Characterization of Oxide Film of Implantable Metals by Electrochemical Impedance Spectroscopy

The oxide film resistance (R-P) and capacitance (C-CPE) diagrams of implantable metals (commercially pure Ti, four types of Ti alloys, Co-28Cr-6Mo alloy, and stainless steel) were investigated by electrochemical impedance spectroscopy (EIS). The thin oxide film formed on each implantable metal surface was observed in situ by field-emission transmission electron microscopy (FE-TEM). The Ti-15Zr-4Nb-1Ta and Ti-15Zr-4Nb-4Ta alloys had higher oxygen concentrations in the oxide films than the Ti-6Al-4V alloy. The thickness (d) of the TiO2 oxide films increased from approximately 3.5 to 7 nm with increasing anodic polarization potential from the open-circuit potential to a maximum of 0.5 V vs. a saturated calomel electrode (SCE) in 0.9% NaCl and Eagle's minimum essential medium. R-P for the Ti-15Zr-4Nb-1Ta and Ti-15Zr-4Nb-4Ta alloys was proportional to d obtained by FE-TEM. C-CPE was proportional to 1/d. R-P tended to decrease with increasing C-CPE. R-P was large (maximum: 13 M Omega.cm(2)) and C-CPE was small (minimum: 12 mu F.cm(-2)s(n-1), n = 0.94) for the Ti-15Zr-4Nb-(0 to 4)Ta alloys. The relative dielectric constant (epsilon(r)) and resistivity (k(OX)) of the oxide films formed on these alloys were 136 and 2.4 x 10(6)-1.8 x 10(7) (M Omega.cm), respectively. The Ta-free Ti-15Zr-4Nb alloy is expected to be employed as an implantable material for long-term use.