Biopotential electrodes FYS 4250 Kap.5 Biopotential electrodes Interface between body and electronics is necessary Ideal to keep currents very small -> electrodes conduct a current betw body and electronics without loss Current is carried in the body by ions

ECG electrodes 1905

Metal / electrolyte = electron / ion transitions Transfer charge between carriers Chemical reactions: C<->C(n+) + ne- A(m-) <-> A+me- N=valence of C and m=valence of A Half cell potential = Electrolyte surrounding the metal is at different electric potential from the rest of the solution Is it possible to measure? NO. Defined hydrogen electrode as ZERO, measure with respect to this electrode

Hydrogen electrode is based on the reaction H2<->2H<->2H(+) + 2e(-) where H2 gas is bubbled over a platinum electrode.

Non-polarizable electrode Ag / AgCl Non-polarizable electrode Half-cell potential is described for non-current situations Current will alter the half-cell potential -> due to the polarization of the electrode Overpotential = Diff betw observed half cell potetntial and the equilibrium zero-current half-cell potential Three basic components: Ohmic overpotential: Resistance of the electrolyte, voltage drop along current path Concentration overpotential: Due to changes in distribution of ions in the electrolyte in the vicinity of the electrode-electrolyte interface. The difference between current induced half-cell potential and equilibrium half-cell potentia = concentration overpotential Activation overpotential: Charge-transfer process in oxidation-reduction reaction are not entirely reversible. (Energy barrier in order to start reaction). Current decides oxidation or reduction predominates -> energy barrier = difference in voltage between electrode and electrolyte All three mechanisms are additive Theoretically two types of electrodes: Perfectly polarizable and perfectly non-polarizable PP: no actual charge cross electrode electrolyte interface when current applied. (Behaves as a capacitor) (Platium, noble metals) PNP: Current passing freely across electrode-electrolyte interface (Ag/Agcl, slightly soluble in water = stable)

Sintret AgCl elektrode Better endurance than electrolytically deposited AgCL electrodes -> Good for repeated usage. (Electrocally flake off under mechanical stress) Metallic silver included to increase electrical conductance

Non-linearity in current conducting electrodes

Electrode induced noise

Motion artefacts

Simple equivalent circuit for metal / elektrolyte interface Polarisation impedance)

Electrodeimpedance, 1cm2, siliconerubber skin + polarisation impedance

Skin Typically use a transparent electrolyte gel containing Cl(-) Epidermis most important

Electrode polarization + skin + deeper layer

Electrode-skin impedance

Surface-electrodes stiff Adv. Robust, long time stable, cheap, easy to use Disadv. Motion artifacts

Suction-electrode

Electrodes With gel-chamber Reduce motion artifacts.Caviyt does not move with respect to the metal disk

Skin electrodes Flexible plates Irregularly shaped body = flexible plates. Important for premature infants X-ray transparent.

Invasive electrodes

Fetal ECG Fetus lies in a bath of amnotic fluid -> penetrate skin of fetus

Implantable electrodes

Micro-electrodes Difficult to build and awkward to use due to the large number of wires

Micro-electrode metal Very fine tip

Micro-electrodes Typically semiconductor based

Liquid filled Glas electrodes Not as small as glass micropipet technique, reproducibility, similar geometric properties, little variation

Micro- elektrodes

Elektriske ekvivalentkretser, metalltip mikroelektrode Rs = resistance of metal Cd= capacitance shaft/metal-rod/liquid fluid Ma= metal-electrolyte interface Mb= reference electrode CW= lead wire Cap Ri = intracellular resistance Re= extracellular resistance Emp= Cell membrane potential

Electrical equivalent circuits, liquid filled glass capillary microelectrode

Stimulation electrodes Practical hints: All that is exposed to elctrolyte must be of the same material Use same materials for pairs of electrodes, the half cell potential is equal. (Minimizes possible saturation effects in high gain direct coupled amplifiers) Provide additional relief from straint by taping lead wire to the skin Make sure the insulation is sufiicient for the use High fidelity recordings, input imedance of amplifier must be higher than the source impedance.