Biopotential electrodes

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Biopotential electrodes

• A complex interface

Summer School Timisoara 2002 R. Hinz
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the 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?
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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
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current influence

• with current flowing the half-cell voltage
  changes

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current influence

• with current flowing the half-cell voltage
  changes
• this voltage change is calledoverpotential or
  polarization

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current influence

• with current flowing the half-cell voltage
  changes
• this voltage change is calledoverpotential or
  polarization

Vp Vr Vc Va
activation, depends on direction of reaction
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polarizable electrode

• perfectly polarizable electrode- only
  displacement current, electrode behave like a
  capacitor

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polarizable electrode

• perfectly polarizable electrode- only
  displacement current, electrode behave like a
  capacitor
• example noble metals like platinum Pt

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nonpolarizable electrode

• perfectly nonpolarizable electrode- current
  passes freely across interface,- no
  overpotential

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nonpolarizable electrode

• perfectly nonpolarizable electrode- current
  passes freely across interface,- no
  overpotential
• examples - silver/silver chloride
  (Ag/AgCl),- mercury/mercurous chloride
  (Hg/Hg2Cl2) (calomel)

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Question!

• How is the current doing this to pass
  freely?Electrons cant live in liquids!Can
  they?

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chemical reactions
silver / silver chloride
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electrical behaviour
equivalent circuit
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electrical behaviour
equivalent circuit


?
?
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electrical behaviour
equivalent circuit


?
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electrical behaviour
equivalent circuit
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equivalent circuit
electrode-electrolyte
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body-surface electrode
anatomy
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again el. behaviour
the skin an additional interface!


?
?
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again el. behaviour
the skin an additional interface!


?
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again el. behaviour
the skin an additional interface!

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again el. behaviour
the skin an additional interface!
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expanded equiv. circuit
dominating parts highlighted
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total equivalent circuit
simplified
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results

• high impedance ZE...
• changing half-cell voltage Vh...

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high impedance

• interference with main power-lines (!!)
• c

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high impedance

• interference with main power-lines (!!)
• potential devider with Rinput
• c

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high impedance

• interference with main power-lines (!!)
• potential devider with Rinput
• frequency dependant. c

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changing half-cell voltage

• influenced by local concentration

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changing half-cell voltage

• influenced by local concentration
• saturation of amplifier

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changing half-cell voltage

• 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
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body-surface electrode
impedance locus diagram