1. Biopotential electrodes
Electrodes are employed to pick up
the electrical signals of the body.
1
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
3. the interface problem
metal electrolyte
M+
A-
e-
I
To sense a signal
a current I must flow !
But no electron e- is
passing the interface!
?
3
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
4. metal cation
leaving into the electrolyte
No current
What’s going on?
4
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
5. metal cation
joining the metal
No current
What’s going on?
5
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
6. 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.
6
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
7. half-cell voltage
No current
7
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
8. half-cell voltage
No current
metal: Li Al Fe Pb H Ag/AgCl Cu Ag Pt Au
Vh / Volt -3,0 negativ 0 0,223 positiv 1,68
8
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
9. Half cell potential
The voltage developed at an
electrode-electrolyte interface is
designated as the half cell- potential.
9
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
10. electrode double layer
No current
10
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
11. electrode double layer
12
No current
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
12. current influence
with current flowing
the half-cell voltage changes
this voltage change is called
overpotential or polarization:
13
Vp = Vr + Vc + Va
activation, depends on direction of reaction
concentration (change in double layer)
ohmic (voltage drop)
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
13. polarizable electrode
“perfectly” polarizable electrode:
- only displacement current,
electrode behave like a capacitor
example: noble metals like platinum
Pt.
Electrodes in which no net transfer of
charge occurs across the metal
electrode interface are called as
perfectly polarized electrodes.
14
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
14. nonpolarizable electrode
“perfectly” nonpolarizable electrode:
- current passes freely across
interface,
- no overpotential
examples:
- silver/silver chloride (Ag/AgCl),
- mercury/mercurous chloride
(Hg/Hg2Cl2) (calomel)
Electrodes in which unhindered
exchange of charge is possible
across the metal electrolyte interface
15
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
15. chemical reactions
16
silver / silver chloride
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
20. high impedance
interference with main power-lines
(!!)
potential devider with Rinput
frequency dependant. c
23
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
21. changing half-cell voltage
influenced by local concentration
saturation of amplifier
motion artefacts by changing the
the gel-skin potential (Vep).
24
simulation:
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
22. body-surface electrode
25
half-cell voltage drift simulated
time / s
ECG - Baselinedrift
20
15
10
5
-5
-10
V
ECG
/
Volt
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai
