Simultaneous incorporation of magnesium and fluorine ions in hydroxyapatite coatings on Ti6Al4V implant.

Abstract

Hydroxyapatite (HA) coated titanium alloys are available as hard tissue implant material. However, the performance is not satisfactory due to insufficient short-term osseointegration and long-term stability of the implant. Studies show that cation incorporation (Mg) into HA resulted in better biological performances, i.e. short-term osseointegration, and anion incorporation (F) into HA decreased the dissolution rate and improved the adhesion strength, i.e. long-term stability. As such, a combination of good short-term osseointegration and long-term stability of the implant is achieved by properly incorporating both cation and anion into HA. This project aims at developing a bi-layer structured Mg and F ions co-substituted HA coating on Ti6Al4V substrate to achieve an integration of short-term osseointegration and long-term stability. Ca10-xMgx(PO4)6F1(OH)1 or MgxFHA in short, Ca9Mg1(PO4)6Fy(OH)(2-y) or MgFyHA in short, and the bi-layer structured HA (BHA in brief) coatings are deposited by a sol-gel dip-coating method. The compositional dependence of co-substitution of Mg and F ions is systematically studied on material structure, in vitro bioactivity, cell response, dissolution rate, and adhesion strength. The results show that as-deposited MgxFHA coatings have a single FHA phase when x ≤ 1.0. A small amount of Mg-substituted β-tricalcium phosphate (β-TCMP) appears together with FHA phase when 1.0 < x ≤ 2.0. The phases of as-deposited MgFyHA coatings depend on F concentration. Without F incorporation, β-TCMP is the main phase, and HA as the secondary phase. When y is 0.5, HA becomes the main phase and only a little β-TCMP presents. Single FHA phase forms when y ≥ 1.0. Grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, fourier transform infra-red spectroscopy results indicate that Ca ions and OH groups in HA crystal structure are substituted with Mg and F ions, respectively. High Mg concentrations (1.0 ≤ x ≤ 2.0) in the MgxFHA coatings improve the in vitro bioactivity a lot. However, F concentration does not influence the bioactivity of the MgFyHA coatings. MG63 cells attach and spread well on all the MgxFHA and MgFyHA coatings. In the MgxFHA coatings, Mg ions show a significant stimulating effect on cell proliferation and late cell differentiation when x is 1.5. F concentration studied does not affect cell numbers grown on the MgFyHA coatings. However, it maintains a positive stimulating on cell differentiation. The incorporation of Mg increases the dissolution rate of MgxFHA coating in the tris-buffered saline (TBS) solution, and the maximum is achieved at x = 1.5. F incorporation decreases the dissolution rate of MgFyHA coating in the TBS solution, especially at the measured fluoridation degree of 0.72 – 1.16. Mg substitution enhances the adhesion strength, but comparable among different Mg concentrations. Greatly enhanced adhesion strength is achieved by F incorporation at the measured F concentration around 1.0. Finally, Mg1.5FHA/MgF1.5HA bi-layer structured HA coating is developed. Single FHA phase forms in the coating with the substitution of Mg and F ions in HA lattice. It shows comparable in vitro bioactivity with pure HA coating, but more significant cell proliferation. The long-term stability of the bi-layer structured HA coating is much better than pure HA coating, exhibiting lower dissolution rate and higher adhesion strength.