We present a numerical model of a surface microdischarge (SMD) in humid air at atmospheric pressure. Our model includes over 50 species and 600 elementary reactions and consists of two, coupled well-mixed regions: a discharge layer with both charged and neutral species and an afterglow region consisting only of neutral species. Multiple time steps employed in our model enable capturing rapid dynamic behaviour in the discharge layer as well as the relatively slow diffusion and reaction in the afterglow. A short duration, high electric field is assumed to be excited at 10 kHz in the discharge region with power density maintained at 0.05 W cm−2. Among the predicted dominant species in the afterglow are O3, N2O5, N2O, HNO3, H2, NO3, H2O2, HNO2 and NO2. The results are in qualitative agreement with Fourier transform infrared absorption spectroscopy. Our simulation results show that density of those reactive species continues to evolve significantly in time, even after ∼15 min of SMD exposure. This result suggests that SMD treatments on the order of minutes or less may involve significant neutral species concentration and flux transients, potentially affecting interpretation of results.

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SummaryAs a result of both the better understanding of complex plasma phenomena and the development of new plasma sources in the past few years, plasma medicine has developed into an innovative field of research showing high potential. While thermal plasmas have long been used in various medical fields (for instance for cauterization and sterilization of medical instruments), current research mainly focuses on application of non-thermal plasmas. Experiments show that cold atmospheric plasmas (CAPs) allow efficient, contact-free and painless disinfection, even in microscopic openings, without damaging healthy tissue. Plasmas influence biochemical processes and offer new possibilities for the selective application of individually designable medically active substances. In dermatology, new horizons are being opened for wound healing, tissue regeneration, therapy of skin infections, and probably many more diseases. First clinical trials show the efficacy and tolerability of plasma in treating infected chronic wounds. A major task will be the introduction of plasma into clinical medicine and, simultaneously, the further investigation of the mechanisms of action of plasma at the cellular level.

dPhysical cold atmospheric surface microdischarge (SMD) plasma operating in ambient air has promising properties for the sterilization of sensitive medical devices where conventional methods are not applicable. Furthermore, SMD plasma could revolutionize the field of disinfection at health care facilities. The antimicrobial effects on Gram-negative and Gram-positive bacteria of clinical relevance, as well as the fungus Candida albicans, were tested. Thirty seconds of plasma treatment led to a 4 to 6 log 10 CFU reduction on agar plates. C. albicans was the hardest to inactivate. The sterilizing effect on standard bioindicators (bacterial endospores) was evaluated on dry test specimens that were wrapped in Tyvek coupons. The experimental D 23°C values for Bacillus subtilis, Bacillus pumilus, Bacillus atrophaeus, and Geobacillus stearothermophilus were determined as 0.3 min, 0.5 min, 0.6 min, and 0.9 min, respectively. These decimal reduction times (D values) are distinctly lower than D values obtained with other reference methods. Importantly, the high inactivation rate was independent of the material of the test specimen. Possible inactivation mechanisms for relevant microorganisms are briefly discussed, emphasizing the important role of neutral reactive plasma species and pointing to recent diagnostic methods that will contribute to a better understanding of the strong biocidal effect of SMD air plasma.