Intrinsic Orientation of Hydroxyapatite Grains on the Surface of Dense Pellets Produced by Uniaxial Pressing

Abstract

Bioceramics synthesized in laboratories must undergo biological tests that consider the final format and all the processing steps involved in the production of the final device. However, in many cases, the synthesized materials are characterized in formats very different from those that will ultimately be implanted into the body. Once living cells are directly cultured on surfaces, a mismatch between bulk and surface microstructural characterizations often leads to the misinterpretation of biological responses. Therefore, our objective in this work was to determine whether and at what level significant microstructural changes can occur between the surface and the bulk of hydroxyapatite pellets produced by uniaxial pressing followed by calcination. Our results showed that the as-synthesized hydroxyapatite crystals had a preferential orientation along the [0 0 1] direction. The calcination process followed by grain growth inhibited this texture feature. However, on the pellet surfaces, the initial orientation was preserved. At the same time, crystallite growth was inhibited, and the cell unit volume of the crystals on the surface was significantly lower than that of the crystals in the bulk. These results demonstrated that the crystallographic features on the surface of hydroxyapatite pellets can be completely different from those observed for the bulk.