Investigation of Apatite Mineralization on Antioxidant Polyphosphazenes for Bone Tissue Engineering

Synthetic bone grafts that promote the natural mineralization process would be excellent candidates for the repair or replacement of bone defects. In this study, a series of antioxidant-containing polyphosphazenes were evaluated for their ability to mineralize apatite during exposure to a solution of simulated body fluid (SBF). All polymers contained ferulic acid (antioxidant), cosubstituted with different amino acid esters linked to the polyphosphazene backbone. Differences in the side groups determined the hydrophobicity or hydrophilicity of the resulting polymers. All of the polymers mineralized monocalcium phosphate monohydrate, a type of biological apatite. However, the mineralization process (the amount of deposition and length of time) was dependent on the hydrophilicity or hydrophobicity of the polymers. The polymer–apatite composites were examined by electron scanning microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravametric analysis. Weight gain data were also obtained. To verify that the nucleation process was due to the presence of calcium and phosphate, two standard solutions were prepared: one solution (NaCl solution) contained only sodium chloride, and the second solution (mSBF) was similar to SBF except without known crystal growth inhibitors such as Mg 2+ and HCO 3 – . No mineralization occurred when the polymers were exposed to the NaCl solution, but mineralization took place upon exposure to mSBF. The apatite phase produced was hydroxyapatite (HAp). The mineralization process in mSBF was much more extensive, with all samples gaining more weight following exposure to SBF. A similar trend was also found (as in the case of SBF), with the amount of deposition and length of deposition time depending on the hydrophilicity/hydrophobicity of the polymer. These results suggest that the nucleation process is due to calcium and phosphate, and the absence of crystal growth inhibitors allows for the rapid nucleation of HAp. In both cases, the mineralization process was favored on hydrophilic surfaces (static water contact angle of 56–65°) versus hydrophobic surfaces (71–86°).