Study on separation of Ni²⁺ of Nitinol modified on its surfaces by the coating of titanium or titanium-niobium alloy

Objective: To investigate whether Ni²⁺ of the Nitinol and Nitinol modified on its surfaces by the coating of metallic titanium and titanium-niobium alloy was separated or not in vivo and in vitro. Methods: In vitro, we collected the specimens from 1 to 7 days after the experiments and determined the concentration of Ni²⁺ by physiologic saline immersing test and by using graphite furnace atomic absorption spectrometry （GFAAS）. In vivo, 18 adult dogs were selected randomly, the cubic implants were imbedded in the muscle by the sides of sacrospinalis and the cylinder in the femurs, 10 pieces of each kinds of implants were imbedded respectively in each dog, drawing the materials by killing them after 12 months from the experiments and assaying the concentration of Ni²⁺ of the hearts, livers, spleens, and so on by GFAAS. Results: In vitro, obvious separation of Ni²⁺ was detected in the group of Nitinol that was not modified, the separating procedure was mainly within the first 4 days, the separation rate （10^-7 μg·cm^-2 ·s^-1 ） from the first to the forth days was 18.2, 3.45, 1.75 and 0.45 respectively, the Ni²⁺ was no more separated from the fifth day. Whereas, Ni²⁺ was not separated in group of Nitinol that was modified by the coating of metallic Ti and TiNb alloy. The recovery rate of the experiment was 94.6%-108.6%. In vivo, the content of Ni²⁺ around implants not modified implants was obvious difference（P＜0.05）compared to modified groups, either in bone or in muscle tissues, either in the same group and the same kind of tissue or in the different groups and the same kind of tissue. Conclusion: Either in vitro or in vivo, the coating by Ti or by TiNb alloy on the surfaces of Nitinol stopped or decreased Nitinol from separating Ni²⁺.