To record biopotentials, an interface is required between the electron-conducting copper wires connected to signal conditioning amplifiers and the ion-conducting environment of living organisms. Electrodes form this interface.
Some of the several kinds of electrodes are better than others in terms of low noise and ease of use. Initial ECG electrodes were non-disposable, nickel-silver plated copper, or stainless steel disks hardwired to the amplifier input leads. Even though conductive gel was employed, this type of electrode had inherently high low-frequency noise due to the complex redox reactions taking place at the metal electrode surfaces. Nonetheless, acceptable ECG and EMG signals could be recorded.
Current practice in measuring ECG and EMG signals from the skin surface is to use disposable sticky patch electrodes. Patch electrodes use a silver | silver chloride interface to a conductive gel containing Na+, K+, and Cl– ions. The gel makes direct, wet contact with the skin. Adhesive for the skin is found in a ring surrounding the electrolyte and AgCl. The copper wire makes contact with the Ag metal backing of the electrode, generally with a snap connector. More lately, skin electrodes have use Ag | AgCl deposited in a thin layer on an approximately 2.5 cm square of thin plastic film. The conductive gel and adhesive are combined in a layer all over the AgCl film. Contact with this low-cost electrode is made with a miniature metal alligator clip on one raised corner.
Related: Types of Bioelectrodes
Silver chloride is used with skin patch electrodes and many other types because of the dc half-cell potential of the Ag | AgCl electrode depends on the logarithm of the concentration of chloride ions. Since the AgCl is in direct contact with the coupling gel, which has a high concentration of Cl– ions, the dc half-cell potential of the electrode remains fairly stable and a has a low impedance, thus low thermal noise.
One more type of electrode employed in neurophysiological research is the saline-filled, glass micropipette electrode used for recording transmembrane potentials in neurons and muscle fibers. Because the tips of these electrodes are drawn down to diameters of a fraction of a micron before filling, their resistances when filled can range from approximately 20 to 103 Mꭥ, depending on the tip geometry, the filling medium, and the surrounding medium in which the tip is placed. Owing to their high series resistances, glass micropipette electrodes create three major problems not seen with other type’s bioelectrodes:
- Because of their high series resistances, they must be used with special signal conditioning amplifiers called electrometer amplifiers, which have ultra-low, input dc bias currents. Electrometer input bias currents are on the order of (10-14 A) and their input resistances are approximately 1015 ꭥ.
- Glass microelectrodes makes tremendous amount of Johnson noise.
- The tips of glass micropipette electrodes have significant distributed capacitance between the electrolyte inside and the electrolyte outside the tip which makes them behave like distributed parameter low-pass filters.
