First-principles study on the hydroxyl migration from inner to surface in hydroxyapatite

Understanding to the surface structure of hydroxyapatite (HAp) is crucial for the design of HAp-based biomaterials. While most bioactive HAp materials have disordered structures, the ordered surface structures relaxed from HAp crystal are commonly used for studying their interaction with biomolecules. Using a combined approach of molecular dynamics simulated annealing simulations and density functional theory calculations, we proposed a disordered surface structure model for HAp. In this model hydroxyls beneath the surface move outward and cover the exposed calcium ions. The destabilization caused by hydroxyl vacancy is compensated by the stabilization of hydroxyl coverage. The Ca-exposed surface with strong electrophilicity changes into the hydroxyl- and phosphate-covered surface with moderate activity. The new surface model would be useful for regulating the surface activity of HAp-based biomaterials.