Abrasion and ion elution of implantable metallic electrode in bending-rotating fatigue testing

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The fatigue properties of stainless steel wire ropes and titanium wires in simulated body fluids at 310 K were investigated in rotating-bending testing as a fundamental study for applications to the FES (Functional Electrical Stimulation) electrode.1.Stainless steel wire rope(1)The new stainless steel (Fe-22Cr-10Ni-6Mn-2Mo-0.4N) wire rope exhibited the higher fatigue strength and wear corrosion resistance than the type SUS3 16L stainless steel wire rope under low maximum bending stress level.(2)The metal ion concentrations in the lactic acid solution increased with increasing of number of cycles in fatigue testing, which agreed with the chemical composition of the stainless steels. On the other hand, in Hanks' solution, they are suppressed at the low level, and the order of the amounts of ion elution was Ni>Cr>Fe.(3)The ferric oxyhydroxides (β-FeOOH) was formed in Hanks' solution as corrosion products during fatigue testing, which might keep iron and chromium ions at low concentration in Hanks' solution.2.Titanium wires(1)The wire made of the new β-type titanium alloy, Ti-14Mo-3Nb-1.5Zr, had excellent fatigue strength compared with the C.P.Ti wire.(2)The ratio of fatigue limit (σ_f) to tensile strength (σ_U) of the C.P.Ti wire was less than 30 percent, which might be caused by the surface flaws introduced during the cold working process.(3)The C.P.Ti wire with diamond like carbon (DLC) coating exhibited the relatively high value of σ_f/ σ_U. This result suggests that the fatigue life of the titanium wire can be improved by the optimization of the DLC coating process.