BME 301: Biomedical Sensors

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BME 301: Biomedical Sensors

Lecture Note 3: Bioelectric Potentials and Biopotential Electrodes

BME 301 Lecture Note 3 - Ali Işın 2013 1

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Bioelectric potentials

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RESTING POTENTIAL-BASIC CONCEPT

• Cell membranes are typically permeable to only a subset of ionic species like

pottasium(K+),Chloride(Cl-) & effectively blocks the entry of sodium(Na+) ions.

• The various ions seeks a balance between inside &

outside the cell according to concentration & electric charge.

• Two effects result from inability of Na+ ions to penetrate membrane-

• Concentration of Na+ ions inside cell is much lower than outside. Hence,outside of cell becomes more positive than inside.

• In an attempt to to balance electric charge,additional K+ ions enters the cell,causing higher concentration of K+ ion inside the cell.

• Charge balance can never be reached.

• Equilibrium is reached with a potential difference across the membrane, negative on inside and positive on outside called Resting Potential.

3

Polarized Cell during RP

BME 301 Lecture Note 3 - Ali Işın 2013

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RESTING POTENTIAL IN NERVE CELL



A nerve cell has an electrical potential, or voltage, across its cell membrane of approximately 70

millivolts (mV). This means that this tiny cell

produces a voltage roughly equal to 1/20th that of a flashlight battery (1.5 volts).



The potential is produced by the actions of a cell membrane pump, powered by the energy of ATP.



As shown in Figure, this membrane protein forces sodium ions (Na+) out of the cell, and pumps

potassium ions (K+) in. As a result of this active transport, the cytoplasm of the neuron contains more K+ ions and fewer Na+ ions than the

surrounding medium. However, the neuron cell membrane is much leakier to K+ than it is to Na+.

As a result, K+ ions leak out of the cell to produce a negative charge on the inside of the membrane.



This charge difference is known as the Resting

Potential of the neuron. The neuron is not actually

"resting" because it must produce a constant

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RESTING POTENTIAL PROPOGATION

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OUTSIDE

INSIDE

K ⁺ = Potassium; Na ⁺ = Sodium; Cl ^- = Chloride; Pr ^- = proteins

Na

+

Na ⁺

K ⁺

K⁺

Force of Diffusion Electrostatic Force

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

- - - -- - -

Cl ^-

Force of Diffusion

Cl^-

Electrostatic Force

Pr ^-

Closed channel open

channel

open channel no

channel 3Na/2K

pump

- 65 mV

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ACTION POTENTIAL-BASIC CONCEPT

 When section of cell membrane is excited by some form of externally applied

energy, membrane characteristics changes

& begins to allow some sodium ions to enter.

 This movement of Na+ ions constitutes an ionic current that further reduces the

barrier of the membrane to Na+ ions.

 Result-Avalanche effect, Na+ ions rush into the cell to balance with the ions outside .

 At the same time K+ ions which were in higher concentration inside the cell during resting state, try to leave the cell but are unable to move as rapidly as Na+ ions.

 As a result the cell has slightly positive potential on inside due to imbalance of K+

ions.

 This potential is called as Action Potential .

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WAVEFORM SHOWING DEPOLARIZATION &

REPOLARIZATION IN ACTION POTENTIAL



The cell that displays an action Potential is said to be depolarized;



The process of changing from resting state to

action potential is called Depolarization.



Once the rush of Na+ ions through the cell

membrane has stopped, the membrane reverts back to its original

condition wherein the

passage of Na+ ions from outside to inside is

blocked



This process is called Repolarization.

7 BME 301 Lecture Note 3 - Ali Işın 2013

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ACTION POTENTIAL PROPOGATION



It “travels” down the axon



Actually, it does not move. Rather

the potential change resulting from

Na+ influx disperses to the next

voltage-gated channel, triggering

another action potential there.

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PROPOGATION OF POTENTIALS IN NERVE IMPULSE

 The Moving Impulse

An impulse begins when a neuron is stimulated by another neuron or by the environment. Once it begins, the impulse travels rapidly down the axon away from the cell body and towards the axon terminals.

 As Figure shows, an impulse is a sudden reversal of the membrane potential. What causes the reversal?

The neuron membrane contains thousands of protein channels or gates, that allow ions to pass through.

Generally, these gates are closed. At the leading edge of an impulse, however, sodium gates open, allowing positively charged Na+ ions to flow inside. The inside of the membrane temporarily becomes more positive than the outside, reversing the resting potential. This

reversal of charges is called an Action Potential. As the action potential, potassium gates open, allowing

positively charged K+ ions to flow out. This restores the Resting Potential so that the neuron is once again

negatively charged on the inside of the cell membrane and positively charged on the outside.

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 A nerve impulse is self-propagating.

That is, an impulse at any point on the

membrane causes an impulse at the next point along the membrane. We might

compare the flow of an impulse to the fall of a row of dominoes. As each domino falls, it causes its neighbour to fall. Then, as the impulse passes, the dominoes set

themselves up again, ready for another

Action Potential.

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Resting and action potentials

 The resting potential is the result of an unequal distribution of ions across the membrane.

 The resting potential is sensitive to ions in proportion to their ability to permeate the membrane.

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Resting potentials

 Forget the membrane and consider what factors determine the movement of ions in solution.

 Aqueous diffusion and

 Electrophoretic movement

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0 mV

Resting potentials

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0 mV

Resting potentials

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-80 mV

Resting potentials

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+

+ +

+

-

- -

-80 mV

Resting potentials

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+

+ +

+

-

- -

- - -

-80 mV

[K+] = 2.5 [Na+] = 125

[Cl-] = 130

A-

[K+] = 135 [Na+] = 7

[Cl-] = 11 A-

Resting potentials

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Resting potentials

Resting membrane

potential is independent of external Na+

concentration

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Resting potentials

Resting membrane

potential strongly depends upon the external K+

concentration

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Summary



The membrane conducts ions very poorly and allows the separation of ionic species.



This results in a potential difference between the outside and the inside of the membrane.



The magnitude of the resting potential is determined by the selective permeability of the membrane to ionic species.



We can quantify the magnitude of the resting potential by

considering both the diffusive and electrophoretic properties.



In order to understand the time dependence and individual

contributions of ionic species to the membrane potential it is

convenient to use an electrical equivalent circuit.

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Resting Membrane Potential

Membrane

outside

inside

Na ⁺

Cl ^-

K ⁺

A ^-

+ + + + + + + + + + +

- - - - - - - - - - -

+ + + + + + + + + + +

- - - - - - - - - - -

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Membrane is polarized

 more negative particles in than out

 Bioelectric Potential

 like a battery

 Potential for ion movement



current ~

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INSIDE

POS

NEG

Bioelectric Potential

OUTSIDE

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Biopotentials

 ECG

 electrocardiogrphy

 EEG

 electroencephalography

 EMG

 electromyography

 ERG

 electroretinograpy

 EOG…

 electrooculography

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Frequencies of Biopotentials

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Electrocardiogram (ECG)

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Recording System EEG

 EEG recording is done using a

standard lead

system called 10- 20 system

 Recall dipole

concept to identify source of brain

activity

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Electromyogram (EMG)

 Measures muscle activity

 Recordintramuscularly through needle electrodes

 Record surface EMG using electrodes on biceps,triceps…

 Use in muscular disorders,muscle based

prosthesis –prosthetic arm, leg

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Electroretinogram (ERG)

 Biopotential of the eye (retina)

 Indicator of retinal diseases such as retinal degenration, macular degernation

 Invasive recording

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KINDS OF ELECTRODES

Electrodes

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Eectrodes

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Figure A disposable surface electrode. A typical surface electrode used for ECG recording is made of Ag/AgCl. The electrodes are attached to the patients’ skin and can be easily removed.

Eectrodes

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Biopotential Electrodes

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The current crosses it from left to right.

The electrode consists of metallic atoms C.

The electrolyte is an aqueous solution

containing cations of the electrode metal C+ and anions A–.

Electrode–electrolyte interface

where n is the valence of C and m is valence of A

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V

_p

= total patential, or polarization potential, of the electrode

E

⁰

= half-cell potential V

_r

= ohmic overpotential

V

_c

= concentration overpotential

V

_a

= activation overpotential

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37 BME 301 Lecture Note 3 - Ali Işın

2013

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A silver/silver chloride electrode, shown in

cross section

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1.73  10 ^–10  142.3 = 2.46  10

BME 301 Lecture Note 3 - Ali Işın

^–8 g

³⁹

2013

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Sintered Ag/AgCI electrode

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E

_hc

is the half-cell potential,

R

_d

and C

_d

make up the impedance associated with the electrode-electrolyte interface and polarization effects,

Rs is the series resistance associated with interface effects and due to resistance in the electrolyte.

Equivalent circuit for a biopotential electrode in contact with an electrolyte

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The electrode area is 0.25 cm2. Numbers attached to curves indicate number of mAs for each deposit. (From L. A. Geddes, L. E. Baker, and A. G. Moore, “Optimum Electrolytic Chloriding of Silver Electrodes,” Medical and Biological Engineering, 1969, 7, pp. 49–56.)

Impedance as a function of frequency for Ag electrodes

coated with an electrolytically deposited AgCl layer

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Experimentally determined magnitude of impedance as a function of frequency for electrodes

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Example

We want to develop an electrical model for a specific biopotential electrode studies in the laboratory. The electrode is characterized by placing it in a physiological saline bath in the laboratory, along with an Ag/AgCl electrode having a much greater surface area and a

known half-cell potential of 0.233 V. The dc voltage between the two

electrodes is measured with a very-high-impedance voltmeter and

found to be 0.572 V with the test electrode negative The magnitude of

the impedance between two electrodes is measured as a function of

frequency at very low currents; it is found to be that given in Figure in

slide 12. From these data, determine a circuit model for the electrode.

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Solution

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Half cell potential of the test electrode E

_hc

= 0.223 V – 0.572 = -0339 V

At frequencies greater than 20 kHz C

_d

is short circuit. Thus R

_s

= 500 Ω = 0.5 kΩ, At frequencies less than 50 Hz C

_d

is open circuit. Thus R

_s

+ R

_d

= 30 kΩ. Thus R

_d

= 30 kΩ - R

_s

= 29.5 kΩ

Corner frequency is 100 Hz. Thus

C

_d

= 1/(2πf R

_d

) = 1/(2π100×29500) = 5.3×10

^-8

F = 0.53×10

^-9

F = 0.53 nF

= -0339 V = 500 Ω

= 29.5 kΩ

= 0.53nF

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Magnified section of skin, showing the various layers

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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.

A body-surface electrode is placed against skin, showing the total electrical equivalent circuit obtained in this situation

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(a) Metal-plate electrode used for application to limbs,

(b) Metal-disk electrode applied with surgical tape,

(c) Disposable foam-pad electrodes, often

used with

electrocardiographic monitoring

apparatus.

Body-surface biopotential electrodes

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A metallic suction electrode is often used as a precordial electrode on clinical electrocardiographs.

A metallic suction electrode

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(a) Recessed electrode with top-hat structure, (b) Cross-sectional view of the electrode in (a),

(c) Cross-sectional view of a disposable recessed electrode of the same general structure shown in figure (c) in slide 17. The recess in this electrode is formed from an open foam disk,

saturated with electrolyte gel and placed over the metal electrode.

Examples of floating metal body-surface electrodes

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(a) Carbon-filled silicone rubber electrode,

(b) Flexible thin-film

neonatal electrode (after Neuman, 1973).

(c) Cross-sectional view of the thin-film electrode in (b).

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Flexible body-surface electrodes

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(a) Insulated needle electrode, (b) Coaxial needle electrode, (c) Bipolar coaxial electrode,

(d) Fine-wire electrode connected to hypodermic needle, before being inserted,

(e) Cross-sectional view of skin and muscle, showing fine-wire electrode in place,

(f) Cross-sectional view of skin and muscle, showing coiled fine-wire electrode in place.

Needle and wire electrodes for

percutaneous measurement of biopotentials

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(a) Suction electrode,

(b) Cross-sectional view of suction electrode in place, showing penetration of probe through epidermis, (c) Helical electrode, that is attached to fetal skin by corkscrew-type action.

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Electrodes for detecting fetal electrocardiogram during labor, by means of intracutaneous needles

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(a) Wire-loop electrode,

(b) platinum-sphere cortical- surface potential electrode, (c) Multielement depth

electrode.

Implantable electrodes for detecting biopotentials

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(a) One-dimensional plunge electrode array (after

Mastrototaro et al., 1992), (b)Two-dimensional array, and (c) Three-dimensional array

(after Campbell et al., 1991).

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Examples of microfabricated electrode arrays

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Capacitance per unit length



₀

= dielectric constant of free space



_r

= relative dielectric constant of insulation material D = diameter of cylinder consisting of electrode plus insulation

D = diameter of electrode L = length of shank

The structure of a metal microelectrode for

intracellular recordings

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(a) Metal-filled glass micropipet.

(b) Glass micropipet or probe, coated with metal film.

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Structures of two supported metal microelectrodes

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(a) Section of fine-bore glass capillary,

(b) Capillary narrowed through heating and stretching,

A glass micropipet electrode filled with an

electrolytic solution

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(a) Beam-lead multiple electrode.

(b) Multielectrode silicon probe after Drake et al.

(c) Multiple-chamber electrode after Prohaska et al.

(d) Peripheral-nerve electrode based on the design of Edell.

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Different types of microelectrodes fabricated using microelectronic technology

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(a) Electrode with tip placed within a cell, showing origin of

distributed capacitance, (b) Equivalent circuit for the

situation in (a),

(c) Simplified equivalent circuit.

Equivalent circuit of metal microelectrode

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(a) Electrode with its tip placed within a cell, showing the origin of distributed capacitance,

(b) Equivalent circuit for the situation in (a),

(c) Simplified equivalent circuit.

^{(From L.}

A. Geddes, Electrodes and the Measurement of Bioelectric Events, Wiley-Interscience, 1972.

Used with permission of John Wiley and Sons, New York.)

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Equivalent circuit of glass micropipet microelectrode

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(a) Constant-current stimulation,

(b) Constant-voltage stimulation.

Current and voltage waveforms seen with electrodes used for

electric stimulation

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Simplified equivalent circuit of a Needle type EMG electrode pair and equivalent circuit of the input stage of an amplifier

Needle type EMG electrode

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Figure shows equivalent circuit of a biopotential electrode. A pair of these electrodes are tested in a beaker of physiological saline solution. The test consists of measuring the magnitude of the impedance between the electrodes as a function of frequency via low-level sinusoidal excitation so that the impedances are not affected by the current crossing the electrode–electrolyte interface. The impedance of the saline solution is small enough to be neglected. Sketch a Bode plot (log of impedance

magnitude versus log of frequency) of the impedance between the electrodes over a frequency range of 1 to 100,000 Hz.

Example

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Solution

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Assume Figure in previous slide models both electrodes of the pair.

The low corner frequency is

Fc = 1/(2RC) = 1/(2·20 k·100 nF) = 80 Hz.

The high corner frequency is Fc = 1/(2 RC)

= 1/(2·20 k||300 ·100 nF) = 5380 Hz.

The slope between the two corner frequencies is –1 on a log-log plot.

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A pair of biopotential electrodes are implanted in an animal to measure the

electrocardiogram for a radiotelemetry system. One must know the equivalent circuit for these electrodes in order to design the optimal input circuit for the telemetry

system. Measurements made on the pair of electrodes have shown that the polarization capacitance for the pair is 200 nF and that the half-cell potential for each electrode is 223 mV. The magnitude of the impedance between the two electrodes was measured via sinusoidal excitation at several different frequencies. The results of this

measurement are given in the accompanying table. On the basis of all of this information, draw an equivalent circuit for the electrode pair. State what each component in your circuit represents physically, and give its value.

Example

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Solution

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BME 301: Biomedical Sensors