Heat treatment of Na2O-CaO-P2O5-SiO2 bioactive glasses: densification processes and post-sintering bioactivity

Because of their excellent bioactivity, bioactiveglasses are increasingly diffused to produce biomedical devi-ces for bone prostheses, to face the dysfunctions that may becaused by traumatic events, diseases, or even natural aging.However, several processing routes, such as the production ofscaffolds or the deposition of coatings, include a thermal treat-ment to apply or sinter the glass. The exposure to high tem-perature may induce a devetrification phenomenon, alteringthe properties and, in particular, the bioactivity of the glass.The present contribution offers an overview of the thermalbehavior and properties of two glasses belonging to the Na2O-CaO-P2O5-SiO2system, to be compared to the standard 45S5BioglassVR. The basic goal is to understand the effect of boththe original composition and the thermal treatment on the per-formance of the sintered glasses. The new glasses, the one(BG_Na) with a high content of Na2O, the other (BG_Ca) with ahigh content of CaO, were fully characterized and sinteringtests were performed to define the most interesting firingcycles. The sintered samples, treated at 880?C and 800?Crespectively, were investigated from a microstructural point ofview and their mechanical properties were compared to thoseof the bulk (not sintered) glass counterparts. The effect of sin-tering was especially striking on the BG_Ca material, whoseVickers hardness increased from 598.9 6 46.7 HV to 1053.4 635.0 HV. The in vitro tests confirmed the ability of the glasses,both in bulk and sintered form, of generating a hydroxyapatitesurface layer when immersed in a simulated body fluid. Moreaccurate biological tests performed on the sintered glassesproved the high bioactivity of the CaO-rich composition even after a heat treatment.