Effects of noble-metal ion implantation on corrosion inhibition and charge injection capability of surgical Ti and Ti-6Al-4V

Studies are described involving effects of noble-metal ion implantation on corrosion inhibition and charge injection capabilities of surgical Ti and Ti-6A1-4V. With surgical alloys, harmful biological responses are principally due to the type and quantity of metal ions released by the corrosion process. One approach to improve long-term biological performance involves surface modifications to significantly reduce degradation rates. With regard to surface-modifications, one of the most effective methods is through ion implantation. Results are presented for ion-implanted Au, Rh, and Ir.

For the static in vitro corrosion properties, the noble-metal ion-implanted Ti-6A1-4V and commercially-pure (CP) Ti were investigated in non-passivating acid and passivating saline solutions. It was postulated that during the early stages of corrosion (or during a corrosion pretreatment) the implanted noble-metal would enrich at the surface and significantly reduce subsequent corrosion rates. The observed behavior for the Ir and Rh implanted materials appeared to follow the postulated mechanism, with both initial and time-dependent improvements in corrosion resistance. However, the Au implanted material yielded early benefits, but the enhanced corrosion resistance deteriorated with time. X-ray photoeletron spectroscopy (XPS) analysis indicated that the implanted Au atoms remained as pure metallic Au, while the Ir and Rh atoms were in some oxide state, which gave the good adhesion of the Ir or Rh enriched surface to the Ti substrate.

For a stimulating neural electrode, the charge density should be as large as possible to provide adequate stimulation of the nervous system while allowing for miniaturization of the electrode. "Activated" Ir has been known as having the highest charge injection capability of any material known. The increase in charge density of pure Ir on continuous cycling is due to the accumulation of the oxide phase (associated with a large surface area) in which the valence state of iridium changes and the double-layer capacitance increases. The charge densities as measured by cyclic voltammetry (second-cycle values) for the Ir implanted materials increased significantly as a consequence of the H₂SO₄ exposure. Initially, the charge injection capability of 5.0 at.% peak-max Ir implanted Ti-6A1-4V after noble-metal surface enrichment was slightly greater in isotonic saline than that of pure Ir, but remained unchanged on continuous cycling because of the limited number of implanted Ir atoms.