Deriving Principles of the Freeze-Foaming Process by Nondestructive CT Macrostructure Analyses on Hydroxyapatite Foams

Abstract

Freeze Foaming is a direct foaming method that aims at manufacturing ceramic cellular scaffolds for diverse applications. Next to porous structures for a potential use as refractories, the focus lies on potential bone replacement material. The main challenge of this foaming method is to achieve a homogeneous and predictable pore morphology. That is why, in a current project, the authors report on the pore morphology formation and evolution of the foaming process by means of nondestructive testing. This contribution primarily compares the effect of the suspension’s temperature on the resulting foam structure (foaming at 5 and 40 °C). As a basis for computed tomographic analysis, a stable and reproducible model suspension was developed that resulted in reproducible foam structures. Characterized by viscosity, foam structure analyses and foaming rate, the resulting Freeze Foams became adjustable with regards to their porosity and pore shape/size. Under certain conditions, we succeeded in achieving a relatively homogeneous pore structure, as proven by computed tomography-derived quantitative analysis.

Hierarchical‐Porous Ceramic Foams by a Combination of Replica and Freeze Technique

“…Ahlhelm et al uses the Freeze Foaming technique for the manufacturing of ceramic foams for possible application in the biomedical technology. They focused on the controlled regulation of the pore morphology by controlling the preparation and process approach.…”

Investigation of the Foam Development Stages by Non-Destructive Testing Technology Using the Freeze Foaming Process

“…To evaluate and verify the findings from the radiographical evaluation, the results were compared to data obtained from the freeze dryer at IKTS (Table 1) [24]. Apart from 5 °C suspensions, their findings support the trend identified at the ILK.…”

“…Due to their high viscosity, this growth is inhibited in 5 °C tempered suspensions. Furthermore, the maximum foaming rate takes place at lower pressures (40–60 mbar) [24]. As a large amount of water evaporates, the pressure reduction rate drops, and the time to reach the target pressure of <5 mbar is too long.…”

“…The fracture behavior clearly exhibits a dependency on suspension temperature (5, 23, and 40 °C) and pretreatment (devolatilized and not devolatilized suspensions during the manufacturing process). Samples with a narrow pore size distribution [24] (5 °C and 23 °C devolatilized [23]) possess a pronounced maximum of force. On the other hand, specimens with a less uniform distribution of pores [24] (40 °C and 23 °C not devolatilized) show a more constant force level, and only reach about half the maximum force when compared to more homogeneous samples.…”

“…The choice of suspension and composition was derived from preliminary tests on the basis of different suspensions which, after Freeze Foaming, resulted in reproducible foam structures [23]. The detailed manufacturing process of the suspension is described in [24]. For the investigations of this contribution and with regard to its possible influence on the foaming process and structure formation, three ceramic suspensions with different temperatures were used (5, 23, and 40 °C).…”