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Cryogenics is a branch of physics concerned with very low temperatures: how to produce the lowest temperatures possible (below minus 30oC), and what effects these low temperatures have on organisms or materials. The prefix cryo is derived from the Greek word kryos, meaning "cold." The person considered by most to be the originator of modern experimental science, Francis Bacon, died as a result of a spontaneous experiment he was conducting on the effects of low temperatures. In 1623, while traveling on a cold and snowy day, Bacon decided to "experiment" to see whether snow would delay the putrefaction of flesh. He stuffed a fowl with snow to observe the effects. In the process, he caught a sudden chill. Over the years, this turned into acute bronchitis, which contributed to his death in 1626. Although some animals are able to lower their body temperatures during hibernation , most animals, like people, cannot tolerate freezing temperatures within their body tissues. Normally, when an organism is exposed to below-freezing temperatures, ice forms in smaller blood vessels and either bursts the blood vessels or stretches them beyond the point where they can function normally. In addition, ice in the blood vessels "captures" the water content, making it impossible for the blood cells to survive. Other types of cells are also damaged during freezing. Frostbite is a common malady caused by cold temperatures; frozen skin and blood cells are damaged from the dehydration due to freezing. Scientists have discovered, however, that some varieties of frogs and turtles can actually survive being frozen. When these animals sense ice on the outsides of their bodies, their livers produce extra glucose (blood sugar), which floods into their cells to protect the cell from freezing and from damage. This also holds the cell's shape so it doesn't collapse upon itself. Nucleating proteins "guide" water out of the cells, allowing the water to go in between the cells and the organs. This allows the water to freeze, but in small pieces, without "spears" that could puncture the cell membrane. Scientists know of only one mammal, the Arctic ground squirrel, that seems to be able to tolerate ice crystals in its bloodstream during a physiological state that falls somewhere between hibernation and freezing. Scientists are studying the "cryoprotectants" of these animals to see whether they have application for the freezing of human organs for transplants. So far they have been successful in freezing only single cells (e.g., sperm cells) and corneas for transplants.



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