1. Biopotential Electrodes

2. Introduction Electrical Contact point Transducer
Biopotential electrodes
Metal (Al, Cu, Fe, Ag,…..)
Non-metal

3. Electrode – Electrolyte Interface
Electrode Electrolyte (neutral charge) M
M+ and A- in solution
Current flow A- M+ M e- M M+ e- A- e- M+
M  M+ + e-
A-  A + e-
M+ : Cation A- : Anion e- : electron

4. Metal Electrolyte Interface
To sense a signal a current I must flow !

5. The Interface Problem To sense a signal a current I must flow !
But no electron e- is passing the interface!

6. Metal Cation
leaving into the electrolyte
No current
What’s going on?

7. Metal Cation: leaving into the electrolyte
No current
One atom M out of the metal is oxidized to form one cation M+ and giving off one free electron e- to the metal.

8. Metal cation: joining the metal
No current
What’s going on?

9. Metal Cation: joining the metal
No current
One cation M+ out of the electrolyte becomes one neutral atom M taking off one free electron from the metal.

10. Half-cell Voltage
No current

11. Half-cell Voltage No current metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au
Vh / Volt negativ positiv

12. Electrode Double Layer
No current ? ? ?

13. Electrode Double Layer
No current ? ?

14. Electrode Double Layer
No current ?

15. Electrode Double Layer
No current
Oxidation or reduction reactions at the electrode-electrolyte interface lead to a double-charge layer

16. 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.

17. Measuring Half Cell Potential
Note: Electrode material is metal + salt or polymer selective membrane

18. Half Cell Potential (Vh)
Iron mV
Lead mV
Copper mV
Platinum mV
Compare to electrophysiological Signals ???
Two Similar electrodes ??? (Ag/Agcl  5 mV and steel 100mV)

19. Difference bet. observed and zero-current half cell potentials
Polarization
If there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization.
Overvoltage
Difference bet. observed and zero-current half cell potentials
Resistance
Current changes resistance
of electrolyte and thus,
a voltage drop results.
Concentration
Changes in distribution
of ions at the electrode-
electrolyte interface
Activation
The activation energy
barrier depends on the
direction of current and
determines kinetics

20. Polarizable and Non-Polarizable Electrodes
Perfectly Polarizable Electrodes
No actual charge crosses the electrode-electrolyte interface when a current is applied. (e.g Platinum electrode)
Perfectly Non-Polarizable Electrode
Current passes freely across the electrode-electrolyte interface. These electrodes see no overpotentials. (e.g. Ag/AgCl Electrode)
Example: Ag-AgCl is used in recording while Pt is use in stimulation

21. Ag/AgCl Electrode Fabrication of Ag/AgCl electrodes
Electrolytic deposition of AgCl
Sintered AgCl: process forming pellet electrodes

22. Electrolysis Process