In vitro bioactivity of silicon-substituted hydroxyapatites

Silicon-containing hydroxyapatites were synthesized by the controlled crystallization method. Chemical analysis, N(2) adsorption, Hg porosimetry, X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to characterize the hydroxyapatite and to monitor the development of a calcium phosphate layer onto the surface of the substrate immersed in a simulated body fluid, that is, in vitro bioactivity tests. The influence of the silicon content and the nature of the starting calcium and phosphorus sources on the in vitro bioactivity of the resulting materials were studied. A sample of silicocarnotite, whose structure is related to that of hydroxyapatite and contains isolated SiO(4) (4-) anions that isomorphically substitute some PO(4) (3-) anions, was prepared and used as reference material for XPS studies. An increase of the unit cell parameters with the Si content was observed, which indicated that SiO(4) (4-) units are present in lattice positions, replacing some PO(4) (3-) groups. By using XPS it was possible to assess the presence of monomeric SiO(4) (4-) units in the surface of apatite samples containing 0.8 wt % of silicon, regardless the nature of the starting raw materials, either Ca(NO(3))(2)/(NH(4))(2)HPO(4)/Si(OCOCH(3))(4) or Ca(OH)(2)/H(3)PO(4)/Si(OCOCH(3))(4). However, an increase of the silicon content up to 1.6 wt % leads to the polymerization of the silicate species at the surface. This technique shows silicon enrichment at the surface of the three samples. The in vitro bioactivity assays showed that the formation of an apatite-like layer onto the surface of silicon-containing substrates is strongly enhanced as compared with pure silicon-free hydroxyapatite. The samples containing monomeric silicate species showed higher in vitro bioactivity than that of silicon-rich sample containing polymeric silicate species. The use of calcium and phosphate salts as precursors lead to materials with higher bioactivity.