• introduction • electrode/electrolyte interface • biopotential electrodes
TRANSCRIPT
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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.
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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
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.
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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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• 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 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)
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.
GBM8320 - Dispositifs Médicaux Intelligents 11
!!
!+
+"+"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;
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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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GBM8320 - Dispositifs Médicaux Intelligents 13 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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* 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.
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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.
V = V + V + V + E P R C A 0
Eo : Standard half-cell potential
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!"
#$%
&+=
'(
)*
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
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!+ +" 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
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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
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Biopotential
Recording Interface Shunt
Capacitances
Interconnect Resistance
Recording Amplifier
• Recording/Stimulating Sites: Thin-film materials such as gold, platinum, and iridium.
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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#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.