Effect of Dielectric and Liquid on Plasma Sterilization Using Dielectric Barrier Discharge Plasma

Abstract

Plasma sterilization offers a faster, less toxic and versatile alternative to conventional sterilization methods. Using a relatively small, low temperature, atmospheric, dielectric barrier discharge surface plasma generator, we achieved ≥6 log reduction in concentration of vegetative bacterial and yeast cells within 4 minutes and ≥6 log reduction of Geobacillus stearothermophilus spores within 20 minutes. Plasma sterilization is influenced by a wide variety of factors. Two factors studied in this particular paper are the effect of using different dielectric substrates and the significance of the amount of liquid on the dielectric surface. Of the two dielectric substrates tested (FR4 and semi-ceramic (SC)), it is noted that the FR4 is more efficient in terms of time taken for complete inactivation. FR4 is more efficient at generating plasma as shown by the intensity of spectral peaks, amount of ozone generated, the power used and the speed of killing vegetative cells. The surface temperature during plasma generation is also higher in the case of FR4. An inoculated FR4 or SC device produces less ozone than the respective clean devices. Temperature studies show that the surface temperatures reached during plasma generation are in the range of 30°C–66°C (for FR4) and 20°C–49°C (for SC). Surface temperatures during plasma generation of inoculated devices are lower than the corresponding temperatures of clean devices. pH studies indicate a slight reduction in pH value due to plasma generation, which implies that while temperature and acidification may play a minor role in DBD plasma sterilization, the presence of the liquid on the dielectric surface hampers sterilization and as the liquid evaporates, sterilization improves.

Examining the Role of Ozone in Surface Plasma Sterilization Using Dielectric Barrier Discharge (DBD) Plasma

“…Plasma sterilization curve, using air and nitrogen as the working gases. that the reduction of ozone (all the air in the chamber is evacuated and filled with N 2 ) leads to insignificant reduction in E. coli concentrations but also proves that N 2 , which is a major component of air and a prominent peak on the spectroscopic signature [3] does not play a dominant role in DBD surface plasma sterilization. However from Figure 9, it is evident that after 2 min of exposing E. coli concentrations to produced ozone, a 3-4 log 10 reduction is obtained, starting from an initial concentration of 10 8 CFU.…”

“…[3] The plasma device consists of a dielectric layer (%1.6 mm thick), either side of which an electrode is embedded. Two types of dielectric material are tested: FR4 (flame retardant 4) and a hydrocarbon ceramic laminate (Rogers W 3003 C), which will henceforth be known as semi-ceramic (SC).…”

“…[1] During plasma generation at atmospheric pressure, the spectroscopic signature exhibits prominent intensity peaks in the wavelength range of 300-400 nm. [2,3] The intensity peaks in this wavelength range pertain mostly to transitions taking place in the 2nd positive system of nitrogen (N 2 ). More importantly, the spectroscopic signature of plasma at atmospheric pressure shows no intensity peaks in the wavelength range of 100-280 nm, which pertains to the UV-C regime.…”

“…[7] The role of each of these agents in plasma sterilization remains highly debated. The type of plasma discussed in this paper and in our previous sterilization experiments [3] is known as dielectric barrier discharge (DBD) plasma. This type of plasma is generated by applying a potential difference between two electrodes embedded on opposite sides of a dielectric (insulator) surface and is characterized by a large number of tiny filamentary micro-discharges.…”

Paper-based plasma sanitizers

“…Plasma-based treatments have the potential to provide comparable sterilization to conventional methods that use heat (17), chemicals (17), or radiation (18-21). Plasma treatments have deactivated a range of microbes, such as Geobacillus stearothermophilus (22), Staphylococcus aureus (MRSA) (23), and adenoviruses (24) on varied substrates [e.g., food (25)]. …”