MEDICAL SUPPLY ELECTRODES 

 

A medical supply electrode is a conductor used to make contact with a nonmetallic part of a circuit (for example; a semiconductor, an electrlyte or a vacuum). The word was coined by the scientist Michael Faraday from the greek words elektron (meaning amber, whence the word electricity is derived) and hodos, a way.

A medical supply electrode in an electrochemical cell – meaning it is a setup used for creating an electromotive force in a conductor separating two reactions; is referred to as either an anode or a cathode. The anode is defined as the electrode at which oxidation occurs, and the cathode is defined as the electrode at which reduction occurs. Each electrode may become either the anode or the cathode depending on the type of reaction occurring in the cell.

A primary cell is a special type of electrochemical cell in which the reaction cannot be reversed, and the identities of the anode and cathode are therefore fixed. It can be discharged but not recharged. While a secondary cell is one in which the reaction is reversible. When the cell is being charged, the anode becomes the positive (+) electrode and the cathode the negative (-). This is also the case in an elecrtolytic cell. When the cell is being discharged, it behaves like a primary or voltaic cell, with the anode as the negative electrode and the cathode as the positive.

To record, medical supply electrodes must be used as transducers to convert an ionic flow of current in the body to an electronic flow along a wire. These are usually made of metal, and have electrode potential and contact impedance. Electrode potential arises because a metal electrode in contact with an electrolyte (body fluids) forms a half-cell with a potential dependent upon the metal in use and the ions in the electrolyte. One might expect that two medical supply electrodes of the same material would produce the same electrode potential, which would cancel out in any recording, but the actual potentials do depend upon the conditions of contact. This might also cause serious problems when amplifying signals in the microvolt region.

The widely used medical supply electrodes for biomedical applications are silver medical supply electrodes, which have been coated with silver chloride by electrolyzing them for a short time in a sodium chloride solution All medical supply electrodes suffer from variations in contact resistance due to movement, and the drying out of any coupling medium. This is improved by setting the electrode back slightly from the surface of the skin (floating electrode) on a quantity of coupling jelly (electrolyte paste).

Further problem may arise if there is direct current flowing through the electrode arising from faulty equipment or from small (microampere) bias currents in the measuring amplifier circuit. These currents can cause chemical changes at the surface of the electrode causing polarization with consequent increase in the electrode potential. This causes drift of the electrode potential and damage to the skin due to the chemical action. Many types of recording medical supply electrodes exist including metal discs, needles, suction medical supply electrodes, glass micromedical supply electrodes, foetal scalp clips or screws, etc.

Medical supply electrodes are also used to inject electricity into the body as in faradism, tens, surgical diathermy and physiotherapy diathermy. They also exist in some analytical apparatus to measure the concentration of specific ions.