Contact-mediated Antibacterial Mechanism in Cu–Ag Composite Coatings Deposited by Brush Plating

This study investigated the antibacterial mechanism of Cu–Ag composite coatings deposited by brushing on AISI 316L stainless steel. High Ag content coatings (∼ 42 wt%) were fabricated using a SIFCO ASC brushing system from acidic electrolytes (pH 0.5) containing 1–5 µm Ag particles. SEM/EDS analysis revealed a uniform distribution of Ag in the Cu matrix without agglomeration, while XRD confirmed a phase separation between metallic Cu and Ag without detectable intermetallic compounds. Zone of inhibition (ZOI) assays against Staphylococcus aureus ATCC 29213 and Escherichia coli ATCC 25922 showed that, unlike pure Cu coatings that produced distinct zones of inhibition, the Cu–Ag composites did not exhibit a measurable ZOI (detection limit < 0.5 mm), but completely suppressed bacterial adhesion on the coating surface (Fig. 4). This shift from diffusion-mediated to predominantly contact-mediated antibacterial activity is due to the physical coating of the copper matrix surface by silver particles, which reduces the exposed copper area available for ion release, combined with the low solubility of silver compounds and the rapid complexation of the released Cu²⁺ ions with chloride ions in agar medium. Notably, galvanic coupling between Cu (E° = +0.34 V) and Ag (E° = +0.80 V vs. SHE) would thermodynamically accelerate Cu dissolution rather than suppress it; therefore, the observed reduction in ion diffusion is primarily explained by the physical surface coating rather than by electrochemical effects. The brush coating allows for selective functionalization of critical areas of the implant without compromising osseointegration surfaces, offering a promising strategy for next-generation antimicrobial medical devices.