Effect of fluorine addition on the corrosion resistance of hydroxyapatite ceramics

Synthetic hydroxyapatite (HA) ceramics have found their important biomedical applications as bone implants and bone substitutes. However, the bioactivity and the biostability of HA still need to be tailored for specific applications. To improve the biostability of HA, fluorine addition to HA ceramics to form fluorine-substituted hydroxyapatite (FHA) is a solution. While the preparation and processing of FHA powders have been found in the literature, this study aimed to investigate the effect of fluorine content on the corrosion resistance (an indication of biostability) of the FHA ceramics. The FHA powders with compositions of Ca10(PO4)6(OH)2(1−x)F2x (x=0, 0.2, 0.4, 0.8) were prepared by a wet precipitation method. By sintering the particle compacts at 1200 °C in air, all the FHA ceramics reached more than 95% of their theoretical densities. The FHA ceramics with different fluorine contents were then immersed in a 5 wt.% citric acid solution for 10 min. Transmission electron microscopy (TEM) results showed that the FHA powders produced were needle-like nanoparticles with a width of 80 nm and a length of 20 nm. Scanning electron microscopy (SEM) results indicated that increasing fluorine content led to increasing corrosion resistance of the FHA ceramics. The increased corrosion resistance of the FHA ceramics compared with the pure HA ceramics suggested the biostability of the FHA ceramics in vitro and in vivo and thus the long-term performance of implants made of the FHA compositions.