Types of Bioelectrodes

Introduction

Bioelectric events have to be picked up from the surface of the body before they can be put to the amplifier for subsequent record or display. This is done by use of electrodes. The potentials produced at different points are measured by placing electrodes at various points on the body. They carry the currents produced due to potential differences to instrumentation amplifiers, where the signals are amplified and further processed by signal processing systems.

Bioelectrodes can be classified as:

• Surface electrodes: These electrodes pick up potentials from the surface of the tissue.
• Deep seated electrodes: These electrodes are inserted inside a live tissue or cell.

When a measurement is made outside the body by placing surface electrodes it is called in vitro measurement. When measurement is made by inserting a needle electrode inside the tissue, it is called in vivo measurement.

Due to the movement of electrodes, noise signals are generated. They are referred to as artifacts. To avoid artifacts and establish a low impedance path, an electrolyte or a jelly is applied to the area where the electrodes make contact. The skin is first cleaned by rubbing alcohol; all hair at that portion is removed. A jelly is then applied and the electrode is placed.

Key Properties of Bioelectrodes

Bioelectrodes should possess the following properties:

• They should be good conductors
• They should have low impedance
• They should not polarize when a current flows through them
• They should establish a good contact with the body and not cause motion
• Potentials generated at the metal electrolyte (jelly) surface should be low.
• They should not cause itching, swelling or discomfort to the patient for example the metal should not be toxic
• They should be mechanically rugged
• They should be chemically inert
• They should be easy clean

The figure below shows how the electrodes make contact with the skin surface:

Electrode Metal and Electrolyte Interface

The metal of the electrode has a tendency to discharge ions into the electrolyte. This voltage is called half-cell potential or offset potential. In certain metals and electrolytes, this potential is significant and in most metals and electrolytes it is significant.

Materials used for Electrodes

The materials used to make Electrodes include:

• Aluminium (Al)
• Copper (Cu)
• Silver (Ag)
• Gold (Au)
• Platinum (Pt)

When we compare these metals to Hydrogen electrode, each metal has some potential as shown in the below figure:

The Skin Contact Impedance

The outer layer of the skin has large impedance which is much greater than the electrical impedance of body tissue beneath the skin. The outer layer skin is responsible for the bulk of the skin contact impedance and therefore, a careful skin preparation is essential in order to obtain best results.

Types of Electrodes used in Biomedical Measurements

The three basic types of biopotential electrodes used in biomedical measurements are:

1. Microelectrodes
2. Skin surface electrodes
3. Needle electrodes

Microelectrodes

These electrodes are designed to measure bioelectric potentials near or within the cell. These electrodes are much smaller in cross-sectional area as compared to the size of the cell in which they are to be inserted so that their penetration should not damage the cell. A cell is rarely larger than 500 microns, therefore these microelectrodes should have tip dimension of about 5 microns and the tip should be strong enough to penetrate the cell without damage.

Microelectrodes can be of two types:

• Metal
• Micropipette

Metal microelectrodes

Metal microelectrodes are formed from a fine needle of a suitable metal down to a fine tip. Then the needle is coated almost to the tip with an insulating material. These electrodes are used to measure the biopotential from the metal-electrolyte interface i.e. the electrode potential is developed across the metal-electrolyte interface which is proportional to the exchange of ions between the metal and the electrolyte of the body.

Micropipette microelectrode

The Micropipette microelectrode is a microcapillary made of glass which is filled with an electrolyte as shown in the figure below:

The metal microelectrodes are used in direct contact with the cell and they have lower resistance. However, these electrodes tend to develop unstable electrode offset potentials. The micropipette microelectrodes have a dual interface; one interface is formed by metal wire in contact with the electrolyte solution filled in the micropipette while the other interface is formed between the electrolyte inside the micropipette and the fluids inside or immediately outside the cell.

The micropipette microelectrodes tend to develop stable electrode offset potentials and thus preferred where steady-state potentials measurements are required however metal microelectrodes have advantages too such as:

• Lower impedance
• Infinite shelf life
• Repeatable and reproducible performance
• Easy cleaning and maintenance

Body Surface Electrodes

These electrodes are designed to measure ECG, EEG, and EMG potentials from the surface of the skin; hence these electrodes are least traumatic. Body Surface electrodes are designed and used to measure bioelectric potentials from the surface of the body. They are available in many forms and sizes. The larger electrodes are usually used for sensing of ECG potentials as these measurements do not depend on the specific localization of the electrodes. However, for sensing of EEG and EMG potentials, smaller electrodes are used as sensing for them depends upon the location of electrode or measurement. Metal plate and suction cup type electrodes are body surface electrodes but they have a common problem or the possibility of slippage or movement. These electrodes are sensitive to movements hence produce incorrect measurements on shifting. To avoid this problem, the floating electrodes are used. The principle of the floating electrode is to eliminate the movement artifacts (false signals) by avoiding any direct contact of the metal electrode with the skin. The contact between the metal electrode and skin is maintained by the electrolyte paste or jelly.

Types of Surface Electrodes

Examples of surface electrodes include:

• Limb electrode
• Disposal electrode
• Pre-gelled electrode
• Circular electrode

More details on Surface electrodes are shown in the figure below:

From above, figure (a), shows a rectangular electrode held together in position by elastic rubber straps. Figure (b), shows a disposal electrode with adhesive to hold the electrode in position. Figure (c) exhibits disposable pre-jelled electrodes. Figure (d) shows a circular electrode, with a rubber bulb which creates a partial vacuum so that the electrode is held in a position and no adhesive is used.

Needle Electrodes

They are generally made of stainless steel. These electrodes are designed to penetrate the skin surface of the body to some depth to record EEG potentials of a region of the brain or EMG potentials of a muscle. These electrodes have to be sharp and small like subdermal needles which help them to easily penetrate the scalp for measuring the EEG potentials. They are required to penetrate up to some surface at certain depth of the skin which is parallel to the surface of the brain or muscle.

You can also read: Sources of Biomedical Signals