Abstract
Hydroxyapatite has been used not only as a bone-repairing material in clinical applications but also as a functional material in the biological field, since it shows high biological affinity and specific characteristics on protein adsorption. It is expected that coating of substrates with a layer of hydroxyapatite will increase its potential applications as the hydroxyapatite will be directly exposed to the surrounding environment, allowing it to show its biological and chemical properties without any intervening effects by coverage with an organic polymer matrix. A biomimetic process for depositing hydroxyapatite on the surfaces of organic polymers provides us with a design of novel materials showing various mechanical properties without losing the functionalities of hydroxyapatite. This paper reviews a biomimetic process for coating of hydroxyapatite layers, through exposure of an organic substrate to a solution that mimics human body fluid. The existence of carboxyl groups effectively triggers the heterogeneous nucleation of hydroxyapatite on the examined organic polymer, such as aromatic polyamide and natural silk protein. The rate of hydroxyapatite formation on the examined organic polymer in the biomimetic solutions is increased by incorporation of calcium ions. The resultant layer formed on the organic substrates consists of nano-sized fine particles of hydroxyapatite with low crystallinity. The fine particles provide a high potential for the adsorption of proteins. The adhesive strength of the hydroxyapatite layer on the organic substrate of the aromatic polyamide increased with increasing content of carboxyl groups. These findings indicate that a coating of hydroxyapatite can be achieved on organic polymers under biomimetic condition by a modification of the polymer with carboxyl groups and calcium ions.
