Electrodes for Biophysical Sensing Bioelectricity is a naturally occurring phenomenon that arises from the fact that living organisms are compared of ions in various different quantities. Ionic conduction involves the migration of ions- positively and negatively charged molecules throughout a region. Electronic conduction involves the flow of electrons under the influence of an electrical field.
Cont… Electrical conduction in medical instrument circuits is electronic. Electrical conduction in the body is ionic. Electrodes provide transduction between ionic and electronic conduction. Biopotential electrodes function as sensor that couple the ionic potentials generated inside the body to an electronic instrument.
Bioelectrodes Bioelectrodes are a class of sensors that transduce ionic conduction to electronic conduction so that the signal can be processed in electric circuits. The usual purpose of bioelectrodes is to acquire medically significant bioelectrica1 signals. Such as Electrocardiographic(ECG), Electroencephalographic (EEG), and Electromyographic (EMG). Bioelectrical signals are acquired from one of three forms of electrode: Surface Electrodes, Indwelling Electrodes, and Micro Electrodes
Electrode or Half Cell Potential The skin and other tissues of higher-order organisms, such as humans, are electrolytic and so can be modeled as an Electrolytic Solution. Imagine a metallic electrode immersed in an electrolytic solution. Immediately after immersion, the electrode will begin to discharge some metallic ions into the solution, while some of the ions in the solution start combining with the metallic electrodes. A gradient charge build up, creating a potential difference, or electrode potential and half cell potential.
Cont… A complex phenomenon is seen at the interface between the metallic electrode and the electrolyte. Ions migrate toward one side of the region or another, forming two parallel layers of ions of opposite charge. This region is called the electrode double layer and its ionic differences are the source of the electrode or half- cell potential. Metallic electrode immersed in an electrolytic solution
Electrode Offset Potential When two dissimilar metals immersed in a common electrolytic solution, they both form the Half cell potential. The differential potential between these two Half cell potential is called an electrode offset potential.
Electrode or Half Cell Potential The Half-cell potential of the electrode depends on: i. The used metals, ii. The electrolyte composition iii. The temperature Different material exhibits different half-cell potentials
Polarizable and Non-Polarizable Electrode I. Perfectly Polarizable Electrodes Perfectly polarizable electrodes are those in which no actual charge crosses the electrode–electrolyte interface when a current is applied. Of course, there has to be current across the interface and the electrode behaves as though it were a capacitor II. Perfectly Polarizable Electrodes or Perfectly Reversible Perfectly non-polarizable electrodes are those in which current passes freely across the electrode–electrolyte interface, requiring no energy to make the transition. Thus, for perfectly non-polarizable electrodes there are no over-potentials. Electrode interface impedance is represented as a resistor.
Medical Surface Electrodes Surface electrodes are those that are placed in contact with the skin of the subject. Also in this category are certain needle electrodes of a sire that prevents their being inserted in the cell. A conductive gel or paste is used to reduce the impedance between the electrode and skin.
Cont… Human skin tends to have a very high impedance compared with other voltage sources. Typically, normal skin impedance, as seen by the electrode. For sweaty skin varies from 0.5 k Ω For dry skin surfaces to more than 20 k Ω Problem skin especially, dry, scaly, or diseased skin, may reach impedances in the 500 k Ω range.
Types of Biopotential Electrodes Bioelectrical signals are acquired from one of three forms of electrode: Body Surface Electrodes, Needle Electrodes Micro Electrodes
Body Surface Electrodes There are four different types of body surface recording electrodes; 1. Column Electrodes 2. Suction Electrodes 3. Floating Electrodes 4. Flexible Electrodes
Column Electrodes The electrode consists of a silver-silver chloride metal contact button at the top of a hollow column that is filled with a conductive gel or paste. This assembly is held in place by the adhesive coated foam rubber disk The use a gel filled or paste filled column that holds the actual metallic electrode off the surface reduces movement artifact. Far this reason the column electrodes are preferred for monitoring hospitalized patients.
Column Electrodes Large surface: Earliest, and still used for ECG. Smaller diameters. Used for ECG, EMG and EEG. Susceptible to Motion artifacts. Disposable foam-pad. Very Cheap. Used for long term recording
Figure (a): Metal-plate electrode used for application to limbs. Figure (b): Metal-disk electrode applied with surgical tape. Figure (c): Column electrodes, often used with ECG.
Suction Cup Electrodes Straps or adhesives not required. Often used for precordial (chest) ECG. For short periods only.
Floating Electrodes Figure (a): Recessed electrode with top-hat structure. Figure (b): Cross-sectional view of the reusable electrode in (a). Metal disk is recessed. Floating in the electrolyte gel. Not directly contact with the skin. Reduces motion artifacts.
Flexible Electrodes Figure (a): Carbon-filled silicone rubber electrode, Figure (b): Flexible thin-film neonatal electrode. Figure (c): Cross-sectional view of the thin-film electrode in (b). Body surface are often irregular. Regularly shaped rigid electrodes may not always work. Special case : infants. Material: polymer or nylon with silver, carbon filled silicon rubber (Mylar film).
Problems with Surface Electrode Several problems are associate with all types of surface electrodes: 1. Adhesive will not stick for long on sweaty or clammy skin surfaces. 2. Fleshy portion of chest and abdomen are selected as electrode site. 3. After 8 hour change the electrode to avoid the ischemia. 4. Movement Artifacts 5. Electrode position slips
Needle Electrode This type of electrode is inserted into the tissue immediately beneath the skin by puncturing the skin at a large oblique angle (i.e.,close to horizontal with respect to the skin surface). The needle electrode is only used for exceptionally poor skin, especially an anesthetized patients, and in veterinary situations. Of course, infection is an issue in these cases, so needle electrodes are either disposable (one time use) or are resterilized in ethylene oxide gas.
Cont… Figure: (a)Insulated needle electrode, (b)Coaxial needle electrode, (c)Bipolar coaxial electrode, (d)Fine-wire electrode connected to hypodermic needle, before being inserted, (e)Cross-sectional view of skin and muscle, showing fine-wire electrode in place, (f)Cross-sectional view of skin and muscle, showing coiled fine-wire electrode in place. Needle and wire electrodes for percutaneous measurement of Biopotentials.
Indwelling Electrode lndwelling electrodes are intended to be inserted into the body. These are not to be confused with needle electrodes, which are intended for insertion into the layers beneath the skin The indwelling electrode is typically a tiny, exposed metallic contact at the end of a long, insulated catheter. In one application, the electrode is threaded through the patient's veins (usually in the right arm) to the right side of the heart to measure the intracardiac ECC waveform.
Cont… Certain low amplitude, high-frequency features (such as the signal from bundle of His) become visible only when an indwelling electrode is used.
Fetal ECG Electrodes Figure (a): Suction electrode, Figure (b): Cross-sectional view of suction electrode in place, showing penetration of probe through epidermis, Figure (c): Helical electrode, that is attached to fetal skin by corkscrew-type action. Electrodes for detecting fetal electrocardiogram during labor.
EEG Electrodes The brain produce bioelectric signals that can be picked up through surface electrodes attached to the scalp. These electrodes will be connected to an EEG amplifier that driver either an oscilloscope or strip chart recorder. Typical Needle electrode is used. The disc electrode have 1cm diameter concave disc made either of silver and gold. The disc electrode in a place by a thick paste that is highly conductive, or by a headband in certain monitoring applications.
Microelectrode The microelectrode is an ultrafine device that is used to measure biopotentials at the cellular level. In practice, the microelectrode penetrates a cell that is immersed in an infinite fluid (such as physiological saline), which is in turn connected to a reference electrode. Although several types of microelectrodes exist, most of them are of one of two basic forms: metallic-contact or fluid-filled. In both cases, an exposed contact surface is about 1to 2 um is in contact with cell.
Microelectrode used to measure the cellular potential
Microelectrodes There are three major types of microelectrode. Solid Metal Electrodes (Tungsten Microelectrodes) Glass-Metal or Supported Metal Electrodes (metal contained within/outside glass needle) Fluid-Filled Electrodes (with Ag-AgCl electrode metal)
Solid Metal Electrode
Glass-Metal Electrodes A very fine platinum or tungsten wire is slip-fit through a 1.5 to 2 mm glass pipette. The electrode can then be connected to one input of the signals amplifier. There are two subcategories of glass-metal electrodes. In the first type, the metallic tip is flush with the end of the pipette taper. In the second type, a thin layer of glass covers the metal point. This glass layer is so thin that it requires measurement in angstroms (1 angstrom = 1.0 × meters) and it drastically increases the impedance of the device.
Fluid-Filled Electrodes The fluid-filled glass microelectrode is shown in the fig. In this type of electrode, the glass pipette is filled with a solution of potassium chloride (KCI), and the large end is capped with an silver-silver chloride (Ag-Ag Cl) plug. The small end need not be capped because the 1 μ m opening is small enough to contain the fluid. The reference electrode is likewise filled with potassium chloride (KCI), but is much larger than the microelectrode. A platinum plug contains fluid on the interface end, while an silver-silver chloride (Ag-Ag Cl) plug caps the other end.
Assignment Biosensor..? Principle and how it works ? Advantages and Disadvantages?