Shining light on cold atmospheric plasmas and their interaction with liquids

Cold atmospheric plasmas, CAPs, operate in air and are a rich source of reactive oxygen and nitrogen species, RONS. Many of these RONS are also produced naturally in cells and can regulate cellular and physiological processes, and as such are relevant to medical science. CAPs are finding an increasing number of medical applications; when applied to living tissue, they can effectively decontaminate wounds covered with bacterial biofilms and destroy, or at least significantly reduce the size of, cancerous tumours. It is currently assumed that the underlying mechanisms by which plasma influences biological activity are defined by the way in which plasma-generated RONS interact with the components of biological liquid, cells and tissue. However, it is unclear how plasma-generated RONS stimulate cell death deep within a biofilm or tumour, which could be micrometres to millimetres in thickness. In the context of cancer treatment, it has been suggested that RONS, generated by plasma at the surface of the tumour, stimulate cellular signalling mechanisms that trigger cell death and that these signals are transmitted deeper into the tissue through cell-to-cell communication, in a manner similar to that seen in other forms of cell stress. While these hypotheses seem credible given that many of the RONS generated directly by plasma are highly reactive, have short lifetimes and can only diffuse over a short distance in real tissues, there is in fact little or no quantitative evidence to back this up - this proposal seeks to address this situation by applying state-of-the-art spectroscopic methods to this problem.
The work will quantify the absolute concentrations of important plasma-generated RONS in the gas phase as they impinge upon pure water and biological interfaces, identifying and determining the kinetics of formation and loss of secondary RONS within the liquid phase, and determining the end point chemistry. These studies will be conducted in real time and with a spatial resolution of a few microns or less, and offer a step-change in our understanding of this application of plasma science. In particular, it will allow the diffusion length of the highly toxic peroxynitrite radical to be determined for the first time, providing crucial evidence to help determine the mechanism of plasma-induced destruction of micro-organisms and cancer cells within biofilms and tumours. The work will determine the penetration depth of plasma-generated RONS into a biologically relevant target, such as agarose, a polysaccharide polymer material which is a surrogate for real tissue, and will explore the dependence of the RONS penetration depth upon the CAP jet exposure time and plasma source-target distance, as well as the composition and thickness of the surrogate tissue. The data will be important for the future development of plasma medical devices and for avoiding unwanted tissue damage. Monitoring the transport of RONS in real time through a biofilm and within the liquid phase will shed light on the hypothesis that plasma may not only stimulate the deactivation of biofilms and tumours at a tissue's surface, but potentially deliver RONS into cells embedded deep within affected tissue.
A detailed theoretical understanding of the mechanisms by which RONS are generated, transported and lost, both within and between the gas and liquid phases, will be provided by development of a state-of-the-art reaction diffusion model which will be optimised by reference to the new experimental data.

In plasma science we are now just starting to mine a rich source of potential applications, in medicine, dentistry and agriculture. For example, plasma induced wound healing, tissue ablation, blood coagulation, and lithotripsy have been demonstrated and in some cases are in use. Furthermore, the antimicrobial effectiveness of plasmas in medicine, dentistry and in food and agriculture has now also been demonstrated. We firmly believe that the work proposed here is a necessary prerequisite for the understanding of the underlying science, i.e. physics, chemistry and biology that will be required for the progress of plasma medicine.
The results from this project will be communicated to the wider scientific community via publication in high quality journals and presentations at international conferences. Novel state-of-the-art instrumentation will be developed in the project which will help to maintain the impact and international leadership of the UK low temperature plasma community. The investigators are active within the professional societies, for example the Royal Society of Chemistry and the Institute of Physics, and the activities within this project will be publicised via our engagements with them. The research will also be communicated to the wider public through talks in Schools, Café Scientifique and through the media.
The research will impact scientists at Universities and research institutes active in the development of sensitive methods for measurements of radicals in the atmosphere and other reactive media, and within the analytical/environmental sciences where measurements of water borne pollutants such as nitrates are paramount. This will clearly inform and impact those sectors which rely on accurate representations in models of the fate and impact of pollutants and emissions (natural and human related) into the biosphere. These include water and air quality policy legislators, industry, government advisory bodies (for example the DEFRA Air Quality Expert Group, DEFRA group on agriculture and water), the transportation and logistics sectors, and environment and health authorities (local and regional).
Palpably, the research has the potential to make a critical contribution to the nation's health and quality of life through better therapeutic strategies to combat the increasing prevalence of biofilms and their resistance to treatment. These strategies are informed by the predictions of models, and validation of chemical mechanisms used in these models will raise confidence in the accuracy of these predictions. Better predictions of plasma medicine and subsequent improvements to therapeutic strategies will directly benefit the general public through improvements to their quality of life, and is central to EPSRC strategy.