The document discusses biopotential electrodes and how they are used to detect electrical signals in the body. It explains that electrodes must form an interface with body tissues or fluids to detect signals, but this interface can cause issues if ions or electrons cannot freely cross. It also discusses different types of electrodes like surface electrodes, depth electrodes, and needle electrodes and how they interface with the body. The half-cell potential, polarization, equivalent circuit models, and sources of noise are also summarized.

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

2. the interface
metal electrolyte
M+
A-
e-
I
To sense a signal
a current I must flow !
2
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

16. electrical behaviour
17
equivalent circuit
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

17. equivalent circuit
18
electrode-electrolyte
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

18. again el. behaviour
20
the skin: an additional interface!
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

19. total equivalent circuit
22
simplified
Cd
Rd
Rep||Rp
Vh* ZE
Vh*
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

23. Depth electrodes
26
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

24. Depth electrode-
27
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

25. Needle electrodes
28
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

26. Needle electrodes – with pins
29
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

27. Inserting a Needle electrode
30
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

28. 31
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

29. 32
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

30. 33
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

31. 34
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

32. 35
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

33. 36
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

34. 37
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

35. 38
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai

36. 39
Prepared by A.Devasena,
Associate Professor,
Dhanalakshmi College of Engg.
Chennai