Inanimate surfaces serve as a permanent reservoir for infectious microorganisms, which is a growing problem in areas in everyday life. Coating of surfaces with inorganic antimicrobials, such as copper, can contribute to reduce the adherence and growth of microorganisms. The use of a DC operated air plasma jet for the deposition of copper thin films on acrylonitrile butadiene styrene (ABS) substrates is reported. ABS is a widespread material used in consumer applications, including hospitals. The influence of gas flow rate and input current on thin film characteristics and its bactericidal effect have been studied. Results from X-ray photoelectron spectroscopy (XPS) and atomic force microscopy confirmed the presence of thin copper layers on plasma-exposed ABS and the formation of copper particles with a size in the range from 20 to 100 nm, respectively. The bactericidal properties of the copper-coated surfaces were tested against Staphylococcus aureus. A reduction in growth by 93% compared with the attachment of bacteria on untreated samples was observed for coverage of the surface with 7 at. % copper.

The different discharge modes that were observed for a plasma jet operated at atmospheric pressure with air or nitrogen by applying a DC high voltage to a microhollow cathode geometry were investigated. Critical parameters that determine the operation in either a selfpulsing or 'true' DC discharge mode were identified based on an electrical circuit model and investigated accordingly. Current limitation by the overall circuit resistance supports either a continuous charging and discharging of stray capacities, together with associated repetitive plasma generation, or a short circuit operation. These different modes can be controlled by gas flow rate and applied high voltage. Consecutive plasma development in the self-pulsing mode resembles characteristics of a transient spark discharge. When the associated memory effects start to last longer than intervals between subsequent electrical breakdowns, eventually a current-limited self-sustained glow-like discharge is supported.

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Atmospheric pressure plasmas are an appealing alternative for surface treatments. An advantage offered by nonthermal plasmas in particular is the modification of temperature sensitive materials. Here, we present copper coatings on acrylonitrile butadiene styrene that were achieved with a nonthermal dc operated air plasma jet.Index Terms-Antimicrobial coating, atmospheric pressure plasma, plasma materials processing.

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