It is a well-known fact that biological material like cells, blood and tissue samples, changes and deteriorates with time. Cryogenic preservation, which involves storage at very low temperatures, is the most effective way of preserving biological materials.
There is a critical temperature for most biological materials. This temperature, known as the glass transition temperature (Tg), is approx. -130°C. When storing biological material at a minimum of 10 to 30 degrees below Tg, i.e. -150-160°C, the samples can be stored almost indefinitely without almost no decomposition.
The refrigeration and freezing of biological materials is a complex process. Biological materials can be preserved in many ways, but low-temperature storage is the only method of preservation that minimises changes to the material. Cryopreservation has been used for decades to ensure the survival of living cells and organisms.
The critical temperature for the long-term survival of biological materials is, in most cases, a temperature that is significantly lower than the nominal fusion temperature. In other words, keeping the sample frozen (in a freezer at -80°C, for example) is simply not sufficient.
Biological samples should be stored at a temperature that is sufficiently below the critical temperature. This is why cells are preserved in special freezers that are cooled using liquid nitrogen. Liquid nitrogen has a temperature of -196°C, and the freezer temperature is therefore -150 -196°C, which is an ideal temperature range.
The cells can either be stored submerged in the liquid nitrogen or in the vapour phase.