Biomimetic mineralization of collagen by combined fibril assembly and calcium phosphate formation

To find new ways for the synthesis of improved bone implant materials, we studied the mineralization of collagen in vitro. Collagen was mineralized by combining the collagen fibril assembly and the formation of calcium phosphate in one process step. Both reactions were initiated simultaneously by mixing an acid, calcium-containing collagen solution with a phosphate-containing neutralization buffer. Under suitable conditions first fibril assembly occurred along with the precipitation of an amorphous calcium phosphate phase. Subsequently, the amorphous calcium phosphate transformed into a crystalline, apatite-like phase, as revealed by IR spectroscopy and X-ray diffraction. In this way, a homogeneously mineralized collagen gel was obtained, consisting of a three-dimensional network of collagen fibrils covered with calcium phosphate. The attachment between the collagen fibrils and the calcium phosphate crystals could be further improved by the addition of polyaspartate to the reaction mixture. In the absence of polyaspartate the calcium phosphate crystals formed clusters loosely bound to the fibrils, while in its presence separate crystals were located on or inside the collagen fibrils. The applied method is useful for studying the mineralization of collagen and offers a promising approach for the development of new bone implant materials.