Download Biopotential Electrodes

Biopotential Electrodes
Introduction
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Electrical Contact point
Transducer
Biopotential electrodes
– Metal (Al, Cu, Fe, Ag,…..)
– Non-metal
Metal Electrolyte
Interface
To sense a signal
a current I must flow !
The Interface Problem
To sense a signal
a current I must flow !
?
But no electron e- is
passing the interface!
Metal Cation
leaving into the electrolyte
No current
What’s going on?
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 eto the metal.
Metal cation:
joining the metal
No current
What’s going on?
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.
Half-cell Voltage
No current
Half-cell Voltage
No current
metal:
Li
Vh / Volt -3.0
Al
Fe
negativ
Pb
H
Ag/AgCl
0
0.223
Cu
Ag
positiv
Pt
Au
1.68
Electrode Double Layer
No current
?
?
?
Electrode Double Layer
No current
?
?
Electrode Double Layer
No current
?
Electrode Double Layer
No current
Oxidation or
reduction
reactions at the
electrodeelectrolyte
interface lead to a
double-charge
layer
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.
Measuring Half Cell Potential
Note: Electrode material is metal + salt or polymer selective membrane
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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Ag/AgCl Electrode
Fabrication of Ag/AgCl electrodes
1. Electrolytic deposition of AgCl
2. Sintered AgCl: process forming pellet
electrodes
Electrolysis Process
Ag  Ag   e 


Ag  Cl  AgCl 