Fretting Fatigue Properties of Ti-6Al-4V Alloy in Pseudo-Body Fluid and Evaluation of Biocompatibility by Cell Culture Method

Plain and fretting fatigue tests of an annealed Ti-6Al-4V alloy were carried out in ca. 150 mL of a pseudo-body fluid. A phosphate-buffered saline without calcium and magnesium salts (PBS(−)) was used as a pseudo-body fluid, and nitrogen gas was bubbled into the saline during the tests in order to reduce the concentration of dissolved oxygen in the saline. The fretting fatigue strength in PBS(−) was lower than those in the air at higher and lower stress amplitudes ranges, while the fatigue strengths in both environments were almost equivalent to each other.Released metallic ions and debris in PBS(−) recovered after fretting fatigue tests under condition A (the stress amplitude, σa is set at 250 MPa) and B (σa is set at 131 MPa) were quantitatively analyzed by graphite furnace atomic absorption spectrophotometry (GFAAS). Before the analysis, some volume of the recovered PBS(−) were filtered through a 0.22 μm pore size filter. The unfiltered fluid of condition A contained iron, manganese, nickel, vanadium, chromium, aluminum and titanium in 42, 110, 110, 130, 970, 1150 and 2420 ppb (μg/L), respectively. Nickel, iron, aluminum and manganese in 24, 4, 3 and 2 ppb, respectively, were found in the filtered fluid of condition B, suggesting that corrosion by passive-active cell occurred between the fresh surface of crack-tips or fretting area and other surface covered by oxide films during the fretting fatigue test.Cytotoxicity of the fluids of conditions A and B was investigated using MC3T3-El cells. Both of the filtered and the unfiltered fluids of the condition B were toxic for the cells, but only the unfiltered fluid of the condition A were toxic for the cells. These facts suggest that metallic substances in small concentrations as 10 ppb have cytotoxicity, and that not merely the amount but also the state of metallic elements affect the extent of cytotoxicity of the fluids.