Does the Immersion Media Influence Phase Development and Properties of CPCs?

Objectives: To evaluate the effect of immersion media on the crystal phase development and compressive strength of a novel bioactive glass-based Calcium Phosphate Cement (CPC) for bone grafting in dentistry.
Methods: Glasses were produced by progressively substituting Sr for Ca on a molar basis. Cements were prepared by mixing the glass with Ca(H₂PO₄)₂ ° H₂O and a 2.5% solution of Na₂HPO₄. 8 cement cylinders were immersed in either 10 mL of Tris Buffer (TB) or Simulated Body Fluid (SBF) for 1h, 1d, 7d and 28d. Compressive strength was measured after immersion. X- Ray Diffraction (XRD) determined the crystal phases, and the immersion solutions were analysed for ions release by ICP.
Results: XRD demonstrated that the immersion media influenced crystal phase formation rate. In TB, OCP (Ca₈H₂(PO₄)_{6 °} 5H₂O) was present at 1h and 1d, followed by its transfomation to Sr-containing hydroxyapatite (Sr_xCa(_10-x)(PO₄)₆(OH)₂, SrHA) after 7d and 28d. SBF delayed this process, with OCP being the main phase even after 7d. Sr-release was lower in SBF than in TB suggesting that more Sr was incorporated into the phases formed following SBF immersion. The immersion media influenced the compressive strength with a reduction in strength correlating with the conversion of OCP to HA. Overall higher values were recorded when SBF was used.

Conclusions: The immersion media influenced the phases formed and the properties of the CPC. SBF delayed the transformation of OCP to HA and resulted in higher compressive strength values compared to TB.
Care must be taken when evaluating properties of a bioactive material after immersion studies as different media used may affect properties obtained.