Tailoring of Hierarchical Porous Freeze Foam Structures

Abstract

Freeze foaming is a method to manufacture cellular ceramic scaffolds with a hierarchical porous structure. These so-called freeze foams are predestined for the use as bone replacement material because of their internal bone-like structure and biocompatibility. On the one hand, they consist of macrostructural foam cells which are formed by the expansion of gas inside the starting suspension. On the other hand, a porous microstructure inside the foam struts is formed during freezing and subsequent freeze drying of the foamed suspension. The aim of this work is to investigate for the first time the formation of macrostructure and microstructure separately depending on the composition of the suspension and the pressure reduction rate, by means of appropriate characterization methods for the different pore size ranges. Moreover, the foaming behavior itself was characterized by in-situ radiographical and computed tomography (CT) evaluation. As a result, it could be shown that it is possible to tune the macro- and microstructure separately with porosities of 49–74% related to the foam cells and 10–37% inside the struts.

Investigation of Targeted Process Control for Adjusting the Macrostructure of Freeze Foams Using In Situ Computed Tomography

“…The DoE was evaluated using 2D (radiographic imaging) and 3D (CT) X-ray scans. During earlier research activities, an in situ freeze foaming examination device (Figure 2) that can be placed inside a V|TOME|X L450 commercial tomography system and that allows for the freeze foaming process to be investigated during the process was developed [15,19]. In the first step, radiographic images of the suspension during foam formation are recorded and examined utilizing the X-ray radiographic rasterization method [19].…”

“…The resulting CT scans have cubic voxels with an edge length of 28.18 µm. The complete test set-up and suspension preparation is described in detail in [15].…”

“…The model suspension developed in [15] is the basis for the research in this contribution. For this model suspension, hydroxyapatite (HAp) (Merck KGaA, Darmstadt, Germany; BET = 70 m 2 /g, d50 = 2.64 m) was chosen as a bioceramic powder.…”

“…After mixing was completed, a part of the suspensions was degassed for 3 min inside the vacuum speed mixer at 1000 rpm and 300 mbar according to the conducted DoE. Details regarding the development of the model suspension are stated in [15].…”

“…In previous work [15], the authors investigated the influence of the suspension composition on macro-and microstructure development, through which both pore hierarchies could be adjusted individually. However, that procedure also implies complex suspension preparation and testing.…”