Principles of transformation of biological and not electric signals in electric. Designs of sensors and electrodes, their basic characteristics

PHOTOELECTRIC TRANSFORMERS

The device for increase of a voltage, a current or capacity of electric fluctuations due to energy of other source is called the amplifier of fluctuations. In medical devices amplifiers of a voltage usually are used, them structure includes a three-electrode electronic lamp.

Photo electronic multipliers (PEA) are applied for strengthening of weak photocurrents. These are devices in which the photo effect and the phenomena of secondary emission of electrons is used.

The device consists from a vacuum photo element with several electrodes. They are covered by substance which is easy let out electrons. Light falls on the cathode and causes photoelectronic emission. Electrons are accelerated by an electric field, get on the first electrode, beat out from it secondary electrons in a lot. And these electrons, being accelerated by a field, get on the second electrode, thus their quantity again increases. There is a strengthening a stream electrons in some hundreds thousand times.

The photoeffect is used also in electron-optical converters (electronic converters of the optical image - ECOI). These devices are intended for strengthening brightness of the image.

THE GENERAL SCHEME OF TRANSFER AND REGISTRATION OF

INFORMATION.

To receive and fix the information on a condition and parameters of medical and biologic system, it is necessary to have the whole set of devices. The primary element of this set a-sensitive element of means of measurements is called as the device of removal.

Devices of removal of the information contacts or cooperates with the system, other elements of set usually are separately from medical and biologic system.

The signal which is given out by the device of removal, amplifies by amplifier and can be measured or registered by the special device.

In some cases the measuring part of system can be removed on significant distance from object of measurements. In this case the strengthened signal on liaison channels is transferred to the receiver. From the receiver the signal goes on a measuring instrument or the registrar. A liaison channel in this set can be a cable or the electromagnetic waves which are let out by the transmitter

The measuring instrument or the registrar is a finishing element of a measuring circuit and displays the information about biological system in the accessible form for direct perception by the observer.

If the measured entrance parameter X, and output size on a measuring instrument or registrar Y, dependence Y = f (X) should be known for calculation of the measured size X.

The sharp response and small inertia of electrical measuring and receiver recorders will be used at electrical measurement of not electrical values. These methods are widely applied in engineering, in biology, in medicine, in the latter case - to measure registration of miscellaneous values describing a condition or activity of bodies of organism: temperature, the pressure, different kind of motions, vibratory processes etc. Basically, in the device of the conforming devices are much in common, for example, behind small exceptions all of them consist of three main bodies: the converter of investigated value in electrical signals or sensor S, amplifier of these signals A and chart recorder R (fig 2).

One of channels of cardiograph is used as amplifiers and the registering device. The sensor is connected to it through a special prefix (fig.3) which harmonizes target parameters of the sensor with entrance characteristics of the amplifier cardiograph.

ELECTRODES.

For reception of the information from biological system it is used devices of removal in the form of electrodes or the sensor.

Electrode - is the conductors of the special form connecting a measuring circuit with biological system. At diagnostics electrodes are used not only for reception of an electric signal but also used for rendering electric influence with the purpose of treatment and at electrostimulation.

Electrodes should have some properties: they should be fixed and removed quickly, have high stability of electric parameters, to be strong, to not create handicapes, to not irritate a biological tissue.

The important physical problem concerning electrodes for removal of a bioelectric signal, consists in minimization of losses of the helpful information, especially on transition resistance-skin.

For this purpose it is necessary increase conductivity of environment between an electrode and a skin, using the gauze napkins moistened by a physiological solution, or electrospending pastes.

At using electrodes can arise, in the first, galvanic electromotive power on contact of electrodes with a biological tissue, in the second, the electrolytic polarization directed against basic electromotive power, as a result of allocation of products of reaction at passage of a current. Influence of these electromotive power can be lowered by methods of electrochemistry. Glass micro-electrodes are used in a microelectrode practice. The glass case of an electrode is an insulator, inside there is a conductor in the form of electrolyte. As the working tip of an electrode has the size of the order 0,5 microns, it can pierce a membrane of a cell and to spend endocellular measurements. Metal a plate basically from lead are used as electrodes in some cases.

SENSORS, THEIR CLASSIFICATION AND CHARACTERISTICS.

Many medical and biologic characteristics cannot be removed by electrodes as they are not reflected by a bioelectric signal: pressure of blood, temperature, sounds of heart and many other things. In these cases sensors are used (measuring converters). In some cases even parameters connected with an electric signal are more convenient for removing by means of sensors. The device, changing the measured or controllable not electric size in an electric signal is called sensor. Use of an electric signal is more preferable, than others as electronic devices allow strengthening rather simply them, to transfer to distance and to register.

Sensors are divided on generating and parametrical.

Generating are sensors, which under influence of a measured signal directly generates a voltage or a current. To such sensors concern:

Piezoelectric, based on piezoelectric effect;

Thermoelectric, based on thermal electricity;

Induction, based on action of an electromagnetic induction;

Photo-electric, based on photo-electric effect.

Piezoelectric sensors are based on the phenomenon of piezoelectric effect. This phenomenon arises at mechanical deformations crystal dielectric. At deformation of elementary crystal cells and shifts подрешеток rather each other there is a polarization. The piezoelectric effect arises in quartz, сегнетовой salts and other crystals. If the plates closed through a neon lamp to put to a crystal possessing piezoelectric properties at impact on a crystal the pressure appears on its sides. The pressure appears and on metal plates, thus the neon lamp flashes. This phenomenon is called direct piezoelectric effect. Mechanical deformation can arise in a crystal placed in an electric floor. This phenomenon is called return piezoelectric effect. Work of the piezoelectric sensor for record of pulse of a beam artery is based on the phenomenon of direct piezoelectric effect. Basic element of the sensor is the plate from ferroelectric which one end becomes stronger on a wrist, and its other end concerns walls of an artery. Fluctuations of a wall of an artery cause deformation of a bend of a plate. The variable potential difference is raised thus on a surface of a plate. The Potential difference repeats under the form of fluctuation of a wall of an artery. It is betraid to the amplifier by means of electrodes and wires, and then to the registering device.

The thermocouple is applied as the thermoelectric sensor. The thermocouple it junction two conductors from various metals. Thermocouples are applied to measurement of temperatures. Indications of the measuring device connected by the free ends of the thermocouple, are proportional to a difference of temperatures of these soldered ends. The thermocouple has advantages in front of the mercury thermometer owing to the big sensitivity and absence of thermal inertia. However on convenience of measurements it is much worse of thermistor. Thermobattery is applied when one thermocouple is insufficiently sensitive. Thermocouples can be made of semiconductors with various conductivity (n-type and p-type).

Mechanical movings of the constant magnet located between two motionless coils (or on the contrary) cause in them an induction current. This phenomenon is used in the induction sensor. The current induced at displacement of coils concerning a magnet, is transferred on the amplifier and the registering device. They apply to a straight line ballistocardiography.

The principle of action of photo-electric sensors is based on dependence of their resistance on illumination and a radiating irradiation.

Sensors in which under influence of mechanical efforts or movings electric parameters change, are called parametrical. Types of these sensors refer to the same as also the phenomena on which they are based:

Resistance - resistive sensors;

Capacity - capacitor sensors;

Inductance - inductive sensors.

These sensors demand the separate power supply.

Sensors of resistance are made of a thin wire, or in the form of a column from substance which resistance strongly changes at a stretching or compression, for example, from the silicone, the pressed metallized pitch. The ends of a wire or a column are fastened to a basis and a mobile element of the sensor. By resistive sensors measure pressure.

Change of a capacitance of the condenser occurs in capacitor sensors. The condenser joins in a measuring circuit, at mutual displacement of its plates.

Inductive sensors consist of an electromagnet with not closed core and the mobile anchor closing the core. Inductive resistance of the coil changes at movings an anchor. The transformer counter, the counter of pressure concerns to them, etc.

Use of crystal diodes and triodes, which sizes do not exceed several millimeters, the devices, containing amplifiers and generators of rather small sizes allow to create. This area of technics name microelectronics.