XPS study of apatite-based coatings prepared by sol–gel technique

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Abstract

Biocompatible layers of hydroxyapatite and fluorhydroxyapatite were grown on Ti substrates by means of dip-coating into sol–gel. In order to improve the adhesion of apatite-based coatings, the substrates were a priori covered with calcium titanate. Therefore, the hydroxyapatite, fluorhydroxyapatite and CaTiO3 coatings were produced and analysed. The chemical composition of the coatings (CaTiO3, CaTiO3+hydroxyapatite, and CaTiO3+fluorhydroxyapatite) was studied by using X-ray photoelectron spectroscopy (XPS). The data of quantitative XPS analysis displayed the different features (cleanness, homogeneity, etc.) of CaTiO3, hydroxyapatite, and fluorhydroxyapatite films after the growth and after the treatment in vitro.

Introduction

Hydroxyapatite Ca10(PO4)6(OH)2 is one of the most innovative and most promising materials for biomedical applications 1, 2, 3, 4, 5, 6. It can be prepared as porous ceramics to prevent loosening of implants, as dense bioactive glass-ceramics or as a bioactive coating on titanium and its alloys 1, 2, 3, 4, 5, 6, 7, 8. The commercial samples of hydroxyapatite on Ti are already available, and mostly are obtained by plasma-spray deposition.

In this work the sol–gel technique is employed for the preparation of apatite-based (HA=hydroxyapatite, FA=fluorhydroxyapatite) coatings. It is based on hydrolysis and condensation of metal alkoxides such as Si(OR)4, Ti(OR)4, where R is an organic radical. These processes lead to the formation of an inorganic network like a sponge, which, after heating, densifies to a solid material with the structure of the corresponding oxides: SiO2, TiO2, etc. The benefits of this technique are various: perfect compositional control, low temperature of the process, good homogeneity of doped gels, easiness of the film deposition on various substrates, etc. 3, 4, 9, 10, 11, 12.

It is widely known that the surface properties of any biomaterial are the most important for the biological response 6, 7, 13, 14, 15, 16. In this paper are reported the results of surface analysis of HA and FA films on Ti (with an intermediate layer of CaTiO3) by using X-ray photoelectron spectroscopy (XPS). A detailed description of the results obtained by using other experimental techniques (AFM, XRD, etc.) will be published elsewhere [17].

Section snippets

Experimental

The films of CaTiO3, HA and FA were prepared from the solutions of the corresponding alkoxides, previously stabilized. The substrates (commercial sheets of Ti foil) were immersed in the solution and the films were deposited by dipping. Then the samples were dried under clean air and subsequently submitted to an appropriate heating treatment. An intermediate layer of CaTiO3 was grown in order to increase the adhesion of apatite-based coatings. Therefore, the samples of CaTiO3, (CaTiO3+HA), and

Results and discussion

In Table 1 the BE values are reported for the principal elements (C 1s, O 1s, Ca 2p, P 2p, F 1s) of the samples investigated: CaTiO3, (CaTiO3+HA), (CaTiO3+FA) before and after the treatments in vitro, also the powders used as reference samples. The BE values for CaTiO3, HA and FA coatings are in good agreement with the data reported in the literature [20].

The XPS measurements indicate that Ti substrates precoated with CaTiO3 are completely covered by biocompatible apatites (HA or FA), because

Conclusions

The sol–gel technique is successfully applied for the preparation of HA and FA coatings on titanium substrates covered by intermediate film of CaTiO3. These coatings are clean compounds (without Na or particular carbonic contaminant species). The HA and FA coatings prepared by sol–gel technique are homogeneous; in fact it is not possible to detect any signal of Ti on the surface of the sample. The best quality (homogeneity and stoichiometry) of HA and FA coatings is achieved when the substrates

Acknowledgements

This work is supported by the National Research Council (CNR) of Italy under the auspices of the project “Special Materials for Advanced Technologies”.

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