Roles of Fluoride on Octacalcium Phosphate and Apatite Formation on Amorphous Calcium Phosphate Substrate

As fluoride is known to affect the biomineralization of calcium phosphates, we examined how the growth at 37 °C of octacalcium phosphate (OCP) and hydroxyapatite (HAp) on an amorphous calcium phosphate substrate is affected by low-dose fluoride (0–2 ppm) in metastable calcium phosphate solution. In the absence of fluoride, highly oriented plate-like OCP crystals grow on the substrate. With 0.7–0.8 ppm fluoride, OCP crystal formation was greatly modulated (the crystals were smaller and their orientation was lower); moreover, tiny plates and needles appeared on the OCP plates. There were two critical fluoride concentrations: (1) at 0.9 ppm, the OCP plates were completely replaced by HAp nanorods; (2) at 1 ppm, the structure of the HAp nanorod assembly changed from an open one to a tightly assembled one. The fluoride suppressed OCP formation, induced HAp nanorod formation, and promoted tight nanorod assembly. The relationship between the fluoride concentration and the formation of OCP and HAp provides insight into the structural design of an OCP/HAp coating layer and should be useful in the application of fluoride to an OCP/HAp coating on any substrate.