Fabrication of thick nanoporous oxide films on stainless steel via DC anodization and subsequent biofunctionalization

Remarkably thick nanoporous anodic films have been formed through simple DC anodization by adding hydrogen peroxide as an oxidant to a sulfuric acid electrolyte. This result was not possible when using the previously reported pulsed polarization technique. While this anodic film's growth was significantly promoted by DC anodization, having only sulfuric acid resulted in the preferential anodic dissolution of the substrate material. Transmission electron microscopy of the anodic film's cross-section confirmed that the film has numerous fine pores with a size distribution ranging from 5 nm to 20 nm. Energy-dispersive x-ray spectroscopic analysis of the anodized specimens revealed that the films were composed of a complex chromium-rich oxide. In addition, coating stainless steel with thick nanoporous oxide films through anodization in the optimized conditions confirmed that it could act as a highly suitable host for synthetic hydroxyapatite coating, resulting in the enhanced growth of natural apatite in a simulated body environment. The enhancement of the apatite deposition is attributed to the availability of the empty pore volume in the thick anodic porous films, formed on stainless steel surfaces. These findings suggest a novel functionality to stainless-steel surface modification. (C) 2016 Elsevier B.V. All rights reserved.