A cryoprotectant is a substance that is used to protect biological tissue from freezing damage (damage due to ice formation). Arctic and Antarctic insects, fish, amphibians and reptiles create cryoprotectants in their bodies to minimize freezing damage during cold winter periods. Insects most often use sugars as cryoprotectants. Arctic frogs use glucose, but Arctic salamanders create glycerol in their livers for use as cryoprotectant.
Conventional cryoprotectants are glycols (alcohols containing at least two hydroxyl groups), such as ethylene glycol, propylene glycol and glycerol. Ethylene glycol is commonly used as automobile antifreeze and propylene glycol has been used to reduce ice formation in ice cream. Dimethyl sulfoxide (DMSO) is also regarded as a conventional cryoprotectant. Glycerol and DMSO have been used for decades by cryobiologists to reduce ice formation in sperm and embryos that are cold-preserved in liquid nitrogen.
Mixtures of cryoprotectants have less toxicity and are more effective than single-agent cryoprotectants. A mixture of formamide with DMSO, propylene glycol and a colloid was for many years the most effective of all artificially created cryoprotectants. Cryoprotectant mixtures have been used for vitrification, i.e. solidification without any crystal ice formation. Vitrification has important application in preserving embryos, biological tissues and organs for transplant. Vitrification is also used in cryonics in an effort to eliminate freezing damage.
Some cryoprotectants function by lowering a solution's or a material's Glass transition temperature. In this way, the cryprotectants prevent actual freezing, and the solution maintains some flexibility in a glassy phase. Many cryoprotectants also function by forming hydrogen bonds with biological molecules as water molecules are displaced. Hydrogen bonding in aqueous solutions is important for proper protein and DNA function. Thus, as the cryoprotectant replaces the water molecules, the biological material retains its native physiological structure (and function), although they are no longer immersed in an aqueous environment. This preservation strategy is most often observed in anhydrobiosis.
Cryoprotectants are also used to preserve foods. These compounds are typically sugars that are inexpensive and do not pose any toxicity concerns. For example, many (raw) frozen chicken products contain a "solution" comprised of water, sucrose, and sodium phosphates.