Operation of a cold DC operated air plasma jet for microbiol decontamination

Non-thermal plasmas offer an effective method for sterilization. For medical applications, such as wound care or plaque removal, the plasma must be cold, i.e. at room temperature. Further it is necessary to conduct a treatment at atmospheric pressure in ambient air. One solution is offered by plasma jets that are generated from discharges operated with noble gases. Alternatively, a cold plasma jet can be generated directly from ambient air in a microhollow cathode discharge geometry. In this configuration a discharge is operated by a dc voltage on the order of 1-2 kV and currents of several milliamps. By flowing air through the discharge channel, a jet is expelled which reaches gas flow rates of about 8 slm room temperature within a few millimeters from the discharge. The efficacy of this setup was recently succesfully demonstrated against different bacteria and yeast1. The microorganisms were plated in 100-mm petri dishes and a 20 mm × 20 mm square was treated by moving the jet in a meander pattern across this area. C. kefyr was the most difficult to inactivate and required an exposure time of 215 s for a reduction of 4-log steps. Whereas for S. aureus a 5.5-log reduction was already achieved in 52 seconds and complete inactivation of 6-log steps in 111 s. Most interestingly it was found that S. aureus and C. kefyr were also affected far outside the immediate treatment area while the effect on other bacteria was limited only to the area directly exposed to the jet. We hypothesize that different interaction mechanisms are responsible for different inactivation rates and are in particular responsible for different inactivation patterns. The most dominant species that was found in the jet's effluent is nitric oxide (NO). Distributions of nitric oxides and different cell susceptibilities might therefore be responsible for the observed inactivation patterns. Accordingly, the topic of our study are nitric oxide concentrations depending on operating parameters, such as power dissipated in the plasma, and gas flow rates. In addition we consider the effect of humidity on the generation of radical species (and on the plasma chemistry in general) and with respect to the observed inactivation kinetics.