Mechanism of incorporation of zinc into hydroxyapatite

The atomic level mechanism of incorporation of Zn2+ into hydroxyapatite (HAp), which is a potential dopant to promote bone formation, was investigated, based on first principles total energy calculations and experimental X-ray absorption near edge structure (XANES) analyses. It was found that Zn2+-doped HAp tends to have a Ca-deficient chemical composition and substitutional Zn2+ ions are associated with a defect complex with a Ca2+ vacancy and two charge compensating protons. Moreover, first principles calculations demonstrated that Zn2+ incorporation into HAp can take place by occupying the Ca2+ vacancy of the defect complex. The Ca2+ vacancy complex is not only the origin of the calcium deficiency in HAp, but also plays a key role in the uptake of trace elements during mineralization.