From Big Medical Encyclopedia

CRYSTALS — the solids which are characterized by the correct periodic arrangement in space of the particles (atoms, molecules, ions) making them.

A crystal structure high cleaning proteins (a myoglobin, insulin, hemoglobin, etc.) have, enzymes (pepsin, ribonuclease, etc.), viruses etc. Many substances which are a part biol it is normal of liquids or at pathology, under certain conditions get a crystal structure that allows to judge presence of these substances at an organism (see. Polarimetry ). Processes of grain formation play an essential role in formation of stones in a gall bladder, kidneys etc. (see. Cholelithiasis , Nephrolithiasis ). The cholesteric plaques arising at atherosclerosis also include crystal educations.

The majority of solids consists of a set accrete small, chaotically located To. (crystal grains, crystallites); separate To. (monocrystals) have the form of regular polyhedrons.

Distinctive properties K. its geometrical form, existence of a certain type of symmetry, anisotropy, i.e. dependence various physical are. properties (electric, mechanical, optical) from the direction in relation to an optical axis (see. Anisotropy ). All characteristic properties To. are defined by its lattice formed by repetition of unit cells. The unit cell is made by atoms (molecules, ions) packed definitely.

To are formed upon transition of substance of thermodynamic less stable state in steadier, napr, of the overcooled fusion or of peresyshchenny solution (see. Crystallization ). Correct many-sided form K. can accept only at uniform and comprehensive inflow of the substances which are postponed for its surfaces and in the absence of external indignations. Geometrical features To. are reflected in laws of a crystallography.

Crystals are widely applied in various areas of human activity. So, by means of crystals of pyroelectricians (e.g., tourmaline, sugar) transformation of heat to electricity and vice versa became possible. These To. apply as sensitive receivers of infrared radiation. The devices intended for direct transformation of heat energy (e.g., energy of the sun) in electric are developed. To. piezoelectric materials (quartz, barium titanate) are deformed under the influence of electric field, and mechanical tension causes in them electric polarization (see). This property of piezoelectric materials is used for registration of a sound and ultrasound, stabilization on the frequency of radiation of radio stations, by differentiations of frequency ranges in high-frequency telephony etc.

In some To. the interrelation of electric and optical properties is quite strongly expressed; these To. call electrooptical. Manufacture optical devices for information transfer with use of a laser bunch of them. With their help it is possible to reject a bunch, to regulate it on intensity, to provide powerful impulses etc. (see. Laser ). Electrooptical To. are applied in light communication, in svetodalnomer, devices of sound recording of sound cinema, in color television, automatic polarimeters, in devices high-speed photo and filmings, in devices to measurement of high tensions, in optical elements of computing systems etc.

Many properties K. find application in medicine, in particular in medical instrument making. E.g., devices for UVCh-and microwave therapy, various spectral devices use at diagnosis of a number of diseases, in forensic medicine etc. Optical properties K. use at production of various luminescent screens (see. X-ray apparatus ). Piezoelectric properties K. find application during the designing hearing aids (see) and others sensors (see). Developed on the basis of a row K. quantum generators can be used in microsurgery, and also to researches in various fields of medicine and biology.

Special group K. make so-called. liquid crystals (see). This type K. it is interesting that is a part of living tissue and plays a part in cell activity. There are data that liquid crystal states and transitions are inherent to cellular structures, in particular membrane (see. Membranes biological ).

Bibliography: Bann H. Crystals, the lane with English, M., 1970; Shaskolskaya M. P Crystallography, M., 1976, bibliogr.; In J. Kristalle, V., 1975.

G.E. Fedorov.