Synthesis of Hydroxyapatite/Collagen Bone-Like Nanocomposite and Its Biological Reactions

Bone is a typical nanocomposite of inorganic and organic substances mainly composed of nanocrystals of hydroxyapatite (Ca10(PO4)6(OH)2, HAp) with nonstoichiometry 20-40 nm in length and type-I collagen molecules 300 nm in length. In microscopic observation of bone, collagen molecules are covered with HAp nanocrystals, and they form nanocomposite fibers in which HAp c-axes are approximately aligned along with collagen molecules (Bacon et al., 1979, Sasaki & Sudoh, 1997). Bone has two important roles in vertebrates; one is as a structural material to maintain vertebrates body structure and to guard important internal organs, such as brain, heart and lung, and another is as an organ to control calcium ion homeostasis by resorbing bone mineral according to deficiency of calcium ion. These two roles are kind of antinomical properties as industrial materials, i.e., TOUGH to endure to external forces for whole life and BREAKABLE to allow calcium release by bone cell functions on demand. However, change our viewpoint about toughness of material, solution of the antinomy is concluded to one property. That is, every materials fatigue by long time use; thus, periodic renewal of material is necessary to maintain enough toughness of bone for whole life. Thus, bone has to be renewed easily by cell functions. That is almost the same meaning as a breakable property of bone. Accordingly, requirement of bone can be translated as follows; bone has to be decomposed by cell functions but be stable without them. Fortunately, our ancestors needed to preserve calcium and phosphate ions in their body when they left from sea. Calcium and phosphate easily form insoluble compound in aqueous solution, brushite (CaHPO4•2H2O), octacalcium phoshate (Ca8H2(PO4)6•5H2O) and HAp, and all calcium phosphates are changed into HAp in aqueous or moistured condition, because it is most stable compound in these conditions. Further, HAp formed in regular