The surface modification of Nitinol superelastic alloy with alkaline-heat treatment and hydroxyapatite/chitosan composite coating for biomedical applications

In the present work, the surface modification of Nitinol was carried out by a two-step process consisting of pretreatment and applying the bioactive composite coating to extend its biomedical applications. For pretreatment, we used a combination of chemical etching, boiling in distilled water and alkaline-heat treatment. According to the results of Raman and grazing-incidence X-ray analysis, the surface layer formed on the pretreated Nitinol was consisted of rutile phase and sodium titanate phases. The pretreatment significantly decreased the concentration of Ni ions released from Nitinol alloy into Ringer's solution during 10 days immersion from 126.6 ppb to 5.3 ppb. Moreover, this pretreatment process did not have the negative effect on the superelasticity of the Nitinol alloy. In the following, we used a composite coating of hydroxyapatite/chitosan on the pretreated Nitinol. This coating was applied using one-step cathodic electrophoretic deposition from suspension containing 5 g/L hydroxyapatite and 0.5 g/L chitosan at different voltages of 30, 40 and 50 V/cm2. A uniform coating with acceptable quality was obtained by electrophoretic deposition at 40 V/cm2 for 120 s. The morphology of this coating was studied using scanning electron microscope and Fourier transform infrared spectroscopy analysis. The findings confirmed fabrication of a crack free morphology, which was consisted of hydroxyapatite and chitosan. Finally, according to the results of potentiodynamic polarization test, the corrosion current density for bare, pretreated and pretreated/coated Nitinol were calculated about 2.63, 1.94 and 0.75 μA/cm2, indicating the effect of pretreatment and applying hydroxyapatite/chitosan coating on decreasing the corrosion rate of Nitinol.