Electrodes

 

Various types of metal and capacitative electrodes are used to sense biopotentials. Biopotentials have been used to evaluate cardiac, muscle or neurological performance through electrocardiography (ECG), electromyography (EMG) and electroencaphlography (ECG) systems.

 

Metal electrodes commonly consist of a metallic material placed in an electrolyte containing its ions (e.g. silver/silver chloride). When the metal electrode is in equilibrium with the electrolyte there is no net current flow. However, contact with skin or some other tissue causes a net current flow across the surface of the electrode. The electrode is now polarized and is no longer in equilibrium with the surrounding electrolyte or tissue; a change in electrode potential is known as the overpotential or overvoltage occurs.

 

The current density is kept to a minimum in sensing electrodes to minimize any effects on surrounding tissues. Electrode skin impedance ranges from a 103 to 105 ohms. Conductive paste is used at the electrode skin interface to reduce the impedance and improve the contact surfaces. Impedance depends on the design of the electrode, the properties of the conductive paste and the frequency of the signal generated.

 

Drift and noise are common problems associated with electrodes. Noise is generated as a result of motion artifact between the electrode and skin or tissue. Baseline drift of the electrode up to a magnitude of 100 mV/min has been reported. Preparation of the skin surfaces, and the use of specially designed electrodes and materials have positive effects in reducing both noise and drift.

 

The capacitative electrode is basically a metal electrode coated with a thin layer of insulating organic or inorganic material. Insulators with high dielectric constants decrease the impedance and increase the capacitance of an electrode, improving performance. Capacitative electrodes have the advantage of being used without skin preparations or conductive pastes; the insulative coating material of capacitative electrodes can be applied directly to the skin. Microelectrodes that have been used to measure biopotentials at the cellular level have been developed using capacitative electrode technology.

 

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Support for the development of this module was provided by the National Science Foundation and The Cooper Union for the Advancement of Science and Art.
 

Please send questions or comments to Professor Ron Adrezin or Professor Daniel Raichel.