• Introduction • Electrode/electrolyte interface • Biopotential electrodes

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March 2011

GBM8320 - Dispositifs Médicaux Intelligents 2

•  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.


•  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

Jenkner et al, “Cell-based CMOS sensor …,” IEEE ISSC, V. 39, 2004.

•  In a transient change of conductivity, the overall ion flux generates an action potential, which is the elementary electrical signal in biological systems.


•  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.


•  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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•  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.


•  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 !


•  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)

Double layer

Webster, J.G., Medical Instrumentation, Wiley, 4Ed, 2009,

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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.


!!

!+

+"+"meAAneCC

m

nGeneral Ionic Equations

•  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-).

a)

b) 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;


•  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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GBM8320 - Dispositifs Médicaux Intelligents 13 Geddes, Principles of Applied Biomedical Instrumentation, Wiley, 1989


•  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.


* Standard Hydrogen electrode

•  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.

*

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•  Electrode material is metal + salt or polymer selective membrane.


•  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.

V = V + V + V + E P R C A 0

Eo : Standard half-cell potential


!"

#$%

&+=

'(

)*

BA

DC

aaaa

nFRTEE ln0

•  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 !++"+ neDCBA dgba

•  The Nernst equation for half-cell potential is

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 These are electrodes in which no actual charge crosses the electrode-electrolyte 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)

Used for recording

Used for stimulation

• Perfectly Non-Polarizable Electrode These are electrodes where current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition. These electrodes see no overpotentials.

Example : Ag/AgCl electrode


!+ +" eAgAg!"+ #+ AgClClAg

AgCl- Cl2

Relevant ionic equations

Governing Nernst Equation

!!"

#

$$%

&+=

'Cl

sAg a

KnFRTEE ln0

Solubility product of AgCl

Fabrication of Ag/AgCl electrodes

1.  Electrolytic deposition of AgCl

2.  Sintering process forming pellet electrodes


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.

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•  Cd : Capacitance of electrode-electrolyte interface •  Rd : Resistance of electrode-electrolyte interface •  Rs : Resistance of electrode lead wire •  Ecell : Half-cell potential for electrode.

Frequency Response

Corner frequency Rd+Rs

Rs


Biopotential

Recording Interface Shunt

Capacitances

Interconnect Resistance

Recording Amplifier

•  Recording/Stimulating Sites: Thin-film materials such as gold, platinum, and iridium.


•  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#Vrms.

•  Biological frequency band: 100Hz-10kHz

•  System noise= Electrode noise + Preamplifier noise

•  Main source of electrode noise is thermal noise:

- RN is noise resistance (real part of probe impedance magnitude).

- !f is recording bandwidth.

fkTRV Nne != 42

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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 left-hand diagram.

Webster, Medical instrumentation: application and design. 3Ed, Wiley 1998.

• Introduction • Electrode/electrolyte interface • Biopotential electrodes

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