Hierachical Study on Heat Transfer and Viability of Cell during Freezing of Biological Tissue

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Freezing can slow down or stop some biological reactions for preserving the biological tissue, and thawing can bring it back to life. It is also true that freezing and thawing are lethal to the living system. During freezing, the extra- and intra-cellular ice formation, osmotic water permeation through cell membrane, deformation of cell and other behaviors occur at microscale and they bring serious injuries connecting to the life-and-death of living cells. These micro behaviors occurred on at least cell size level seem to be due to the physicochemical complexity of biological materials. Water is chemically restrained in the states of aqueous solution and colloidal solution, and those are physically confined in small spaces or compartmentalized by the cross-linked network structures.In this research project, firstly, simplified freezing model of cell element was proposed, and the extra- and intra-cellular ice formation, osmotic water permeation through cell membrane and other microscale behavior during the freezing of biological cell were discussed as a function of temperature.Secondly, viability of the cell was studied in relation with the micro-behaviors during freezing and thawing. In experiments, microscopic observation of wheat protoplasts was performed, and the cell viability was inspected by the fluorescence test. The osmotic conditions encountered during freeze-thaw process were also realized by osmotic manipulations. Summarizing these results, the cell damage during freezing process was estimated taking account of "solution effect injury" and "intracellular ice injury".Thirdly, the freezing process of biological tissue was numerically simulated using heat transfer and thermodynamic model. On the basis of these results, viability of the cell during freezing of biological tissue was discussed in conjunction with the heat transfer process.