Surface structural change on pellets of calcium ethyl phosphate in biomimetic solutions: Effects of carbonate concentrations

Abstract

Hydroxyapatite (HAp) with similarity in natural bone mineral is called bone-like apatite (BAp), and the formation of BAp on material surfaces in the body environment substantially contributes to materials to make bonding to living bone. It is proposed that designed materials, namely salts of calcium and phosphate esters (SCPEs), possess the property of enhancing BAp formation in simulated body fluid (SBF) modified with alkaline phosphatase (ALP). Such materials are called bioresponsive ceramics. The crystalline phase of BAp is generally identified as HAp by X-ray diffraction analysis. Since several-types of SBF have been reported, in this study, HAp formation on calcium ethyl phosphate (CaEtP) pellets, which is one of SCPEs, was examined in the solutions mimicking SBF, that is, modified biomimetic solution (mBS) and evolved biomimetic solution (eBS). The critical difference between these solutions and conventional SBF is carbonic acid concentrations in the solutions. It was found that capability of HAp formation of CaEtP pellets in mBS and eBS was lower than that in mBS and eBS containing ALP. The morphology of the HAp formed on the pellets was distinctive in mBS and eBS containing ALP: i.e., formation of wool-ball-like shaped particles in mBS containing ALP, while formation of fine spherical particles in eBS containing ALP. These differences, namely nucleation, crystal growth and morphology, are likely governed by concentrations of carbonate ion species in the reaction solution. It can be concluded that controlling the concentration of carbonate ions in the biomimetic solutions enables controlling the formation behavior of HAp as well as its morphology. These findings provide basic knowledge about nucleation and crystal growth of HAp in biomimetic solutions as well as basic understanding of the reactivity on bioresponsive ceramics.