Biopotential Electrodes Biopotential Electrodes Micro Electrode Skin Surface

Biopotential Electrodes

Biopotential Electrodes • Micro Electrode • Skin Surface Electrode • Needle Electrode

Metal Microelectrodes C Microns! R

Ag-Ag. Cl Electrode

Electrode – Electrolyte Interface Electrode C Electrolyte (neutral charge) C+, A- in solution Current flow ee. C+ : Cation C C C+ A- C+ AA- : Anion e- : electron Fairly common electrode materials: Pt, Carbon, …, Au, Ag, … Electrode metal is use in conjunction with salt, e. g. Ag-Ag. Cl, Pt-Pt black, or polymer coats (e. g. Nafion, to improve selectivity)

Electrode Double Layer Oxidation or reduction reactions at the electrodeelectrolyte interface lead to a double-charge layer

Cd : capacitance of electrode-eletrolyte interface Rd : resistance of electrode-eletrolyte interface Rs : resistance of electrode lead wire Ehc : cell potential for electrode

Equivalent Circuit Cd Rd Rs Ecell Rd+Rs : capacitance of electrode-eletrolyte interface : resistance of electrode lead wire : cell potential for electrode Corner frequency Rs Frequency Response

Contact (Half Cell) Potential • Depends on: • The metal, • Concentration of ions in solution and • Temperature. • Half cell potential cannot be measured without a second electrode. • The half cell potential of the standard hydrogen electrode has been arbitrarily set to zero.

Some half cell potentials Standard Hydrogen electrode Note: Ag-Ag. Cl has low junction potential & it is also very stable -> hence used in ECG electrodes!

Electrode Skin Interface Ehe Electrode Cd Rd Gel 100 m Stratum Corneum Epidermis 100 Dermis and subcutaneous layer m Nerve endings Capillary Ce Rs Sweat glands and ducts Ese EP Re CP RP Alter skin transport (or deliver drugs) by: Pores produced by laser, ultrasound or by iontophoresis Ru Skin impedance for 1 cm 2 patch: 200 kΩ @1 Hz 200 Ω @ 1 MHz

Motion Artifact Why When the electrode moves with respect to the electrolyte, the distribution of the double layer of charge on polarizable electrode interface changes. This changes the half cell potential temporarily. What If a pair of electrodes is in an electrolyte and one moves with respect to the other, a potential difference appears across the electrodes known as the motion artifact. This is a source of noise and interference in biopotential measurements Motion artifact is minimal for non-polarizable electrodes

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

Commonly Used Biopotential Electrodes Metal plate electrodes – Large surface: Ancient, therefore still used, ECG – Metal disk with stainless steel; platinum or gold coated – EMG, EEG – smaller diameters – motion artifacts – 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 Metal disk Electrolyte gel in recess (a) (b) Snap coated with Ag-Ag. Cl Plastic cup Foam pad External snap Gel-coated sponge Plastic disk 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(a) Carbon-filled silicone rubber electrode. (Mylar film) (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 in place. BION – implanted electrode for muscle recording/stimulation

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

Metal Microelectrodes C Microns! R 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!

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

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 amplifier!

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

Stimulating Electrodes Features – Cannot be modeled as a series resistance and capacitance (there is no single useful model) – The body/electrode has a highly nonlinear response to stimulation Platinum electrodes: – Large currents can cause Applications: neural – Cavitation stimulation – Cell damage – Heating Modern day Pt-Ir and other exotic Types of stimulating electrodes 1. Pacing 2. Ablation 3. Defibrillation metal combinations to reduce polarization, improve conductance and long life/biocompatibility Steel electrodes for pacemakers and defibrillators