Cryogenics is the study and use of materials at extremely low temperatures. Such low temperatures cause changes in the physical properties of materials that allow them to be used in unusual engineering, industrial, and medical applications. For example, in the cryogenic temperature range, air becomes a liquid—or even a solid—and living tissue freezes instantly. Matter behaves strangely at the lowest temperatures of the cryogenic range. Electric currents never stop flowing, liquids run uphill, and rubber becomes as brittle as glass. In medicine, cryogenic cooling is used in some diagnostic techniques, such as magnetic resonance imaging (MRI). Cryosurgery uses liquid nitrogen to kill unhealthy tissue by freezing it. Cryogenics is expected to play an important role in the development of better procedures for preserving human organs for transplant.
Certain vaccines must be stored at cryogenic temperatures. For example, the Pfizer–BioNTech COVID-19 vaccine must be stored at temperatures of −90 to −60 °C (−130 to −76 °F).