Download Electrodes – Part I

GBM8320
Dispositifs Médicaux Intelligents
Electrodes – Part I
Mohamad Sawan et al.
Laboratoire de neurotechnologies Polystim!
!
!
http://www.cours.polymtl.ca/gbm8320/!
[email protected]!
M5418!
April 2013
Integrated Microelectrodes : Course outline
•  Introduction
−  Electrogenic cell
•  Electrode/electrolyte interface
− 
− 
− 
− 
Electrical double layer
Half-cell potential
Polarization
Electrode equivalent circuits
•  Biopotential electrodes
− 
− 
− 
− 
− 
− 
Body surface electrodes
Internal electrodes
Implantable electrodes
Electrode arrays
Microfabricated electrodes
Microelectrodes.
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Electrogenic cells
•  Many types of cells in the body have the ability to undergo a
transient electrical depolarization and repolarization
•  These are either triggered by external depolarization (in the heart) or
by intracellular, spontaneous mechanisms
•  Cells that exhibit the ability to generate electrical signals are called
electrogenic cells
•  The most prominent electrogenic cells include brain cells or neurons
and heart cells or cardiomyocytes. (e.g. cardiac pacemaker cells).
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Electrogenic cells
•  Electrogenic cells such as neurons contain ion channels, selectively
enable the permeation of certain ions such as sodium or potassium
•  In a transient change of conductivity,
the overall ion flux generates an action
potential, which is the elementary
electrical signal in biological systems.
Jenkner et al, “Cell-based CMOS sensor …,” IEEE ISSC, V. 39, 2004.
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Electrogenic cells
•  Electrical activity is explained by
differences in ion concentrations
within the body (sodium, Na+;
cloride, Cl–; potassium, K+)
•  A potential difference occurs
between 2 points with different
ionic concentrations
•  Cell membranes at rest are more
permeable to some ions (e.g. K+,
Cl–) than others (e.g. Na+)
–  Na+ ions are pumped out of the cells, while K+ ions are pumped in
–  Due to a difference in rates of pumping, a difference in positive ion
concentration results
–  A negative potential (–70 mV ) is established between the inside and
outside of the cell.
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Electrogenic cells – Action potentials
•  When a cell is electrically stimulated, the permeability of the cell
membrane changes
–  Na+ ions rush into the cell, and K+ ions rush out
–  Again, due to a difference in rates of flow, the ion concentration changes (more
positive ions inside cell than outside)
–  Cellular potential becomes positive (40 mV)
–  Cell is said to be depolarized.
•  After the stimulus, the permeability of the cell membrane returns to its
original value, and the rest potential is restored
–  Due to unequal pumping rates of ions
–  Time taken for restoration is called the refractory
period
–  Cell is said to be repolarized during this time
•  The resulting variation in cellular potential
with time is known as the action potential.
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Integrated Microelectrodes : Course outline
•  Introduction
−  Electrogenic cell
•  Electrode/electrolyte interface
− 
− 
− 
− 
Electrical double layer
Half-cell potential
Polarization
Electrode equivalent circuits
•  Biopotential electrodes
− 
− 
− 
− 
− 
− 
Body surface electrodes
Internal electrodes
Implantable electrodes
Electrode arrays
Microfabricated electrodes
Microelectrodes.
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Biopotential sensors/electrodes
•  Biopotential electrodes convert ionic conduction to electronic
conduction so that biopotential signals can be viewed and/or stored
•  Different electrodes types include surface macroelectrodes, indwelling
macroelectrodes & microelectrodes (cuff or other shapes)
•  Skin and other body tissues act as electrolytic solutions !
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Electrode - electrolyte interface
•  Charge carriers in electrode
materials:
–  Metals (e.g. Pt) : electrons
–  Semiconductors (e.g. n-Si) :
electrons and holes
–  Solid electrolytes (e.g. lanthanum
fluoride - LaF3) : ions
–  Insulators (e.g. SiO2): no charge
carriers
–  Mixed conductors (e.g. IrOx) : ions
and electrons
–  Solution (e.g. 1 M NaCl in H2O):
solvated ions.
Inner Helmholtz plane (IHP)
Outer Helmholtz plane (OHP)
Gouy-Chapman layer (GCL)
Webster, J.G., Medical Instrumentation, Wiley, 4Ed, 2009,
Double layer
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Electrode - electrolyte interface
•  Electrode discharges some metallic ions into electrolytic solution
–  Increase in # free electrons in electrode
–  Increase in # positive cations (electric charge) in solution;
OR
•  Ions in solution combine with metallic electrodes
–  Decrease in # free electrons in electrode
–  Decrease in # positive cations in solution.
•  As a result, a
charge gradient
builds up between
the electrode and
electrolyte and this
in turn creates a
potential difference.
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Electrode - electrolyte interface
General Ionic Equations
a)
C ↔ C n + + ne −
b)
Am − ↔ A + me −
where n and m are les valences
•  If the electrode is of the same material as the cations, then this
material gets oxidized and enters the electrolyte as a cation and
electrons remain at the electrode & flow in the external circuit;
•  If anion can be oxidized at the electrode to form a neutral atom, one or
two electrons are given to the electrode.
•  The dominating reaction can be inferred from the following :
- Current flow from electrode to electrolyte : Oxidation (Loss of e-)
- Current flow from electrolyte to electrode : Reduction (Gain of e-).
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Electrode - electrolyte interface
The electrical double layer
•  For both mechanisms, (Oxidation = Loss of e-, and Reduction = Gain
of e-), two parallel layers of oppositely charged ions are produced;
i.e. the electrode double layer :
- e.g. when metal ions
recombine with the
electrode.
•  The excess of negative
anions is replaced with
positive cations in the
case of metal ions
discharging into
solution, and Vh is then
< 0.
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Electrode - electrolyte interface
The electrical double layer
Geddes, Principles of Applied Biomedical Instrumentation, Wiley, 1989
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Electrode - electrolyte interface
The half-cell potential
•  A characteristic potential difference established by the electrode and its
surrounding electrolyte which depends on the metal, concentration of ions in
solution and temperature.
•  Reason for half-cell potential : Charge separation at interface :
Oxidation or reduction reactions at the electrode-electrolyte interface lead to a
double-charge layer, similar to that which exists along electrically active
biological cell membranes.
•  Half-cell potential cannot be measured without a second electrode.
The half-cell potential of the standard hydrogen electrode has been arbitrarily
set to zero. Other half cell potentials are expressed as a potential difference
with this electrode.
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Electrode - electrolyte interface
Half-cell potential of materials
• 
Convention: The hydrogen
electrode is defined as
having a half-cell potential
of zero.
• 
The half-cell potentials of all
other electrode materials is
measured with respect to
this hydrogen electrode.
• 
Eo : Standard half-cell
potential.
*
* Standard Hydrogen electrode
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Electrode - electrolyte interface
Half-cell potential of materials :
Measurement
•  Electrode material
is metal + salt or
polymer selective
membrane.
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Electrode - electrolyte interface
Polarization
•  If there is a current between the electrode and electrolyte, the observed half-cell
potential is often altered due to polarization, then an overpotential occurs:
Overpotential
Difference between observed and zero-current half-cell potentials
Resistance
Current changes resistance
of electrolyte and thus,
a voltage drop results.
Activation
The activation energy
barrier depends on the
direction of current and
determines kinetics
Concentration
Changes in distribution
of ions at the electrodeelectrolyte interface
VP= VR+ VC+ VA + E0
Note: Polarization and impedance of the electrode are two of the most important
electrode properties to consider.
Eo : Standard half-cell potential
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Electrode - electrolyte interface
Polarization
•  When two aqueous ionic solutions of different concentration are separated by
an ion-selective semi-permeable membrane, an electric potential exists
across this membrane.
•  For the general oxidation-reduction reaction
a A + b B ↔ gC + dD + ne −
•  The Nernst equation for half-cell potential is
γ δ
&
RT
a
a #
E = E0 +
ln $ Cα Dβ !
nF % a A a B "
where Eo and E are Standard & half-cell potentials, a : Ionic activity (generally same
as concentration)‫‏‬, and n : Number of valence electrons involved.
Note: for a metal electrode, 2 processes can occur at the electrolyte interfaces:
–  A capacitive process resulting from the redistribution of charged and polar particles with no
charge-transfer between the solution and the electrode
–  A component resulting from the electron exchange between the electrode and a redox
species in the solution termed faradaic process.
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Electrode - electrolyte interface
Polarizable and Non-polarizable electrodes
• Perfectly Polarizable Electrodes
Used for stimulation
These are electrodes in which no actual charge crosses the electrodeelectrolyte interface when a current is applied. The current across the
interface is a displacement current and the electrode behaves like a
capacitor.
Example : Platinum Electrode (Noble metal)
• Perfectly Non-Polarizable Electrode
Used for recording
These are electrodes where current passes freely across the electrodeelectrolyte interface, requiring no energy to make the transition. These
electrodes see no overpotentials.
Example : Ag/AgCl electrode
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Electrode - electrolyte interface
Non-polarizable electrodes (Ag/AgCl)
Relevant ionic equations
Ag ↔ Ag + + e −
Ag + + Cl − ↔ AgCl ↓
Cl2
AgCl-
Governing Nernst Equation
RT & K s #
0
E = E Ag +
ln $
!
nF $% aCl − !"
Solubility
product of
AgCl
Fabrication of Ag/AgCl electrodes
1.  Electrolytic deposition of AgCl
2.  Sintering process forming pellet electrodes
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Electrode - electrolyte interface
Motion artifact
What
•  If a pair of electrodes is in an electrolyte and one moves with
respect to the other, a potential difference appears across the
electrodes known as the motion artifact. This is a source of noise
and interference in bio-potential measurements.
Why
•  When the electrode moves with respect to the electrolyte, the
distribution of the double layer of charge on polarizable electrode
interface changes. This changes the half-cell potential temporarily.
Note
•  Motion artifact is minimal for non-polarizable electrodes
(Measurement electrodes – AgCl).
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Electrode - electrolyte interface
Electrode equivalent circuit
Rd+Rs
Corner frequency
Rs
Frequency Response
• 
• 
• 
• 
Cd : Capacitance of electrode-electrolyte interface
Rd : Resistance of electrode-electrolyte interface
Rs : Resistance of electrode lead wire
Ecell : Half-cell potential for electrode.
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Electrode - electrolyte interface
Electrode equivalent circuit (Cont’d)
•  Recording/Stimulating Sites: Thin-film materials such as gold, platinum,
and iridium.
Recording
Interface
Biopotential
Interconnect
Resistance
Shunt
Capacitances
Recording
Amplifier
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Electrode - electrolyte interface
Electrode noise characteristics
• 
Extracellular action potentials have amplitude in the range of 50-500µV =
Very low-level input signals
• 
Total system input-referred noise should be < 20µVrms.
• 
Biological frequency band: 100Hz-10kHz
• 
System noise= Electrode noise + Preamplifier noise
• 
Main source of electrode noise is thermal noise:
Vne2 = 4kTRN Δf
- RN is noise resistance (real part of probe impedance magnitude).
- Δf is recording bandwidth.
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Body Surface Recording Electrodes
Electrode-skin interface
•  A body-surface electrode is
placed against skin, showing
the total electrical equivalent
circuit obtained in this situation.
•  Each circuit element on the
right is at approximately the
same level at which the
physical process that it
represents would be in the lefthand diagram.
Webster, Medical instrumentation: application and design. 3Ed, Wiley 1998.
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