Spark Plasma Sintering and Characterization of Bioceramics

Bulk hydroxyapatite- and bioactive glass-based ceramics are produced in this work by Spark Plasma Sintering (SPS). The main results are briefly summarized in what follows. Densification and crystallization phenomena taking place when an innovative CaO-rich bioactive glass (BG_Ca/Mix) and conventional 45S5 Bioglass® are processed by SPS. Fully dense and wholly amorphous BG_Ca/Mix products are obtained at 730 °C after 2min dwell time. Temperatures equal or higher than 830 °C are needed to induce crystallization of CaSiO3 in the parent glass. Conversely, Na6Ca3Si6O18 crystals are formed in sintered 45S5 samples produced after 2 min at 550°C, although the glassy character is preserved. Products from BG_Ca/Mix powders display higher hardness and local elastic modulus. SPS was also employed to produce highly dense and bioactive composites from different mixtures of hydroxyapatite (HA) and BG_Ca/Mix glass. The produced samples are significantly denser with respect to the counterparts obtained by conventional sintering. Thanks to the low tendency to crystallize of BG_Ca/Mix, it was possible to reduce the devitrification of the glassy phase in the samples, with beneficial effects in terms of the resulting in vitro bioactivity. Detailed in-vitro experiments concerning SaOS-2 human osteoblasts cultivated on the surface of dense samples obtained by SPS from three different HA powders are finally carried out. The composition and microstructures characterizing the sintered products significantly affect the biological response of the cells. In particular, mineralization phenomena are observed to occur only in samples exclusively made of HA, particularly where submicrometer sized grains are involved. Correspondingly, an apatite layer with a trabecular-like microstructure was formed on the scaffolds surface. In contrast, mineralization is suppressed, and so does the formation of the apatite phase, when cells were seeded on specimens composed of β-TCP.