Effects of Non-Collagenous Matrix Proteins, Fatty Acid Derivatives, etc. on the Nucleation and Growth of Calcium Phospha
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EFFECTS OF NON-COLLAGENOUS MATRIX PROTEINS, FATTY ACID DERIVATIVES, ETC. ON THE NUCLEATION AND GROWTH OF CALCIUM PHOSPHATE ON HYDROXYAPATITE A.T.-C. Wong, J.T. Czernuszka Department of Materials, University of Oxford, Oxford, U.K ABSTRACT The effects of various non-collagenous bone matrix bio-chemicals on the deposition of calcium phosphate on hydroxyapatite surfaces have been studied. Guanidine-HCI bone extract, guanidine-EDTA bone extract and phosphatidylserine all showed an overall inhibitory effect on calcification. Phosphoserine promoted secondary nucleation but hindered crystal growth. Serum albumin altered the lattice structure of the crystals, thus inducing a significant calcium deficiency. This structural change also resulted in a different fracture behaviour. INTRODUCTION Hard tissues in general consist of minerals deposited at specific sites on a collagen base. It is known that various non-collagenous matrix proteins and lipids participate in the in vivo mineralisation process. Calcium phosphate (Ca-P), the main constituent of hard tissue minerals, has attracted vast interest in biomaterials research as it exhibits promising potential as a replacement material for bone. The scope of these studies has recently been extended to composite materials comprising collagen and Ca-P. The effects of the presence of the above bio-chemicals on the in vitro calcification of collagen must be taken into consideration when developing a collagen / Ca-P based bio-composite for orthopaedic applications. In vitro collagen calcification has been extensively studied 1 -6, but due to the complex chemistry of the various Ca-P phases and the affinity of collagen for phosphate radicals, a general agreement on the r8les played by these compounds is still to be reached. Apart from the induction of mineral deposition on the organic collagen base network, these organic compounds also influence the growth behaviour of Ca-P on existing crystals. These effects may differ from those in collagen / Ca-P systems as organics may have different conformations if bound to collagen. In vitro growth experiments on hydroxyapatite [CalO(PO 4) 6 (OH) 2 , HAP] substrates provide a means to study the influence exerted by different additives. HAP, among all the different phases of Ca-P, is used because it resembles most closely the actual mineral phase in hard tissues. These growth experiments eliminate the uncertainty arising from the complex collagen architecture and should, therefore, give a clearer view of the mineral growth phenomenon. Indeed, seeded growth has been performed using various proteins, lipids, etc.7 -14 but unfortunately, inconsistencies among different researches still exist. It is therefore the aim of the present work to reassess the possible part played by certain organic bone-matrix constituents under identical experimental conditions in every case. Phosphoproteins are significantly present in bone and teeth, and both inhibitory and promotional effects on mineralisation have been reported 15- 17. The main distinctive feature of p
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