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

Abstract

Deep space missions will bring new challenges, beyond our experience so far with International Space Station, to life support systems including water supply. The complexity of these missions might leave spacecrafts and facilities uncrewed for several months. In this situation, biofilm growth can deteriorate the quality of stored water and cause water supply system failure during reinitiation, threating the mission success. Antimicrobial coatings have been used for biofilm mitigation in various conditions. A successful coating to control biofilm formation in deep space mission, among other things, must have long lifetime considering the duration of such missions. In this study, a solution was provided to the biggest drawback of silver nanoparticles as antimicrobial coating; short lifetime. Passivating with sulfide was tested to control silver ion release from silver nanoparticles, hence, prolonging antimicrobial activity. Stainless steel bellow pieces, as the most prone parts to biofilm growth, was chosen as the substrate. The pieces were coated with silver and passivated silver with different passivation degree to find the optimum condition. The substrates were exposed to Pseudomonas aeruginosa in M9 medium for 12 months for biofilm formation. The bacteria count on the bellow pieces as a representative of biofilm as well as bacteria count and silver ion concentration in M9 medium were measured at 1.5, 3, 6, and 12-month timepoints. Passivation slowed down silver ion release rate from silver nanoparticles, however, biofilm mitigation at the end of the experiment for one passivated coating was the same as silver coating, which means the passivated coating can last longer by releasing less antimicrobial agent, silver ions. Besides performance in biofilm mitigation, we demonstrated 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 application of passivated silver coating for space missions.