| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2001. === Includes bibliographical references. === Historically, using suitable mechanical replacements for bone has been a priority in designing permanent, load-bearing orthopedic implants. As a result, the biomaterials used in these implants have been largely limited to bioinert titanium-based alloys, as well as to polycrystalline alumina and zirconia ceramics. However, analysis of implants incorporating these traditional biomaterials indicated that most failures involved an unstable implant-tissue interface and/or a mismatch of the mechanical behavior of the implant with the surrounding tissues. As a result, up to 20% of patients receiving permanent, load-bearing implants may undergo a revision operation. The objective of this research was to develop an alternative biomaterial that combined both mechanical resilience and an osteoconductive surface to provide a stable interface with the surrounding connective tissue so that the need for revision operations may be significantly reduced. In the effort to address the issue of mechanical strength and bioactivity simultaneously, hydroxyapatite (HAP) has generated considerable interest. Though a commonly used bioceramic, HAP has been limited by its processability. This material is sensitive to non-stoichiometry and impurities during synthesis and processing due to its complex composition and crystal structure (Ca10(P04)6(OH)2, P63/m). === (cont.) Consequently, conventionally processed HAP materials lack phase purity and homogeneity. Densification of HAP requires high temperatures that result in grain growth and decomposition into undesired phases with poor mechanical and chemical stability. To circumvent densification at high temperatures, glassy additives have been introduced to promote liquid-phase sintering at a lower temperature. However, the presence of a secondary glassy phase gave rise to poor mechanical characteristics. Hence, clinical applications of HAP have been limited to powders, coatings, porous bodies, and non-load-bearing implants. To overcome the deficiencies of conventionally processed HAP, nanostructure processing was applied, which allowed for materials design from the molecular level. By using an aqueous chemical precipitation technique, a fully dense, transparent, nanostructured HAP-based bioceramic that exhibited superior mechanical properties and enhanced tissue bonding was obtained. Processing parameters affecting the molecular and structural development of HAP were used to tailor HAP stoichiometry, crystallite size, morphology and surface chemistry for optimal thermal stability and sinterability. Unlike conventionally processed HAP, the stoichiometric, equiaxed, nanocrystalline HAP powders demonstrated significantly enhanced sinterability by fully densifying at a remarkably low temperature of 900ʻC with pressure-assisted sintering. === (cont.) Furthermore, high-resolution electron micrographs illustrated that the sintered compact possessed a uniform and ultrafine microstructure with an average grain size of -100 nm, with no glassy or amorphous interfaces along the grain boundaries. The crystallinity of the HAP grains and grain boundaries and the minimal flaw sizes could be credited for the superior strength of nanostructured HAP compared to conventional HAP. Compared to polycrystalline HAP, nanocrystalline HAP also provided greater osteoblast function. In vitro experiments indicated that nanocrystalline HAP surfaces enhanced cell attachment, proliferation and mineralization. The larger grain boundary volume resulting from the ultrafine microstructure might have enhanced protein adsorption, ... === by Edward Sun Ahn. === Ph.D.
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