Sviluppo e caratterizzazione di materiali innovativi a base di idrossiapatite e vetro bioattivo

The difference in purity, particle size, microstructure, and thermo-chemical stability of three commercially available hydroxyapatite powders are found to play an important role during their consolidation using Spark Plasma Sintering (SPS) as well as strongly affect the characteristics of the resulting sintered bodies. A fully dense material without secondary phases was obtained by SPS at 900°C, when using the relatively small sized, with refined grains and high purity powders. The sintered product, consisting of sub-micrometer sized hydroxyapatite grains, displayed optical transparency and good mechanical properties. In contrast, the higher temperature levels (up to 1200 °C) needed to sinter powders with larger particles, or finer ones which contain additional phases, lead to products with coarser microstructures and/or significant amount of β-TCP as a result of HAp decomposition. Optical characteristics, hardness and elastic modulus of the resulting sintered samples are correspondingly worsened. Thanks to the favorable response bioglass materials display during their interaction with biological tissues, they have become one of the most important and investigated class of ceramics for biomedical application. In particular, as a consequence of the several chemico-physical phenomena involved at the glass-physiological fluids interface, a bone-like hydroxycarbonate apatite (HCA) layer is formed on the biomaterial surface, so that a strong bond between the latter one and tissues can be established. It is well known that mechanical and biological properties of bioactive glasses are significantly affected by the relative amounts among the different oxide constituents as well as the sintering conditions adopted to consolidate the initial amorphous powders. One of the most important aspect to consider in this regard is represented by the tendency of the material to crystallize during heat treatments, as the extensive crystallization of the bioglass negatively influence its bioactivity properties. In particular, the onset time for the formation of the HCA layer during glass exposure to a simulated body fluid (SBF) solution is delayed by crystallization.