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Hydroxyapatite (HAp)-20vol% β-tricalcium phosphate (β-TCP) composites were prepared. The effect of β-TCP size on bone-like layer growth and adhesion of osteoblast-like cells on the composites was systematically studied in vitro. When the composites were soaked in a simulated body fluid, the formation and growth rates of the bone-like layer increased with increasing β-TCP size, even if the volume fraction of β-TCP was constant. Moreover, selective dissolution of β-TCP phase and formation of the bone-like layer around the phase were frequently observed. Higher Ca 2+ concentration due to the fast dissolution of β-TCP beneath the sample surface resulted in faster formation and growth of the bone-like layer, especially in the samples containing β-TCP powders larger than 100 μm. MC3T3-E1 osteoblast-like cells preferentially adhered to β-TCP phase in HAp/β-TCP composites because of the enrichment of Ca 2+ ions around β-TCP.

Calcium-deficient hydroxyapatite (DAp) was synthesized by hydrolysis of alpha-TCP at different pH. Ca/P ratio of DAp increased with increasing pH during the hydrolysis. In addition, DAp which was synthesized at lower pH, showed higher solubility in the simulated body fluid (SBF) and had a more negatively charged surface than that at higher pH. Bone-like crystals were formed on DAp which have been soaked in SBF at 309.5 K in vitro. DAp demonstrated faster formation of a bone-like layer than stoichiometric hydroxyapatite (HAp). Moreover, the formation of the crystals on DAp synthesized at lower pH was much faster than that at higher pH. The adhesion of MC3T3-E1 osteoblast-like cells was also influenced by pH during the synthesis of DAp; the number of adhering cells increased with a decrease in the pH. Therefore, high solubility and negative charge of DAp, especially that synthesized at lower pH resulted in fast formation of bone-like crystals and good cell adhesion.