Transcript Biopotential electrodes

Biopotential electrodes
A complex interface
Summer School Timisoara 2002
R. Hinz
the interface problem
metal
e-
electrolyte
I
?
2
M+
-
A
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
One atom M out of the metal
is oxidized to form
one cation M+ and giving off
one free electron eto the metal.
4
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
metal:
Li
Vh / Volt -3,0
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Al
Fe
negativ
Pb
H
Ag/AgCl
0
0,223
Cu
Ag
positiv
Pt
Au
1,68
electrode double layer
No current
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current influence
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with current flowing
the half-cell voltage changes
this voltage change is called
overpotential or polarization:
Vp = Vr + Vc + Va
activation, depends on direction of reaction
concentration (change in double layer)
ohmic (voltage drop)
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polarizable electrode

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“perfectly” polarizable electrode:
- only displacement current,
electrode behave like a capacitor
example: noble metals like platinum Pt
nonpolarizable electrode
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“perfectly” nonpolarizable electrode:
- current passes freely across
interface,
- no overpotential
examples:
- silver/silver chloride (Ag/AgCl),
- mercury/mercurous chloride
(Hg/Hg2Cl2) (calomel)
chemical reactions
silver / silver chloride
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electrical behaviour
equivalent circuit
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equivalent circuit
electrode-electrolyte
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again el. behaviour
the skin: an additional interface!
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total equivalent circuit
simplified
Cd
Vh*
Rep||R p
Rd
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Vh*
ZE
results
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high impedance ZE:
...
changing half-cell voltage Vh*:
...
high impedance
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interference with main power-lines (!!)
potential devider with Rinput
frequency dependant. c
changing half-cell voltage

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
influenced by local concentration
saturation of amplifier
motion artefacts by changing the
the gel-skin potential (Vep).
simulation:
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body-surface electrode
half-cell voltage drift simulated
ECG - Baselinedrif t
VECG / Volt
20
15
10
5
-5
-10
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time / s