Plasma-Based Strategies to Control the Release of Ag⁺ on Short- and Long-Term Periods from Ag-Based Antibacterial Coatings

Recently, the prevention of hospital-acquired infections (HAIs) has increased to a great extent, due to an augmented awareness on the importance to reduce the spread of bacteria on everyday surfaces located, e.g., in hospital rooms [1]. In this sense, the use of antibacterial (AB) coatings can be an important adjunct in the fight against HAIs. More specifically, silver (Ag)-based AB coatings are biologically active when they disperse Ag⁺ against Gram-positive and -negative bacteria, without any risk of bacterial resistance [2]. However, their success as a long-term AB coating hinges on the ability to control the release of Ag⁺ [3]. Thus, based on a low-pressure plasma platform, this study evaluated the feasibility to efficiently modulate the release kinetics of Ag by controlling its quantity and oxidation state, choosing an adequate matrix, such as hydrogenated amorphous carbon (a-C:H) for its excellent mechanical properties, and using an additional fluoropolymeric (CF_x) barrier to improve the long-term release behavior. In this work, the different approaches have been tested, the surface composition and morphology characterized, and the impact on the release kinetic evaluated.<br/>To do so, stainless steel 316L substrates were introduced into a low-pressure plasma enhanced chemical vapor deposition - physical vapor deposition (PECVD-PVD) reactor, to simultaneously deposit Ag/AgO NPs and the a-C:H film. Subsequently, a-C:H:Ag and a-C:H:AgO samples were placed into a pulsed plasma polymerization reactor to deposit a fluorocarbon (CF_x) barrier. The coatings were then characterized using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and microwave plasma – atomic emission spectroscopy (MP-AES).<br/>The XPS results showed that a-C:H films were mainly formed by carbon and oxygen. The modified film a-C:H:Ag displayed 2.8 at. % Ag and an O/Ag ratio of 0.2 ± 0.1, whereas the a-C:H:AgO coating showed 2.5 at. % Ag and an O/Ag ratio of 1.4 ± 0.3. The further addition of CF_x is evidenced through an increase in fluorine content, with an F/C ratio of 1.44 ± 0.02 for a-C:H/CF_x, 0.7 ± 0.5 for a-C:H:Ag/CF_x, and 0.6 ± 0.5 for a-C:H:AgO/CF_x. Concerning the surface morphology, the a-C:H films exhibited a smooth topography in a nanoscale with a R_RMS between 0.76 ± 0.04 and 0.69 ± 0.08 nm. The incorporated Ag NPs showed a nanometric sized oval shape, whereas AgO NPs displayed more dense clusters. Instead, the addition of the CF_x barrier evidenced the occurrence of defects (1.38 μm) on the surface. These defects can be attributed to a chain scission/etching process, which breaks the polymer and exposes the underneath a-C:H:Ag/AgO matrix. Regarding the amount of Ag in the coating, it was observed that by changing the power applied in the PVD system, Ag content can be modulated. For the same amount of Ag within the coating, it has been demonstrated that the release profile varied depending on the strategy. Indeed, a-C:H:Ag displayed a slow release of Ag⁺ eventually reaching a concentration plateau after reaching its maximum concentration at 2 hours with 22 ppb. For a-C:H:AgO, a high burst up to 125 ppb in day 3 followed by a continuous decrease of Ag content was obtained. while with polymeric barriers, herein CFx, the release Ag⁺ was slowed down without reaching a concentration plateau [3].<br/>By combining these approaches, it is possible to tune the release profile of the coating, and thus its AB activity. In conclusion, this study presents a low-pressure plasma platform with different strategies to tune the release kinetics of Ag⁺: modifying Ag concentration and oxidation, using an a-C:H matrix, and adding an additional CF_x polymeric barrier. Combined, they represent an innovative multifunctional coating with potential long-term AB stability to help reduce HAIs.<br/>[1] Khan et al., J Trop Biomed 2015 (5) 509.<br/>[2] Bonilla-Gameros et al., Nanomedicine: NBM 2020 (24) 102142.<br/>[3] Cloutier et al., Trends Biotechnol 2015 (33) 637.