1. 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

2. ECG electrodes 1905

3. 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

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

5. 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)

6. 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

7. Non-linearity in current conducting electrodes

8. Electrode induced noise

9. Motion artefacts

10. Simple equivalent circuit for metal / elektrolyte interface
Polarisation impedance)

11. Electrodeimpedance, 1cm2, siliconerubber skin + polarisation impedance

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

13. Electrode polarization + skin + deeper layer

14. Electrode-skin impedance

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

16. Suction-electrode

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

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

19. Invasive electrodes

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

21. Implantable electrodes

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

23. Micro-electrode metal
Very fine tip

24. Micro-electrodes
Typically semiconductor based

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

26. Micro-
elektrodes

27. 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

28. Electrical equivalent circuits, liquid filled glass capillary microelectrode

29. 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.