Biomonitoring electrodes have progressed quite a long way from early research into how they function during the 19th century. With ongoing innovations in both sensor and medical technology, as well as further understanding of the human body, these devices continue to evolve. This article briefly looks at the history of ECG sensors through to their future.
Since 1838, when Carlo Matteucci, professor of physics at the University of Pisa, showed that an electric current accompanies each heartbeat, sensors for monitoring electricity in the human body have changed dramatically. Matteucci used a preparation known as a “rheoscopic frog” in which the cut nerve of a frog's leg was used as the electrical sensor and twitching of the muscle was used as the visual sign of electrical activity.
The Rheoscopic Frog
Matteucci also experimented with an astatic galvanometer for the study of electricity in muscles. At the time, this was typically performed by inserting one galvanometer wire in the open end of a dissected muscle and the other on the surface of the muscle.
In 1887, British physiologist Augustus D. Waller published the first human electrocardiogram. His earliest experiments involved placing the paws of his bulldog into saline-filled glass containers and using a brass studded collar. His saline sensors were connected to crude galvanometers by conductive wire.
In 1901, Willem Einthoven refined Waller’s technology by using a fine quartz string coated with silver in a device called the string galvanometer. This device weighed over 600 lbs. and took five technicians to operate it. The sensors were buckets filled with saline solution connected with wire to the string galvanometer. Three buckets—one for each hand and one for the left leg—were arranged in an equilateral triangle. In 1912, Einthoven presented this method to the Chelsea Clinical Society in London; it would be known as the “Einthoven Triangle.” Einthoven later won the Nobel Prize for inventing the electrocardiograph.
Today’s Biomonitoring Electrodes
Today’s biomonitoring electrodes for use in ECG, EEG, Tens, and other applications are comprised of a plastic substrate covered with a silver/silver chloride ionic compound. (Silver chloride is only very slightly soluble in water, so it remains stable.) The electrode is assembled with an electrolyte gel in which the principle anion is Cl-. Cl- is an attractive anion for electrode applications since the skin interface contains an excess of chloride ions in solution (perspiration).
|Modern biomonitoring electrodes, sensors and components|
Silver on the electrode surface oxidizes to silver ions in solution at the interface. These ions combine with Cl- already in solution to form the ionic compound AgCl. Silver chloride is only very slightly soluble in water, so most of it precipitates out of the solution onto the silver electrode and contributes to a silver chloride deposit. Sensors are also converted from metallic Ag to Ag/AgCl by electrolytic or chemical conversion processes.
The Ag/AgCl electrode is the most widely used for all applications of biological electrode systems. Early versions of sensors were fabricated from solid silver, silver coated brass, and other materials, such as tin and nickel. The frequency dependence of conductive materials other than Ag/AgCl is less desirable, yielding a high DC offset and a low cutoff frequency compared to the Ag/AgCl electrode. Today, nearly all biomonitoring electrodes used to monitor and record biopotentials are Ag/AgCl.
Biopotentials are electrical potentials inside the living body that are created by ionic activity in living cells. Excitable cells of the heart are the origin of the ECG signal. Due to the conductivity of the human body, these potentials are manifested on the body’s surface. Ionic activities must be converted to electron currents with an electrode as the transducer. The skin, which has a dry dielectric surface, impairs the transfer of ions from the tissue to electrons in the electrode. Typically, a chloride gel or saline solution is used as a conductive bridge from the patient’s skin to a silver/silver chloride sensor. In addition to the skin impedance, the electrical transducer comprises the resistance of the electrolytic gel and the double layer at the electrode-electrolyte interface, as well as half-cell potentials caused by different energies of the electrode, electrolyte, and skin.
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Today’s sensors are fabricated by first making a plastic injection molded substrate and then coating it with a very thin layer of silver. The outer layer of the silver is converted to silver chloride. Various methods of coating are being used today, but all processes require strict controls to maximize the effective use of silver. Because gels dry out and only last a few days after being placed on a patient, it has been proposed that future electrodes may be fabricated with dry conductive composites. Today, conductive carbon fiber loaded ABS plastic is being used to reduce silver and eliminate stainless steel, brass, and nickel used in electrode components. These engineered resins reduce or eliminate the risk of burns in Magnetic Resonance Imaging (MRI) applications as well as corrosion or galvanic reactions. The electrode-skin impedance is governed by contact area and skin properties as well as the materials used in manufacture and presents specific challenges to designers of cost effective disposable electrodes. Biomonitoring sensors and electrodes of the future will employ the use of nano composites, microscopic circuitry, low power wireless, RFID, and other technologies.
Salvatore Emma, Jr. is vice president and general manager of Micron Products Inc.  He has been given the task of guiding efforts in strategy, operations, innovation, and continuous improvement. Emma, Jr. can be reached at 978-345-5000 or firstname.lastname@example.org .