Cryogenics, or ultracold freezing, is not a new idea, and with a couple of exceptions, the equipment used is similar to that of 25 years ago. Liquid nitrogen, or occasionally liquid helium, is retained in a cryovessel or dewar. Plastic tubes containing material to be preserved are chilled either by immersion in the liquid or by being kept in the gas or vapor phase, which results from the boiling of the liquid. There are a variety of devices for holding and retrieving these samples, and the form of these devices determines how big the opening in the dewar is and therefore, how long the dewar will last.
The size of the dewar not only determines how many samples can be accom-modated but also the type of material from which the dewar will be constructed. Smaller, laboratory-scale dewars are manufactured from aluminum, whereas lar-ger vessels or freezers are usually made of stainless steel. In either case, dewar construction is very similar to a vacuum thermos bottle.
An inner vessel is wrapped with insulating tape, which is used to prevent con-ductive loss. To this end, aluminum foil insulating tape is never allowed to come into contact with another part of the tape, which is also aluminum. This contact would cause conductive loss or a cryo leak. Instead, aluminum foil only comes into contact with paper.
The only points of contact between the inner vessel and outer vessel are the neck and a small pin at the bottom. The pin, which is often constructed from fiberglas, is hollow and prevents the inner vessel from swinging within the outer skin. The neck, then, is the major point of contact between the two layers. Also constructed of fiberglas, the neck bears the entire weight of the liquid nitrogen, samples, and sample organizers. It is the Achilles heel of all cryovessels. The larger the neck, the larger the Achilles heel.
So why are all necks not made as small as possible? Convenience. Larger sample organizers make keeping track of samples easier. Modern cryovessels, then, are a compromise between convenience and dewar lifetime.
The space between inner and outer vessels is evacuated to less than 10 mtorr. But the exact amount that is below 10 mtorr varies among manufacturers and determines the performance of the dewar. This performance is reflected in the static holding time of the vessel or how long it takes for a filled tank to empty with the cap left in place. Vacuum performance is determined by how long a manufacturer is willing to leave the vessel attached to a vacuum pump. This is critical because it limits the number of cryovessels a manufacturer can make in any given time. Pumping times of 2 days are normal for laboratory-scale vessels.
Cryogenic vessels are as prone to breakage as were the vacuum bottles many of us carried to school in earlier days. Despite the availability of wheeled accessories, cryovessels last longer when allowed to remain in place. The acceleration of liquid nitrogen and samples by movement of the dewar creates torque on the fragile neck, which eventually allows the passage of atmospheric gases or liquid nitrogen (if the dewar is overfilled) into the space between the inner and outer vessel. When the vacuum is compromised, the performance rapidly deteriorates and labile samples are at risk. This eventuality is the reason for the popularity of level monitors that measure the level of liquid nitrogen inside the dewar without the need to remove the cap. These devices work on a variety of principles, including thermocouples and sonar.
There is still no substitute for the measurement of liquid level by the insertion of a nonconductive rod or ruler. Liquid level is measured from the frostline, which forms after the probe is removed from the vessel.
Whether samples are kept in a liquid or gas phase, the goal is to keep biochemicals active or cells viable. Recently, the science of how freezing affects viability or stability has received some attention. Recipes for freezing cells in varying concentrations of glycerol or dimethyl sulfoxide have appeared, and the rate at which freezing occurs has been addressed by a number of companies that markets a controlled-rate freezing apparatus.
