Visualising the evolution of crystallisation and mineralisation of bioactive glasses

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Bioactive glasses are known to be excellent materials for bone regeneration and tissue engineering. Moreover, certain ions, like copper or silver ions, can be incorporated into the bioactive glass composition, providing additional therapeutic benefits, like stimulation of angiogenesis (blood vessel formation) or antibacterial effects. Despite their excellent biocompatibility and bioactivity, there are still several issues, which are not fully understood and limit bioactive glass clinical application.One key issue is their crystallisation behaviour. Generally, glassy materials can easily be shaped by sintering, to form coatings or complex 3D structures such as porous scaffolds. One special drawback of typical bioactive glasses, however, is their rapid crystallisation at elevated temperatures. This is known to inhibit dense sintering of the most commonly used composition, 45S5, and results in its scaffolds possessing poor mechanical properties. In order to improve sintering of these glasses, the most important questions to be answered are: How do crystallisation and the type of crystal phases forming affect sintering? How does this vary with composition and glass structure? And what is the influence of crystallisation on in vitro bioactivity and cell compatibility? It is known that the crystallisation of bioactive glasses is not per se a negative thing. However, we need a deeper understanding of the processes taking place in order to control crystallisation and obtain bioactive glass systems, which allow for densification during sintering, and create mechanically more stable scaffolds which are highly bioactive. Thus, the main aim of this project is to investigate the crystallisation processes in bioactive glasses and glass-ceramics, synthesised by different routes (melt-quench vs. sol-gel) and of varying composition. To achieve this, the novelty of this research lies in the use of X-ray microscopy (XRM) in addition to conventional characterisation techniques like electron microscopy (SEM and TEM) und X-ray diffraction. XRM offers important advantages in comparison, mainly with regard to non-destructive sample preparation and the possibility to get a 3D bulk visualisation of the sample with nanometre resolution. This allows us to observe changes during crystallisation on the same specimen at different stages of heat treatment. The evolution of crystallisation processes from the initial nucleation step to the final crystallisation stage will be monitored by the combined techniques mentioned above. XRM results will hereby provide new insight into the 3D structure of these materials and will help to clarify open questions related to crystallisation, sintering and high-temperature processing as well as their influence on in vitro bioactivity.