The mechanisms of crystallization and dissolution of caclium phosphates atsurfaces

The generation of calcium phosphates at the surfaces of ceramic implants ma y be beneficial since the facilitation of bone formation permits their fixa tion. A key to the development of successful biomaterials is therefore an u nderstanding of the factors that control crystal growth and dissolution in aqueous solution. The Constant Composition method has been used to investig ate the influence of factors such as solution composition, ionic strength, pH and temperature on the crystallization and dissolution of the calcium ph osphates, brushite (DCPD), octacalcium phosphate (OCP), hydroxyapatite (HAP ) and fluorapatite (FAP). In parallel with these studies, contact angle mea surement along with surface tension component theory was employed to invest igate the roles of interfacial free energy in mineralization and deminerali zation. The exploitation of these factors is illustrated in studies of coat ing specific calcium phosphate phases on titanium metal and alloy surfaces and nucleation and growth of OCP on ceramic components including anatase, r utile, silica, alumina and zirconia surfaces. In all these reactions involv ing calcium phosphates, concomitant dissolution reactions are often involve d. Constant Composition kinetic studies have shown that the reaction rates decrease markedly with time despite the sustained driving force, eventually approaching zero even though the crystals remain in contact with undersatu rated solutions. Surface dissolution could be reinitiated by exposing the c rystals to the solutions of different undersaturations. These results sugge st that dislocation sizes on the surfaces play a significant role in the di ssolution kinetic processes.