Fabrication and characterization of hydroxycarbonate apatite with mesoporous structure

Mesoporous hydroxycarbonate apatite (MHCAp) was converted directly from calcium carbonate particles by treatment with a phosphate buffer solution (PBS). After soaking in PBS, the calcium ions are released from CaCO₃ particles and react with phosphate ions to form hydroxycarbonate apatite nanoparticles. The aggregation of the nanoparticles not only produces apatite plates, but also forms mesopores within the plates. The phases of CaCO₃ play an important role in the conversion rate because of the different solubility products. If the main phase is vaterite, CaCO₃ particles are converted completely to MHCAp after soaking in PBS for 24 h. However, if the main phase is calcite, the percentage of the unreacted CaCO₃ particles is ∼19.7%. In addition, the conversion percentages of MHCAp increase with prolonging immersion time in PBS and increasing temperatures. The most of pore sizes of MHCAp are distributed around 4.1 ± 0.3 nm, which are not affected by the experimental conditions such as the phases of calcium carbonate, immersion time (6-24 h) and temperatures (25-50 °C). Simulated body fluid (SBF) immersion tests reveal that MHCAp exhibits a superior in vitro bone-forming bioactivity. It is hypothesized that the conversion mechanism of CaCO₃ to MHCAp may be rationalized in terms of a dissolution-precipitation process.