Electrode-skin impedance

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. 

Figure 7.30(b) shows an electrode with solid contact gel. The gel is sticky and serves both as contact electrolyte and for electrode fixation. For this electrode, EEA = EA. The electrolyte conductivity is rather low because the solution is a gel with low ionic mobility. Electrolyte series resistance may be the dominating factor of electrode/skin impedance at high frequencies. This may introduce problems for use (e.g., for impedance plethysmography around 50 kHz). With this electrode type the skin is not wetted. With such constructions it is... 

McAdams, E.T., Jossinet, J., 1991b. The impedance of electrode-skin impedance in high resolution electrocardiography. Automedica 13, 187—208. 

For example, if a subject s electrode-skin impedance Zgicctmie is this value is low enough... 

The impedance of the skin is dominated by the SC at low frequencies. It has generally been stated that skin impedance is determined mainly by the SC at frequencies below 10 kHz and by the viable skin at higher frequencies (Ackmann and Seitz, 1984). This will of course be dependent on factors such as skin hydration, electrode size and geometry etc., but it may nevertheless serve as a rough guideline. A finite element simulation on a concentric two-electrode system used by Yamamoto et al. (1986) showed that the SC accounted for approximately 50% of the measured skin impedance at 10 kHz but only approximately 10% at 100 kHz (Martinsen et al., 1999). 

Yamamoto and Yamamoto (1976) measured skin impedance on the ventral side of the forearm with a two-electrode system and an AC bridge. They used Beckman silver/silver chloride electrodes filled with gel and measured 30 min after the electrodes had been applied. The skin was stripped with cellulose tape 15 times, after which the entire SC was believed to have been removed. Impedance measurements were also performed between each stripping so that the impedance of the removed layers could be calculated. The thickness of the SC was found to be 40 pm, which is more than the common average values found elsewhere in the literature. For example, Therkildsen et al. (1998) found a mean thickness of 13.3 pm (minimum 8 pm/maximum 22 pm) when analyzing 57 samples from nonfriction skin sites on Caucasian volunteers. However, the moisture increase caused by electrode occlusion and electrode gel has most certainly significantly increased the SC thickness. 

In low-frequency applications (<100 Hz), the skin impedance is very high compared with the polarization impedance of wet electrodes and deeper tissue impedance. The SC consists of dead and dry tissue, and its admittance is very dependent on the state of the superficial layers and the water content (humidity) of the surrounding air in contact with the skin before electrode onset. In addition, the sweat ducts shunt the SC with a very variable DC conductance. Sweat fills the ducts and moisturizes the surrounding SC. Therefore, the state of the skin and measured skin admittance at the time of electrode... 

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 value of the impedance between two skin surface electrodes is usually dominated by the contribution of the skin. However, the skin impedance may be negligible if... 

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. ...

In some skin applications, the electrode polarization impedance may still he a source of error. With solid gel contact electrolytes, the series resistance of the contact medium may he disturbing at higher frequencies. When the stratum corneum is highly penetrated hy electrolytes (Figure 7.5), the skin impedance is so low (50 kO) that the electrode polarization impedance becomes important. 

Even a light abrasion of the skin surface is a surprisingly effective way of lowering the skin impedance the first half-hour or so. This is because dry stratum corneum contributes more than 99% of the total electrode impedance (10 Hz, see Figure 7.6). Because the stratum corneum is very thin, about 10—15 pm except on palmar and plantar sites, removing just 5 pm of surface layers can be quite effective. 

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

In a two-electrode unipolar impedance system, it may be difficult to estimate a possible contribution from the neutral electrode. Often only a zone of the current path proximal to the measuring electrode is of interest (e.g., when measuring skin admittance). In such cases, the distal volume segment of the current path is a disturbing part. It may also be difficult to work with a sufficiently large neutral electrode in some situations. By adding a third electrode. Figure 7.24, it becomes easier to control the measured tissue zone. The... 

Now the point is that the noise current does not go to M, so it is not measured, it is canceled A condition is that A is able to give the necessary current and the necessary output voltage to overcome the voltage drop in the skin impedance under the CC electrode. 

Two electrode pairs are used to set up two different current paths crossing each other in the target tissue volume. Each pair is supplied by a separate oscillator, adjusted to, for example, 5000 and 5100 Hz. The idea is that the target volume is treated with the frequency difference, 100 Hz. The advantage is the possible selective choice of a limited treated volume deep in the tissue, together with lower electrode polarization and skin impedance, plus less sensation in the skin and the tissue outside the treated volume. 

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

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. 

In addition to changes in abdominal impedance itself, also the measurement setup can influence the results, for example by movement of electrodes or poor electrode-skin contact... 

Figure 7.6 Clinical prototype for treatment with low-frequency ultrasound. 12 W of 55 kHz ultrasound is applied to a skin area of 0.8 cm 2 until the impedance is below 10 KX2. The hand grip serves as the return electrode for the impedance measurement. The coupling media and ultrasonic horn are within the handpiece housing. Reprinted with permission from Ref. 9. Copyright 2004 Mary Ann Liebert, Inc. publishers.

Yamamoto and Yamamoto [8,9] showed that not only is the skin s impedance profile quite complex, but, in addition, it was demonstrated that the skin s impedance changes in a very complex manner. In particular, the skin s impedance depends on the season, the time of day at which the impedance is determined, the state of the subject studied, the site at which the impedance is measured, and the electrode paste used to make the impedance measurements. Other investigators have confirmed and amplified on these observations [10-12,14,15,17]. For example DeNuzzio and Berner [12] demonstrated the important influence that the type of electrolyte present around the electrodes have on the skin s impedance Clar et al. [10] showed that the time variation occurred not only during the day but from day to day and Panescu et al. [14] demonstrated that the average impedance of the forearm was less than that for the palm. 

Because at the millimeter scale different regions of the skin have very different electrical characteristics [14,19-22], electrode dimensions should be on the order of 1 cm. For human skin in vivo, this is due in part to the presence of sweat glands, which can represent low-impedance pathways when activated [17]. 

Electrolysis at the electrodes needs to be minimized, since this will alter electrolyte composition. Partly for this reason, impedance measurements should be made at current densities on the order of 1 xA/cm or less [17]. Making impedance measurements at 1 pA/cm or less has the added advantage of minimizing iontophoresis and electroosmosis, which can decrease the skin s impedance with time [16,17]. 

Impedance spectroscopy has been used in the assessment of skin diseases such as eczema and psoriasis [7]. However, these studies suffer in that these diseases exhibit cracking of the skin. This cracking implies that defective electrode contact is probably made during impedance measurement [7]. 

Grimnes, S. Impedance measurement of individual skin surface electrodes. Med. Biol. Eng. Comput. 27 750, 1983. 

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