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Biopotential Electrodes

Biopotential Electrodes. Biopotential Electrodes. Micro Electrode Skin Surface Electrode Needle Electrode. Metal Microelectrodes. C. Microns!. R. Ag-AgCl Electrode. Electrode – Electrolyte Interface. Electrode Electrolyte (neutral charge). C+, A- in solution. C. Current flow. C.

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Biopotential Electrodes

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  1. Biopotential Electrodes

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

  3. Metal Microelectrodes C Microns! R

  4. Ag-AgCl Electrode

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

  6. Electrode Double Layer Oxidation or reduction reactions at the electrode-electrolyte interface lead to a double-charge layer

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

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

  9. 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.

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

  11. Ese EP Re Ce RP CP Electrode Skin Interface Ehe Alter skin transport (or deliver drugs) by: Pores produced by laser, ultrasound or by iontophoresis Electrode Cd Rd Sweat glands and ducts Rs Gel 100 m Stratum Corneum Epidermis 100 m Dermis and subcutaneous layer Ru Skin impedance for 1cm2 patch: 200kΩ @1Hz 200 Ω @ 1MHz Nerve endings Capillary

  12. 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

  13. Body Surface Recording Electrodes Electrode metal Electrolyte • Metal Plate Electrodes • Suction Electrodes • Floating Electrodes • Flexible Electrodes

  14. 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

  15. 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

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

  17. 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).

  18. 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

  19. Fetal ECG Electrodes 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.

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

  21. Microelectrodes Why Measure potential difference across cell membrane Requirements • Small enough to be placed into cell • Strong enough to penetrate cell membrane • Typical tip diameter: 0.05 – 10 microns Types • Solid metal -> Tungsten microelectrodes • Supported metal (metal contained within/outside glass needle) • Glass micropipette -> with Ag-AgCl electrode metal Intracellular Extracellular

  22. 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!

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

  24. Ag-AgCl wire+3M KCl has very low junction potential and hence very accurate for dc measurements (e.g. action potential) Glass Micropipette heat pull 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. Fill with intracellular fluid or 3M KCl Intracellular recording – typically for recording from cells, such as cardiac myocyte Need high impedance amplifier…negative capacitance amplifier!

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

  26. Electrical Properties of Glass Intracellular Microelectrodes Glass Micropipette Microelectrode

  27. 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 – Large currents can cause –Cavitation –Cell damage –Heating Platinum electrodes: Applications: neural stimulation Modern day Pt-Ir and other exotic metal combinations to reduce polarization, improve conductance and long life/biocompatibility Steel electrodes for pacemakers and defibrillators • Types of stimulating electrodes • Pacing • Ablation • Defibrillation

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