Behaviour of calcium phosphate ester salts in a simulated body fluid modified with alkaline phosphatase: a new concept of ceramic biomaterials

Ceramic biomaterials that promote bone tissue regeneration are vital in the treatment of various bone-degenerative diseases and have stimulated intense bioceramic-related research. Salts of the calcium ion and phosphate esters (SCPE) can be transformed into calcium phosphate in aqueous environments containing alkaline phosphatase (ALP). Inspired by the formation process of hydroxyapatite (HAp) in our bones, these SCPEs are promising novel ceramic biomaterials showing a biomolecule-responsive function for bone repairing applications, because our bone tissues contain ALP. In this study, the behaviour of calcium phenylphosphate (CaPP), a typical SCPE, under conditions mimicking an in vivo environment were investigated. A simulated body fluid (SBF) modified with ALP was used as reaction media. The transformation from CaPP to HAp in the SBF containing ALP occurred, while no reaction occurred without ALP present. The transformation process of CaPP can be concluded as follows: first, soaked CaPP dissolves into the SBF and forms calcium ions and phenylphosphate ions. Subsequently, phosphate ions are generated by hydrolysis of phenylphosphate ions. This hydrolytic reaction is mediated by ALP. Finally, HAp is formed by the reaction of calcium, phosphate, and hydroxide ions in the SBF. Interestingly, the transformation reaction of CaPP occurred under conditions outside the optimal pH ranges for ALP. The findings of this study demonstrate that SCPEs are good candidates for a new class of ceramic biomaterials that respond to trigger biomolecules, namely ALP, in this case.