A study of the physicochemical pathways in an atmospheric surface barrier discharge for microbial inactivation

A major cause for concern in the healthcare sector is the treatment of chronic infections caused by pathogenic microorganisms, particularly those in the form of a biofilm. Conventional biofilm decontamination techniques such as antibiotic treatment rapidly become ineffective due to the emergence of resistant organisms. It is estimated that up to 80% of human bacterial infections are caused by biofilms, therefore there is an urgent global need to develop an effective method for biofilm decontamination. [1] Atmospheric pressure non-thermal plasma generated in ambient air have proven to be a highly effective biocide, their low temperature nature presents a number of opportunities for the healthcare sector. Within such plasmas a plethora of charged and neutral reactive nitrogen and oxygen species (RONS) are created, such as O, OH, O3, H2O2, NO etc… It is the nature of these RONS that drive applications within the healthcare sector, as the presence of certain species can cause microbial death. Given that these types of plasmas are generated at near room temperature they provide a means to treat materials that are thermosensitive, which is particularly important when it comes to the decontamination of invasive medical equipment that can be damaged through current sterilisation techniques that employ high temperatures (i.e. autoclaving). This thesis focuses on the surface barrier discharge (SBD), a class of device based on the widely used dielectric barrier discharge configuration. SBD’s are known to be extremely stable whilst operating in ambient air and can be generated over large areas. A key disadvantage of the SBD system is the spatial separation between the plasma generation region and the target sample, which limits the transport of highly reactive and highly antimicrobial RONS such as O, N and OH to the target. In this work the underpinning discharge mechanisms involved in the generation and transportation of reactive chemistries from a surface barrier discharge are explored. Efforts focus on the interplay between electrode geometry and the breakdown characteristics of the discharge, as decreasing the area of a circular discharge gap below a certain threshold is shown to cause an exponential increase to the breakdown voltage, while also impacting several other discharge characteristics including deposited power and the EHD induced flow. The dielectric temperature effects on species generation is shown, as the rate at which the transition into ozone poisoning mode occurs can be altered and essentially negated entirely through dielectric temperature manipulation, while the influence of air feed rate into the discharge chamber is also presented alongside as a viable means of transition control. Thus, enabling the density of specific RONS to be tailored for the needs of an application. Through both experimental investigations and simulations, the transport dynamics of key reactive nitrogen species (RNS) that are relevant in biofilm decontamination are found to be carried several cm from the active discharge domain, closely following the plasma induced flow. While a method to exert a degree of control over the induced flow is also established, through the introduction of a phase shift between the high voltage signals applied to a linear electrode configuration, without compromising the chemistry of the downstream reactive species. Finally, the impact of plasma-liquid interactions on biofilm decontamination efficacy is assessed, as the composition of the liquid phase species is demonstrated to have significant influence over the bactericidal efficacy of plasma activated water (PAW). Several possible underpinning mechanisms responsible for microbial inactivation are identified, that include the production of reactive nitrogen species under acidic conditions. The work presented in this thesis adds to the field by giving an insight into the reactive chemistries and dynamics of species generated by non-thermal discharges. Several techniques are demonstrated to essentially tailor the physical and chemical nature of SBDs to the needs of future applications