Electrode current carrying

Fig. 2.4j is a simplified diagram of an amperometric detector. Three electrodes are used, called working, auxiliary and reference electrodes (we ae and re). The we is the electrode at which the electroactivity is monitored, and the re, usually a silver-silver chloride electrode, provides a stable and reproducible voltage to which the potential of the we can be referenced. The ae, usually stainless steel, is a current-carrying electrode. 

The counter electrode is the current carrying electrode and it must be inert and larger in dimension. Platinum wire or foil is the most common counter electrode. For work with micro- or ultramicroelectrode where the maximum current demand is of the order of few microamperes, the counter electrode is not necessary. At very low current, a two-electrode system with the reference electrode can function as the current-carrying electrode with very little change in the composition of the reference electrode. Many commercial glucose sensors and on-chip microcells have such electrode configuration. 

At anodic potentials Anthracene oil, i.e. cresote, formed insoluble, insulating films on current-carrying electrodes (Figure 4). Figure 4a. shows oxidation traces from three successive cyclicvoltammograms with switching potential,... 

D.c. conductivity measurements require current-carrying electrodes. If these electrodes polarize, K can still be measured If I is kept constant. However, it is mostly more convenient to measure the a.c. conductivity, see sec, 4.5e. For capillaries the total conductivity equals the sum of the contributions of the bulk, and that of the surface, 2K /a if a is the capillary radius (see the denominator of [4.3.41]) ... 

Characteristics ofa Current-Carrying Electrode on an Oxide Electrolyte 399... 

An EC based on a LE is shown schematically in Fig.l. It is composed of a liquid that conducts ions and two current-carrying electrodes, an anode and a cathode (Ei, E2). An electrode can be composed of more than one piece of one kind of metal as when using a sacrificial anode. Additional electrodes (E,-ef) that do not carry currents are added in most cases to serve as reference electrodes in voltage measurements and analysis of cell operation. 

E2 - current-carrying electrodes Eref - reference electrode. 

Two current-carrying electrodes in an electrolyte are the source and sink of electrons—from electrons of the metal to ions or uncharged species of the electrolyte. 

A current carrying electrode is used to trigger (excite, pace) a nerve. The electrode may be on the skin (transcutaneous excitation), as a needle inserted through the skin, as a catheter electrode, as a part of an active implant, or as a microelectrode into an exposed nerve or axon. 

Important electrical characteristics of an electrode/tissue system are determined solely by the geometrical configuration. To clarify this important function, the systems to be treated in Chapter 6 are simple models suited for basic analysis and mathematical treatment as well as computational approaches such as finite element analysis (Section 6.5). In bioimpedance systems, the biomaterial is usually an ionic wet conductor, and the current carrying electrodes are polarized. However, in fliis chapter, the models are idealized in several ways. Biomaterial is considered homogeneous and isotropic. An electrode is considered isoelectric (superconducting metal). Only DC systems without polarization phenomena and frequency dependence are considered. Then a potential difference between two points in tissue space is equal to the voltage difference found between two circuit wires connected to the same two points,... 

In addition, the relative contribution of the deeper layers will increase. Even so, the sensitivity is proportional to the current density squared, so a given volume of tissue proximal to the electrodes is more important for the result flian the same volume in the deeper layers. By varying the distance between the electrodes, it is therefore possible to control the measuring depth. Ollmar and Nicander (1995) varied the measurement depth by inserting a third active current carrying electrode between the two measuring electrodes. 

Figure 6.22 Illustration showing current paths and sensitivity in a simulation of a tetrapolar impedance measurement system. Four electrodes on top of the model and a spherical object in the center of the model below the electrodes (a) Current paths for the two current-carrying electrodes are the dark lines to the left, and the current paths for the reciprocal currents are the gray to the right, (b) Sensitivity distribution in a slice through the model in the same level as a spherical object inside the model. The darkest regions indicate negative sensitivity. Courte of Fred Johan...

In the solution, the current spreads out from the current carrying electrodes, and we must use current density [A/m ] as quantity instead of current (see Eq. (2.3)). A flux is the flow rate through a cross-sectional area, so current density is a flux density. But the total sum of ionic charges passing through the solution is the current density integrated over the whole cross-sectional area, and per second this must equal the electronic current I in the external electrode wire. 

If the effect of the zones proximal to the current carrying electrodes and the polarization impedance of the electrodes themselves are to be reduced, the four-electrode system is preferred. Such four-electrode systems correspond to a two-port, four-terminal network equivalent (see Section 8.1). Because there are two ports, these systems actually measure transfer parameters between the ports. This means that if for instance impedance is measured to 0 O, this does not necessarily imply high-conductivity tissue, but rather no signal transfer from CC to PU electrodes. 

Current-carrying electrode. CC electrodes are polarized because they are current carrying. 

A monopolar current carrying electrode system has one small electrode as the... 

Plethysmography and body composition estimation. In Figure 7.31, the usual configuration is shown with the current carrying electrodes CC and CC as the outer electrodes. 

A DC or pulse current polarizes the electrode, and from the electrolytic basic experiment described in Section 2.2 it is also clear that faradaic current flow changes the chemical environment at the electrode surface. Current carrying electrodes are used in such different applications as nerve stimulation, pacemaker catheter stimulation and defibrillation with 50 A passing for some milliseconds. Often a square wave pulse is used as stimulation waveform (e.g., pacemaker), and the necessary overvoltage is of great interest (see Section 9.1). In such applications a clear distinction must be made between tissue nonlinearity (Section 8.4.3) and electrode nonlinearity (this section). Nonlinearity network theory is treated in Section 7.9.3. 

In a practical case when current carrying electrodes are used with tissue, it may also be difficult to differentiate between the nonlinearity of the electrode processes and the tissue processes. 

To analyze the situation with a tetrapolar electrode system in contact with, for example, a human body, we must leave our simplified models and turn to lead field theory (see Section 6.4). The total measured transfer impedance measured is the ratio of recorded voltage to injected current according to Eq. 6.39. The impedance is the sum of the impedance contributions from each small volume dv in the measured volume. In each small volume, the resistance contribution is the resistivity multiplied by the vector dot product of the space vectors (the local current density from a unit reciprocal current applied to the recording electrodes) and (the local current density from a unit current applied to the true current carrying electrodes). With disk-formed surface electrodes, the constrictional resistance increase from the proximal zone of the electrodes may reduce sensitivity considerably. A prerequisite for two-electrode methods is therefore large band electrodes with minimal current constriction. 

If the system is reciprocal, the swapping of the recording and current carrying electrode pairs shall give the same transfer impedance. It is also possible to have the eleetrode system situated into the volume of interest, for example, as needles or catheters. Sueh volume calculation, for example, of cardiae output, is used in some implantable heart pacemaker designs (see Seetion 10.12.3). 

Cheng K. S., Simske S. J., Isaacson D., Newell J. C., and Gisser D. G., Errors due to measuring voltage on current-carrying electrodes in electric current computed tomography, IEEE Trans. Biomed. Eng., vol. 37, no. 60, pp. 60-65,1990. 

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