Development of electrochemically deposited surfaces based on copper and silver with bactericidal effect

Abstract

Environmental surfaces play a major role in the transmission of nosocomial pathogens. Surfaces that prevent bacterial adhesion or exert a microbiocidal effect can be integrated into the existing disinfection practices to increase surface hygiene and reduce the incidence of healthcare-associated infections. The high antibacterial efficacy of copper alloy surfaces is due to the so-called contact killing, which is controlled by the redox activity of copper and the toxic action of copper ions. The redox activity of copper induces electrochemical reactions with the alloying elements and the surrounding environment, and this can, positively or negatively, influence the antibacterial activity of copper alloys. Bacterial-metal contact is crucial for establishing conditions of contact killing, so that the antibacterial efficacy of copper alloys can be optimized if the surface area to volume ratio is increased. These chemical and physical characteristics can be used to produce copper alloys with increased antibacterial efficacy. As such, an electroplated copper-silver alloy coating was developed with a high surface area to volume ratio, using the galvanic coupling of copper and silver to trigger electrochemical reactions, when in contact with bacterial cells. The purpose of the present PhD project was to demonstrate the antibacterial activity of a newly developed copper-silver alloy coating obtained by electroplating, to explain its antibacterial properties with a materials science-based approach and to evaluate relevant conditions potentially influencing the efficacy. In the light of an increasing global demand for antimicrobial coatings to improve surface hygiene, this coating could contribute to reducing transmission of bacteria from surfaces, and the present work aimed to put it in the context of the copper-based antibacterial strategies. The antibacterial and antiadhesive properties of the alloy coating were demonstrated in wet conditions (e.g. buffer and nutrient broth) and its antibacterial efficacy in dry conditions. In liquid environments, copper-silver alloy coated surfaces released copper ions in the bacterial suspension: copper dissolved rapidly due to the presence of silver in the alloy, according to the principle of galvanic corrosion. This explained the higher antiadhesive activity of the coppersilver alloy in buffer, even as compared to copper or silver alone. Bacterial numbers were reduced by 5-6 log units in suspension with copper-silver alloy coated surfaces in buffer. Nutrient broth neutralized copper ions in solution and protected bacterial cells, thus there was neither antibacterial nor antiadhesive efficacy in these conditions.Under dry conditions, contact killing of bacteria on copper-silver alloy coated surfaces was evaluated using the U.S. EPA test methods for copper alloys. More than 99.9% reduction in numbers was achieved both after 2 hours exposure and over 24 hours of continuous bacterial contamination. A modified live/dead staining technique in combination with confocal laser scanning microscopy was applied in order to visualize in situ the killing of bacterial biofilms at the copper-silver alloy coated surfaces. S. aureus biofilm was inactivated more quickly than P. aeruginosa biofilm. Furthermore, in situ pH monitoring at the copper-silver alloy coated surfaces revealed a fast local pH raise due to the electrochemical reactions induced by potential differences between silver, copper and bacterial cells, when in contact with the alloy. Chlorides and phosphates, commonly present in chemical detergents and disinfectants, can influence the antibacterial efficacy of copper alloy surfaces by reacting with copper and its alloying elements. Chlorides established conditions for active dissolution of copper and stabilized copper ions in solution, explaining the highest antiadhesive efficacy of the coppersilver alloy coating in chloride containing media. In contrast, copper did not dissolve in the suspension with only phosphates, therefore no antibacterial activity was observed in these conditions. Other indoor environmental factors, such as surface oxidation over time, build-up of organic material and abrasion of the surfaces can affect the antibacterial properties of copper alloy surfaces, and these factors are highly dependent on the specific application. Hence, two field tests of the copper-silver alloy as coating for door handles were carried out to evaluate the antibacterial performance and durability in clinical settings. Reference uncoated door handles had approx. twice as high microbial load as compared to the copper-silver alloy coated door handles. These data confirmed previous results of other copper and copper alloys-based antibacterial strategies. The lifetime of the copper-silver alloy coating was estimated to be at least one year in such applications, prior to re-coating interventions. Lastly, bacterial tolerance or resistance to the copper-silver alloy coating and cross-resistance to other antimicrobials did not appear as primary concern, as indicated by an adaptive laboratory evolution study in which S. aureus was exposed to the copper-silver alloy coating. The industrial scalability and production of the copper-silver alloy coating is possible, and it could be applied on already existing objects limiting the costs. Therefore, the copper-silver alloy coating qualifies as promising surface solution strategy to limit infectious diseases in healthcare settings, and microbial contamination in biopharmaceutical industry and food production environments.