Dual Constant Composition Kinetics Studies of the Demineralization of Ceramic Plasma Coated Apatite Surfaces
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DUAL CONSTANT COMPOSITION KINETICS STUDIES OF THE DEMINERALIZATION OF CERAMIC PLASMA COATED APATITE SURFACES
E.P. Paschalis and G.H. Nancollas Department of Chemistry, Biophysical Sciences and Biomaterials State University of New York at Buffalo Buffalo, New York 14214 USA ABSTRACT Calcium phosphate phases such as hydroxyapatite (HAP) and octacalcium phosphate (OCP) have long been considered as model systems for biological minerals. Moreover such phases are involved in ceramic and plasma coated titanium (HPCTI) prosthetic devices. Most of these mineral preparations are mixtures of calcium phosphate phases, as evidenced by X-ray analysis. However, many mineralizing systems involve the formation of metastable intermediates which may subsequently undergo phase transformations. There is therefore considerable interest in investigating the simultaneous growth and dissolution of multiple mineral phases. Based on the Constant Composition (CC) method , the new Dual Constant Composition (DCC) technique has been developed for kinetics studies of the dissolution of the mixed calcium phosphate phases frequently encountered in-ceramic and plasma coated apatite surfaces.
INTRODUCTION The biological importance of ceramic calcium phosphate phases such as hydroxyapatite (HAP), octacalcium phosphate (OCP) and tricalcium phosphate (TCP) is now well established. There
is now little doubt that many biomineralization processes involve the formation of metastable intermediates which may subsequently transform into thermodyhamically more stable phases. Many studies have been aimed at elucidating the mechanisms of mineralization and demineralization reactions of such materials which may therefore involve the simultaneous growth and/or dissolution of multiple calcium phosphate phases. More recently, two additional apatitic phases have received much attention. These are ceramic hydroxyapatite (CHAP) and hydroxyapatite plasma-sprayed titanium alloy implants (HPCTI). Despite the fact that the extensive use of these materials in implantology is well documented, little basic research has been aimed at elucidating the criteria which determine the successful application of these surfaces for implantology. Since in almost every case multiple calcium phosphate phases are involved, kinetic studies of simultaneous crystallization reactions are important but have been severely limited in conventional kinetics methods because of the decreasing driving force or supersaturation with respect to each of phases as the reactions proceed. Based on the CC method (1-7), the recent development of the DCC method (8,9) has enabled studies to be made of
Mat. Res. Soc. Symp. Proc. Vol. 252. ©1992 Materials Research Society
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concurrent dissolution and/or mineralization of mixtures of calcium phosphate phases. One important question that should be addressed with regard to the nature of CHAP and HPCTI surfaces is whether they are If this is not the case, it is pure (apatitic) phases. important to determine the nature of the other phases present In the present s
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