Speaker
Description
Cryopreservation technique of living tissue and cells is used in long term storage and transportation. Human ES / iPS cells could be useful in regenerative medicine. During cryopreservation of human ES cells, there is a problem that the survival rate after freezing and thawing process is low compared with mouse ES cells. The survival rate of these cells after cryopreservation process (cooling and thawing) depends on the cooling rate and the type and concentration of cryoprotectant. There are two options to improve the cell survival rate by the control of the cooling rate. One is in the slow cooling rate under conditions where the rate of occurrence of the intracellular and the extracellular freezing is optimal. Another is in the high cooling rate that causes the vitrification freezing. When the cooling rate is very high, such as the direct immersion in liquid nitrogen, the cells freeze in the vitrification state and the ice crystals don't form. However, in the case of the cooling by the direct immersion in liquid nitrogen, film boiling occurs and the vapor layer covers the surface of cooling object. As the result, the heat transfer characteristics from the cooling object to liquid nitrogen deteriorate.
From this background, the heat transfer characteristics during cooling in the cryopreservation have been clarified in order to further improve the cooling rate in the case of the direct immersion in liquid nitrogen. It has been reported in our previous studies that the cooling rate can be improved with the microstructure in cooling surface and it is confirmed by the temperature measurement at 1 mm from the inner surface of the cooling object that the changes in surface condition can effect on the temperature variation near the inner surface. On the other hand, the mechanism of the heat transfer promotion by the surface microstructure is not clarified though it is considered that the heat transfer promoted by the increase in surface area and the change in the behavior of the vapor layer and bubbles due to the hydrodynamic instability during boiling in the liquid nitrogen.
In the present study, the effect of the surface microstructure on the heat transfer characteristics in the cooling object using cryopreservation is investigated and the variation of the cooling rate with the surface condition is examined. A microstructure imitating the shape of stainless steel mesh (60 mesh/inch) is manufactured on the surface of the cooling object (polypropylene cryo-vial) using a computer numerical control lathe (CNC lathe). Agar is used as a simulated living tissue and is filled inside the cooling object. The temperature sensor is installed near the inner wall in the agar. For the variation of the vapor behavior with the surface condition, the visual observation of the vapor bubbles is conducted by use of the high-speed video camera. It is confirmed that in the case of the cooling object with the surface microstructure, the cooling rate is improved by about 75 % compared with the no-machining condition. The mechanism of the heat transfer promotion through the surface microstructure is examined.
| Submitters Country | Japan |
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