Transformation from calcium sulfate to calcium phosphate in biological environment

The formation of a nano-apatite surface layer is frequently considered a measure of bioactivity, especially for non-phosphate bioceramics. In the present study, strontium-doped calcium sulfate, (Ca,Sr)SO4, was used to verify the feasibility of this measure. The (Ca,Sr)SO4 specimen was prepared by mixing 10% SrSO4 by weight with 90% CaSO4·½H2O powder by weight. A solid solution of (Ca,7.6%Sr)SO4 was then produced by heating the powder mixture at 1100 °C for 1 h. The resulting (Ca,Sr)SO4 specimen was readily degradable in phosphate solution. A newly formed surface layer in the form of flakes was formed within one day of specimen immersion in phosphate solution. Structural and microstructure–compositional analyses indicated that the flakes were composed of octacalcium phosphate (OCP) crystals. An amorphous interface containing OCP nanocrystals was found between the newly formed surface layer and the remaining (Ca,Sr)SO4 specimen. The specimen was also implanted into a rat distal femur bone defect. In addition to new bone, fibrous tissue and inflammatory cells were found to interlace the (Ca,Sr)SO4 specimen. The present study indicated that a more comprehensive evaluation is needed to assess the bioactivity of non-phosphate bioceramics.