| NiTi alloy has attracted more interest as biomedical implants due to its high super-elasticity,elastic modulus closed to the bone,etc.However,corrosion of NiTi alloy is easy to occur during the long term implantation,which will cause Ni ions release.Excessive concentration of Ni will lead to several negative effects,such as allergy,dyspnea and potentially carcinogenic effects.In addition,NiTi alloy is a kind of bio-inert material,which could not chemically bond with human bone,and may fall off after implantation,causing the failure of the operation and dramatic pain for patients.Therefore,suitable surface modification technique is of great significance for NiTi alloy to improve its biological properties.In this paper,Al2O3 coating has been fabricated on NiTi alloy through cathodic plasma electrolytic deposition?CPED?.In order to improve the bio-compatibility and bio-activity of NiTi alloy,hydroxyapatite?HA?crystal could be prepared on the Al2O3 coating via introducing Ca,P elements into Al2O3 coating or polydopamine assisted biomimetic mineralization.Firstly,Al2O3 coating has been fabricated by CPED technique using the electrolyte composed of Al?NO3?3·9H2O and absolute ethanol.The coating consisted of?-Al2O3 and?-Al2O3 crystal phase,was tightly adhered to the NiTi alloy,and the coating thickness was about 80120?m.Both the coating thickness and coating roughness increased with the increasing CPED treatment time and concentration of Al?NO3?3,while the bonding strength increased at first and then decreased after the maximum.The optimum preparing process of the Al2O3 coating was 45 min CPED treatment time,0.2 mol/L of Al?NO3?3,450 V treatment voltage,100 Hz frequency and 15%duty cycle.The results of corrosion resistance test showed that the corrosion resistance of the sample with the Al2O3 coating was enhanced by 37.4times compared to that of the uncoated NiTi alloy,and the current density was reduced to 1.13×10-6 A/cm2.Ca,P-containing Al2O3 coating was fabricated on NiTi alloy through CPED technique with the addition of calcium glycerophosphate?Ca-Gp?.The coating thickness and coating roughness increased with the increasing concentration of Ca-Gp.When the concentration of Ca-Gp was 0.02 mol/L,HA crystal could formed on the coating surface.The results of corrosion resistance test indicated that the corrosion resistance of the sample with the Al2O3 coating was enhanced by 6.9 times compared to that of the uncoated NiTi alloy,whose current density was reduced to7.84×10-7 A/cm2.Although the Ca,P concentration in the coating could increase with the increasing concentration of Ca-Gp,the solubility of Ca-Gp in ethanol was rather low and the enhancement of Ca,P concentration was limited.In addition,the bonding strength between the coating and substrate significantly decreased with the increasing concentration of Ca-Gp.Therefore,the preparation of Ca,P-containing Al2O3 coating through the addition of Ca-Gp was not an ideal surface modification technique for NiTi alloy.In order to increase the concentration of Ca and P elements in the coating,Ca?NO3?2·4H2O and triethyl phosphate?TEP?with higher solubility was used as the source of Ca and P elements.And then,the HA-containing Al2O3 coatings were prepared on NiTi alloys through a CPED process,followed by hydro-thermal treatment?HT?.When the concentration of Ca?NO3?2·4H2O was 0.04 mol/L and the concentration of TEP was 0.4 mol/L,the concentrations of Ca and P elements were8.15%and 4.74%respectively,and the Ca/P ratio was 1.72,which was closest to that of natural HA?1.67?.The results of the electrochemical properties test and Ni ions release test showed that this Ca,P-containing Al2O3 coating could improve the corrosion resistance of NiTi alloy and reduce the Ni ions release significantly.The current density of the Ca,P-containing Al2O3 coating was reduced to 1.07×10-8A/cm2.The Ni ion release rate in simulated body fluid?SBF?after 56 days was reduced to 0.44?g/cm·2day,which was 25.7 times lower than that of NiTi alloy.In addition,SBF soaking could further promote the HA mineralization on the coating surface.HA/Al2O3 composite coating was fabricated through the polydopamine?PDA?induced HA biomimetic mineralization on Al2O3 coating.This coating could improve the corrosion resistance of NiTi alloy and reduce the Ni ions release significantly.The current density of the HA/Al2O3 composite coating was reduced to1.17×10-8 A/cm2.The Ni ion release rate in simulated body fluid?SBF?after 56days was reduced to 0.38?g/cm·2day,which was 30.0 times lower than that of NiTi alloy.In addition,the cell growth and cell proliferation of human bone marrow mesenchymal stem cells?hMSCs?showed that the Al2O3 coating,Ca,P-containing Al2O3 coating and HA/Al2O3 composite coating could promote the cell adhesion and cell proliferation,and the performance of HA/Al2O3 composite coating was the best.Compared with Al2O3 coating,the Ca,P-containing Al2O3 coating and HA/Al2O3composite coating could promote the osteogenic differentiation of hMSCs,and the HA/Al2O3 composite coating was better.Therefore,the HA/Al2O3 composite coating has better bio-compatibility and bio-activity. |
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