Abstract
The oxide film resistance (RP) and capacitance (CCPE) 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. RP for the Ti–15Zr–4Nb–1Ta and Ti–15Zr–4Nb–4Ta alloys was proportional to d obtained by FE-TEM. CCPE was proportional to 1/d. RP tended to decrease with increasing CCPE. RP was large (maximum: 13 MΩ·cm2) and CCPE was small (minimum: 12 μF·cm−2·sn−1, n = 0.94) for the Ti–15Zr–4Nb–(0 to 4)Ta alloys. The relative dielectric constant (εr) and resistivity (kOX) of the oxide films formed on these alloys were 136 and 2.4 × 106–1.8 × 107 (MΩ·cm), respectively. The Ta-free Ti–15Zr–4Nb alloy is expected to be employed as an implantable material for long-term use.
Highlights
Stainless steels, Co–Cr–Mo alloys, commercially pure Ti (C.P.-Ti), and Ti alloys have been widely used for orthopedic implants
We examined the electrochemical stability of oxide films formed on highly biocompatible Ti–15Zr–4Nb–4Ta, Ti–15Zr–4Nb–1Ta, and Ta-free Ti–15Zr–4Nb alloys and other implantable metals by field-emission transmission electron microscopy (FE-TEM) and electrochemical impedance spectroscopy (EIS)
Thin oxide films of approximately 4.2 ± 0.2, 3.2 ± 0.2, 1.7 ± 0.2, and 2.3 ± 0.1 nm thicknesses were observed on Ti–15Zr–4Nb–1Ta, Ti–6Al–4V, Co–28Cr–6Mo, and high-N stainless steel surfaces after anodic polarization up to 0 V vs. saturated calomel electrode (SCE), respectively
Summary
Co–Cr–Mo alloys, commercially pure Ti (C.P.-Ti), and Ti alloys have been widely used for orthopedic implants. The mechanical strength of stainless steel is increased by adding nitrogen (N) and 20% cold working [1]. C.P.-Ti is classified into grade (G)-1, G-2, G-3, and G-4. The mechanical strength of C.P. G-4 Ti is increased by 20% cold working [2,3]. Among the Ti alloys, Ti–6Al–4V alloy (hereafter, alloy compositions are expressed in mass%) is widely used. Low-cost manufacturing processes for highly biocompatible Ti–15Zr–4Nb–4Ta, Ti–15Zr–4Nb–1Ta, and
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