Published online by Cambridge University Press: 10 February 2011
The generation of minerals such as calcium phosphates on the surfaces of dental and joint replacement implants is beneficial since the facilitation of bone formation permits their fixation. In contrast, the prevention of crystallization is desired on other surfaces such as kidney and cardiac valve prostheses. A key to the development of successful biomaterials is therefore an understanding of the factors that control crystal nucleation, growth and dissolution in aqueous solution. The Constant Composition method was used to investigate the influence of factors such as solution composition, ionic strength, pH and temperature on the crystallization and dissolution of the calcium phosphates, brushite (DCPD), octacalcium phosphate (OCP), hydroxyapatite (HAP) and fluorapatite (FAP). In parallel with these studies, a contact angle method along with surface tension component theory was employed to investigate the roles of interfacial free energy in mineralization and demineralization. Values of the interfacial tensions, -4.2, 4.3, 10.4 and 18.5 mJm-2 obtained from contact angle measurements for DCPD, OCP, HAP and FAP, respectively, compare well with those calculated from dissolution kinetics experiments and provide information concerning the growth and dissolution mechanisms. The exploitation of these approaches is illustrated in studies of the coating of specific calcium phosphate phases on titanium metal and alloy surfaces and nucleation and growth of OCP on polymer surfaces modified by silanization to produce amine- and carboxylterminated end groups. In all these reactions involving the calcium phosphates, concomitant dissolution reactions are often involved. Constant Composition kinetic studies have shown that the rate of these reactions decrease markedly with time despite a sustained driving force, and eventually, the rates approach zero even though crystals remain in the undersaturated solutions. Dissolution can be reinitiated by exposing the crystals to the solutions of different undersaturations. These results suggest that dislocation sizes play a significant role in the dissolution kinetic processes.