In the present study, the preparation of dense hydroxyapatite (HAp) materials at relative low temperatures using a spark plasma sintering was carried out. The bioactivity of HAp samples prepared by a spark plasma sintering method was investigated by in vitro tests and compared with HAp obtained by a conventional hot-pressing method. No growth of bone-like HAp crystals on surface of HAp sintered by a conventional hot-pressing method at 1200 degrees C was observed after immersion in a simulated body fluid (SBF) for 2 days. However, many large bone-like HAp crystals were observed on the surface of HAp samples prepared by a spark plasma sintering at 1200 degrees C after 2 days in the SBF immersion test. Especially, the negatively charged surface of the HAp samples prepared by spark plasma sintering was covered with larger HAp crystals compared with the positively charged surface. The electric poling of HAp was measured using a thermally simulated current technique. This rapid growth of bone-like HAp crystals of the HAp samples made by spark plasma sintering was believed to be related with the OH(-) and/or Ca(2(+) ) ion deficiency at the grain boundaries of the HAp matrix grains as well as a small electric poling effect resulting during the spark plasma sintering process.
To promote the osteogenic properties of osteoblasts, we synthesized a hydroxyapatite (HAp) with beta-tricalcium phosphate (beta-TCP) biphasic calcium phosphate containing Nb ions (NbTCP/HAp). NbTCP/HAp was prepared by annealing precipitates obtained by coprecipitation of an aqueous solution of Ca(NO(3))(2) and a mixture of (NH(4))(2)HPO(4) and aqueous Nb solution. The precipitates can be regarded as a calcium-deficient HAp, the PO(4) sites of which are partly occupied by Nb ions. NbTCP/HAp was successfully synthesized by thermal decomposition of the precipitates. NbTCP/HAp enhanced the calcification of normal human osteoblasts (NHOst), and the amount of calcified tissue increased in proportion to the Nb ion concentration in the NbTCP/HAp. The alkaline phosphatase (ALP) activity of NHOst was also enhanced by NbTCP/HAp. Because Nb ions significantly enhance the ALP activity of NHOst, calcification by NbTCP/HAp is considered to be due to enhancement of ALP activity induced by Nb ions dissolved from NbTCP/HAp. These results indicate that NbTCP/HAp can be an effective bone repair material.
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