Calcium phosphate bioceramics with improved properties: chemical synthesis

Abstract

As compared to metallic skeletal restorations, calcium phosphate ceramics offer superior properties, including: strong chemical resistance to environmental and microbial corrosion, compositionally controllable in vivo biological performance (bioactivity, solubility), high wear resistances, and excellent mechanical properties. Unfortunately, these ideal properties have not been obtained because commercial calcium phosphate ceramics vary notably in purity, crystal structure, and particle size. As a novel processing solution, a two-step processing technique was developed to result in a wide compositional range of pure, stoichiometric calcium phosphates [xCaO/spl middot/P/sub 2/O/sub 5/; x=3.0-3.3], The first step involved the low temperature (/spl les/5/spl deg/C) reaction between CaO and high purity phosphoric acid [H/sub 3/PO/sub 4/, Na-2 ppb]. After eliminating soluble reactants and impurity calcium phosphate phases, a crystalline calcium orthophosphate [CaHPO/sub 4/] powder was obtained in 61.1% yields. Thermal analysis (TGA, DTA), X-ray diffraction (XRD) and optical pyncnometry indicated that this product endothermically transforms at 460/spl deg/C [92.7% yield] into a highly pure, crystalline calcium pyrophosphate (2CaO/spl middot/P/sub 2/O/sub 5/) with an average particle size of 3.8 /spl mu/m (2-15 /spl mu/m). In the second step, 2CaO/spl middot/P/sub 2/O/sub 5/ was intimately blended with low alkali impurity CaCO/sub 3/ (1,1,3 mole%) and calcined in pure oxygen atmospheres to result in controlled composition calcium phosphate ceramics (xCaO/spl middot/P/sub 2/O/sub 5/). To demonstrate the potential of this technique, crystalline oxyapatite ceramics (3.33CaO/spl middot/P/sub 2/O/sub 5/) were obtained in high yields (85.8%) at temperatures exceeding 1000/spl deg/C with average crystalline particle sizes of 3.8 /spl mu/m (2-20 /spl mu/m). Petrographic, refractive index characterization of the ceramic precursor powders indicate that in all stages of processing, high purity, spherical powders of uniform crystallinity and particle size were obtained. In contrast to conventional calcium phosphate ceramics, this novel processing technique results in crystalline precursor powders of high purity, controlled composition, uniform crystal structure, and homogeneous particle size distribution. These ceramic precursor powders can be used with conventional ceramic processing techniques to result in products with the improved properties required for clinical success.