Review of Octacalcium Phosphate Materials for Bone Tissue Engineering

At present, reconstructive bone surgery makes wide use of calcium phosphate materials. They have the ability of filling and maintaining the volume of bone defects, are mechanically integrable with body tissues, are biocompatible, and have osteocompatible potential. The main problems of their application in bone tissue engineering are low constructional properties and insufficient biological activity in repairs of critical size defects. Osteoplastic materials based on resorbable calcium phosphates have been introduced into clinical practice relatively recently. These materials are octacalcium phosphate (OCP, Ca8(HPO4)2(PO4)4 ⋅ 5H2O). It is believed that OCP is one of the possible precursors of formation of biological hydroxyapatite. Hydroxyapatite is a crystal structure similar to hydroxyapatite (HA, Ca10(PO4)6(OH)2) and dicalcium phosphate dihydrate (DCPDH, CaHPO4 ⋅ 2H2O), with alternating layers of these calcium phosphates. OCP contains five water molecules as part of the crystalline unit cell. These features make OCP a better option than other calcium phosphates; in particular, they provide it with high reactive capability to rearrange into an apatite-like structure under the influence of the biological environment. The problem of creating OCP materials and products is that OCP is thermally unstable, which does not allow using conventional ceramic production methods (high-temperature processing). It is a relevant task to search for and develop alternative methods of generating porous materials from thermally unstable calcium phosphates, such as OCP, with preset composition, microstructure, and properties. This review is devoted to the development of the physicochemical basics of OCP calcium phosphate materials with widely variable chemical and phase composition and to the determination of their structuring regularities and properties, including additive technologies of creating personalized implants for bone tissue engineering.