Solution Combustion Synthesis as a Route for the Preparation of Hydroxyapatite and Tricalcium Phosphates: A Review

Calcium phosphates (CPs) are biomaterials of significant interest in bone tissue engineering due to their close chemical and structural similarity to the inorganic component of natural bone. Among this family, hydroxyapatite (HAp) and tricalcium phosphates (TCPs, β-TCP, α-TCP) are the most widely used phases. These materials differ in their Ca/P molar ratio (1.67 for HAp and 1.50 for TCPs), crystal structure, and thermodynamic stability, etc. They share excellent bioactivity and biocompatibility, making them suitable for a wide range of biomedical applications. Solution combustion synthesis (SCS) has emerged as an alternative to conventional synthesis routes, such as solid-state reactions and coprecipitation, offering a rapid and energy-efficient pathway to obtain calcium phosphate phases with tailored properties. It has been demonstrated that both the fuel-to-oxidizer ratio and the nature of the fuel are key parameters in controlling phase formation during SCS. Moreover, these parameters strongly influence the resulting microstructural features, including morphology, porosity, crystallite size, and degree of agglomeration. Despite the widespread application of SCS in the synthesis of inorganic oxides, its use for the preparation of calcium phosphate phases remains comparatively underexplored, and comprehensive reviews specifically addressing this topic are scarce. There is a notable lack of systematic analyses correlating reaction parameters with phase selectivity and microstructural evolution in HAp and TCPs synthesized by SCS. Furthermore, there remains a clear need for robust and standardized protocols that enable precise control over material properties while ensuring reproducibility. In this context, the present review examines recent advances in the solution combustion synthesis of calcium phosphates, systematically consolidating the research conducted to date on SCS-derived calcium phosphates and elucidating the relationships between synthesis parameters, phase evolution, and microstructural development.