Nitrogen Shielding of an Argon Plasma Jet and Its Effects on Human Immune Cells

Atmospheric pressure plasmas are widely used in research for biomedical or clinical applications. Reactive oxygen species and reactive nitrogen species (RNS) produced by plasmas are thought to be of major significance in plasma-cell interactions. Different applications, however, may demand for different plasma properties. Tailoring plasma devices by modulating the supply gas or the surrounding is a suitable way to alter reactive species composition, vacuum ultra violet emission, or temperature. Treatment regimens involving availability of oxygen or humidity may lead to increased hydrogen peroxide deposition in liquids and thus will be toxic to cells. Using an atmospheric pressure argon plasma jet, we applied a nitrogen gas curtain to its visible effluent during treatment of human immune cells. The curtain deprived the plasma of molecular oxygen. This excluded gas-phase oxygen plasma chemistry and led to generation of highly energetic metastables. Planar laser-induced fluorescence spectroscopy verified laminar gas flow and complete elimination of surrounding air by the gas curtain. We used human immune cells to monitor cytotoxic effects as they are highly relevant in potential clinical plasma applications, e.g., treatment of chronic wounds. Air curtain plasma treatment led to significantly higher cytotoxicity compared with nitrogen curtain plasma treatment. Scavenging of hydrogen peroxide abrogated cell death in both gas curtain conditions. This indicated a negligible contribution of highly energetic metastables or increased gas temperature to cytotoxicity. Finally, the results suggested an oxygen-independent generation of hydrogen peroxide pointing to an indirect role of UV or RNS in plasma-mediated cytotoxicity.