Star-Like Pattern Formation during Plasma-Hydrogel Interactions and Plasma-Enabled Biofilm Inactivation

Abstract

Cold atmospheric pressure plasmas (CAPs) have received a lot of attention in recent decades due to their wide applications including surface treatment of heat sensitive surfaces, water disinfection and food preservation. Cold plasma is a source of abundant reactive oxygen and nitrogen species which are known to have a biological effect on living matter which promoted its use in inducing cancer cell apoptosis and wound healing. The penetration and distribution of plasma produced reactive species in the tissues is critical for understanding the biochemical processes involving plasma medicine. A gelatin-based model tissue has been commonly used by researchers to understand the plasma-tissue interaction. The distribution of reactive species was correlated with the star like pattern formed on the surface of the gel. In this work, the parameters influencing the formation of this star-like pattern were investigated. A pulsed dc helium-based plasma jet was used for these studies. The size of the plasma-induced pattern and the surrounding evaporation zone increased with the voltage in correspondence with the increase in charge deposition on the gel surface. The charge deposition was larger for positive polarity compared to negative polarity discharge despite similar total energy which correlated well with observed pattern formation. There was no pattern observed in humidified helium plasma treatment although the charge was found to be similar as in helium-oxygen plasma treatment which showed the formation of a pronounced pattern indicating the role of drying as a major factor for pattern formation in addition to the charging effect. The surface ionization waves, although very weak due to gel’s finite conductivity were observed using time-resolved imaging and resembled a gear-like structure made of surface streamers which matched exactly with the number of filaments on the star-like pattern, indirectly linking the plasma morphology to the pattern structure. Electrical forces resulting from charging were found to be sufficient to cause gel deformation. The wound healing process is often hampered by infection or biofilm formation. The second part of this work focuses on the CAP treatment of a Pseudomonas aeruginosa bacterial biofilm. Biofilms are different from the freely suspended planktonic bacteria in terms of physiology and environmental stress tolerance causing higher resistance to antibiotic drugs. A RF driven Ar + O2 plasma jet was used for direct and indirect wet treatments of saline covered biofilms. A complete inactivation was observed within 90 s exposure time to plasma in wet direct treatment as opposed to a gradual inactivation in wet indirect treatment indicating the additive effect of short-lived species in the former. The inactivation of bacteria was found to correlate with the generation of ClO- in the solution formed through the reaction of plasma-produced O radicals with the Cl- present in the solution. The concentration of these chlorine compounds was observed to be also dependent on the pH of the solution.