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Cryopreservation is the preservation of  cells and tissues by cooling to very low temperatures (less than - 190 Deg C). At very low temperature, all biological activity which could cause decay of  the cell structure stops, so the cells can be revived after a long time without much damage.

During cryopreservation, the cells and the tissues may be damaged by the formation of  ice within the cells and on the surface of  the cells. Some tissues may tolerate damage caused by the external ice formation, but internal ice formation will cause the death of  the cell. To prevent damage to cells cryoprotectants are used.

Cryopreservation is used for preserving human organs like the liver and heart for transplant and blood. Sometimes, the human body is preserved, so that it may be revived at a later date, when technology is more advanced.

Freeze substitution is the process by which tissues are rapidly frozen below 0 deg C and then immersed in a low temperature solution (substituting fluid) for a period of  a week or more to dissolve the ice gradually. Care is taken to ensure that the cell structure of the tissue remains undamaged.

The substituting fluid usually contains both a fixing agent and a solvent for ice, so that the tissue is preserved without any damage to the cells. After all the ice has been dissolved, the excess fluid is washed out. The frozen tissue can then be used for further examination.