Biopotential Electrodes III

Electrodes Recording Stimulating

Recording Electrodes Surface Inserted Solid metal Supported metal Array Electrodes Microelectrodes Solid metal Supported metal Glass micropipette Metal Plate Suction Floating Flexible Needle Wire

Surface: Metal Plate Electrodes Metal-plate electrode used for application to limbs. Metal-disk electrode applied with surgical tape. )Disposable foam-pad electrodes, often used with ECG

Surface: Suction Electrodes No straps or adhesives Precordial (chest) ECG Can only be used for short periods

Surface: Floating Electrode Double-sided Adhesive-tape ring Insulating package Metal disk Electrolyte gel in recess (a) (b)

Surface: Flexible Electrodes Body contours are often irregular Material : (a) Polymer or nylon with silver (b) Carbon filled silicon rubber (Mylar film) (c) Cross-sectional view of the thin-film electrode in (b).

Inserted (Internal) Electrodes Insulated needle, (b)Coaxial needle, (c) Bipolar coaxial, (d) Fine-wire electrode connected to hypodermic needle, before insertion,

Fetal ECG Electrodes Electrodes for detecting fetal ECG 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.

Microelectrodes 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

Metal Microelectrode

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

Glass Micropipette Heat and pull

Electrical Properties of Microelectrodes Metal Microelectrode

Electrical Properties of Glass Intracellular Microelectrodes Glass Micropipette Microelectrode

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

Stimulating Electrodes Features – Net current across electrode-electrolyte interface is not zero – The body/electrode has a highly nonlinear response to stimulation – Large currents can cause – Chemical reaction – Cavitation – Cell damage – Heating Platinum electrodes: Applications: neural stimulation Steel electrodes for pacemakers and defibrillators Applications of stimulating electrodes Pacing Ablation Defibrillation

Practical Hints Electrode and lead wire (exposed to the electrolyte) must be of the same material. Use two similar electrodes when measuring differentials. Lead electrode interface failure. Electrode insulation material. Input impedance of Amplifier.