M.Ashok A.Ciby L.Srinath

INTRODUCTION  Cryogenics is the study and use of materials at extremely low temperatures.  Such low temperatures cause changes in the physical properties of materials that allow them to be used in unusual engineering, industrial, and medical applications.  For example, in the cryogenic temperature range, air becomes a liquid—or even a solid—  and living tissue freezes instantly.  Matter behaves strangely at the lowest temperatures  of the cryogenic range.

 Electric currents never stop flowing, liquids run uphill, and rubber becomes as brittle as glass.  In medicine, cryogenic cooling is used in some diagnostic techniques, such as magnetic resonance imaging (MRI).  Cryosurgery uses liquid nitrogen to kill unhealthy tissue by freezing it.  Cryogenics is expected to play an important role in the development of better procedures for preserving human organs for transplant.

The Cooling Process A substance is normally cooled by placing it next to something colder. To make the substance supercold, however, heat must also be removed and the substance must be insulated (encased). An important method of cryogenic supercooling involves liquefying gases and using these gases to cool other substances. One technique is to convert to liquid form a gas that can be liquefied by pressure alone.

Then a gas requiring a lower temperature to become a liquid is placed in a container and immersed (dipped) in the first. The gas that is already liquefied cools the second and converts it to a liquid. After several repetitions of this process, the targeted gas is liquefied. A Dewar flask is normally used to store such very low temperature liquefied gases.

If the disorderly spin of electrons in a substance could be slowed down, then the substance would cool down. In cooling by demagnetization, a strong magnetic force is used to give the outside energy required to line up the molecules of a paramagnetic substance (one made up of paramagnetic ions). This also raises the temperature. At the same time the substance is cooled in liquid helium. When the substance cools down to its starting temperature, the magnetic field is removed.

This causes the ions to resume their disorderly alignment (order). The energy the ions use to move comes from the heat energy of the substance, which causes the temperature of the substance to drop. Liquid nitrogen is one of the safest cooling agents available. In medicine it is used to kill unhealthy tissues by freezing them. Cryogenic processes are also used to supply "banks" storing eye corneas, blood, and sperm for future surgical procedures. Some embryos have also been frozen and stored for later implantation (surgical placement) in women.

Cryosurgery Cryosurgery is relatively bloodless because the low temperatures used constrict the blood vessels, stemming the flow. Special instruments are used that have freezing tips to kill the damaged tissue and shields to protect surrounding tissue. Cooper used cryosurgery to freeze and destroy damaged tissue in the brains of patients with Parkinson's disease (a degenerative illness).

Since then, cryosurgery has found many applications. It is used to repair detached retinas and to remove cataracts. It is also used to treat liver cancer and prostate cancer. Cryosurgery is also widely used in the fields of dermatology, gynecology, plastic surgery, orthopedics, and podiatry. Cryosurgery has also been used successfully for more than 30 years in veterinary medicine.

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