Dewars and Cryopreservation
Liquid nitrogen is a unique substance that can be both harmful and helpful to mankind. As a liquid that exists at a very low temperature of -196˚C, liquid nitrogen has the ability to freeze human flesh rapidly. Despite the dangers of frostbite, over-pressurization, and asphyxiation, liquid nitrogen can be used for various other needs such as medical procedures, food preparation, and cryogenics. Liquid nitrogen can be stored in a dewar, a non-pressurized vacuum container that can maintain a consistent temperature allowing the nitrogen to remain in liquid form. Dewars range in capacities from 3 to 300 liters.
When liquid nitrogen is placed within a suitable dewar, the process of cryopreservation is possible. Cryopreservation allows for the preservation of biological constructs at very low temperatures used to form cryogenic liquids. At such low temperatures, any chemical activity that might cause damage to delicate biologicals is halted. Reaching low temperatures without the formation of ice halts any additional damage to the living organism, allowing the organism to live forever and be used as needed.
What Can Be Stored in a Dewar?
By inhibiting all biochemical activity, cryopreservation allows for the storage of long-term sensitive biological materials. Cell lines, cell cultures, tissues, and other biological samples can be stored in liquid nitrogen dewars to ensure that their lives are extended forever. The pharmaceutical industry, researchers, and numerous laboratories that are concerned with biological sample preservation regularly resort to the use of dewars to secure the vitality of their organisms.
For instance, stem cells, which are living organisms that cannot be replaced, may be kept alive at the lowest consistent storage temperature because of the dewar. The cells are place in cryovials in order to prevent direct contact with liquid nitrogen and can be later used to enhance medical procedures and research.
What are the Guidelines for Dewar Storage?
Numerous guidelines exist for the handling of liquid nitrogen itself; however there are no existing guidelines regarding liquid nitrogen dewar storage practices. The overall goal to ensure the vitality of organisms being stored is simply to maintain the correct temperature within the dewar. There are two important elements to maintaining the correct temperature in dewars: use of a calibrated temperature monitoring system that can accurately record temperatures to -196C and ensuring that liquid nitrogen is always present in the dewar.
The Significance of -196˚C
There is no fixed point on the temperature scale that indicates when cryogenics takes place. Scientists assume that a gas is cryogenic if liquefication begins at or below -150˚C, whereas the U.S. National Institute of Standards and Technology (NIST) considers gases to be cryogenic if liquefication occurs below -180˚C. Since most boiling points of permanent gases lie below -180˚C, this point on the temperature scale is a sensible estimation.
At atmospheric pressure, the boiling point specifically for liquid nitrogen is -196˚C. If this temperature is maintained, then the so-called permanent gas, nitrogen, will remain a liquid making goals of cryopreservation possible. This should be handled with care as the liquid to gas expansion ratio of nitrogen is 1:694, thus liquid nitrogen can rapidly boil and turn into a cloud of nitrogen gas or explode if exposed to temperatures higher than -196˚C.
How Often to Refill the Dewar
With the rapid phase transition from liquid to gas, the length of time in which the liquid nitrogen runs out varies. The dewar holding the liquid nitrogen will maintain its temperature as long as liquid nitrogen is present within it. Different levels of quality among dewars cause some to hold liquid nitrogen longer than others. A standard quality dewar should hold liquid nitrogen for a good amount of time, but liquid nitrogen is constantly evaporating and will continuously be lost.
Most manufacturers now publish the daily evaporation and static holding time values on the product pages and instruction manuals. Static holding time, the amount of time it takes for all of the liquid nitrogen to evaporate out of its containers, depends on every dewar’s specific capacity, evaporation rate, and mechanical design. A standard time cannot exist to refill liquid nitrogen for all dewars. There is a large range for static holding times as dewar specifications can differ greatly. The times can be found easily and should be closely followed so as to ensure the safety and vitality of the stored biologicals within the dewar.
A Parting Reminder
Since the security, damage, and spoilage of cells with a limited life span are major concerns, all liquid nitrogen dewars require an accurate and reliable temperature monitoring device. Our liquid nitrogen monitoring devices are a guaranteed way to ensure that your cryo-needs are well managed.