A cryoprotectant is a substance used to protect biological tissue from freezing damage (i.e. that due to ice formation). Arctic and Antarctic , fish and create cryoprotectants (antifreeze and antifreeze proteins) in their bodies to minimize freezing damage during cold winter periods. Cryoprotectants are also used to preserve living materials in the study of biology and to preserve food products.
For years, glycerol has been used in cryobiology as a cryoprotectant for blood cells and bull sperm, allowing storage in liquid nitrogen at temperatures around −196 °C. However, glycerol cannot be used to protect whole organs from damage. Instead, many biotechnology companies are researching the development of other cryoprotectants more suitable for such uses. A successful discovery may eventually make possible the bulk cryogenics storage (or "banking") of organ transplant human and xenobiotic organs. A substantial step in that direction has already occurred. Twenty-First Century Medicine has vitrified a rabbit kidney to −135 °C with their proprietary vitrification cocktail. Upon rewarming, the kidney was successfully transplanted into a rabbit, with complete functionality and viability, able to sustain the rabbit indefinitely as the sole functioning kidney.
Mechanism
Cryoprotectants operate by increasing the solute concentration in cells. However, in order to be biologically viable they must easily penetrate and must not be toxic to cells.
Glass transition temperature
Some cryoprotectants function by lowering the glass transition temperature of a solution or of a material. In this way, the cryoprotectant prevents actual freezing, and the solution maintains some flexibility in a glassy phase. Many cryoprotectants also function by forming
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 utilized in
cryptobiosis.
Toxicity
Mixtures of cryoprotectants have less
toxicity and are more effective than single-agent cryoprotectants.
A mixture of
formamide with DMSO (dimethyl sulfoxide), 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 crystal ice formation). Vitrification has important applications in preserving embryos,
biology tissues and organs for
Organ transplant. Vitrification is also used in
cryonics, in an effort to eliminate freezing damage.
Conventional
Conventional cryoprotectants are glycols (alcohols containing at least two
hydroxide groups), such as
ethylene glycol ,
propylene glycol and
glycerol.
Ethylene glycol is commonly used as automobile
antifreeze; while
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
cryobiology to reduce ice formation in
spermatozoon,
,
and
that are cold-preserved in
liquid nitrogen. Cryoconservation of animal genetic resources is a practice that involves conventional cryoprotectants to store genetic material with the intention of future revival.
Trehalose is non-reducing sugar produced by yeasts and insects in copious amounts. Its use as a cryoprotectant in commercial systems has been patented widely.
Examples in nature
Arctic fish use antifreeze proteins, sometimes appended with sugars, as cryoprotectants.
Insects
Insects most often use
or
as cryoprotectants. One species that uses cryoprotectant is
Polistes exclamans (a wasp). In this species, the different levels of cryoprotectant can be used to distinguish between morphologies.
Amphibians
Cold-adapted arctic
, such as
, and some other
in polar and subpolar regions naturally produce
glucose,
but southern brown tree frogs and Arctic
create
glycerol in their
to reduce ice formation.
When glucose is used as a cryoprotectant by arctic frogs, massive amounts of glucose are released at low temperature and a special form of insulin allows for this extra glucose to enter the cells. When the frog rewarms during spring, the extra glucose must be rapidly eliminated, but stored.
Food preservation
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 sucrose and
solution in water.
Common
See also
-
Antifreeze protein
-
Cryoconservation of animal genetic resources
-
Cryoconservation of plant genetic resources
-
Cryopreservation
-
Cryostasis (clathrate hydrates)
-
List of emerging technologies
-
Freeze drying
7. Urmatskikh A.V. "Method of cryopreservation of cells, organs, tissues and organisms." RU 2804972 C2, 04.05.2022.
