Anisotropic expansion effect of Sr doping on the crystal structure of hydroxyapatite

Abstract

Sr doping in hydroxyapatite (HA) biomaterials has attracted continuous interest due to its well-known positive effects on bone formation. Investigating the effects of Sr on the HA crystal structure is of primary importance for understanding the underlying mechanisms of structure–property relationship. However, the effect of Sr on the HA crystal structure is still controversial and incomplete. Herein, the anisotropic effects of Sr doping on the HA crystal structure are determined by experimental, theoretical and simulative investigations. The good agreement of the results suggests that Sr preferentially replaces Ca(1) sites in the HA structure with a Sr content of 10 mol%, and causes significantly larger expansion in the c-axis direction than in the a-axis direction. The relative expansion rate in the c-axis direction is about 2.22 times that in the a-axis direction, which induces an increased aspect ratio of Sr-HA nanocrystals. When Sr replaces Ca(1) sites, the Sr–O bond length in the SrO₆ configuration increases compared with that in Ca(1)O₆, giving rise to the anisotropic lattice expansion due to the different projection lengths in different directions of this configuration. Moreover, reaction process investigations show that Sr is incorporated in HA by multi-step reactions from Sr-DCPD to Sr-DCPA and to Sr-HA. These findings shed new light on the anisotropic effect of Sr ions on the HA crystal structure, and provide a theoretical basis for development of ion-modified HA biomaterials.