Sol-gel preparation and properties of fluoride-substituted hydroxyapatite powders

Hydroxyapatite (HA) and fluor-hydroxyapatite (FHA) powders were synthesized by a sol-gel method for usage as bone filler and drug carrier. Calcium nitrate and triethyl phosphite were used as precursors under an ethanol-water-based solution. Different amounts of ammonium fluoride (NH₄F) were incorporated for the preparation of FHA powders. With heat treatment above 400°C, a characteristic apatite phase was observed for all the sol-gel powders. However, the crystallization temperature decreased with increasing fluoride addition. The tricalcium phosphate (TCP) phase formed in the pure HA powder above 800°C was attenuated in the FHA powders, confirming an enhanced phase stability of the FHA powders. Increasing the F^- addition improved crystallinity and increased the crystallite size, as was determined from X-ray diffraction (XRD) analyses. The lattice parameters of the heat-treated powders varied corresponding to the fluoride addition, i.e., a gradual decrease in the a-axis, while little change in the c-axis was observed with increasing fluoride addition, indicating a nearly complete substitution of fluoride within the apatite lattice. However, little difference was observed with heat-treatment temperatures (400°-1000°C). The powders substituted with fluoride exhibited reduced dissolution rates in an in vitro solution as compared with the pure HA powder, suggesting the possibility of tailoring bioactivity with fluoride substitution.