Particle-size-dependent octacalcium phosphate overgrowth on β-tricalcium phosphate substrate in calcium phosphate solution

Abstract The particle size of ceramics affects their performance in both in vitro and in vivo reactions. We evaluated the effects of the particle size of β-tricalcium phosphate (β-Ca3(PO4)2; β-TCP) on octacalcium phosphate (Ca8(HPO4)2(PO4)4·5H2O; OCP) overgrowth on a β-TCP substrate by using two types of β-TCP substrate, one composed of micrometer-sized particles and one composed of nanometer-sized particles (micro-TCP substrate and nano-TCP substrate), under physiological conditions. When the β-TCP substrate was immersed in a simple calcium phosphate solution, it was quickly covered with OCP. The morphology and size of the OCP crystals, as well as the structure, thickness, and crystal density of the overgrown OCP layer, depended on the β-TCP particle size. When the nano-TCP substrate was immersed, string-like (S) precipitates were initially deposited, and then flake-like (F) crystals formed on them. Plate-like (PL) OCP crystals grew on the flake-like crystals, and a three-layer structure (S-layer/F-layer/PL-layer) was formed, while no such structure was observed on the micro-TCP substrate. Small amounts of tiny OCP crystals and HAp-nanofibers precipitated in the micro-TCP substrate, whereas only HAp-nanofibers were observed in the nano-TCP substrate. These findings will facilitate the structural design of OCP-coating layers on a β-TCP scaffold.