Calcium level-responsive in-vitro zinc release from zinc containing tricalcium phosphate (ZnTCP)

The in vitro Zn release from tricalcium phosphate containing Zn (ZnTCP; 0.63, 6.17, and 12.05 Zn w/w%) was investigated. The rates of release from ZnTCP powders were measured in 25 mL of simulated body fluid (SBF) containing 10 mg/100mL Ca (SBF/H), 5 mg/100mL Ca (SBF/L), or no Ca (SBF/-) at pH 7.25, 37.0+/-0.1 degrees C. The release from 6 and 12% ZnTCP was initially very fast. The rate of release from ZnTCP decreased as the concentration of Ca in the dissolution media increased, but increased as concentration of Zn in TCP increased. The dissolution kinetics of ZnTCP followed the Hixon-Crowell equation at the initial stage of dissolution, and the initial dissolution rate constant (IDR) was calculated by the least-squares method. The effect of Ca concentration on percent IDR of ZnTCP suggested that Zn release from 0.6% ZnTCP was significantly high compared to that from 6 or 12% ZnTCP. The relationship between the amount of Ca precipitated and Zn release of various ZnTCP samples suggested that the release from 0.6% ZnTCP was significantly different compared to that from 6 and 12% ZnTCP, consistent with the data for percent Zn IDR. X-ray diffraction data suggested that 0.6% ZnTCP contained 5% hydroxyapatite, a low solubility material, which acted as seed crystal during the dissolution test.