A comparative study of the dissolubility of pure and silicon substituted hydroxyapatite from density functional theory calculations.

Introduction of silicon into hydroxyapatite (HA) is one of the effective ways to modulate the bioactivity of HA-based biomaterials. The bulk and surface structures of silicate-substituted HA (Si-HA) were characterized by using density functional theory calculations. The energetically favorable structures were identified from a number of candidate structures. Particular attention was paid to the surface structures of Si-HA, whose bioactivity is closely relevant to their surface atoms. Compared to the surface of pure HA, the Si-HA surface has similar surface energy but different charge distribution. Under the implicit solvent model, the exposed calcium/oxygen atoms become more positive/negative in net charge, resulting in a considerable change in the surface electrostatic potential at van der Waals distances. However, changes in the dissolution of surface calcium ions are not remarkable, as depicted by their activation energy leaving from the surface. Our calculations reveal that the surface structures and properties of HA were changed to some extent by silicate substitution, which provides some hints for understanding the experimentally observed changes in bioactivity and biodegradability of Si-HA that still remain controversial in many aspects.