BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 5

usually specify the electrode o sets that are commonly present for the application covered

by the standard. For example, the standards issued by the Association for the Advancement
of Medical Instrumentation (AAMI) specify that electrocardiography (ECG) ampli

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fi ers must

tolerate a dc component of up to 300 mV resulting from electrode-skin contact.

Commercial ECG electrodes have electrode o sets that are usually low enough, ensur-

ing little danger of exceeding the maximum allowable dc input of

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ff set speci fi cations of the

standards. However, the design of a biopotential ampli fi er must consider that there are

times when the dc o set may be much larger. For example, neonatal ECG monitoring

applications often use sets of stainless-steel needle electrodes, whose of

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sets are much
higher than those of commercial self-adhesive surface ECG electrodes. In addition, many
physicians still prefer to use nondisposable suction cup electrodes (which have a rubber
squeeze bulb attached to a silver-plated brass hemispherical cup). After the silver plating
wears o

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ff , these brass cup electrodes can introduce very large off, ff sets.

LOW-POLARIZATION SURFACE ELECTRODES

Silver (Ag) is a good choice for metallic skin-surface electrodes because silver forms a
slightly soluble salt, silver chloride (AgCl), which quickly saturates and comes to equilib-
rium. A cup-shaped electrode provides enough volume to contain an electrolyte, including
chlorine ions. In these electrodes, the skin never touches the electrode material directly.
Rather, the interface is through an ionic solution.
One simple method to fabricate Ag/AgCl electrodes is to use electrolysis to chloride a
silver base electrode (e.g., a small silver disk or silver wire). The silver substrate is
immersed in a chlorine-ion-rich solution, and electrolysis is performed using a common 9-
V battery connected via a series 10-k potentiometer and a milliammeter. The positive ter-

minal of the battery should be connected to the silver metal, and a plate of platinum or silver
should be connected to the negative terminal and used as the opposite electrode in the solu-
tion. Our favorite electrolyte is prepared by mixing 1 part distilled water (the supermarket
kind is okay), 1/2 part HCl 25%, and FeCl 3 at a rate of 0.5 g per milliliter of water.
If you want to make your own electrodes, use re

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ned silver metal (99.9 to 99.99% Ag)
to make the base electrode. Before chloriding, degrease and clean the silver using a con-
centrated aqueous ammonia solution (10 to 25%). Leave the electrodes immersed in the
cleaning solution for several hours until all traces of tarnish are gone. Rinse thoroughly
with deionized water (supermarket distilled water is okay) and blot-dry with clean

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lter
paper. Don't touch the electrode surface with bare hands after cleaning. Suspend the elec-
trodes in a suitably sized glass container so that they don't touch the sides or bottom. Pour
the electrolyte into the container until the electrodes are covered, but be careful not to
immerse the solder connections or leads that you will use to hook up to the electrode.
When the silver metal is immersed, the silver oxidation reaction with concomitant sil-
ver chloride precipitation occurs and the current jumps to its maximal value. As the thick-
ness of the AgCl layer deposited increases, the reaction rate decreases and the current
drops. This process continues, and the current approaches zero. Adjust the potentiometer
to get an initial current density of about 2.5 mA/cm 2 , making sure that no hydrogen bub-
bles evolve at the return electrode (large platinum or silver plate). You should remove the
electrode from the solution once the current density drops to about 10

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A/cm 2 . Coating
should take no more than 15 to 20 minutes. Once done, remove the electrodes and rinse
them thoroughly but carefully under running (tap) water.
An alternative to the electrolysis method is to immerse the silver electrode in a strong bleach
solution. Yet another way of making a Ag/AgCl electrode is to coat by dipping the silver metal
in molten silver chloride. To do so, heat AgCl in a small ceramic crucible with a gas

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flame until
it melts to a dark brown liquid, then simply dip the electrode in the molten silver chloride.

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