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.