Bioelectric signals:

 Bioelectrical signals are very low amplitude and low frequency electrical signals. These signals can be measured from biological beings, for example, humans. Bioelectrical signals are generated from the complex self-regulatory system and can be measured through changes in electrical potential across a cell or an organ. Their basic source is the cell membrane potential which under certain conditions may be excited to generate an action potential. The electric field generated by the action of many cells constitutes the bio-electric signal. The most common examples of bioelectric signals are the ECG (electro cardio graphic) and EEG (electro encephalo graphic) signals.

Bioacoustic

 Signals: The examples of such signals are: flow of blood in the heart, through the heart’s valves and flow of air through the upper and lower airways and in the lungs which generate typical acoustic signal.

Biomechanical Signals: 

The movement of the chest wall in accordance with the respiratory activity is an example of this type of signal.

Biochemical Signals:

 The signals which are obtained as a result of chemical measurements.

Biomagnetic 

Signals: Extremely weak magnetic fields are produced by various organs such as the brain, heart and lungs.

Bio-impedance Signals: 

The measurement of galvanic skin resistance is a typical example of this type of signal.

Bio Electrodes: 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.

The materials used to make Electrodes include:

• Aluminium (Al)

• Copper (Cu)

• Silver (Ag)

• Gold (Au)

• Platinum (Pt)

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

1. Microelectrodes

2. Skin surface electrodes

3. Needle electrodes

These electrodes are designed to measure bioelectric potentials near or within the cell. 

(1) Microelectrodes can be of two types:

• Metal

• Micropipette

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. 


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


Skin 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





Examples of surface electrodes include:
• Limb electrode
• Disposal electrode
• Pre-gelled electrode
• Circular electrode







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.