Feed gas humidity versus ambient humidity — What matters more in plasma medicine and what makes cells care?

Summary form only given. Humidity plays an important role in plasma chemistry and influences the production of a variety of reactive species like OH, H₂O₂ or O₃. This is especially true in the fast growing field of plasma medicine where patients are supposed to be treated under ambient conditions with changing humidity concentrations. Also when humidity is present in the feed gas of the plasma source - unwanted or intended - a change in the plasma produced active species is obtained. In this paper the influence of ambient humidity and feed gas humidity on the effluent of an atmospheric pressure argon plasma jet is investigated by means of optical emission spectroscopy, Fourier transformed infrared spectroscopy inside a white cell with an effective absorption length of 19.2 m, as well as absorption spectroscopy in the UV and IR spectral range. With these techniques gas phase production rates of H₂O₂, O₃ and NO₂ as well as spatial distributions of excited plasma species like Ar, O, N₂ and OH are determined in dependence of the humidity concentration. On basis of the presented results it is shown that feed gas humidity is more relevant to consider than ambient humidity. However, when constant feed gas humidity concentrations are experimentally provided changes in ambient humidity become relevant again. In order to give an impression of how crucial feed gas humidity influence is on indirect plasma treated human skin cells results of a viability assay are presented¹. These results are correlated to the liquid phase production of H₂O₂ molecules as well as O₂and OH radicals in liquid cell growth medium. Applied measurement techniques are colorimetric assays for determination of the H₂O₂ density and electron spin resonance spectroscopy for O₂and OH radical densities. A remarkable correlation of the cell viabili- y with H₂O₂ concentration was found whereas O₂ and OH do not seem to have a direct effect.