Development of a Freeze-Drying Stage for In-Situ µ-CT Measurements

Mathias Hilmer,Petra Foerst,Sebastian Gruber

doi:10.3390/pr8070869

Abstract

This paper shows the development of a freeze-drying stage for in-situ μ-CT measurements. The stage can operate in a temperature range of −40 °C up to 70 °C, and a pressure range from atmospheric pressure to 7 Pa at the sample holder. To get the best visualization of the probe, it is fundamental that the materials around the sample holder are not absorbing most of the radiation. For this reason, we built an axial symmetrical stage built out of polyetheretherketon (PEEK). A test of the stage by different freeze-drying experiments with maltodextrin and sucrose particles and solutions demonstrated its suitability to visualize the freeze-drying processes in-situ. It was possible to track the drying front during the process by radiographic and tomographic measurements, as well as to visually resolve the ice crystals and porous structure in tomographic measurements. Using different samples and process parameters, we showed that the freeze-drying stage is not only suitable for in-situ µ-CT measurements, but also allows us to use the stage for other imaging methods such as neutron imaging, and for any sample where a controlled environment is needed.

Highlights

Freeze-drying is a commonly used method for the preservation of pharmaceutical, biotechnological and food materials
Freeze-drying is considered a process that retains most of the properties of the material, such as structure, color and valuable substances like vitamins [1]
The same group visualized the structure development during freeze-drying by using μ-CT measurement [5]. They used freeze-drying, microwave-drying and serial combinations of these two drying methods to show the impact of the drying process on the structural changes during the drying of carrot discs

Summary

Introduction

Freeze-drying is a commonly used method for the preservation of pharmaceutical, biotechnological and food materials. As microstructural changes are a local phenomenon, which usually take place in the vicinity of the sublimation front, it is important to understand the structural formation during freeze-drying. A valuable tool to observe microstructural changes during a drying process is μ-CT measurement. The same group visualized the structure development during freeze-drying by using μ-CT measurement [5] They used freeze-drying, microwave-drying and serial combinations of these two drying methods to show the impact of the drying process on the structural changes during the drying of carrot discs.

Methods

Results

Conclusion