Effects of Sr²⁺/Zn²⁺ co-substitution on crystal structure and properties of nano-β-tricalcium phosphate

The incorporation of therapeutic ions like Sr²⁺, Si⁴⁺, Zn²⁺ and Li⁺ into biomaterials has become a promising approach to promote bone regeneration. However, the effects of Sr²⁺ and Zn²⁺ co-substitution on the crystal structure and properties of β-tricalcium phosphate (β-TCP) have not been elucidated well. In this study, Sr²⁺/Zn²⁺ co-substituted β-tricalcium phosphate (SrZnTCP) nano-powders with different extents of substitution (0–4.8 mol%) were synthesized by poly(ethylene glycol)-assisted co-precipitation and subsequent heat treatment. The as-synthesized SrZnTCP nano-powders were characterized by x-ray diffraction, Fourier transform infrared spectroscopy, elemental analysis, Rietveld refinement and differential scanning calorimetry. The results showed that the conversion of calcium-deficient apatite to β-TCP was achieved after heat-treatment above 800 °C. The a-axis and c-axis lattice parameters gradually decreased with increasing level of Sr²⁺/Zn²⁺ co-substitution in β-TCP lattice. Sr²⁺ and Zn²⁺ preferentially occupied the ninefold coordinated Ca (4) sites and the sixfold coordinated Ca (5) sites, respectively. The co-substitution of Sr²⁺ and Zn²⁺ for Ca²⁺ significantly improved the thermal stability of β-TCP. The release rate of Zn²⁺ from SrZnTCP depended on Ca²⁺ concentration over 63-day immersion in PBS solution while that of Sr²⁺ was not affected by Ca²⁺ concentration. The amount of Sr²⁺ released increased with increasing Sr²⁺ content in SrZnTCP. Collectively, SrZnTCP showed great promise as a Sr²⁺/Zn²⁺-releasing biomaterial for bone repair, although no obvious mineralization was observed on β-TCP and SrZnTCP disc samples during 56 days of immersion in simulated body fluid.