Comparison of Carbonate Hydroxyapatite with and without Sodium Co-Substitution

Abstract

Carbonate hydroxyapatite (CHA) bioceramics can be synthesised to contain sodium ions as a co-substituted ion, or as sodium-free compositions. It is unclear, however, which composition would produce the optimum biological response. The aim of this study was to find a reliable method to produce sodium co-substituted and sodium-free CHA compositions that would have the same level of carbonate substitution, and to characterise the effects of the two different substitutions on the structure of the CHA samples. After sintering at 900oC in a CO2 atmosphere, all samples contained approximately equal amounts of carbonate groups on the A- and B-sites, as observed by FTIR. The sample produced with NaHCO3 and the sodium-free sample (CHA1) have comparable carbonate contents, whereas the sample produced with Na2CO3 contains significantly more carbonate, probably due to the excess sodium ions allowing more carbonate co-substitution. The sodium-free CHA sample, however, has significantly smaller unit cell parameters compared to both sodium co-substituted CHA samples, and also to HA. This characterisation of the samples shows that the sodium-free CHA sample (CHA1) and the sample produced with NaHCO3 would provide CHA compositions for biological testing with similar carbonate contents and distributions, but with structural differences due to the sodium substitution.