Electrode-skin interface

Safety is an important factor when determining the quality of any iontophoresis electrode. During transdermal iontophoretic delivery using metal electrodes, an applied DC current will induce pH changes on the electrode/skin interface [178], pH measurement is used to eliminate the possibility of unsafe pH changes (chemical bums). It has been reported that the pH shift caused by platinum electrodes has a significant influence on the permeation and stability of insulin [175],... 

Cooper, G., Barker, A.T., Heller, B.W., Good, T., Kenney, L.P.J., Howard, D., 2011. The use of hydrogel as an electrode-skin interface for electrode array FES appHcations. Medical... 

As indicated in Fig. 1, a transdermal iontophoretic system requires that two electrode assemblies contact the patient s skin. The donor electrode (also known as the delivery or active electrode) contacts the drug reservoir. The counter electrode (also known as the return or receptor electrode) contacts the counter reservoir and completes the electrical circuit by providing a path for the current. The two reservoirs are separated from each other and contact skin over a fixed area. The electrodes apply an electric field across the skin by converting electric current supplied by the battery into ionic current moving in the skin and body. In doing so, a Faradaic reaction takes place at the electrode/ electrolyte interface. As described previously in this chapter, there is generally a linear dependence of the rate of drug delivery on this current. 

Cortical Motor cortex interface electrode Skin... 

Electrodes for external defibrillation are connected to the defibrillator and are of two types hand held and self-adhesive. The hand-held ones, shown in Figure 14.5, are most often used by medical personnel and have a metal surface area between 70 and 100 cm in. They must be coupled to the skin with an electrically conductive gel material that is specifically formulated for defibrillation to achieve low impedance across the electrode-patient interface. Hand-held electrodes are reusable. 

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. 

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. 

The fact that useful spectra can be obtained from polymers in various forms, from fibers which cannot be studied by transmission techniques, from other intractable materials, from aqueous solutions, etc., should make this technique useful in many disciplines. The use of ATR for the study of the chemistry of surfaces should be further explored in biochemical applications, for example, deposition of monolayers from solution (see, for example, Sharpe, 1961, 1965). The ATR technique has been used for analysis of bacterial cultures (Johnson, 1966) and in forensic science (Denton, 1965). It has also been applied to a great variety of substances molecular species present at electrode interfaces (Hansen et al., 1966 Mark and Pons, 1966) carbohydrates (Parker and Ans, 1966) a single crystal of pentaerythritol (Tsuji et al., 1970) cosmetics on the skin (Wilks Scientific Corp., 1966) pesticidal traces (Hermann, 1965a) water-alcohol mixtures (Malone and Flournoy, 1965) nitrate ion (Wilhite and Ellis, 1963) leather (Pettit and Carter, 1964) and blood spectra from within the human circulatory system (Kapany and Silbertrust, 1964)l The last-mentioned application requires special equipment. 

The metal/electrolyte interface area is called the electrode area (EA). EA may be the plane area or include a surface roughness or fractal factor. The interface area of the contact medium and the tissue is called the effective electrode area (EEA). Skin surface electrodes are often EEA > EA (Eigure 7.30). Electrolyte-fiiied glass micro-electrodes are often EEA < EA. 

According to electrochemical theory, the kinetics of an electrochemical reaction is controlled by the potential drop between the solid and solution phases [133-136]. A dynamic zone extending in both directions from the electrified interface over which this drop exists is called the double layer (DL) of charge. The DL in the solution is made up of adsorbed and solvated ions (molecules) and solvent. Its dense part, which is referred to as the Helmholtz layer (HL), plays the major role in the interfacial processes. At low ion concentration, there is also a diffuse layer Gouy layer) in the solution. The countercharged part of the DL in a metal electrode is comprised of a skin layer with an excess or a deficit of electrons. The DL in a semicondnctor electrode is called the space charge layer. It consists of an accumulation, depletion, or inversion layer with an excess or a deficit of electrons or holes and ionized donor or acceptor states, depending on... 

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. 

The best bioelectric interfaces are combinations of metals and their metallic salts, usually chlorides. The metal salt is used as a coating on the base metal and acts as an intermediary in the electrode-electrolyte processes. Silver in combination with a chloride coating is the most widely used biopotential recording electrode. Silver chemically reacts in chloride-bearing fluids such as saline, skin sweat, and body fluids containing potassium and sodium chloride. After a few hours of immersion, a silver electrode will become coated with a thin layer of silver chlorides. This occurs by the spontaneous reaction ... 

The third type of movement detection is electrical. These control interfaces sense bioelectric signals. Switches of this type require no force, and a common example of this type of interface is capacitive switches used on some elevator buttons. Bioelectrical switches detect muscle electrical activity (EMGs) or eye movement (EOG) by attaching electrodes to the skin. 

The classic, high-quality electrode design consists of a highly conductive metal (silver) interfaced to its salt (silver chloride) and connected via an electrolytic gel to the human body [21]. Silver-silver chloride-based electrode design is known to produce the lowest and most stable junction potentials [1,20]. Junction potentials are the result of the dissimilar electrolytic interfaces and are a serious source of electrode-based motion artifacts. Therefore, additionally, an electrolytic gel typically based on sodium or potassium chloride is applied to the electrode. A gel concentration on the order of 0.1 M (molar concentration) results in a good conductivity and low junction potential without causing skin irritation. 

Hg Hg2Cl2 electrode preparation (see Figure 4.11). Method I (28). On adding the Hg2Cl2 to the dry mercury it will rapidly form a pearly skin on the surface. Once the surface is completely covered, the addition of Hg2Cl2 is stopped, otherwise the electrode will be sluggish. This interface must be prepared prior to the introduction of filling solution for a stable potential to develop. 

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