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Which position within a freezing sample is most representative for measuring the cooling rate?
ABSTRACTS, 24th ANNUAL cycle is - 175°C. Drying is achieved by programmed microprocessor control of the thermocouple heater circuit. Condenser surfaces during sample drying are the sample chamber wall and the cryopump. The mean free path of water molecules within the system is 60 m. Thus, both condenser surfaces are directly available to the water molecules. Residual gas analyses of the vacuum contents during the drying cycle detect water peaks initially at - 128°C. Following drying, the sample chamber walls are dried by allowing slow evaporation of the LN, in the surrounding dewar. Electron microscopic analyses of the prepared samples reveal structural integrity without indications of ice crystal formation in the vitrified surface of tissue. 28. Which Position within a Freezing Sample Is Most Representutive for Measuring the Cooling Rate? U. HARTMANN, M. JOCHEM, CH.
K~RBER, AND G. RAW (Helmholtz-Institut fur Biomedizinische Technik an der RWTH Aathen, D-5100 Aachen, West Germany). It is a common practice to measure the thermal history of a freezing sample by using a temperature sensor which often is placed in the geometrical center of the sample. But many investigators have shown in their experimental and theoretical studies that local cooling rates can vary drastically both in time and in space inside a single freezing volume. Normally the cooling rate at the periphery of a sample is lower than in the center. A computer model was developed to calculate the transient heat transfer and solidificationprocess within a complete freezing container. This model takes into account the influences of the external cooling conditions, the container walls, and the freezing bag as well as the nonplanar solidification of an aqueous solution. The calculations yield a description of the conditions of temperature and concentration within the container and the propagation kinetics of the dendrite tips and the basal plane of the solidification front. Selective parameter variations were carried out to clarify the influence of external cooling conditions and of the container geometry on the nonuniform distribution of cooling rates within different regions of the freezing container. It could be demonstrated that the determination of the cooling rate in the center of the sample is frequently not representative for the cooling conditions within the entire container. The calculations for plate geometries showed that for a single measurement of the cooling rate (which should be as representative as possible for the whole sample) the position x = 213* d/2 (X = space coordinate, originating at the inner surface of the freezing bag; d = sample thickness) seems to be the best place. This result was found to be valid for a wide range of external cooling conditions and container dimensions.
MEETING
551
29. Thermal Design of a Freezing Container for Platelet Preservation by Direct Submersion into Liquid Nitrogen (LN2). U. HARTMANN,
M. JOCHEM, CH. K~RBER, M. W. SCHEIWE' AND G. RAU (Helmholtz-Institut fur Biomedizinische Technik an der RWTH Aachen, D-5100 Aachen, West Germany). The aim of this work was to demonstrate that for some cell types, complicated high-performance and hence very expensive automatic freezing devices are not necessarily required. The exact knowledge of the heat transfer processes during freezing may be used to construct thermally appropriate containers, generating the desired cooling protocols inside the sample by simple submersion into LN2. In a first step it was necessary to get detailed information by experiments about the heat transfer during boiling in LN2. In a second step a computer model was developed to simulate the transient heat transfer and solidification processes within a plate-like freezing container. In a third step the thermal contact resistance impeding the heat transfer between the freezing bag containing the cell suspension and the container wall was quantified. In this combination it was possible to precisely predict the thermal behavior of a complete freezing container during quenching in LN2. For the application of platelet preservation with hydroxyethyl starch (HES) a parameter variation was carried out using this computer program. The aim of these calculations was to find a container which generates automatically the optimal cooling rate for platelets in presence of HES (which lays in the range between 8 and 25 K/min) simply by immersion into LN2. The calculations led to a container design with an internal thermal insulation of the container walls. This insulation reduces the heat flux out of the container and raises the heat capacity of the whole freezing device. So it was possible to create the desired cooling rates inside the whole sample volume. To test the applicability of this new container comparative freezing of platelets was carried out by using both the new container as well as a microprocessor-controlled automatic freezing apparatus. The survival of the cells frozen with the container, and their functional recovery after thawing were found to be equally good, in part slightly better than those obtained with the automatic freezing device. Hence the concept of relatively inexpensive freezing containers for direct submersion into LN2 seems very promising for use in routine work. 30. Video-Microscopy of Freezing in Aqueous tions and Blood Cell Suspensions.
Solu-
CH. K~RBER, S. ENGLICH, AND G. LIPP (Helm-
’ Present Address: Nordmark D-2082 Uetersen, West Germany.
Werke GmbH,
K~RBER, AND G. RAW (Helmholtz-Institut fur Biomedizinische Technik an der RWTH Aathen, D-5100 Aachen, West Germany). It is a common practice to measure the thermal history of a freezing sample by using a temperature sensor which often is placed in the geometrical center of the sample. But many investigators have shown in their experimental and theoretical studies that local cooling rates can vary drastically both in time and in space inside a single freezing volume. Normally the cooling rate at the periphery of a sample is lower than in the center. A computer model was developed to calculate the transient heat transfer and solidificationprocess within a complete freezing container. This model takes into account the influences of the external cooling conditions, the container walls, and the freezing bag as well as the nonplanar solidification of an aqueous solution. The calculations yield a description of the conditions of temperature and concentration within the container and the propagation kinetics of the dendrite tips and the basal plane of the solidification front. Selective parameter variations were carried out to clarify the influence of external cooling conditions and of the container geometry on the nonuniform distribution of cooling rates within different regions of the freezing container. It could be demonstrated that the determination of the cooling rate in the center of the sample is frequently not representative for the cooling conditions within the entire container. The calculations for plate geometries showed that for a single measurement of the cooling rate (which should be as representative as possible for the whole sample) the position x = 213* d/2 (X = space coordinate, originating at the inner surface of the freezing bag; d = sample thickness) seems to be the best place. This result was found to be valid for a wide range of external cooling conditions and container dimensions.
MEETING
551
29. Thermal Design of a Freezing Container for Platelet Preservation by Direct Submersion into Liquid Nitrogen (LN2). U. HARTMANN,
M. JOCHEM, CH. K~RBER, M. W. SCHEIWE' AND G. RAU (Helmholtz-Institut fur Biomedizinische Technik an der RWTH Aachen, D-5100 Aachen, West Germany). The aim of this work was to demonstrate that for some cell types, complicated high-performance and hence very expensive automatic freezing devices are not necessarily required. The exact knowledge of the heat transfer processes during freezing may be used to construct thermally appropriate containers, generating the desired cooling protocols inside the sample by simple submersion into LN2. In a first step it was necessary to get detailed information by experiments about the heat transfer during boiling in LN2. In a second step a computer model was developed to simulate the transient heat transfer and solidification processes within a plate-like freezing container. In a third step the thermal contact resistance impeding the heat transfer between the freezing bag containing the cell suspension and the container wall was quantified. In this combination it was possible to precisely predict the thermal behavior of a complete freezing container during quenching in LN2. For the application of platelet preservation with hydroxyethyl starch (HES) a parameter variation was carried out using this computer program. The aim of these calculations was to find a container which generates automatically the optimal cooling rate for platelets in presence of HES (which lays in the range between 8 and 25 K/min) simply by immersion into LN2. The calculations led to a container design with an internal thermal insulation of the container walls. This insulation reduces the heat flux out of the container and raises the heat capacity of the whole freezing device. So it was possible to create the desired cooling rates inside the whole sample volume. To test the applicability of this new container comparative freezing of platelets was carried out by using both the new container as well as a microprocessor-controlled automatic freezing apparatus. The survival of the cells frozen with the container, and their functional recovery after thawing were found to be equally good, in part slightly better than those obtained with the automatic freezing device. Hence the concept of relatively inexpensive freezing containers for direct submersion into LN2 seems very promising for use in routine work. 30. Video-Microscopy of Freezing in Aqueous tions and Blood Cell Suspensions.
Solu-
CH. K~RBER, S. ENGLICH, AND G. LIPP (Helm-
’ Present Address: Nordmark D-2082 Uetersen, West Germany.
Werke GmbH,