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Saturday, 25 January 2020

Electrodes in Biomedical Instrumentation

Electrodes are devices which are used to convert ionic voltages into electronic voltages. Two electrodes can be used as a transducer.

Electrode Paste: Electrode Paste is an electrically conducting material employed as an interface between the electrode and surface of the body.

Types of Electrodes in Biomedical Instrumentation:

A wide variety of electrodes can be used to measure bio-electric events, but nearly all can be classified as belonging to one of the three basic types.

(a) Microelectrodes
(b) Skin surface electrodes
(c ) Needle electrodes


Electrodes which are used to measure bio electric potentials near or within a single cell are called micro electrodes.

Microelectrodes are generally of two types. (i) Metal electrode (ii) Micropipette

i) Metal electrodes

Metal electrodes are formed by electrolytic ally etching the tip of a fine tungsten or stainless steel wire to the desired size. Then the wire is almost coated to the tip with an insulating material. Fig shows a commercial type of metal electrode.

ii) Micropipette

The micropipette type of microelectrode is a glass micropipette with the tip drawn out to the desired size (usually about 1 micron (1μm) in diameter.) The micropipette is filled with an electrolyte compatible with the cellular fluids. A commercial type of microelectrode is shown in fig. In this electrode, a thin film of precious metal is bonded to the outside of a drawn glass electrode. All the above types of micro electrodes are used to take measurement within the cell or near the cell.


Electrodes which are used to measure ECG, EEG and EMG potentials from the surface of the skin are called skin surface electrodes.

The larger electrodes are usually associated with ECG where as smaller electrodes are used in EEG and EMG measurements. Following are various types of surface electrodes.

(i) Metal plate electrodes. (Limb electrodes)
(ii) Suction cup electrodes. (chest electrodes)
(iii) Adhesive tape electrodes.
(iv) Floating electrodes.
(v) Disposable electrodes.
(vi) Ear clip electrodes.

(i) Metal plate electrodes. (Limb electrodes)

The most common type of electrodes routinely used for recording ECG are rectangular or circular surface (metal plate) electrodes. The material used is German silver, nickel silver or nickel plated steel. They are applied to the surface of the body with electrode jelly. The electrodes are held in position by elastic straps. They are also called limb electrodes as they are most suitable for application on the four limbs of the body. They are reusable and last several years. These are generally preferred for use during surgery and are not suitable for use in long term patient monitoring.

(ii) Suction cup electrodes

Suction cup electrode is commonly used to record the uni- polar chest leads. The electrode is popular for its practicality, being easily attachable to fleshy parts of the body. Electrode jelly forms the vacuum seal. This electrode suffers by electrode slippage or movement after a sufficient length of time.

(iii) Adhesive tape electrode (Pre-jelled electrodes)

The adhesive tape electrodes reduce movement artifact by limiting electrode movement and reducing interface impedance. The pressure of the surface electrode against the skin may squeeze the electrode paste out. To avoid this problem, adhesive tape electrode is used.It consists of a light weight adhesive metallic screen backed by a pad for electrode paste. The adhesive backing holds the electrode in place.

(iv)Floating electrodes

The principle of this electrode is to practically eliminate movement artifact by avoiding any direct contact of the metal with the skin.

The floating electrode consists of a rigid plastic cup which contains the metal electrode and holds the electrode jelly. These are generally attached to the skin by means of two sided adhesive rings which adhere to both the plastic surface of the electrode and the skin. During long term monitoring or exercise testing this electrode is an important part of the system.

(v) Disposable electrodes

Various types of disposable electrodes have been introduced in recent years to eliminate the requirement for cleaning and care after each use. In general Disposable electrodes are of the floating type with simple snap connectors by which the leads, which are reusable, are attached.

(vi) Ear clip electrodes

A special ear clip electrode was developed for use as a reference electrode for EEG measurement.


Electrodes for electromyography work are usually of the needle type. Needle electrodes are used in clinical electromyography, neurography and other electrophysiological investigations of the muscle tissues underneath the skin and in the deeper tissues. The material of the needle electrode is generally stainless steel.

Needle electrodes are of various types. Some are,
(i) Monopolar
(ii) Bipolar.

(i) Monopolar needle electrode

The monopolar needle electrode usually consists of a Teflon coated stainless steel wire which is bare only at the tip.

(ii) Bipolar needle electrode

A concentric (coaxial) needle electrode contains both the active and reference electrode within the same structure. It consists of an insulated wire contained within a hypodermic needle. The inner wire is exposed at the tip and this forms one electrode. This needle electrode looks like a medicine dropper. Some electro-encephalography use small sub dermal needles to penetrate the scalp for EEG measurement. They are not inserted into the brain. They merely penetrate the skin.

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