While scientists keep trying to broaden the potential of the human organism, their efforts were crowned by the end of the 20th century with the development of biosensory devices. What is more, thanks to combinations of appropriate technologies and our organs of sense (or senses), we could soon be able to visualize with the help of skin receptors images in various electromagnetic fields and also monitor our own internal conditions.

Our sensory organs respond to electromagnetic fields in the visible (vision) and infrared (sense of temperature) ranges of the spectrum; to mechanical disturbances-audio waves (hearing), gravity (gravitational sensibility), mechanical pressure (sense of touch), to chemical signals (taste and sense of smell). But are these six senses enough or are the potentialities of the human body much broader?

According to some of the Moscow experts in this field-Dr. Sergei Varfolomeyev and Dr. Yuri Yevdokimov, and Academician Mikhail Ostrovsky - there is no one simple answer to this question. On the one hand, the scientists are certain that man's sensory systems possess an utmost degree of sensitivity permitted by the laws of chemistry and physics. And we can also regulate our sensitivity levels. At the same time the specialized receptor cells- which pick up the signal and translate it into an electric response-as a rule exceed but by a very small margin the energy levels of molecular heat noise. And it is these cells that, directly, or through intermediate neurons, encode and transmit sensory data by electric signals to the brain. And only after the brain has processed the message, it issues appropriate commands, or inputs the data into the memory for subsequent use.

This being so, the receptor cells can be described as highly specialized "pickup" sensors which register signals from without and also from within the body. The former include light, all kinds of mechanical disturbances (like sounds, etc.); chemical effects, changes of temperature, electric and magnetic fields. The latter include changes in blood pressure, strains in muscles and tendons, body temperature variations...

In their turn sensor receptors are subdivided into static and dynamic ones- those which monitor and transmit to the brain on a continuous basis and with permanent frequency data on some physical parameter (such as blood pressure), and those which respond only to changing signal strength (say of a visual cell-to changing light intensity, or tactile ones-to pressure on the skin).

Several methods have been identified which make it possible for the receptor cell to "measure" the input within a range of many orders of magnitude. First of all, there is a set of them with different scale of sensitivity (let us say, it is especially high in the eye retina cells). On the other hand, the non-linear nature of the conversion scale provides for one and the same relative sensitivity for signals of different strength. And, thirdly, the sensitivity of a receptor varies depending on the value of the input.

The understanding of these and other facts has brought about a situation in the 1990s when sensor technologies achieved unprecedented growth. And they were oriented at the development of analytical devices which make it possible to obtain data on the parameters of various media in the form of electric signals. In such cases scientists single

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out the substance or physical field under investigation from the mass of similar ones in order to transform data into an electrical response recorded in a digital or analogue formats. The greatest progress has been achieved with analytical devices used as the selecting elements in a biomolecule - biosensors.

Most of these are intended for the analysis of biological fluids. The thing is that blood contains thousands of different components, which means that the main task consists in determining in a rapid and effective way the concentration of the one most needed. For diabetic patients, for example, a clinical glucose test is a matter of vital importance.

A biosensor usually consists of two functional elements: a "sensor" containing what we call a selector material, and a physical converter, or transducer, which translates the input into an electric signal. Acting as the former are all types of biological structures-enzymes, antibodies, receptor nucleic acids and even living cells. Used in biosensors are electrochemical or optical converters, gravitational, calorimetric and resonance systems. Combinations of enzyme- catalytic and chemical reactions has made it possible to develop such of them which can be used for assays of glucose, amino acids, lactic sugar, urea and other metabolites.

The simplest design of an enzyme biosensor provides for the substrate or the product of an enzymatic reaction to be electrochemically active, that is it should be capable of rapid oxidation or reduction on the electrode upon the application of a corresponding potential. For example, in glucose tests one makes use of the glucose oxygenation reaction with the formation of hydrogen peroxide. And since the physical chemistry of different states of nucleic acids (isotropic, crystalline and liquid crystal) has been studied well enough, designing biosensors on the basis of liquid crystals of double-stranded DNA will make it necessary to take into account the properties inherent in these molecules only-S. Varfolomeyev, Yu. Yevdokimov and M. Ostrovsky say.

In their opinion, one can trace a chain of steps for the development of biosensors. The first is the formation of cholesteric liquid crystals from DNA molecules-in the circular dichroism spectrum in the absorption band of nitrogen bases there appears an intense line. The second step: after the treatment of liquid crystals with stained biologically active compounds two lines emerge in the same spectrum: one of them is located in the area of active absorption of nitrogen bases. With its secondary structure intact, the DNA represents an "inner standard" which reflects the quality of the newly formed liquid crystal. The other line is located in the area of absorption of the biologically active substance which forms a complex with the pairs of DNA bases; the amplitude of this line is proportional to the concentration of the compound itself. And, third-the above compounds, while reacting with the pairs of bases and being differently positioned with respect to the long axis of the helical DNA molecule, generate optical signals in different bands of the circular dichroism spectrum. Finally, an optical signal, as different from an electrical one generated by the interaction of the biologically active substance, can be transmitted without interference to the receptor.

At the gestation stage now is a basically new type of biosensors on the basis of DNA - the sensor element is incorporated into the polymer chain which joins the adjacent molecules. This method, called "molecular designing", makes it possible to build polyfunctional liquid-crystal sensors containing sensor elements of different chemical nature. These are capable of identifying groups of compounds which modify the properties of the "sensitive" elements located between DNA molecules.

One of the latest achievements of biotechnology and bioengineering is linked with the development of methods providing for immobilization of living cells within polymers and solid carriers of different nature. This makes it possible to obtain cells which retain close upon 100 percent of enzymic activity and which are capable of functioning for a long time. They retain essential structures and possess great stability, ensuring in some cases the viability and activity of enzymatic systems for as long as several years.

The area of application of cellular biosensors is large enough. They can be used for selective measurements of the levels of phenols, proline, glutamine, tyrosine, lactic and ascorbic acids, glucose and also for rapid assay of drinking water. With their help specialists can determine the biological consumption of oxygen by analyzing aggregate organic compounds some of which can be used by microorganisms. And the very procedure takes only a couple of minutes.

Biosensors and biosensoric technologies already provide for the detection and analysis of signals which lie far beyond the scope and scale of the human senses. Such are some of the hard facts of science at the start of the 21 st century.

S. Varfolomeyev, Yu. Yevdokimov, M. Ostrovsky, "Sensor Biology, Sensor Technologies and Development of New Human Sensory Organs", VESTNIK RAN, (Bulletin of the Russian Academy of Sciences), Vol. 70, No. 2, 2000.

Prepared by Sergei PSHIRKOV

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