Cryopreservation of living cells is a necessary part of many medical procedures such as organ and tissue transplants, conservation of reproductive and stem cells, and etc. However, cells and tissues can be damaged by the cryopreservation process itself. There are two competitive conditions of cooling. A slow cooling causes freezing of the extracellular fluid, whereas the intracellular fluid remains unfrozen for a while. This results in an increase in the concentration of salt in the extracellular solution, which leads to the cellular dehydration and shrinkage due to the osmotic flow through the cell membrane. If cooling is rapid, the water inside the cells forms small, irregularly-shaped ice crystals (dendrites) that are relatively unstable. If the cells are subsequently thawed out too slowly, these crystals will aggregate to form larger, more stable crystals which may cause damage. Maximum viability is obtained by cooling at a rate in a transition zone in which the combined effect of both these mechanisms is minimized. There are arguments that better results can be obtained, if the temperature falls not monotonically in time, especially in the range were the latent heat is being released. Thus, one can speak about optimal cooling protocols. A positive effect can be achieved by creating temperature gradients in the freezing area or by forcing ice nucleation through mechanical vibration or some temperature chock localized in a small area (seeding). Another control tool is cryoprotective agents which vary eutectic properties of solutions.
