Sterilization by Non-thermal Plasma

The prototype manufactured by Sominex (Bayeux, France) consisted of a stainless steel vacuum chamber 35 L (Popot and Gelle, 2012). A high vacuum state was obtained by using two pumps (Agilent TRISCROLL 300 and Agilent V301). The plasma discharge was generated directly inside a sealed bag (SüdPack^®Medica, Germany) by a radio-frequency (RF) polarization plate (Ø 300 mm; RF generator: 13.56 MHz; 300 W) coupled with two magnetic coils (0–14 Gauss; SEF, Labege, France) located at the top and the bottom of the vacuum chamber (Popot and Gelle, 2012; Ben Belgacem et al., 2017). Three mass flow controllers were connected to the gas lines (O₂, Ar, and N₂) to control the flow rate (0.5 sccm). Contaminated glass slides were placed inside the bag, which was sealed and then set on the RF polarization plate. When the low pressure reached 1.45 × 10^-4 mbar, the gases were injected (0.5 sccm) through the first Tyvek^®membrane and the excess gas was released through the second Tyvek^®membrane into the vacuum chamber. Then, the discharge was induced by the RF polarization (25 W) inside the bag and the plasma was densified by the magnetic field (14 Gauss) for 5, 15, and 120 min. Controlling the pressure difference between the vacuum chamber and the bag, the plasma is kept confined inside the bag.

At the end of the process, the pumping system was stopped and nitrogen was injected into the vacuum chamber until the system returned to atmospheric pressure. To evaluate the antimicrobial efficiency, untreated bacteria were used as controls for each independent experimentations. These controls consisted of non-exposed samples to plasma as well as samples exposed only to low pressure.