Coating complex metallic surfaces with passivated silver nanoparticles for long-term biofilm control

Abstract

Inferring from our experience so far with the International Space Station, deep space missions are bound to encounter new challenges in life support systems, including water supply. The complexity of these missions might demand spacecraft and facilities to be uncrewed for several months. In this situation, biofilm growth can deteriorate the quality of stored water, leading to failure of water supply system during reinitiation and thus threatening the success of such missions. Antimicrobial coatings have been used for biofilm mitigation under various conditions. A successful coating to control biofilm formation in deep space missions, must have a long lifetime considering the duration of such missions. In this study, a solution to the short lifetime of silver nanoparticles as an antimicrobial coating is provided. Passivation with sulfide was performed to control the release of silver ions from silver nanoparticles, thereby prolonging the antimicrobial activity. Stainless steel bellow pieces, as the most prone parts to biofilm growth, were chosen as the substrate. The pieces were coated with silver and passivated silver at different passivation degrees to find the optimum condition. The substrates were exposed to Pseudomonas aeruginosa in an M9 medium for 12 months for the biofilm formation. The bacteria count on the bellow pieces as a representative of the biofilm as well as the bacteria count and silver ion concentration in the M9 medium were measured at 1.5, 3, 6, and 12 month time points. It was observed that passivation slowed down the silver ion release rate from silver nanoparticles. However, biofilm mitigation at the end of the experiment for one passivated coating was the same as that of the silver coating, which means that the passivated coating can last longer by releasing less amount of silver ions, which are the antimicrobial agents. Besides investigating the performance in biofilm mitigation, we demonstrate that the bellows can be coated homogeneously in a continuous reactor and passivation can enhance the stability of the coating to mechanical stress during expansion/retraction of the bellow, paving the way for the application of the passivated silver coating for space missions.