# Biological Decontamination by Microplasma
## 2 Materials and Methods
### Microplasma Electrode  
The microplasma electrode was facing the target to be sterilized.  
Figure 1: Experimental setup. (**a**) Schematic image of the experimental setup; (**b**) image of the microplasma-electrode support system.  
Microplasma was generated at atmospheric pressure by a surface DBD-type electrode using a configuration with a dielectric material of 25 \(\upmu\)m thickness between a grid-shaped wire with 1.2 mm width on the high voltage side and 0.2 mm width on the ground side, as shown in Figure 2a,b. A back dielectric layer of 50 \(\upmu\)m thickness covers the high-voltage electrode. The electrode has a diameter of 30 mm and the grid-shape is arranged with an interval of 3.5 mm.  
The electrode has 31 perforated holes, with a diameter of \(\mathcal{O}\) 2 mm, to allow the gas to flow and furthermore bring the active species generated by microplasma to a specific target. Microplasma discharge appeared near the grid of the grounded electrode, as shown in Figure 2b,c.  
Previously, using a similar type of electrode, we have reported the sterilization results obtained using a gas-flow rate of 2 L/min [16]. _Staphylococcus aureus_ was inactivated using pulse and AC voltages to energize microplasma electrodes. Air and Ar/O\({}_{2}\) mixtures were used as discharge gases.  
Figure 2: Microplasma electrode system. (**a**) An image of the microplasma electrode system with a grounded front-side electrode and high-voltage back-side electrode; (**b**) a schematic image of the cross-section of the microplasma electrode system; (**c**) an image of the microplasma electrode during discharge.  
The output of the Marx generator energized the electrodes at a negative-pulse voltage with a pulse width of 16.9 us and a pulse rising time of 548 ns, as shown in Figure 3. The measured discharge current is the total current, \(I_{tot}\),  
\[I_{tot}=I_{disp}+I_{cond}, \tag{1}\]  
where \(I_{disp}\) is the displacement current that occurs due to the charging of the capacitance formed by the microplasma electrodes, and \(I_{cond}\) is the conduction current that flows through the plasma.  
The displacement current, \(I_{disp}\), is as follows  
\[I_{disp}=C\frac{dV}{dt}, \tag{2}\]  
where \(V\) is the applied voltage. The capacitance of the microplasma electrode \(C\) was calculated considering the current at the discharge voltage of \(-400\) V, where the measured current is the displacement current \(I_{disp}\), and there is no microplasma discharge [35; 36]. Thus, \(I_{tot}=I_{disp}\), and for the microplasma electrode \(C=37\) pF was calculated. After knowing \(C\), we have calculated:  
\[I_{cond}=I_{tot}-C\frac{dV}{dt}. \tag{3}\]  
The microplasma's instantaneous power is obtained by multiplying the voltage \(V\) and conduction-current \(I_{cond}\) waveforms. The microplasma discharge power was calculated by integrating the power waveform in time and divided with the period corresponding to the frequency of 500 Hz. The calculated discharge power considering the conduction current \(I_{cond}\) at the discharge voltages \(-1.5\) kV and \(-1.7\) kV was 2.52 W and 2.91 W, respectively.