Understanding how dielectric tube geometry impacts the atmospheric pressure plasma jet production of reactive oxygen and nitrogen species in water and in a poly(vinyl alcohol) hydrogel

This study compares the production of reactive oxygen and nitrogen species (RONS) in water and in a poly(vinyl alcohol) (PVA) hydrogel. Treatments were performed with a pulsed-direct current (DC) plasma jet operated using three different inner diameter (ID) quartz tubes (1 mm, 2 mm and 3 mm). It was found that the larger (2 mm and 3 mm) ID tubes were more effective at producing RONS in water, whereas the smaller (1 mm) ID tube was more effective at producing RONS in the PVA hydrogel. These results show that larger ID tubes enhance plasma processes important for producing RONS in water but not for delivering RONS into soft materials such as the PVA hydrogel. This is because the PVA hydrogel was shown to present a significant barrier that inhibits the penetration of RONS during plasma jet treatment. Although the smaller 1 mm ID tube was not as time-efficient at producing RONS, other plasma processes such as gas flow dynamics and a more intense electric field through a confinement effect leads to a more focused plasma jet capable of penetrating the PVA hydrogel barrier. Therefore, considering factors such as the dielectric tube geometry of the plasma jet may be more important than assessing overall RONS production, which is usually measured in water, when developing plasma jets to deliver RONS into the subsurface of materials. These findings are useful in the future development of plasma jets and associated protocols for activation of hydrogels (in plasma activated hydrogel therapy, PAHT) or other applications such as treatment of solid cancer tumours.