Incorporation of Ca²⁺ ions in gelatin-siloxane hybrids through a sol-gel process

Several gelatin-siloxane hybrids incorporating Ca²⁺ ions were synthesized through a sol-gel process starting with gelatin, 3-(glycidoxypropyl) trimethoxysilane (GPSM), and Ca(NO₃)₂. Amino acid analysis, ²⁹Si NMR spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, and dynamic mechanical thermal analysis were used to assess the hybrid structure. It has been indicated that the grafting degree of epoxy groups of GPSM to gelatin and the extent of polymerization of methoxysilane end groups of GPSM strongly depended on the fraction of GPSM (f_G) in the hybrids, but not on incorporation of Ca²⁺ ions. Hence, the hybrids with the same f_G value, independent of incorporation of Ca²⁺ ions, had a similar cross-link density. However, the glass transition temperature (T_g) and storage modulus (E′) around T_g decreased with incorporation of Ca²⁺ ions. Through the deconvolution of amide I bands of FT-IR spectra, flexibility may be interpreted by that the percentage of gelatin having a random structure increased with incorporation of Ca²⁺ ions. The Ca²⁺-containing hybrids can spontaneously deposit apatite when they were soaked in a simulated body fluid of the Kokubo recipe, showing their bioactivity.