 more than about 65% of the total body water freezes. Research exploring the phenomenon of "freezing frogs" has been performed primarily by the Canadian researcher, Dr. Kenneth B. Storey.
Freeze tolerance, in which organisms survive the winter by freezing solid and ceasing life functions, is known in a few vertebrates: five species of frogs (Rana sylvatica, Pseudacris triseriata, Hyla crucifer, Hyla versicolor, Hyla chrysoscelis), one of salamanders (Hynobius keyserlingi), one of snakes (Thamnophis sirtalis) and three of turtles (Chrysemys picta, Terrapene carolina, Terrapene ornata). Snapping turtles Chelydra serpentina and wall lizards Podarcis muralis also survive nominal freezing but it has not been established to be adaptive for overwintering. In the case of Rana sylvatica one cryopreservant is ordinary glucose, which increases in concentration by approximately 19 mmol/l when the frogs are cooled slowly.
One of the most important early theoreticians of cryopreservation was James Lovelock. In 1953, he suggested that damage to red blood cells during freezing was due to osmotic stress, and that increasing the salt concentration in a dehydrating cell might damage it. In the mid-1950s, he experimented with the cryopreservation of rodents, determining that hamsters could be frozen with 60% of the water in the brain crystallized into ice with no adverse effects; other organs were shown to be susceptible to damage. This work led other scientists to attempt the short-term freezing of rats by 1955, which were fully active 4 to 7 days after being revived.
Cryopreservation was applied to humans beginning in 1954 with three pregnancies resulting from the insemination of previously frozen sperm. Fowl sperm was cryopreserved in 1957 by a team of scientists in the UK directed by Christopher Polge. During 1963, Peter Mazur, at Oak Ridge National Laboratory in the U.S., demonstrated that lethal intracellular freezing could be avoided if cooling was slow enough to permit sufficient water to leave the cell during progressive freezing of the extracellular fluid. That rate differs between cells of differing size and water permeability: a typical cooling rate around 1 °C/minute is appropriate for many mammalian cells after treatment with cryoprotectants such as glycerol or dimethyl sulphoxide, but the rate is not a universal optimum.
The first human body to be frozen with the hope of future revival was James Bedford's, a few hours after his cancer-caused death in 1967. Bedford is the only cryonics patient frozen before 1974 still preserved today.
Storage at very low temperatures is presumed to provide an indefinite longevity to cells, although the actual effective life is rather difficult to prove. Researchers experimenting with dried seeds found that there was noticeable variability of deterioration when samples were kept at different temperatures – even ultra-cold temperatures. Temperatures less than the glass transition point (Tg) of polyol's water solutions, around −136 °C (137 K; −213 °F), seem to be accepted as the range where biological activity very substantially slows, and −196 °C (77 K; −321 °F), the boiling point of liquid nitrogen, is the preferred temperature for storing important specimens. While refrigerators, freezers and extra-cold freezers are used for many items, generally the ultra-cold of liquid nitrogen is required for successful preservation of the more complex biological structures to virtually stop all biological activity.
Phenomena which can cause damage to cells during cryopreservation mainly occur during the freezing stage, and include: solution effects, extracellular ice formation, dehydration and intracellular ice formation. Many of these effects can be reduced by cryoprotectants. Once the preserved material has become frozen, it is relatively safe from further damage. However, estimates based on the accumulation of radiation-induced DNA damage during cryonic storage have suggested a maximum storage period of 1000 years.
- Solution effects
- As ice crystals grow in freezing water, solutes are excluded, causin