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 equilibrium. 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-kQ potentiometer and a milliammeter. The positive terminal 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 solution. Our favorite electrolyte is prepared by mixing 1 part distilled water (the supermarket kind is okay), 1/2 part HCl 25%, and FeCl3 at a rate of 0.5 g per milliliter of water.
If you want to make your own electrodes, use refined silver metal (99.9 to 99.99% Ag) to make the base electrode. Before chloriding, degrease and clean the silver using a concentrated 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 filter paper. Don't touch the electrode surface with bare hands after cleaning. Suspend the electrodes 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 silver chloride precipitation occurs and the current jumps to its maximal value. As the thickness 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 2.5mA/cm2, making sure that no hydrogen bubbles 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 |A/cm2. 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 flame until it melts to a dark brown liquid, then simply dip the electrode in the molten silver chloride.
Warning! The materials used to form Ag/AgCl electrodes are relatively dangerous. Do not breathe dust or mist and do not get in eyes, on skin, or on clothing. When working with these materials, safety goggles must be worn. Contact lenses are not protective devices. Appropriate eye and face protection must be worn instead of, or in conjunction with, contact lenses. Wear disposable protective clothing to prevent exposure. Protective clothing includes lab coat and apron, flame- and chemical-resistant coveralls, gloves, and boots to prevent skin contact. Follow good hygiene and housekeeping practices when working with these materials. Do not eat, drink, or smoke while working with them. Wash hands before eating, drinking, smoking, or applying cosmetics.
If you don't want to fabricate your own electrodes, you can buy all sorts of very stable Ag/AgCl electrodes from In Vivo Metric. They make them using a very fine grained homogeneous mixture of silver and silver chloride powder, which is then compressed and sintered into various configurations. Alternatively, Ag/AgCl electrodes are cheap enough that you may get a few pregelled disposable electrodes free just by asking at the nurse's station in the emergency department or cardiology service of your local hospital.
Recording gel is available at medical supply stores (also from In Vivo Metric). However, if you really want a home brew, heat some sodium alginate (pure seaweed, commonly used to thicken food) and water with low-sodium salt (e.g., Morton Lite Salt) into a thick soup that when cooled can be applied between the electrodes and skin. Note that there is no guarantee that this concoction will be hypoallergenic! A milder paste can be made by dissolving 0.9 g of pure NaCl in 100 mL of deionized water. Add 2g of pharmaceutical-grade Karaya gum and agitate in a magnetic stirrer for 2 hours. Add 0.09 g of methyl paraben and 0.045 g of propyl paraben as preservatives and keep in a clean capped container.
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