Hydroxyapatite (HAP) is a naturally occurring form of calcium phosphate that constitutes the largest amount of inorganic components in human bones. The chemical formula of HAP is Ca10(PO4)6(OH)2 with a Ca/P ratio

of 1.67. HAP is the most stable calcium phosphate with low solubility in physiological environments defined by temperature,pH, body fluids, etc.

Nanoscale HAp crystals possess excellent sintering ability due to their high surface energy, and the HAp

nano-ceramics with enhanced mechanical properties can be fabricated using Hap nano-powders as raw materials.Moreover, HAp nanobioceramics exhibit better bioactivity and higher resorbability than those in microscale sizes. HAp nanoparticles can be used for cell targeting and diagnosis, and drug/gene delivery