article posted 22 March 2016
Murilo Crovace received a bachelor's degree at Engenharia de Materiais from Universidade Federal de São Carlos (2006) and master's at Material and Metallurgical Engineering from Universidade Federal de São Carlos (2009). Has experience in Material and Metallurgical Engineering, acting on the following subjects: glass-ceramics, microstructure, scaffolds, biosilicate and anisotropy.
The mixed alkali effect (MAE) in the sintering of Bioglass 45S5-based compositions
M.C. Crovace1*, V.O. Soares1, A.C.M. Rodrigues1, O. Peitl1 & E.D. Zanotto1
1 Vitreous Materials Laboratory, Department of Materials Engineering, Federal University of São Carlos, São Carlos – SP, Brazil
2 Department of Sciences, State University of Paraná, Goioerê – PR, Brazil
Bioactive glasses haven been studied for over four decades, since Bioglass 45S5 was found to form a direct bond with living tissues. Bioglass is currently known worldwide and is being considered for different uses involving the regeneration of both soft and hard tissues due to its unique combination of biological properties such as osteoconduction, osteoinduction, angiogenesis and antibacterial activity. However, the commercial availability of Bioglass is still limited to coarse glass particles for bone regeneration or fine powders for the treatment of dentin hypersensitivity, given the difficulty of obtaining more complex shapes. In some cases, the understanding of the sintering and crystallization behavior is critical, e.g. in the fabrication of scaffolds. The vast majority of the Bioglass-based scaffolds reported in the literature are sintered at temperatures above 1000°C, where the particles are already fully crystallized. It has been reported that full surface crystallization of Bioglass 45S5 particles occurs prior to significant densification by viscous flow. Since sintering by viscous flow has clear advantages over solid-state sintering, the search for new strategies to increase the sinterability of Bioglass is of great importance. In this work, the complex phenomena of sinter-crystallization and the in vitro
bioactivity of some Bioglass 45S5 compositions with gradual substitutions of Na2
O by K2
O and of CaO by MgO were studied. The sintering is addressed in terms of both glass stability and viscosity. Powdered glass pellets were analyzed in an optical dilatometer and the relative density (rho/rho0
) as a function of temperature was followed. In both cases, a mixed alkali effect (MAE) was observed. The MAE effect was also identified in the viscosity of the two sets of compositions. Overall, substitution of CaO by MgO was more effective in increasing the sinterability of Bioglass 45S5. However, while the replacement of Na2
O by K2
O does not affect the onset time for hydroxycarbonate apatite (HCA) formation, the replacement of CaO by MgO hinders the crystallization of the amorphous calcium phosphate layer into HCA.
Figure 1: Calculated relative density curves as a function of temperature for the compositions 100Na/0K (Bioglass 45S5), 75Na/25K, 50Na/50K, 25Na/75K and 0Na/100K. The sintering experiments were performed in an optical dilatometer. The particle size distribution was in the range 25–75 µm and the heating rate was 80°C/min.
Figure 2: Calculated relative density curves as a function of temperature for the compositions 100Ca/0Mg (Bioglass 45S5), 75Ca/25Mg 50Ca/50Mg, 25Ca/75Mg and 0Ca/100Mg. The sintering experiments were performed in an optical dilatometer. The particle size distribution was in the range 25–75 µm and the heating rate was 80°C/min.