Development and Characterization of a Cold Atmospheric Plasma Source for Clinical Application
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The aim of this thesis was the development and characterization of a cold atmospheric plasma source (CAP) tailored for clinical applications. During experiments, various materials were tested in a long-term plasma afterglow environment for their suitability in CAP decontamination and as components for CAP sources, and afterwards a novel, adaptable CAP source was built and characterized. For the suitability-study, materials were chosen from different material groups, namely metals (stainless steel), glass (borosilicate glass) and polymers (polypropylene, rigid polyvinyl chloride and fluorinated ethylene propylene), representing typical materials used in the healthcare sector.Here, different degrees of surface modification were found. While fluorinated ethylene propylene showed almost no modifications, slight to significant changes in free surface energy, surface morphology, surface composition and oxidation was found for the other materials, especially stainless steel. Additionally, sporicidal efficacy of the CAP source was proven, with a log10 reduction of Bacillus atrophaeus endospores of 4.3-6.2 within 15 minutes. Here, hydrophobic properties of the tested materials and hence layering and shielding of the bacteria showed the greatest limiting effect in spore reduction.
Based on these findings, the prototype of a CAP source was designed and constructed. Aim for this design was to create an adaptive, low-cost, efficient and durable plasma source. Hence, sputter-coating of the electrodes on a flexible dielectric material was tested. As dielectric, polyethylene naphthalate (PEN) foil was chosen because of its high dielectric strength and excellent chemical resistance. By sputter-coating, different electrode materials and electrode thicknesses were deposited onto the PEN foil and tested for their resistance against material modifications and for their endurance during plasma ignition. Based on its performance, palladium was chosen, with a thickness of 110 nm for both electrodes.
For this prototype (palladium-sputtered electrodes with thickness 110 nm on both sides), different characterization tests were done. Firstly, ozone measurements revealed plasma parameters for ozone and nitrogen mode. Secondly, laser microscopy showed the development of a border area on the edge of the grounded electrode. Thirdly, plasma diagnostical approaches indicated electron temperature, electron density and vibrational temperature for nitrogen mode, and plasma power was evaluated. Lastly, bactericidal efficacy was measured for different ozone mode plasma parameters and for nitrogen mode, using different CAP source prototypes. Here, within 5 minutes, a log10 reduction of 6.7 (nitrogen mode) and 5.3 (ozone mode) was achieved, respectively.
Endurance of the different plasma mode prototypes showed to differ, depending on the used plasma mode. While the nitrogen mode prototype burned through after almost 150 minutes, corresponding to 50 plasma ignitions, the two ozone mode prototypes still worked reliably after the end of the experiments.
An evaluation of the performance of the newly developed CAP prototype shows, that reduction efficacy is in the same order of magnitude, but with shorter required treatment intervals than the plasma source used during the material tests. At the same time, the newly developed prototype works with an power density, which is one order of magnitude lower than that used for the previous plasma source, indicating improved effectiveness.
To sum it all up, the experimental studies conclude, that the treatment with cold atmospheric plasma is a non-destructive and adaptable cleaning method for various materials. Combining CAP treatment with conventional sterilization methods could enhance the pathogen reduction, especially with regard to geometrically complex or heat-sensitive materials. PEN foil as dielectric has promise because of its characteristics and sputter-coating technique shows to be an adaptable and low-cost possibility to produce flexible and highly effective cold atmospheric plasma sources.