ELEC 408 – Fall 2009
ELEC 408 – Electrodes and Electrode Theory
Action potentials are produced by the movement of ions and can be recorded from outside
excitable cells (extracellular recording) because of fields generated close to the active fibres. In order to
observe these potentials using electronic instrumentation, they must be transduced into ohmic potentials.
This process is mediated by metal conductors.
The electrode-electrolyte interface: when a metal disk is placed in an electrolyte solution, an ion-electron
exchange occurs – metal ions enter the solution and electrolyte ions combine with the metal of the
electrode resulting in a charge distribution at the metal-electrolyte interface:
+
+
+
+
+
+
+
+
metal
+
 +
 +
+
 +
+
+
 +
The exact form of the charge
distribution depends on the
characteristics of the electrode
and the electrolyte.
electrolyte
A potential, called the half-cell potential is generated as a result of the charge layer. The potential at the
electrode interface can only be measured with respect to another electrode; half-cell potentials are
measured with respect to a standard electrode called the hydrogen electrode. The hydrogen electrode is
formed by platinum in contact with a solution of hydrogen ions and dissolved molecular hydrogen1. The
hydrogen electrode is defined as having a half-cell potential of zero. If an electrode of one metal is
connected to an electrode of another metal, a non-zero potential will exist between them. For example, at
25C Ni has a half-cell potential E0 = 0.23V and Ag has a half-cell potential E0 = 0.799V. If a Ni and Ag
electrode are connected together, the potential difference will be 0.230.799 = 1.029V. If a biological
potential is recorded using electrodes of different metals, the dc offset may swamp the much smaller
biological signal. Thus it is important to use electrodes made of the same metal in contact with the same
electrolyte.
When current flows between the electrode and electrolyte, the half-cell potential will change. The
difference between the potential with current and the equilibrium zero-current half-cell potential is called
the over-potential. Three components contribute to the over-potential – the ohmic over-potential, the
concentration over-potential, and the activation over-potential. The ohmic over-potential is the voltage
drop between two electrodes in an electrolyte solution, where current is flowing through the solution
between the electrodes and the solution has a finite resistance. The resistance of the electrolyte may vary
with current, thus the ohmic over-potential may not be linearly related to current. The concentration overpotential arises because the concentration of ions at the electrode-electrolyte interface will change when
current passes across the interface. The activation over-potential is due to the difference in activation
energy (or energy barrier) that must be overcome for a metal atom to oxidize and enter the electrolyte
solution as a cation, and the activation energy required for a cation to be reduced and deposit a metal atom
on the electrode surface.
There are two theoretical types of electrodes in terms of the reaction at the charge layer when current
flows between the electrode and the electrolyte1:
 perfectly polarized electrodes – do not allow transfer of charge across the electrode-electrolyte
interface; current flows as a displacement current
 perfectly nonpolarizable electrodes – allow free exchange of charge across the interface.
The properties of real electrodes lie between these theroetical limits.
Electrode Theory
E. Morin, Queen's University
1
ELEC 408 – Fall 2009
If two electrodes of the same metal are placed in solution, a large fluctuating potential may exist between
the electrodes, because of a small amount of contaminant on one or both electrodes.
s(t)
Two electrodes of
the same metal in an
electrolyte solution.
The signal, s(t) has a
large fluctuating
potential.
Silver (Ag) electrodes can be stabilized by electrolytically coating the electrode with a layer of silverchloride (AgCl); Ag-AgCl electrodes are widely used in biopotential recording.
s(t)
The noise in s(t) has
been reduced.
Ag electrodes coated
with AgCl, in an
electrolyte solution
containing Cl–.
The AgCl layer stabilizes the electrode, effectively attenuating the noise between the electrodes, as long
as the Ag-AgCl electrode is in contact with an electrolyte solution containing a relatively high
concentraion of Cl– ions.
Electrode Impedance
The electrode-electrolyte interface has an impedance, Ze, which depends on the nature of the
charge layer. Since the charge layer has charges of different sign separated by a small distance, there is a
capacitance at the interface which may be large.
The Warburg model of the electrode-electrolyte interface:
Electrolyte
Electrode
Rw
Rw and Cw are the
Warburg resistance
and capacitance.
Cw
Since it is known that dc current can cross the electrode-electrolyte interface, a parallel resistor must be
added; a dc source is added to model the half-cell potential.
Rf
At high frequencies:
[ωCw]1 << Rf and Ze = Rf║Rw
Electrode
Electrolyte
Rw
Electrode Theory
E. Morin, Queen's University
Cw
At low frequencies:
[ωCw]1 >> Rf and Ze ≈ Rf
E½
2
ELEC 408 – Fall 2009
The magnitude of Rf, Rw and Cw depends on the electrode metal and surface area, composition of the
electrolyte, temperature, the density of the current crossing the electrode-electroltye boundary and
frequency. For example, Rf decreases with increasing current density:
Rf ()
20k
10k
20
40
J (mA/cm2)
A complete model for two electrodes in contact with an electrolyte is shown below.
Rf1
Electrode 1
Electrolyte
Rw1
Cw1
E1½
Rw2
Cw2
E2½
Re1
Electrode 2
Rf2
Recording biopotentials from the skin surface
The fields generated by activity in excitable tissues (nerves or muscles) in the body can spread
through the aqueous environment inside the body to appear at the skin surface. Electrodes placed on the
skin can detect these potentials. Electrodes are coupled to the skin via an electrolyte – either sweat or an
electrolyte solution applied between the electrode and skin.
Anatomy of the skin:
Image from:
http://www.unm.edu/~lkravitz%0A/
Kinesiology/skinanatomy.html
Electrode Theory
E. Morin, Queen's University
3
ELEC 408 – Fall 2009
The skin comprises three main layers – the subcutaneous fat (the deepest layer), the dermis and the
epidermis (at the surface). The deeper layers of the skin contain blood vessels, nerves, sweat glands and
hair follicles. The uppermost layer, the epidermis, is comprised of three sub-layers: the stratum
germinativum, stratum granulosum and stratum corneum. The stratum corneum is a layer of dead cells on
the surface which protects the underlying layers. New cells are continuously generated in the stratum
germinativum and migrate to the stratum corneum, which replace old cells that have been worn off or
shed. Because the stratum corneum is composed of dead cells, it has a high impdance. As well, the tissues
under the stratum corneum contain dissolved ions. When an electrode is coupled to the skin via an
electrolyte the stratum corneum can be considered a semi-permeable membrane, so a potential difference
exists across the skin. Thus a model for an electrode attached to the skin surface is:
Rf
Cs
Ese
Rb
Rel
Rw
Dermis &
subcutaneous
tissue
Cw
E½
Rs
Epidermis
Electrolyte
Electrode
The properties of the electrode-skin interface result in two issues with regard to recording biological
signals:
 high skin impedance can result in poor signal detection
 relative movement between the electrode and the skin produces an artifact called motion artifact
If the skin impedance is very large, it is difficult for the field potentials generated by the biological signal
to cross this barrier and signal amplitude will be low. Other interfering signals will become larger. For
example, 60 Hz or power line interference will appear` as a large signal over the surface of the body. This
signal is generally attenuated by differential amplification, since it s a common mode signal, but if it is
large, it can result in a serious artifact particularly since the common mode rejection ration (CMRR) in a
differential amplifier is never perfect.
Reducing or short-circuiting the skin impedance has the effect of improving signal fidelity, because the
signal does not have to cross a high impedance barrier. Skin impedance can be reduced by:
 removing the stratum corneum by abrasion or puncturing the skin
 hydrating the stratum corneum using an electrolyte gel or paste.
There are two sources of motion artefact in surface electrodes: i) mechanical disturbance of the electrode
charge layer occurs because relative movement between the electrode and the skin disturbs the charge
layer at the electrode-electrolyte interface altering the half-cell potential; and ii) if the skin under the
electrodes is stretched or deformed, the skin potential is altered. Skin potential can change by as much as
5-10mV with stretching.
Electrode Theory
E. Morin, Queen's University
4
ELEC 408 – Fall 2009
+++++++

++ +++ + +
Electrode
Electrolyte gel
Epidermis
A mechanical disturbance such as
movement, will disturb the charge
layer, changing the half-cell potential
and causing an artifact to appear in
the signal recording.
Dermis
The first type of motion artefact is greatly attenuated in recessed electrodes, in which the electrodeelectrolyte interface is separated from the skin surface by a layer of conductive gel or paste. Any
mechanical disturbances caused by relative motion between the electrode and the skin are damped by the
intervening gel layer, and their effect on the signal is limited. The second type of motion artefact is not
attenuated by the use of recessed electrodes, but can be reduced by reducing the skin impedance. Tam and
Webster [69] suggest reducing the skin impedance by removing the upper layers of the skin via abrasion
and Burbank and Webster [9] found that skin impedance is lowered by puncturing the skin. Both
techniques have drawbacks including determining the proper level of abrasion or depth of puncture, the
time required, and the possibility of skin irritation and infection. Cleansing the skin with solvents (e.g.
rubbing alcohol) and rubbing a conductive paste into the skin is recommended for reducing skin
impedance.
References
Geddes, L.A. and Baker, L.E., Principles of Applied Biomedical Instrumentation, 3rd ed., Wiley, 1989.
Webster, J.G. (ed.), Medical Instrumentation: Application and Design, 4th ed., Wiley, 2009.
Electrode Theory
E. Morin, Queen's University
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