α-Tricalcium phosphate: Synthesis, properties and biomedical applications

Nowadays, α-tricalcium phosphate (α-TCP, α-Ca3(PO4)2) is receiving growing attention as a raw material for several injectable hydraulic bone cements, biodegradable bioceramics and composites for bone repair. In the phase equilibrium diagram of the CaO–P2O5 system, three polymorphs corresponding to the composition Ca3(PO4)2 are recognized: β-TCP, α-TCP and α′-TCP. α-TCP is formed by heating the low-temperature polymorph β-TCP or by thermal crystallization of amorphous precursors with the proper composition above the transformation temperature. The α-TCP phase may be retained at room temperature in a metastable state, and its range of stability is strongly influenced by ionic substitutions. It is as biocompatible as β-TCP, but more soluble, and hydrolyses rapidly to calcium-deficient hydroxyapatite, which makes α-TCP a useful component for preparing self-setting osteotransductive bone cements and biodegradable bioceramics and composites for bone repairing. The literature published on the synthesis and properties of α-TCP is sometimes contradictory, and therefore this article focuses on reviewing and critically discussing the synthetic methods and physicochemical and biological properties of α-TCP-based biomaterials (excluding α-TCP-based bone cements).