Skin-electrode resistance

For example, for a 500kW 500pF unit with p s = 4 x 10-7Q Hz-1 2, Pe is 0.02W at 0.1 MHz and 7kW at 500 MHz. Therefore it is evident that below 1 MHz the major contribution to heat generation is dielectric loss, whilst at higher frequencies a significant proportion is due to electrode resistance, and that, because of the skin effect, this resistance cannot be reduced by making the electrodes or leads thicker than a small fraction of a millimetre. However, the thicker the electrodes and leads are the better is the heat transfer from the capacitor. 

A DC voltage connected to a skin electrode pair generates a DC current in the electrode wires. The resistance R = V/I, but V comprises both the tissue voltage drop and the... 

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. 

Three major problems with this approach are skin irritation, interface problems at the wood-metal interface, and varying electrode resistance as a function of moisture content in the balsa layer. 

Zirconium oxide (ZrO ) is the most common compound of zirconium found in nature. It has many uses, including the production of heat-resistant fabrics and high-temperature electrodes and tools, as well as in the treatment of skin diseases. The mineral baddeleyite (known as zirconia or ZrO ) is the natural form of zirconium oxide and is used to produce metallic zirconium by the use of the Kroll process. The KroU process is used to produce titanium metal as well as zirconium. The metals, in the form of metaUic tetrachlorides, are reduced with magnesium metal and then heated to red-hot under normal pressure in the presence of a blanket of inert gas such as helium or argon. 

Q.22.2 When most people lie or make an untrue statement their sympathetic nervous system becomes active (fight or flight response). This leads to increased sweat secretion and thus decreased skin resistance. In a lie detector, a current is passed between two silver chloride electrodes held at 5 v. A current of 4.2 mA is measured when the subject gives his name. Later in the interview he lies and a current of 8.9 mA is measured. What is the skin resistance in the (a) honest and (b) dishonest state ... 

These inmitive concepts stand at the foundation of the electrical SHM methods for composite materials. This approach is deemed self-sensing because it relies entirely on measuring a material property (i.e., electrical characteristic) and does not require an additional transduction sensor the only instrumentation that needs to be installed on the composite structure consists of the electrodes. In the case of composite transport aircraft, the conductive screen skins currently used to mitigate lightning strike could potentially also serve as the measuring electrodes. Electrical SHM methods range from the simple measurement of the electrical resistance measurements up to more sophisticated methods such as electrical potential mapping, dielectric measurement, and electrochemical impedance. 

To avoid burns under dispersive electrodes, the lEC Standard for HF Surgical Devices [5] requires that HF surgical equipment having a rated output power of more than 50 W shall be provided with a continuity monitor or contact quality monitor. The most common of these is the contact quahty monitor. A contact quality monitor consists of a circuit to measure the impedance between the two sides of a split dispersive electrode and the skin. This impedance is inversely proportional to the actual area of contact between the patient and the dispersive electrode. A small high-frequency current flows from one section of the dispersive electrode through the skin to the second section of the dispersive electrode. If the impedance between these two sections exceeds a certain threshold, or increases by a certain percentage, the patient contact area has unacceptably decreased, an audible alarm sounds, and the ESU output is disabled. The cable continuity monitor is less common. Unlike the contact quality monitor, this monitor only checks the continuity of the cable between the ESU and the dispersive electrode and sounds an alarm if the resistance in that conductor is greater than 1 2. 

Knowing the SC thickness enabled the authors to calculate the parallel resistivity and relative permittivity of the removed SC. Furthermore, the resistivity and relative permittivity of the viable skin was calculated by assuming homogenous electrical properties and using the formula for constrictional resistance (cf. Figure 6.3). The resistance of the disk surface electrode is (Eq. 6.17) R = p/4a. Because RC = pcj-eo, the relative permittivity of the viable skin can be calculated from the measured capacitance with a similar formula ... 

Without the skin and with the living body tissue considered purely resistive, the resistance, R, of a body segment is determined by the mean resistivity, p mean length, L and mean cross-sectional area. A, according to R = pL/A. The resistance can be measured by a four-electrode technique (Freiberger, 1933 Grimnes, 1983a). 

Figure 4.26 Body segment resistance distribution (no skin contribution, no current constriction). Values presented are as found with a four-electrode technique 500 Q is a one-finger contribution. Linear values according to Eq. 2.2 are not very dependent on current density levels.

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. 

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... 

Without the protective action of the skin, the internal body resistance may be divided into a constrictional zone resistance with increased current density near an electrode, and segmental resistances of each body segment with rather uniform current density. With small area electrode contact, the constrictional zone resistance will dominate (see Section 6.2.2). The segmental resistance may be estimated from the equation Rsr = L/aA. With constant a, the segmental resistance depends on the ratio L/A, and accordingly varies according to body or limb size. 

Placement of electrodes directly on the skin will show a large electrical resistance, often in the megohm (10 ) region. This is due mostly to the surface layer of dead and dehydrated skin cells on the epidermis, known as the corneum. Below the epidermis, the next sublayer is the living part of the skin, known as the dermis. It exhibits a complex electrical behavior having a resistance, capacitance, and potential generators similar in some respects to that of electrodes. 

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. 

Whenever a person is connected to an electrical device by a grounded conductive pathway that is low resistance, such as through biopotential electrodes, there is a concern about electrical safety. Electric voltages that ordinarily would be harmless to casual skin contact can become dangerous or even lethal if someone happens to be well grounded. Wetted skin can provide low resistance, and from this stems the old adage about the inadvisability of standing in pools of water when around electric appliances. 

---