Sintering and crystallisation of 45S5 Bioglass® powder
Introduction
45S5 Bioglass® is a commercially available inorganic material, which has been used as bone replacement for more than 20 years.1 This is the first bioactive glass developed by Hench et al. in 1969 2 and has the following chemical composition: 45 wt.% SiO2, 24.5 wt.% Na2O, 24.5 wt.% CaO and 6 wt.% P2O5. This glass is highly bioactive and it is both osteoinductive and osteoconductive, enabling its application in bone tissue engineering.3, 4 Moreover, it has been shown that the ionic dissolution products of 45S5 Bioglass® may enhance new bone formation (osteogenesis) through a direct control over genes that regulate cell induction and proliferation.5
45S5 Bioglass® has been used for fabrication of scaffolds for bone tissue engineering by sintering powders of particle size < 5 μm and employing the foam replication technique.6 The success of the scaffold fabrication process depends on the sintering ability of the Bioglass® powder since this particular bioactive glass composition is prone to crystallisation at the high temperature required for sintering, as also investigated by several authors.7, 8, 9, 10, 11 Therefore, the requisite for optimising the fabrication of Bioglass® scaffolds is to understand the sintering conditions of Bioglass® particles and the interaction between sintering and crystallisation of the material. By knowing the structural transformations which occur during the heat treatment of Bioglass®, the scaffold fabrication process can be tailored, e.g. in terms of achieving the highest possible density of the foam struts and the required crystallinity which itself controls the material bioactivity.12
Previous studies have shown that there are five structural transformations during the heating of Bioglass® up to 1000 °C: a first glass transition, a glass-in-glass phase separation, two crystallisation processes and a second glass transition.12, 13, 14 Modelling of the sintering behaviour of bioactive glass scaffolds has been also considered recently15 but there is still a lack of knowledge about the effect of different process variables on the final microstructure of the partially crystallised Bioglass®.
The aim of this work was to study the sintering process of 45S5 Bioglass® powder by using different thermal analysis methods; including dilatometry, differential thermal analysis (DTA) and, for the first time, heating microscopy to investigate sintering anisotropy effects. The crystalline phase characterisation after the crystallisation and sintering process was carried out by Fourier Transformed Infrared Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD) techniques.
Section snippets
Materials
Bioglass® (type 45S5) powder (NovaMin USA) of mean particle size <5 μm was used in this investigation. The chemical composition was given in Section 1. This glass is being used for fabrication of tissue engineering scaffolds by the foam replica method,6 as mentioned above.
Heating microscopy
The sintering process of Bioglass® powder compacts was directly observed by heating microscopy. This technique allows the quantification of sintering variables by measuring the variation of the sample dimensions during the
Heating microscopy
Typical heating microscope silhouettes of cubic Bioglass® samples during sintering at a heating rate of 20 °C/min and at different characteristic temperatures are shown in Fig. 1. The reduction of the sample dimensions during the sintering process can be noticed. The dimensional changes of the sample can be clearly appreciated by comparing the two images at 50 and 1100 °C. The sample seems to keep the cubic shape up to 1100 °C (Fig. 1a–e). At 1100 °C the sample started to melt and the shape of the
Conclusions
The sintering process of 45S5 Bioglass® powder was studied by using different thermal analysis and microscopic methods. The Bioglass® powder sinters in two major steps: the first one in the temperature range 500–600 °C and the second one between 850 and 1100 °C. After sintering at 1050 °C for 140 min, considered to be the optimal isothermal sintering conditions, the main crystalline phase is Na2Ca2Si3O9 but other crystalline phases are possible under different heat treatment conditions, which are
Acknowledgements
The authors acknowledge experimental assistance of Ms Tania Hoffer (RWTH Aachen) with the heating microscopy investigation and financial support from the EU via the Marie Curie fellowship scheme (Grant MEIF-CT-2005-024248).
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Current address: Materials Science and Chemical Engineering Department, Politecnico di Torino, Italy.