Investigation of the structure formation of Freeze Foams on the example of biocompatible ceramics

The objective of the presented work is a detailed investigation of the so-called Freeze Foaming process, which is a relatively new shaping method to produce cellular, open porous and interconnected components, e.g. ceramics for potential bone substitutes. Freeze Foaming is based on the ambient pressure reduction of an aqueous suspension in a freeze dryer. Due to the decreasing pressure, the suspension inflates and forms a proto foam until it crosses the triple point at which it consolidates by instantaneous freezing, followed by sublimation to a dry ceramic foam. The aim of a recent project is to identify process-influencing factors and their effects on the pore morphology of Freeze Foams to derive the very principles of the foaming process. For evaluating individual effects on the foam structure, a stable and reproducible model suspension was developed using hydroxyapatite as ceramic material. Characterization was carried out with respect to solid content and viscosity. This model suspension enables a stable foaming behavior resulting in a reproducible porosity of shaped Freeze Foams. In a first series of experiments to evaluate the effect of particular pore forming mechanisms the air content of the suspension and the pressure reduction rate of the freeze dryer were varied. Process monitoring by vacuum pressure and temperature recording inside the foams allowed clearly distinguishing between two pore forming factors: air and water vapor. Micro computed tomography as a nondestructive testing method and mercury porosimetry were carried out to investigate porosity, pore size and shape. Scanning electron microscopy was used to analyze the microstructure of shaped foams. As result, not only rising dissolved and entrapped air as well as vapor were individually identified as pore formers, but also ice crystals, which seem to be crucial for the development of a Freeze Foam's microstructure. Due to sublimation during freeze-drying, microporous struts are formed, which (inter)connect the foam cells. The morphology of these micropores was found to strongly depend on the air content of the suspension. Whereas foams made of a degassed suspension showed dendritic micropores usually obtained by freeze casting of ceramics, non-degassed suspensions resulted into a more globular pore morphology. Thus, a targeted adjustment of these properties would allow directly influencing the biocompatibility and mechanical stability of potential ceramic bone substitutes.