Comparison of metal release from various metallic biomaterials in vitro

To investigate the metal release of each base and alloying elements in vitro, SUS316L stainless steel, Co–Cr–Mo casting alloy, commercially pure Ti grade 2, and Ti–6Al–4V, V-free Ti–6Al–7Nb and Ti–15Zr–4Nb–4Ta alloys were immersed in various solutions, namely, α-medium, PBS(−), calf serum, 0.9% NaCl, artificial saliva, 1.2 mass% l-cysteine, 1 mass% lactic acid and 0.01 mass% HCl for 7 d. The difference in the quantity of Co released from the Co–Cr–Mo casting alloy was relatively small in all the solutions. The quantities of Ti released into α-medium, PBS(−), calf serum, 0.9% NaCl and artificial saliva were much lower than those released into 1.2% l-cysteine, 1% lactic acid and 0.01% HCl. The quantity of Fe released from SUS316L stainless steel decreased linearly with increasing pH. On the other hand, the quantity of Ti released from Ti materials increased with decreasing pH, and it markedly attenuated at pHs of approximately 4 and higher. The quantity of Ni released from stainless steel gradually decreased with increasing pH. The quantities of Al released from the Ti–6Al–4V and Ti–6Al–7Nb alloys gradually decreased with increasing pH. A small V release was observed in calf serum, PBS(−), artificial saliva, 1% lactic acid, 1.2% l-cysteine and 0.01% HCl. The quantity of Ti released from the Ti–15Zr–4Nb–4Ta alloy was smaller than those released from the Ti–6Al–4V and Ti–6Al–7Nb alloys in all the solutions. In particular, it was approximately 30% or smaller in 1% lactic acid, 1.2% l-cysteine and 0.01% HCl. The quantity of (Zr+Nb+Ta) released was also considerably lower than that of (Al+Nb) or (Al+V) released. Therefore, the Ti–15Zr–4Nb–4Ta alloy with its low metal release in vitro is considered advantageous for long-term implants.