Electrode theory Basic electrode Electrodes for ECG n

Electrode theory. Basic electrode, Electrodes for ECG n Electrode theory n Need, types of Bio-potential electrodes. n Basic electrode theory n Various Electrodes for ECG.

ELECTRODE - ELECTROLYTE & ELECTROLYTETISSUE INTERFACE

Electric equivalent circuits

Electrode theory- Basic electrode, n Need & Types of Bio-potential electrodes n n n Surface electrodes Deep seated electrode Factors to be consider while measurements: Ø Careful and suitable selection of electrodes for satisfactory record of bioelectric signal. Ø Comfortable for the patients to wear over long period Ø They should not produce any moving artefacts Ø Convenient in practical applications Ø Good contact with skin (for low contact impedance)

Cont. . n The characteristics of a metalic/nonmetalic surface electrodes depends uponn Condition at metal-electrolyte interface n Electrolyte –skin interface n Quality of electrolytes n Electrode properties Why electrode jelly is required while measurements of biopotential signals?

Cont. . n Electrode Potentials All electrode potentials are measured wrt a ref. Electrodes n Ref. Electrodes-Hydrogen electrode (H absorbed on platinum back -Calomel electrode Electrode potentials of few metal electrodes wrt Hydrogen n Metal Ionic symbol Electrode Potential(Volt) Aluminium Al+++ -1. 66 Iron Fe++ -0. 44 Lead Pb++ -0. 12 Hydrogen H+ -0. 0 Copper C++ +0. 34 Silver Ag+ +0. 80 Platinum Pt+ +1. 2 gold Au+ +1. 69

Cont. . q potential between Electrode s in electrolyte Electode Metal Electrolyte Potential difference between Electrode (m. V) Stainless steel Saline 10 silver Saline 9. 4 Silver-silver chloride Saline 2. 5 Silver-silver chloride (11 mm disc) ECG paste 0. 47 Silver-silver chloride (sponge) ECG paste 0. 2

L 6: Electrodes for ECG, EEG, EMG n Electrodes for ECG. n Electrodes for EEG, n Electrodes for EMG, n Biochemical Electrodes

Electrodes for ECG, EEG, EMG Electrodes are used to pickup the biopotentials from the surface of the body of from inside the cells. Basic electrodes are classified as: v v v Skin surface electrodes Needle electrodes Microelectrodes

Body Surface Recording Electrodes Electrode metal Electrolyte 1. Metal Plate Electrodes (historic) 2. Suction Electrodes (historic interest) 3. Floating Electrodes 4. Flexible Electrodes

Commonly Used Biopotential Electrodes Metal plate electrodes n Large surface: Ancient, therefore still used, ECG n Metal disk with stainless steel; platinum or gold coated n EMG, EEG n smaller diameters n motion artifacts n Disposable foam-pad: Cheap! (a) Metal-plate electrode used for application to limbs. (b) Metal-disk electrode applied with surgical tape. (c)Disposable foam-pad electrodes, often used with ECG

Commonly Used Biopotential Electrodes Suction electrodes - No straps or adhesives required - precordial (chest) ECG - can only be used for short periods Floating electrodes - metal disk is recessed - swimming in the electrolyte gel - not in contact with the skin - reduces motion artifact Suction Electrode

Commonly Used Biopotential Electrodes Insulating package Double-sided Adhesive-tape ring (a) Metal disk Electrolyte gel in recess (b) Snap coated with Ag-Ag. Cl External snap Gel-coated sponge Plastic cup Plastic disk Foam pad Reusable Disposable Dead cellular material Tack Capillary loops Germinating layer (c) Floating Electrodes

Commonly Used Biopotential Electrodes Flexible electrodes - Body contours 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) (a) Carbon-filled silicone rubber electrode. (b) Flexible thin-film neonatal electrode. (c) Cross-sectional view of the thin-film electrode in (b).

Internal Electrodes Needle and wire electrodes for percutaneous measurement of biopotentials (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 coiled fine-wire electrode place. The latest: BION –inimplanted electrode for muscle recording/stimulation Alfred E. Mann Foundation

Fetal ECG Electrodes for detecting fetal electrocardiogram during labor, by means of intracutaneous needles (a) Suction electrode. (b) Cross-sectional view of suction electrode in place, showing penetration of probe through epidermis. (c) Helical electrode, which is attached to fetal skin by corkscrew type action.

Electrode Arrays Contacts Ag/Ag. Cl electrodes Contacts Insulated leads Ag/Ag. Cl electrodes Base Insulated leads (a) Tines Exposed tip Base (b) Base (c) Examples of microfabricated electrode arrays. (a) One-dimensional plunge electrode array, (b) Two-dimensional array, and (c) Three-dimensional array

Microelectrodes Why Measure potential difference across cell membrane Requirements Intracellular n Small enough to be placed into cell n Strong enough to penetrate cell membrane Extracellular n Typical tip diameter: 0. 05 – 10 microns Types n Solid metal -> Tungsten microelectrodes n Supported metal (metal contained within/outside glass needle) n Glass micropipette -> with Ag-Ag. Cl electrode metal

Metal Microelectrodes C Microns! Extracellular recording – typically in brain where you are interested in recording the firing of neurons (spikes). Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier! R

Metal Supported Microelectrodes (a) Metal inside glass metal (b) Glass inside

Glass Micropipette heat pull Fill with intracellular fluid or 3 M KCl Ag-Ag. Cl wire+3 M KCl has very low junction potential and hence very accurate for dc measurements (e. g. action potential) A glass micropipet electrode filled with an electrolytic solution (a) Section of fine-bore glass capillary. (b) Capillary narrowed through heating and stretching. (c) Final structure of glass-pipet microelectrode. Intracellular recording – typically for recording from cells, such as cardiac myocyte Need high impedance amplifier…negative capacitance

Electrical Properties of Microelectrodes Metal Microelectrode Metal microelectrode with tip placed within cell Use metal electrode+insulation -> goes to high impedance amplifier…negative capacitance amplifier! Equivalent circuits

Electrical Properties of Glass Intracellular Microelectrodes Glass Micropipette Microelectrode

Electrodes for ECG, EEG, EMG n Various Electrodes used for the measurement or recording of ECG signal. n v v v v Immersion electodes(not used now) Limb electrodes Suction cup electrodes Floating electrodes Spray –on electrodes Pregelled disposable electrode Pasteless (dry) electrodes Air –jet ECG electrodes

ECG plate electrode. The electrode is usually Fastened to the arm or leg with a perforated Rubber strap which keeps it in position during ECG recording Light weight floating Electrode with press Stud for long term monitoring ECG Electrodes

ECG Electrodes

Disposable pre-gelled ECG Electrode. A porous tape overlaying placed over the electrode resists perspiration and Ensures positive placement Under stress conditions Air –Jet ECG Electrodes

Suction cup electrode (Unipolar chest ECG Electrode. ) Floating type skin surface ECG Electrodes 29

Electrodes for ECG, EEG, EMG n Various Electrodes used for the measurement or recording of EEG signal. v v v Scalp surface electrodes (chlorided silver discs) Ear-clip electrodes (reference electrode) Small needle electrodes Silver ball or pellet electrodes(exposed cortex, high resistance) Pad electrode(silver ball is belled out at the end and padded with a sponge) Chlorided silver wire in plastic cup electrodes

EEG Electrode which can Be applied to the surface Of the skin by an adhesive tape. Ear-clip Electrodes & EEG scalp surface Electrodes EEG Electrodes

EEG Electrodes

Electrodes for ECG, EEG, EMG n Various Electrodes used for the measurement or recording of EMG signal. v Needle electrodes v v n Monopolar Bipolar Concentric core needle Multi-element needle electrodes Biochemical Electrodes n n n Reference electrode p. H electrodes Blood Gas electrode

EMG Electrodes

Micro Electrodes

Metal micro-electrodes

Biochemical Electrodes n Biochemical Electrodes n n n Reference electrode p. H electrodes Blood Gas electrode v p. O 2 Electrode v p. CO 2 Electrode

L 7, L 8: Nernst eq. & Skin contact impedance n Nernst equation n Skin contact impedance measurement n Motion artifacts

Nernst Equation: Dept. of Electronics and Telecommunication Engineering

SKIN CONTACT IMPEDANCE MEASUREMENT: Dept. of Electronics and Telecommunication Engineering

: Dept. of Electronics and Telecommunication Engineering