JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICINE 9 (1998) 779 — 783 Synthesis and characterization of carbonate hydroxyapatite J. C. MERRY, I. R. GIBSON, S. M. BEST, W. BONFIELD IRC in Biomedical Materials, Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK E-mail: j.c.merry@qmw.ac.uk Substituted apatite ceramics are of clinical interest as they offer the potential to improve the bioactive properties of implants. Carbonate hydroxyapatite (CHA) has been synthesized by an aqueous precipitation method and precipitates with two different levels of carbonate, processed as powders. Sintering experiments were performed to establish the influence of carbonate in significantly reducing the temperature required to prepare high-density ceramics when compared with stoichiometric hydroxyapatite (HA). High-temperature X-ray diffraction was used to characterize the phase stability of the apatites on sintering. Increasing carbonate content was shown to reduce the temperature at which decomposition occurred, to phases of CaO and -TCP. Mechanical testing, performed using biaxial flexure, showed that the CHA specimens had strengths similar to stoichiometric HA.  1998 Kluwer Academic Publishers 1. Introduction Hydroxyapatite (HA; Ca  PO  )  (OH)  ) closely re- sembles the mineralized phase of bone and tooth, the structure of which corresponds principally to a crys- tallographic form of apatite [1]. HA is a bioactive material and when implanted in vivo it is able to bond with the host tissue by stimulating a specific biological response at the host/biomaterial interface. However, biological apatites differ chemically from stoichiometric HA in that they contain a number of additional trace elements substituted into the HA lattice. One of the major substituents is carbonate (CO  ) which in bone mineral occurs at levels, typi- cally, of 5—8 wt % [2, 3]. Therefore, simulating the chemistry of mineralized tissue in bioceramic materials may be a means of improving the level of biological response of the im- plant to the host in vivo [4, 5]. In HA, this can be achieved by selectively substituting ions into the crys- tal structure during synthesis. The wider aims of this study were to produce a range of carbonate-substituted apatites, that rep- licated the levels of carbonate found in biological apatites, and to assess their suitability as enhanced bioactive materials. The work presented in this paper seeks to characterize the effect of carbonate on the synthesis, processing and mechanical strength proper- ties of this material. 2. Materials and methods 2.1. Synthesis and processing The method of carbonate hydroxyapatite (CHA) precipitation used in this study was based on previously reported work involving the aqueous pre- cipitation of both stoichiometric and carbonated HA [6, 7]. Analytical grade reagents (BDH AnalaR, Merck Ltd, Lutterworth, UK) were used to prepare calcium nitrate 4-hydrate (Ca(NO  )  ) 4H  O) and di- ammonium hydrogen orthophosphate ((NH  )  HPO  ) solutions with concentrations in the ratio of 1 : 0.6 M. Two levels of carbonate substitution were achieved by adding 0.2 mol (CHA1) and 0.4 mol (CHA2) sodium hydrogen carbonate to the di-ammonium hydrogen orthophosphate solutions prior to reaction. The precipitations of CHA took place over 2—3 h as the diammonium hydrogen orthophosphate solution was added dropwise to the calcium nitrate solution. Throughout the reaction, the pH was stabilized at * 11 using ammonium hydroxide solution and the temperature controlled at 20 °C. The resulting suspen- sion was then aged for 24 h and washed thoroughly in deionized water. To form powders, the precipitates were first filtered under vacuum. The dried filter cakes were then ground in a pestle and mortar before being ball milled for 90 min. The milled powder was then sieved for 60 min using BS410:86 sieves (Endecotts Ltd, UK), and only the powder which had passed through a 75 m mesh was used for further experimental study. 2.2. Powder characterization The particle-size distribution of the processed pow- ders was assessed using laser diffraction with a Mal- vern Mastersizer X (Malvern Instruments, Malvern, UK). The specific surface area of the powders was measured by the Brunauer—Emmett—Teller (BET) gas 0957—4530  1998 Kluwer Academic Publishers 779