Enhanced bioactivity and osteoconductivity of hydroxyapatite through chloride substitution

The effect of chloride-substitution on bioactivity and osteoconductivity of hydroxyapatite (OHAp) was newly investigated. Chloride-substituted hydroxyapatites (ClAp) with low and high chloride concentrations were synthesized by reacting Ca(OH)2 and H3 PO4 with NH4 Cl of low and high concentrations, with subsequent sintering. As a control, pure OHAp was prepared under the same conditions but without addition of NH4 Cl. The ClAp showed markedly enhanced bioactivity in simulated body fluid (SBF) as the chloride substitution was increased. In contrast, OHAp did not show any bioactivity at all within the testing period. The solubility tests in deionized water also showed that the higher the chloride-substituting amount, the higher the dissolution amounts of the constituent elements of apatite, which directly affect bioactivity by increasing the degree of supersaturation of apatite in SBF. In addition, ClAp also showed noticeably higher osteoconductivity within the 4 weeks of implantation in calvarial defects of New Zealand white rabbits, compared with that of OHAp. The total system energy of the apatite calculated by the ab initio method showed that the higher the chloride-substituting amount, the higher the total system energy, which suggests that the ClAp was energetically less stable compared with OHAp. This result demonstrates the higher solubility of ClAp over that of OHAp in SBF and deionized water. The improved solubility of the OHAp enhances its bioactivity and consequent osteoconductivity. Taken together, it can be concluded that ClAp has encouraging potential for use as a bone grafting material due to its highly enhanced bioactivity and osteoconductivity compared with pure OHAp.