EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The unique combination of foaming gas and freezing ice during the Freeze Foaming (developed by Fraunhofer Institute for Ceramic Technologies and Systems) of biocompatible materials results in a hierarchical porous structure consisting of macro-structural foam cells and micro-structural porous struts. Porous calcium phosphate ceramics are highly suitable as bone substitute materials due to their proven biocompatibility. In a successfully completed DFG-project main process influencing parameters for controlled foaming of hydroxyapatite ceramics were identified.
In the follow-up project it is of great interest to control both pore hierarchies, the foam cells, and the strut pores, with the aim of developing tailored foams for biomedical applications. Following this aim, a model-suspension with stable foaming behavior was chosen to investigate the influence of most important process parameters: the water content of the suspension, temperature, air content and the pressure reduction rate during foaming. For this purpose, a full factorial DOE (Design of Experiments) was established. Special molds made of flexible thermoplastic polyurethane were developed by 3D-printing, because of a greater design freedom in comparison to rubber molds. Also, they had to show a low absorption of X-ray radiation.
Foam cells were examined by computer tomography analysis and strut pores with mercury Porosimetry and electron microscopy. An in-situ CT device (developed at Institute of Lightweight Engineering and Polymer Technology of Technical University of Dresden) allows material phenomenological investigation and foam structure analysis during the foaming process.
Deep insights in the mechanisms during foam growth, but also during freezing of the foam could be gained. A discrete investigation of macro- and microstructure development was possible for the first time. With the help of the gained knowledge, it will be possible to manufacture tailored Freeze Foams for specific biomedical applications, but also others like catalyst support or thermal insulators if different materials are used.