Metal release from stainless steel, Co–Cr–Mo–Ni–Fe and Ni–Ti alloys in vascular implants

A seven-day immersion test using several solutions was conducted on stainless steel, Co-based alloy, and Ni–Ti alloy, which are used for stents and stent grafts. The quantitative data on the release of each metal ion and the correlation between metal ion release rate and pH were obtained. The quantities of Fe and Ni released from stainless steel gradually decreased with increasing solution pH (pH 2–7.5). For Co–Cr–Mo–Ni–Fe alloy, the quantity of Cr released steadily increased as pH decreased (pH ≤ 6) and reached nearly zero at pHs higher than 6 (pH 6–7.5). Co release was slightly affected by a variation in pH. The quantities of Ni and Ti released from Ni–Ti alloy markedly increased with decreasing pH (pH ≤ 4) and they leveled off from pH 4 (pH 4–7.5). Although the rapid increases were observed at approximately pH 2, the quantities were even higher than that of Co released from the Co–Cr–Mo and Co–Cr–Mo–Ni–Fe alloys. For further investigation of the rapid increase in the quantities of metals released at pH 2, an anodic polarization test was employed to study the passive and transpassive behaviours of Ni–Ti alloy. The critical current density for the passivation of Ni–Ti alloy markedly increased as pH decreased (pH ≤ 4) and was low (1.4 μA/cm²) at pH higher than 4 (pH 4–7.5). The potential at a current density of 10 μA/cm², by contrast, markedly rose with decreasing pH (pH ≤ 2), and was 1.2 V from pH 2 (pH 2–7.5).