Biopotential Electrodes

1
Biopotential Electrodes
2
Introduction

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

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Electrode Electrolyte Interface
Electrode Electrolyte (neutral
charge)
M and A- in solution
M
Current flow
A-
M
M
e-
M
M
e-
A-
M
e-
M ?? M e- A- ?? A e-
M Cation A- Anion e- electron
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Metal Electrolyte Interface
To sense a signala current I must flow !
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The Interface Problem
To sense a signala current I must flow !
But no electron e- ispassing the interface!
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Metal Cation
leaving into the electrolyte
No current
Whats going on?
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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.
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Metal cation joining the metal
No current
Whats 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.
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Half-cell Voltage
No current
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Half-cell Voltage
No current
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Electrode Double Layer
No current
?
?
?

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Electrode Double Layer
No current
?
?

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Electrode Double Layer
No current
?

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

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Measuring Half Cell Potential
Note Electrode material is metal salt or
polymer selective membrane
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Half Cell Potential (Vh)

• Iron -440 mV
• Lead -126 mV
• Copper 337 mV
• Platinum 1190 mV
• Compare to electrophysiological Signals ???
• Two Similar electrodes ??? (Ag/Agcl ? 5 mV
  and steel ?100mV)

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Polarization
If there is a current between the electrode and
electrolyte, the observed half cell potential is
often altered due to polarization.
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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
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Ag/AgCl Electrode

• Fabrication of Ag/AgCl electrodes
• Electrolytic deposition of AgCl
• Sintered AgCl process forming pellet
  electrodes

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Electrolysis Process