Skin Electrodes

The area of the skin wetted or in contact with the electrolyte solution is called the effective electrode area of the electrode. EEA is a dominating factor determining electrode/skin impedance. EEA may he much larger than the metal area in contact with the solution (EA), which determines the polarization impedance. The electrode is fixed with a tape ring outside the EEA. Electrolyte penetrating the tape area increases EEA, hut reduces the tape sticking area. Pressure on the electrode does not squeeze electrolyte out on the skin surface because of the rigid container construction. 

Band electrodes are used as large-surface, low-impedance electrodes with the additional attractive feature of reducing the current constrictional effect found with smaller disk electrodes. This is important in two electrode apphcations such as impedance. 

Consider the consequence of swapping the current carrying and recording electrodes. 

The problem with a plate electrode in some apphcations is the covering effect on the tissue. With a multiple-point electrode, humidity for instance can escape from the surface. Dependent on tip sharpness and electrode pressure the points may penetrate the superficial layers. 

Yamamoto et al. (1986) used a two-electrode quasibipolar system, with the large electrode having about 10 times the area of the small one. The concentric electrodes were positioned spring mounted in a tube to secure a constant electrode mechanical pressure on the tissue 

---

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. 

FIGURE 4.4 Examples of different skin electrodes (a) metal plate electrodes, (b) suction electrode for EGG, (c) metal cup EEG electrode, (d) recessed electrode, (e) disposable electrode with electrolyte-impregnated sponge (shown in cross section), (f) disposable hydrogel electrode (shown in cross section), (g) thin-film electrode for use with neonates (shown in cross section), and (h) carbon-filled elastomer dry electrode. 

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. ... 

Transplanted organs, such as heart or kidney, can be followed with impedance measurements using skin electrodes or implanted leads or catheters (Ollmar 1997). 

Figure 7.8 shows a capacitive electrode system. One half-cell is an ordinary indifferent electrolyte-skin electrode. In the other half-cell, the electrolyte has been replaced by a dielectric so that there is no galvanic coupling between the electrode metal plate and the skin. The dielectric is usually a thin layer of a chloride or oxide of the metal in the electrode plate. The metal plate functions as a substrate for the thin dielectric that can be made from anodizing or oxidizing silver, aluminum, silicon, or titan. The dielectric must be robust and endure humidity and sweat arriving from the skin. The capacitance between the metal plate and the skin is dependent on contact area, dielectric thickness, and... 

Polarization comprises both dynamic and static properties. An AC voltage u connected to a skin electrode pair generates an AC current in the electrode wires. The impedance is Z = uH and it comprises both the tissue impedance and the double electrode polarization impedances. They are physically coupled in series. Can we avoid the electrode... 

Figure 7.33 Skin electrode wires. Courtesy Tormod Martinsen.

The skin potential (SP) method measures potential difference between two skin electrodes without current flow. It is the endogenous potential level and response generated by the body itself. Usually the palmar and plantar skin sites are active during a response other sites are passive if the person is relaxed. Both SP electrodes must not be placed on active sites, because if they are equally active no change in potential difference will occur. If both sites are more or less active, the measured potential difference may be small or even with reversed polarity. One electrode is placed at an active site (e.g., the hypothenar), and the other must be at a passive skin site (e.g., the forearm). See more about passive skin sites in Chapter 7.11, Electrode Terminology. 

The afferent pain nerves have a higher threshold and rheobase than sensory and motor nerves. Thus it is possible to stimulate sensory and motor nerves without ehciting pain. Very short pulses of 10—400 ps duration are used, with constant amplitude current up to about 50 mA and treatment duration of 15 min or more. The skin electrodes may be bipolar or monopolar. The position is in the pain region an electrode pair may, for example, be positioned on the skin on the back of the patient, or implanted with thin leads out through the skin. The electrode pair may also be positioned outside the pain area (e.g., at regions of high afferent nerve fiber densities in the hand). 

Jossinet, J., McAdams, E.T., 1991. The skin/electrode interphase impedance. Innov. Technol. Biol. Med. 12 (1), 22-31. 

Depending on the electrode composition and its area, electrode-electrolyte resistance is on the order of a few hundred ohms with typical ECG skin electrodes and thousands to millions of ohms with small wire electrodes and microelectrodes. Electrode interface resistance is usually not large compared with other resistance in the electrode-biological circuit. 

This bridges the electrode surface to the skin. The gel is an aqueous chloride-bearing solution that will hydrate the skin, reduce the impedance of the comeum, and produce a more uniform medium for charge transfer. Skin-electrode impedance can drop below S kfl across the hydrated skin and abraded comeum. High impedances in the range of 20 to 100 kfl are not uncommon if the skin is not adequately prepared, i.e., abraded and clean. 

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... 

In practice, this interference cancellation process works fairly well however, the assumption that the power-line-induced signal is common mode (uniform) over the body does not hold perfectly in all situations. Slight differences in its phase or amplitude over the subject s body when in close proximity to some electric field source or unbalanced electrode impedances can cause this cancellation process to be less than perfect. Some line noise may still pass through into the recording. Usually, improvements in skin electrode preparation can remedy this problem. 

Presence of biological interference (from skin, electrodes, motion, etc.)... 

Serological reagents (73), Laboratory equipment and reagents (10), Test systems (67), Tumour-associated antigen immunological test systems (5) Neurology devices (106) Diagnostic Skin Skin electrodes... 

Respiratory rate is measured by techniques based on either measuring thoracic expansion or based on measuring changes in skin impedance. For the former technique, most systems use strain gauges made from piezoresistive material combined with textile structures. Hertleer et al. reported a fabric sensor made of SS yam knitted in spandex belt [18]. For the latter technique, noninvasive skin electrodes are placed on the thorax, and the variation of the electrical impedance can be detected during respiration cycles. 

Nerve conduction Stressors such as lead and some solvents whidi can affect nervous activity - uses skin electrodes and an oscilloscope... 

Amatneek, K. F. and Simenhoff, M. L., 1966, A 24-hour monitoring skin electrode, Proc. 19th ACEMB, San Francisco, p. 34. 

Morris, T. W., 1967, Skin-electrode impedance of long term electrodes, Proc. 20th ACEMB, Boston, p. 15.5. 

Transcutaneous pacing, also referred to as external or noninva-sive pacing, involves the delivery of electrical Impulses through externally applied cutaneous electrodes. The electrical impulses are conducted through an intact chest wall using skin electrodes placed in either anterior-posterior or sternal-apex positions. (An anterior-posterior placement is shown here.)... 

Skin <1 day Examinatioiv surgery Adhesive skin electrode, urethra catheter, surgical drape Acrylic electroconductive gel silicone, polyvinyl chloride, etc. polyethylene, polyurethane, cellulose, alginic acid gel, etc. 

---