Antimicrobial Activity of Anodic Porous Alumina with Controlled Surface Structures

The antimicrobial properties of anodic porous alumina (APA) formed by the anodization of Al substrates were evaluated. APA surfaces with needle-like projections fabricated under controlled preparation conditions exhibited high antibacterial activity against Escherichia coli and Staphylococcus aureus. In antibacterial tests using APA with different interpore distances, all APA surfaces showed high antibacterial activity against E. coli, regardless of the interpore distance. In the case of S. aureus, in contrast, the smaller the interpore distance in APA is, the higher the antibacterial activity. These results indicate that different bacterial species require different optimal surface structures for antimicrobial activity. The needle-like projections formed on the surface of APA became finer as the interpore distance decreased; the finer the projections, the more efficiently the soft cell membrane of S. aureus is disrupted and the higher the antibacterial activity. On an APA with an interpore distance of 60 nm, the colony formation rate of S. aureus and E. coli was reduced to less than 1/100,000 compared to that of a nonanodized Al substrate. The fabrication process described in this article is expected to make it possible to impart antimicrobial properties to the surfaces of various Al products.