Cryogenics is a field of science that focuses on the production and behavior of materials at extremely low temperatures. The goal of this discipline is to achieve and maintain cold states that fundamentally alter the properties of matter. For biological preservation, this extreme cold halts all processes of decay and biological degradation. This ability to place living material into a state of suspended animation makes cryogenic freezing a powerful tool in medicine and research.
Defining the Cryogenic Range
The cryogenic temperature range begins below -150°C (-238°F). This threshold is a practical demarcation because it is colder than the boiling points of common refrigerants used in conventional cooling. To reach and maintain temperatures this low, specialized cooling agents called cryogens are necessary. Cryogens are liquefied gases, such as nitrogen and helium, which possess extremely low boiling points. Working within this range allows scientists to observe unique physical phenomena, like superconductivity, and achieve the biological stability required for long-term preservation.
The Target Temperature: Liquid Nitrogen
The standard temperature for long-term preservation, known as cryopreservation, is -196°C (-320.8°F). This is the boiling point of liquid nitrogen, which is the most common and cost-effective cryogen used in biological storage. Samples are typically stored submerged in liquid nitrogen or within its vapor phase to maintain this stable, ultra-cold environment. This temperature is well below the glass transition temperature of water, which is around -130°C. Below this transition point, water molecules in a cell enter a solid, glass-like state, preventing the movement and chemical reactions that cause cellular damage over time. At -196°C, all metabolic and enzymatic activity effectively ceases, ensuring the viability of the stored cells or tissues.
The Cooling Process: Vitrification
Achieving the required ultra-low temperatures without destroying biological material relies on a technique called vitrification. When water freezes slowly, it forms sharp, destructive ice crystals that can puncture cell membranes. Vitrification bypasses this damage by converting cellular water into a non-crystalline, glass-like solid. This solidification is accomplished using high concentrations of cryoprotective agents (CPAs) and extremely rapid cooling rates. CPAs are specialized chemicals, such as dimethyl sulfoxide (DMSO) or ethylene glycol, that penetrate cells and replace a significant portion of the intracellular water. By lowering the freezing point and increasing the viscosity of the remaining solution, the CPAs enable the sample to solidify into a glass without the formation of damaging ice crystals.
Practical Applications of Extreme Cold
The ability to achieve and maintain the intense cold of -196°C has broad applications across medicine and biotechnology. In reproductive medicine, cryopreservation is routinely used to store embryos, eggs, and sperm for in vitro fertilization (IVF) procedures. Tissue banks rely on the same technology for the long-term storage of stem cells, blood components, and other biological specimens. The field of cryosurgery utilizes localized, extreme cold, often supplied by liquid nitrogen, to destroy unhealthy tissue, such as skin lesions or certain tumors. Beyond established medical uses, the theoretical goal of preserving entire human bodies for future revival, known as cryonics, also depends on maintaining the biological stasis afforded by storage at the liquid nitrogen temperature.