Atmospheric Pressure Plasma Sources for Plasma Medicine

Abstract

Several low temperature, atmospheric pressure plasma sources have been developed in recent years, opening new possible applications for plasmas in environmental and biomedical fields. Low temperature atmospheric pressure plasma treatment has recently been shown to kill bacteria, heal chronic diabetic wounds, and selectively kill cancer cells. The biomedical outcomes are primarily attributed to the production of reactive oxygen and nitrogen species, which include OH, HO2, and NO. Tailoring the composition of reactive species produced by the plasma that solvate in the liquid is critical to providing a consistent outcome in a given application. Two models are used to investigate this problem. GlobalKin is a 0-D plasma kinetics model which considers the plasma chemistry to be a well stirred reactor. GlobalKin has been upgraded to include a liquid module, which considers well-stirred liquid chemistry coupled with the plasma chemistry. This model runs quickly, so large parameter spaces and complex chemistries can be explored. nonPDPSIM is a 2-dimensional plasma hydrodynamics model which solves Poisson’s equation, charged species transport, and the chemistry of charged and neutral species. Photoionization, fluid dynamics effects, and solvation into liquid are all included. Updates to nonPDPSIM include improvements in the liquid solvation calculation, performance improvements based on switching from numerical to analytical derivatives, and the ability to more tightly couple the fluid dynamics calculation and the plasma chemistry calculation. A GlobalKin study of air dielectric barrier discharge treatment of liquid covered tissue has highlighted the role of HO2NO2 in decaying to deliver HO2 and NO2 at long timescales. Gas flow can be used to tailor the ratio of various reactive species delivered to the liquid, particularly decreasing HNOx at higher gas flow rates. A study of water droplets and aerosols treated in an air plasma has also shown the importance of droplet size and density in controlling the densities of species in the liquid based on their Henry’s law constants (their proclivity to solvate in water). nonPDPSIM has been used to model helium atmospheric pressure plasma jets (APPJs) operating in humid air. The plasma in these devices propagate as an ionization wave, which lasts for 10s to 100s of ns and is repetitively pulsed. The effects of nearby grounds and impurities in the helium, which are often not controlled, were revealed in this study. Other parameters such as gas composition and electrode configuration were also examined for their effect on ionization wave propagation and the resulting reactive species production. The behavior of a unique APPJ source, referred to as a multi-jet, is also explored. This relies on the propagation of an ionization wave through holes in a dielectric tube to form an array of jets. Qualitative agreement has been shown with fast camera imaging of the ionization wave. Another study has shown the cause of plasma-induced flow instabilities to be the expansion of gas after localized gas heating during the discharge pulse (10s of ns). An acoustic wave propagates out of the plasma jet and disturbs the shear layer between the high velocity helium and the slow ambient air, causing a shear instability. Experimental measurements of spatially and time resolved electron densities in a He APPJ contacting a dielectric surface were performed in a well-controlled environment. The observed electron densities and ionization wave behavior were compared to modeling, and highlight the capabilities and limitations of this model.