First-principles study of hydroxyapatite surfaces and water adsorption

Hydroxyapatite (HA) is the main mineral component of natural bone and an important synthetic biomaterial. The surfaces of this material are critical as the biological action takes place at the interface between the HA and the biological medium. Ab initio total energy methods are used to study the atomic structure and surface chemistry of HA. Low index HA surfaces in vacuum, the adsorption of water on these surfaces, and the loss of Ca from them are considered. All surfaces are found to react strongly with water, and after adsorption of water (001) and Ca-rich (010), surfaces are found to be energetically favored. The loss of Ca from the surface in exchange for two H is also very favorable. The calculated energies of the various surfaces shed light on the morphologies of HA crystallites found in dissolution studies.