Shining light on cold atmospheric plasmas and their interaction with liquids

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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.