Cryopreservation refers to the storage of a living organism at ultra-low-temperature such that it can be revived and restored to the same living state as before it was stored. Indefinitely long storage times require that the organism be maintained below the glass transition temperature of aqueous solutions, approximately -130ºC, the temperature at which frozen water no longer sublimes and recrystallizes. Therefore -150°C freezers or liquid nitrogen are required for longer storage times.
In order to make liquid out of gasses, scientists in the 19th century discovered insulated bottles for storage and transport. This type of vessel is still being used today. For years cryopreservation in liquid nitrogen at -196°C has been the standard for long term preservation.
During the years it became clear that cross contamination became a real risk for this kind of preservation. Not only through existing ice, but from sample to sample as well. Scientific evidence has been delivered in the past about examples of Hepatitis cross contamination.
That was the reason why storage in vapor phase became more popular. Valuable samples were not stored in the liquid nitrogen anymore, which eliminated the risks of cross contamination.
There are disadvantages of storage in vapor phase though:
- loss of storage capacity; 1/3 of the vessel can not be used any more
- the vapor phase temperature is very much depending on the level of liquid nitrogen
- large vertical temperature gradients, can put valuable samples at greater risk during long term storage.
To protect long term sample storage from cross contamination and the risk of vertical gradients, dry storage systems like -150°C ultra low temperature freezers and the Isothermal concept have been developed.