \(\upmu\)L)** & **(k/J/50 \(\upmu\)L)** & **(k/J/mL)** \\ \hline _E. coli_ & 7.88 & 157.5 & 4.49 & 89.9 & 0.28 & 5.6 \\ _P. aeruginosa_ & N.A. & N.A. & N.A. & N.A. & N.A. \\ _S. aureus_ & 0.99 & 19.9 & N.A. & N.A. & N.A. & N.A. \\ \hline \hline \end{tabular}  
* N.A.—not applicable.  
\end{table} 
Table 2: D values in Joule after microplasma treatment at \(-1.5\) and \(-1.7\) kV. using a flexible DBD device similar with ours at a discharge voltage of 3 kV. The discharge power reported was 0.5 W/cm\({}^{2}\) and the electrode surface area was 2 cm\({}^{2}\). The authors used bacteria-inoculated and dried glass coverslips to measure the surviving bacterial load, and 20 \(\mu\)L of the bacterial culture were placed onto each coverslip. Considering that for the inactivation of 1 mL of bacterial liquid the surface area of the electrode will increase 50 times, the calculated D value in Joule is 1.55 kJ/mL. In our experiments, the bacteria to be inactivated was placed 2 mm from the microplasma electrode, while the results from [PERSON] et al. [12] were obtained with the bacteria-inoculated glass coverslip directly in contact with the discharge. The microplasma reported in this paper was generated at only \(-\)1.7 kV with a flexible electrode as well, thus making it a safer and economical alternative than other types of plasma for biological decontamination. The results shown in [9] were obtained using plasma jet at 1 kW input power; thus, compared with results obtained with microplasma at 2.91 W input power, microplasma is recommended as a more efficient alternative to the plasma jet, obtained with bulky power supplies. Plasma jet has the disadvantage that it raises the temperature of the material that is in contact with, and also, due the electrical charge, some damage might occur in cases in which the sterilization process is applied to human skin. A DBD-type electrode system was used in [17; 30], but the discharge voltages were over 10 kV and the input power was 60 W and 50 W, respectively. In these cases, the discharge voltage and input power also require bulkier power supplies and a voltage difficult to insulate in comparison with microplasma. The microplasma electrodes are scalable to any surface area, while the discharge voltage will remain the same only the input power needs to be adjusted.