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Sinterability, Mechanical Properties and Solubility of Sintered Fluorapatite-Hydroxyapatites

Mechanical assemblies within biological systems occur widely combining different material systems and occasionally similar crystal structures. Mechanical assemblies were produced by sintering mixtures of hydroxyapatite and fluorapatite. It was found that these led to lower densities compared to solid solutions of the same composition and accompanying lower mechanical properties. Investigation of lattice parameters revealed that sintering submicron mixtures produced homogeneous mixtures. Further work on larger crystal sizes will reveal how such interfaces provide integrity for mechanically assembled apatites in biological systems and the ability to produce in-situ roughening after insertion of smooth surfaces. Introduction A mechanical assembly of chemically enriched apatites is a common occurrence within biological systems. Examples of these include the outer fluorapatite layer on hydroxyapatite in teeth or the chemical enrichment of inorganic crystals in bone subjected to therapeutically administered solutions or resorbable biomaterials. Mechanical blends of different apatites have not been used as biomaterials. These can be employed in biomedical devices to improve mechanical properties or administer controlled quantities of specific chemicals from a chemically mapped biomaterial surface. The use of fluoride in apatite and bioapatites provides a means of modifying the physicochemical and mechanical properties. The natural occurrence of fluoride within apatites signifies an important function within the body. As an ion in solution, it can be used to stimulate bone growth, however, excessive quantities can lead to damaging effects. Fluoride concentrations increase in the bone with age, with apatite providing an effective storage medium. The use of fluoride for biomedical applications needs to be well designed to offer a controlled amount of this ion. The slow release of fluoride is possible by inclusion of low concentrations in bioactive materials. Slow resorption can lead to release of fluoride ions that become integrated into the surrounding tissues. This work will investigate the sinterability, mechanical properties and solubility of sintered mechanical mixtures of fluorapatite and hydroxyapatite with reference to fluorhydroxyapatite solid solutions. Comment will be provided on the clinical usefulness of apatite ceramics produced by closely located, but chemically different apatite regions. Materials and Methods Hydroxyapatite and fluorapatite were synthesized using the semi-bath approach by controlled addition of an ammonium nitrate to a calcium nitrate solution. The precipitate was aged, dried in ethanol, calcined at 900 C. Mechanical powder blends were produced by combining fluorapatite at concentrations of 0, 20, 40, 60, 80 and 100% to hydroxyapatite in an agate ball mill with ethanol, followed by drying in an oven at 120 C. Powder blends were uniaxially compressed to form cylindrical shaped pellets, then vacuum sealed and cold isostatic pressed at 193 MPa to provide a Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 39-42 doi:10.4028/www.scientific.net/KEM.254-256.39 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,19:46:55) Title of Publication (to be inserted by the publisher) more uniform densification of the cylindrical pellets. The pellets were sintered at 1200 C for two hours within a tube furnace supplied with an atmosphere of air saturated with water vapour at 90 C. Pellets were ground on SiC paper using 500, 800, 1200 and 2400 grades followed by polishing on a short napped cloth with 1 micron diamond suspension for a period of three minutes under a load of 2.5 kg. Pellets were then assessed for density, hardness, elastic modulus and fracture toughness using micro-indentation. The degree of diffusion was assessed by examination of the lattice parameters for mixtures containing more than 40% fluorapatite. Solubility was assessed on samples by immersion in a 0.1M potassium acetate buffer at pH 5, 37 C for 24 hours before removal for microstructure assessment. Results and discussion Mechanical mixtures of 20, 40, 60 and 80% fluorapatite in hydroxyapatite powder revealed a decrease in density at a sintering time of 2 hours with a minimum at 80% fluorapatite, Fig 1. The different chemistry in crystallites has produced a lower density for mechanical blends of fluorapatite and hydroxyapatite. The tendency for the minimum to be offset towards the fluorapatite composition indicates greater difficulty in sintering of a small amount of hydroxyapatite particles with fluorapatite. This can be explained in the different packing in the unit cells. Fluorapatite is a denser structure and cannot readily accomodate the larger OH group, thus leading to a lower mobility of this ion. Conversely, movement of the smaller F ion in a less densely packed hydroxyapatite unit cell is significantly easier attributed to the small decrease in density from hydroxyapatite. The small decrease does suggest that the presence of fluoride can retard the densification of hydroxyapatite powders, and a report of fluoride concentration in hydroxyapatite formulations is a recommended addition to the requirement to report heavy metals, such as As, Cd, Hg, Pb. Solid solutions show a different trend indicating that a minimum is only observed when fluoride and hydroxyl ions are present at similar concentrations. The lower density in the mechanical mixtures can thus be interpreted as a combination of the solid solution effect and the presence of apatites with different packing density. The lattice parameters of the mechanical blends indicated a decrease in the a-lattice parameter, an indication that fluoride is substituted into the lattice. The lack of foreign peaks in an X-ray diffraction pattern (not shown here) indicates that diffusion between the two apatites occurs completely at sintering at 1200 C for 2 hours. Fig 1. Density of tablets sintered at 1200 C for 2 hours The hardness follows the same trend in the density, where the trend is mainly attributed to the porosity within the sample. Comparison of the 20% and 100% fluoridated apatite, that exhibit comparable densities, indicated that the fluoride content gives rise to an increase in hardness, Fig 2. 0 20 40 60 80 100 90 92 94 96 98 100 Mech. mixture Solid solution