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Electrogenic cell
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Electrical double layer
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Half-cell potential
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Polarization
!
Electrode equivalent circuits
•
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!
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Body surface electrodes
!
Internal electrodes
Implantable electrodes
Electrode arrays
Microfabricated electrodes
Microelectrodes.
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•   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 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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•
•
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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•   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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•
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Electrogenic cell
•
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Electrical double layer
!
Half-cell potential
!
Polarization
!
Electrode equivalent circuits
•
!
Body surface electrodes
!
Internal electrodes
!
Implantable electrodes
!
Electrode arrays
!
Microfabricated electrodes
!
Microelectrodes.
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•
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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•   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 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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General Ionic Equations a)
C
C n + + ne !
b) A m !
A + where n and m are les valences me !
•   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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•   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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Geddes, Principles of Applied Biomedical Instrumentation, Wiley, 1989
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•   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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•
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 material is metal + salt or polymer selective membrane.
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•   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.
Concentration
Changes in distribution of ions at the electrode- electrolyte interface
Activation
The activation energy barrier depends on the direction of current and determines kinetics
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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•   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 " g C + d D + ne !
•   The Nernst equation for half-cell potential is
E = E 0 +
RT nF ln
&
$ a *
C a )
D 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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• 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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Relevant ionic equations
Ag +
Ag "
+ Cl #
Ag +
"
+ e !
AgCl !
Cl
2 Governing Nernst Equation
AgCl
-
E = E 0
Ag
+
RT nF ln
&
$
K a
Cl s
'
#
!
Fabrication of Ag/AgCl electrodes
1.
Electrolytic deposition of AgCl
2.
Sintering process forming pellet electrodes
Solubility product of
AgCl
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What
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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.
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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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Recording/Stimulating Sites: Thin-film materials such as gold, platinum, and iridium.
Recording
Interface
Interconnect
Resistance
Shunt
Capacitances
Biopotential
Recording
Amplifier
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•   Extracellular action potentials have amplitude in the range of 50-500 # V
!
Very low-level input signals
•   Total system input-referred noise should be < 20 # V rms
.
•   Biological frequency band: 100Hz-10kHz
•   System noise= Electrode noise + Preamplifier noise
•   Main source of electrode noise is thermal noise:
V 2 ne
= 4 kTR
N
!
f
- R
N
is noise resistance (real part of probe impedance magnitude).
- !
f is recording bandwidth.
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•   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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