Comparison of techniques for the synthesis of hydroxyapatite

Hydroxyapatite (HA) is a widely researched calcium phosphate biomaterial that is used primarily in bone replacement due to its chemical similarity to bone mineral. Many synthesis methods have been reported for the production of stoichiometric HA; however, processing parameters vary widely for each technique. Herein, the authors present a comparison of nanosized HA produced by aqueous precipitation, solvothermal synthesis and two self-propagating combustion synthesis (SPCS) methods using urea or citric acid combined with nitric acid. Powders were characterised via scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis, which enabled subtle compositional differences identified via powder X-ray diffraction to be clearly distinguished. Phase transformation of ‘HA’ prepared by aqueous precipitation to β-tricalcium phosphate and HA above 600°C revealed as-synthesised samples were in fact calcium-deficient HA. Mixed-phase particles formed spontaneously from solution via SPCS were shown to exhibit a larger average particle size (5·8–11·4 µm) and degree of agglomeration compared with aqueous precipitation (0·6 µm) and solvothermal (2 µm) samples. Ultimately, such physicochemical variations could significantly influence key properties such as solubility, bioactivity and potentially also biocompatibility. This paper provides evidence that the selection of a synthesis method and conditions to produce HA cannot be deemed as a trivial process.