Biocompatible whiskers with controlled morphology and stoichiometry
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Hydroxyapatite whiskers have been prepared by the hydrothermal method. The crystals had diameter, length, and aspect ratio in the range of 1-10 fim, 30-50 /xm, and 5-20, respectively. Their Ca/P molar ratio varied from 1.59 to 1.62. The morphology of the crystals can easily be controlled by the concentrations of species in the starting solution, while the Ca/P ratio is almost independent of them. Through the reaction with calcite powder at 600 °C, the Ca/P ratio of the whiskers has been improved even to the stoichiometric value of 1.67. Taking into account morphology and chemical composition of the HAp whiskers, they should not be health hazardous and may find applications as substitutes for asbestos and other fibrous materials which presently have restricted use because of their carcinogenic natures.
I. INTRODUCTION Inorganic fibers and whiskers have been used for many years, finding applications mostly as reinforcements in composites and thermal insulation. Various fibrous materials, such as glass, carbon, SiC, Si 3 N 4 , A12O3, ZrC>2, and many others1 have been prepared for such purposes. Among natural fibers, asbestos has been widely used due to its high tensile strength, chemical stability, and low cost.2 Unfortunately, an application of the fibrous materials may be connected with a health risk. Asbestos is reported to be a cause of many serious diseases including lung cancer.2 The toxicity of widely used silicon carbide whiskers is stated to be only slightly lower.3 Stanton et al.A and Pott5 reported that the carcinogenic effect of the fibrous materials was restricted to long and thin fibers (diameter < 1 fim, length > 10 /mm). Many commercially available whiskers and fibers may be safe from this point of view; however, their chemical compositions must also be taken into account. Alumina, zirconia, titania, silicon carbide, and silicon nitride are known as bioinert materials.6 [Types of implant-tissue response (after Hench 6 ): If the material is toxic, the surrounding tissue dies. If the material is nontoxic and biologically inactive (bioinert), a fibrous tissue of variable thickness forms. If the material is nontoxic and biologically active (bioactive), an interfacial bond forms.] These compounds are not toxic, but their biocompatibility is doubtful. (Biocompatibility denotes acceptance of
a)Present
address: National Institute of Materials and Chemical Research, 1-1 Higasaki, Tsukuba 305, Japan. J. Mater. Res., Vol. 10, No. 3, Mar 1995
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the implant to the tissue surface. This broad term also includes nontoxicity, noncarcinogenity, chemical inertness, and stability of the material in the living body.55) On the other hand, bioactive calcium phosphates exhibit excellent biocompatibility due to their chemical and crystallographic similarities with the mineral constituents of bones and teeth.7 Among them hydroxyapatite [HAp, theoretical formula Cai 0 (PO 4 ) 6 (OH) 2 ] is known as the most biocompatible material. It has been used in medicine for many yea
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