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  Theory of Electrode and Transducers

Electrode:

The electrode is the critical interface between a stimulating and measuring device and the entity to be stimulated or measured. Electricity flows via wires by electron flow. It flows via tissue or fluid by ion flow. To generate an ion from an electron, a chemical reaction is essential. This reaction takes place in a micron of metal surface. This is a different reaction for an anode than for cathode. It generally depends much more strongly on metal selected for electrode than on fluid or tissue electrolyte in which the electrode is immersed. In all situations, the reaction pair is strongly dependent on quiescent voltage level of the system and on the magnitude of voltage excursions around that level.

Practical Electrodes for the Biomedical Measurements:

A) Body-Surface Biopotential Electrodes:

This category comprises electrodes which can be positioned on the body surface for recording the bioelectric signals. The integrity of skin is not compromised when such electrodes are applied, and they can be employed for short-term diagnostic recording like taking a clinical electrocardiogram or long-term chronic recording such as takes place in cardiac monitoring.

B) Intracavitary and Intratissue Electrodes:

Electrodes can be positioned in the body for biopotential measurements. Such electrodes are usually smaller than skin surface electrodes and do not need special electrolytic coupling fluid, as natural body fluids serve this function. There are lots of various designs for such internal electrodes. Basically such electrodes can be categorized as needle electrodes that can be employed to penetrate the skin and tissue to reach the point where the measurement is to be made, or they are electrodes which can be located in a natural cavity or surgically generated cavity in tissue.

C) Microelectrodes:

The electrodes explained in the previous paragraphs have been applied to studying bioelectric signals at organism, organ, or tissue level however not at the cellular level. In studying the electric behavior of cells, electrodes which are themselves smaller than the cells being studied require to be used. Three kinds of electrodes have been explained for this purpose: micropipette electrodes, etched metal electrodes and metal-film-coated micropipette electrodes. The metal microelectrode is basically a subminiature version of the needle electrode. In this situation, a strong metal wire like tungsten is utilized. One end of this wire is etched electrolytically to provide tip diameters on the order of a few micrometers. This structure is insulated up to its tip, and it can be passed via the membrane of a cell to contact cytosol. The benefit of this kind of electrode is that it is both robust and small and can be employed for neurophysiologic studies. Its principal drawback is the complexity encountered in its fabrication and its high source impedance.

Transducer:

A transducer is an electronic device which converts energy from one form to the other. Common illustrations comprise microphones, thermometers, loudspeakers, position and pressure sensors, and antenna. Though not usually thought of as transducers, photocells, LEDs (that is, light-emitting diodes), and even general light bulbs are transducers.

Efficiency is a significant consideration in any transducer. Transducer effectiveness is stated as the ratio of power output in the desired form to total power input. Mathematically, when P symbolizes the total power input and Q symbolizes the power output in the desired form, then the efficiency E, as the ratio between 0 -1, is as:

E = Q/P

When E% symbolizes the efficiency as a percentage and then:

E% = 100Q/P

No transducer is 100 % efficient; some power is forever lost in the conversion procedure. Generally this loss is manifested in the form of heat. Certain antennas approach 100-percent efficiency. The well-designed antenna supplied with 100 watts of radio frequency (that is, RF) power radiates 80 or 90 watts in the form of an electromagnetic field. Few watts are dissipated as heat in the antenna conductors, feed line conductors and dielectric, and in objects close to the antenna. Among the worst transducers, in terms of efficiency, are the incandescent lamps. A 100-watt bulb radiates just a few watts in the form of visible light. Mainly, the power is dissipated as heat; a small amount is radiated in UV (ultraviolet) spectrum.

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