Biomimetic growth of hydroxyapatite on gelatin films doped with sodium polyacrylate

Gelatin films were used as biomimetic substrates for the nucleation of hydroxyapatite from simulated body fluid (SBF). Stretching and presence of sodium polyacrylate appear to be essential factors for the specific nucleation and growth of hydroxyapatite crystals inside the films. After soaking in 1.5SBF for periods longer than 4 days, all the films display a completely calcified surface. However, the spherical aggregates on the film surfaces do not give any X-ray diffraction effect and exhibit a low Ca/P molar ratio, typical of amorphous calcium phosphate. The ordered deposition of crystalline hydroxyapatite has been verified to take place only in stretched polyacrylate--gelatin films. The crystals grow as tablets about 2 microns thick among the gelatin layers, with their crystallographic c-axes preferentially oriented parallel to the direction of orientation of the collagen molecules, thus resembling the parallel orientation of apatitic crystals and collagen fibrils in calcified biological tissues.