Coherent surface structure induces unique epitaxial overgrowth of metastable octacalcium phosphate on stable hydroxyapatite at critical fluoride concentration

The phase transformation from soluble calcium phosphates to less-soluble hydroxyapatite (HAP) is a thermodynamically natural route. This process is irreversible, and effective use of poorly reactive HAP to repair teeth that have no cellular metabolism remains challenging. However, this thermodynamically controlled transformation may apparently be reversed through the fast nucleation and growth of metastable phases, leading to a reactive HAP surface. Here, the assembled HAP-nanorod phase is demonstrated to change into the metastable octacalcium phosphate (OCP) phase in a calcium phosphate solution containing 0.8 ppm fluoride. Grown OCPs display parallel surface streaks and their 1 1 0 and 00 l ( l : odd) electron-diffraction spots are often not visible. The streaked, elongated OCP gradually grows into large plates with flat surfaces that exhibit an intense 1 1 0 spot. Crystal-structure models reveal that the unique epitaxial overgrowth of OCP on HAP occurs since both materials share coherent {1001 faces, resulting in the distinctive disappearance of 1 1 0 and 00 l OCP spots. A polysynthetic twin model that reliably explains this disappearance is proposed for the growth of OCP. This apparent reverse phase transformation produces hybrid calcium phosphates consisting of HAP cores and highly reactive outer OCP layers that are promising for the repair of dentin caries.

Automated summary

The transformation of soluble calcium phosphates to metastable octacalcium phosphate (OCP) in specific conditions provides a new approach for repairing teeth. The hybrid calcium phosphates produced by this transformation, consisting of HAP cores and highly reactive outer OCP layers, show promise for treating dentin caries.