Heat treatment of Na₂O‐CaO‐P₂O₅‐SiO₂ bioactive glasses: Densification processes and postsintering bioactivity

Abstract

Because of their excellent bioactivity, bioactive glasses are increasingly diffused to produce biomedical devices for bone prostheses, to face the dysfunctions that may be caused by traumatic events, diseases, or even natural aging. However, several processing routes, such as the production of scaffolds or the deposition of coatings, include a thermal treatment to apply or sinter the glass. The exposure to high temperature may induce a devetrification phenomenon, altering the properties and, in particular, the bioactivity of the glass. The present contribution offers an overview of the thermal behavior and properties of two glasses belonging to the Na2O-CaO-P2O5-SiO2 system, to be compared to the standard 45S5 Bioglass(®). The basic goal is to understand the effect of both the original composition and the thermal treatment on the performance of the sintered glasses. The new glasses, the one (BG_Na) with a high content of Na2O, the other (BG_Ca) with a high content of CaO, were fully characterized and sintering tests were performed to define the most interesting firing cycles. The sintered samples, treated at 880°C and 800°C respectively, were investigated from a microstructural point of view and their mechanical properties were compared to those of the bulk (not sintered) glass counterparts. The effect of sintering was especially striking on the BG_Ca material, whose Vickers hardness increased from 598.9 ± 46.7 HV to 1053.4 ± 35.0 HV. The in vitro tests confirmed the ability of the glasses, both in bulk and sintered form, of generating a hydroxyapatite surface layer when immersed in a simulated body fluid. More accurate biological tests performed on the sintered glasses proved the high bioactivity of the CaO-rich composition even after a heat treatment.

Processing and in vitro bioactivity of high-strength 45S5 glass-ceramic scaffolds for bone regeneration

“…However, the foam spectrum shows peaks representative of the crystalline phases: Na 6 Ca 3 Si 6 O 18 (associated with Na 2 CaSi 2 O 6 JCPDS-ICDD 01-077-2189) and Na 2 Ca 4 (PO 4 ) 2 SiO 4 (JCPDS-ICDD 29-1193). The same has been identified by other researchers in previous studies of sintered bioactive glasses of the same composition [11,[15][16][17]. This clearly indicates that a process of crystallization is present during the sintering of foams.…”

Functional bioactive glass topcoats on hydroxyapatite coatings: Analysis of microstructure and in-vitro bioactivity

“…In addition, some new weak peaks could be detected, which may be associated with a calcium silicate, Ca 2 SiO 4 (larnite). It is interesting to note that in the relatively Na 2 Opoor BG_Ca glass a calcium silicate, instead of a sodium-calcium silicate developed, which is the typical outcome of the crystallization of 45S5 Bioglass® and other Na 2 O-rich bioglasses [26]. Moreover, well-defined peaks could be attributed to the HA layer underneath the glass topcoat.…”

“…8b), after one day in SBF the glass topcoat underwent a through-thickness reaction and only silicon and oxygen could be detected by the X-EDS analysis (Table 2), thus suggesting a silica gel layer developed on the surface. The very high reactivity of the topcoat was due to the concomitant effect of the intrinsic bioactivity of the BG_Ca glass [26] and of the porous microstructure of the layer. In fact the rich porosity of the topcoat favoured the penetration of SBF in depth and greatly increased the specific surface area available for the chemical reactions.…”

“…As described in a previous contribution [28], the bioactive glass (BG_Ca) [25,26] was produced by the conventional melt-quenching method. Appropriate raw materials were mixed and then molten in a platinum crucible at 1450°C.…”

“…The BG_Ca glass (having composition, in wt.%: 4.7 Na 2 O, 42.3 CaO, 6.1 P 2 O 5 , and 46.9 SiO 2 ) was selected since it combines high bioactivity with a relatively low tendency to crystallize at high temperature [25,26].…”