Commercial skin electrodes

Commercial skin electrodes are usually packaged in metal foil wrappers that are impervious to moisture. These wrappers help prevent evaporation of the gel and should not be removed until the electrodes are to be used. The electrodes should always be examined for adequate gel prior to... 

Instruments are available for measuring impedance between electrode pairs. The procedure is recommended strongly as a good practice, since high impedance leads to distortions that may be difficult to separate from actual EEG signals. In fact, electrode impedance monitors are built into some commercially available EEG devices. Note that standard DC ohmmeters should not be used, since they apply a polarizing current that can result in a buildup of noise at the skin-electrode interface. 

Skin electrodes have the largest commercial product volume, most of them are pregelled ready-to-use nonsterile products. Some of them have a snap-action wire contact others are prewired, for instance, adapted for babies. There is a contact electrolyte between the skin and the electrode metal. Dry SC is a poor conductor and this easily results in poor (high impedance) contact and noise. The contact area with the skin should be as large as practically possible, and reducing the SCs thickness by sandpaper abrasion is useful. Hydrating the skin with contact electrolyte or by the covering effect of the electrode will usually reduce the contact impedance with time (minutes to hours). 

FIGURE 10.8 Polarization impedance by Bode plot. Two electrodes face to face, contribution of one electrode. Commercial ECG skin electrode with wet gel and AgCl metal surface of 0.7 cm. ... 

The presence of polymer, solvent, and ionic components in conducting polymers reminds one of the composition of the materials chosen by nature to produce muscles, neurons, and skin in living creatures. We will describe here some devices ready for commercial applications, such as artificial muscles, smart windows, or smart membranes other industrial products such as polymeric batteries or smart mirrors and processes and devices under development, such as biocompatible nervous system interfaces, smart membranes, and electron-ion transducers, all of them based on the electrochemical behavior of electrodes that are three dimensional at the molecular level. During the discussion we will emphasize the analogies between these electrochemical systems and analogous biological systems. Our aim is to introduce an electrochemistry for conducting polymers, and by extension, for any electrodic process where the structure of the electrode is taken into account. 

Figure 4.20 Skin impedance as a function of time for an electrocardiogram (ECG) electrode of the commercial, long-term, wet gel, strong electrolyte type. From Grimnes (1983a), by permission.

The mechanical or viscous properties of the contact medium are important, and often the electrolyte is thickened by a gel substance or contained in a sponge or soft clothing. Commercial electrocardiogram (ECG) electrodes are often delivered as pregelled devices for single use, and the medium may contain preservatives to increase storage life, or quartz particles for abrading purposes on the skin. 

Figure 7.5 Spectra of skin impedance plus electrode polarization and the effect of wet contact electrolyte-skin penetration. Values just after electrode onset (a) and after 1 (b) and (c) 4 h. One commercial pregelled ECG electrode on forearm, skin wetted area 3 cm. ...

Figure 7.6 Skin surface electrode geometry and its equivalent electric model. Right Typical values at 10 Hz for a commercial wet gel ECG electrode. Note that before contact electrolyte penetration is >99% of the total impedance.

Figure 7.19 Electrode polarization impedance for 5 cm skin contact area hydrogel/aluminum electrode. Two ECG commercial electrodes front to front contribution of...

A depth-selective skin electrical impedance spectrometer (formerly called SCIM) developed by S. Ollmar at the Karoiinska Institute is an example of a commercial instrument intended for quantification and classification of skin irritation. It measures impedance at 31 logarithmically distributed frequencies from 1 kHz to 1 MHz, and the measurement depth can to some extent be controlled by electronically changing the virtual separation between two concentric surface electrodes (Ollmar, 1998). 

Commercial electrode gels usually use relatively high concentrations of potassium or sodium chlorides at a neutral pH. Since these concentration levels can irritate the skin, there are different types of gels are on the market offering trade-offs of low resistance versus gentleness to the skin. 

Mercury-mercurous chloride. This is probably the most widely used reference electrode. It is reversible to chloride ion and is usually made up in saturated aqueous potassium chloride solution, although Irnoldm " and O.lmoldm solutions are also common. In commercial electrodes, the solution is often retained with a porous plug or ceramic frit saturated aqueous KCl, being very dense, easily leaks out. A separate compartment will therefore be necessary for the reference electrode if chloride ions must be kept out of the working solution. Calomel electrodes can easily be prepared by shaking clean dry mercury with the powdered mercurous chloride which forms a skin around the mercury. The chloride ion solution is then carefully poured on top to complete the electrode. Home-made calomel electrodes can have a very low resistance and high performance. 

Treatment of certain polymeric surfaces with excited inert gases greatly improves the bond strength of adhesive joints prepared from these materials. With this technique, called plasma treatment, a low-pressure inert gas is activated by an electrode-less radio-frequency discharge or microwave excitation to produce metastable species which react with the polymeric surface. The type of plasma gas can be selected to initiate a wide assortment of chemical reactions. In the case of polyethylene, plasma treatment produces a strong, wettable, cross-linked skin. Commercial instruments are available that can treat polymeric materials in this manner. Table 7.10 presents bond strength of various plastic joints pretreated with activated gas and bonded with an epoxy adhesive. 

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