Atmospheric pressure dielectric barrier discharges interacting with liquid covered tissue

The interaction of plasmas with liquids is of increasing importance in biomedical applications. Tissues treated by atmospheric pressure dielectric barrier discharges (DBDs) in plasma medicine are often covered by a thin layer of liquid, typically a blood serum like water with dissolved gases and proteins up to hundreds of micrometres thick. The liquid processes the plasma-produced radicals and ions prior to their reaching the tissue. In this paper, we report on a computational investigation of the interaction of DBDs in humid air with a thin water layer covering tissue. The water layer, 50-400 mu m thick, contains dissolved O-2aq ((aq) means an aqueous species) and alkane-like hydrocarbons (RHaq). In the model, the DBDs are operated with multiple pulses at 100 Hz followed by a 1 s afterglow. Gas phase reactive oxygen and nitrogen species (RONS) intersect the water-vapour saturated air above the liquid and then solvate when reaching the water. The photolysis of water by plasma-produced UV/VUV plays a significant role in the production of radicals. Without RHaq, O-2aq(-), ONOOaq-, NO3aq- and hydronium (H3Oaq+) dominate the water ions with H3Oaq+ determining the pH. The dominant RONS in the liquid are O-3aq, H2O2aq, and HNOxaq. Dissolved O-2aq assists the production of HNO3aq and HOONOaq during the afterglow. With RHaq, reactive oxygen species are largely consumed, leaving an R.(aq) (alkyl radical) to reach the tissue. These results are sensitive to the thickness of the water layer.