Plasma-surface engineering for biofilm prevention: evaluation of anti-adhesive and antimicrobial properties of a silver-nanocomposite thin film
Résumé
Biofilms can be generally defined as a complex and dynamic ecosystem, constituted by a community of microorganisms adherent to a substrate and often embedded within a selfproduced extracellular polymeric matrix. Biofilm formation is a well-recognized phenomenon, especially in medical and food industries and often leads to undesirable effects (nosocomial infections, energy losses, accelerated corrosion, food spoilage, and spread of foodborne diseases…). In this framework, surface engineering for preventing biofilm formation is a challenging question, which has fuelled an explosion of research in surface science for the development of antimicrobial and/or anti-adhesive materials by physical or chemical modifications. The surface treatment can prevent biofilm formation by limiting the initial microbial adhesion and/or by killing microorganisms as they come in close contact with the solid surface. Among the different approaches considered, a growing interest is focused on thin silver coatings, like silver-based composite materials, due to their extended time-release properties. In the present work, plasma-mediated thin films (~170 nm), containing silver nanoparticles embedded in an organosilicon matrix, were deposited onto stainless steel. The process originality relies on a dual strategy, associating silver sputtering and simultaneous Plasma Enhanced Chemical Vapour Deposition, in an argon-hexamethyldisiloxane plasma, using an asymmetrical radiofrequency discharge at 13.56 MHz. SEM demonstrated the nanoparticle-based morphology of the deposited layer. X-ray photoelectron spectroscopy confirmed the presence of metallic silver nanoparticles embeddedin the organosilicon matrix. The film anti-adhesive potentialities were evaluated in vitro towards the model yeast Saccharomyces cerevisiae by performing shear-flow induced detachment experiments, under well-controlled hydrodynamic and physico-chemical conditions. The maximal effect was achieved for the organosilicon matrix alone. When silver nanoparticles were incorporated, yeast detachment was less pronounced, probably due to the strong affinity of embedded silver for biological groups of the cell wall surface. The presence of methyl groups in the matrix network could also promote enhanced hydrophobic cell/coating interactions. An antifungal action of released silver (Ag+ ions and/ornanoparticles) at the immediate vicinity of the coating surface occurred, since a 1.4 log reduction in viable counts was observed, compared to control conditions with bare stainless steel. TEM observations of the yeast ultrastructure demonstrated morphological and structural damages. The presence of electron-dense silver clusters was also detected not only on the cell surface but also within the cell. In parallel, the coating antimicrobial properties against bacteria were assessed (reduction in viable counts of 1.5 and 2.4 log and for Escherichia coli and Staphylococcus aureus, respectively).
