Bio Potential

Biomedical Instrumentation – EE 4385

BI Lecture Series Homer Nazeran PhD, CPEng (Biomed.)

Biopotential Electrodes • To measure biopotentials (voltages) across the body (generated in the

body due to ionic current flow), we need some sort of interface between the body and the electronic measuring device to convert ionic current flow to electronic current flow

• In any practical voltage measurement situation, current, i, flows in the

measuring device for at least a fraction of the time the measurement is being made. Ideally, i should be zero, but practically it is not!

i Bio-source Tissue interface • Transducers (sensors) that serve as the interface between the body and

the electronic measuring device are known Biopotential Electrodes. These sensors must conduct a current between the body and the electronic device

• Electrodes carry out a transduction function. They convert ionic current

flow in the body to electronic current in the measuring device To understand how biopotential electrodes work, we need to: • Look at the basic mechanisms of the transduction process and discuss

how these may affect the electrode characteristics • Examine principle electrical characteristics of the biopotential electrodes • Discuss the electrical equivalent circuits (electrical models) for the

electrodes • Focus on biopotential electrodes used in clinical measurements

Measurement

Device ♥



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Electrode-electrolyte Interface Review of basic electrochemistry The simple diagram below shows the charge distribution at the electrode-electrolyte interface C C+ A- C+ C C+ A- C+ e- C C+ A- A- C C+ A- C C+ A- e- C C+ A- C C+ A- C C+ A-

Electrode Electrolyte Interface Current flow Note: • C represents the metal atom (electrode) • C+ represents the cation of the metal • A- represents an anion (i.e. Cl-) • e- represents an electron • No free electrons in the electrolyte • No free anions and cations in the electrode • For charge to cross the interface something must occur at the interface

that transfers the charge between these carriers • What actually occurs is chemical reactions at the interface

C ↔ Cn+ + ne- Am- ↔ A + me- n = valence of cation m = valence of anion



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• Electrode is made of some atoms (metal) of the same material as cations

(i.e. Ag) Reactions are reversible and reduction reactions can also occur (Note: oxidation reactions are from left to right and reduction reactions are from right to left) • When no current flows across the interface:

Rate of oxidation reactions = rate of reduction reactions

so the net transfer of charge across the interface is = 0. • When current flow is from electrode to electrolyte, oxidation reactions

dominate and the reaction goes to the right • When current flow is from electrolyte to electrode, reduction reactions

dominate and the reaction goes to the left Electrode-electrolyte Interface Characteristics To understand the electrode-electrolyte characteristics, let’s look at the following situation: Metal C Solution contains C+ (ions of the metal) and an equal number of A- to maintain charge neutrality



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• When metal comes into contact with the solution, the following reaction begins immediately:

C ↔ Cn+ + ne- Initially the reaction goes predominantly to the right or to the left depending on the following: • The concentration of Cn+ • The equilibrium conditions for the reaction, therefore local concentration

for the Cn+ at the interface changes and neutrality of charge is not maintained in this region

• Electrolyte surrounding the metal is at a different potential from the rest

of the solution • A potential difference known as the half-cell potential is determined by

the metal involved, the concentration of its ions in the solution and temperature

The distribution of charge at the vicinity of the interface has been of great interest to electrochemists and several theories have evolved. Without worrying about these theories at this stage, we accept their findings that : • Some sort of separation of charges at the interface results in an electric

double layer, wherein one type of charge is dominant and the opposite charge is distributed in excess in the immediately adjacent vicinity. This electric double layer generates the half-cell potential

• It is not possible to measure the half-cell potential of an electrode

because unless we use a second electrode we can not provide a connection between the electrolyte and one terminal of the potential measuring device. Therefore we end up measuring the potential difference between the half-cell potential of the metal and that of the second (reference ) electrode



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• By international agreement: The H2 electrode is used as the reference electrode.

The arrangement to measure half-cell potential for electrode X. H2 (reference) electrode is shown on the left.

• Table5.1 shows the half-cell potentials for common electrode materials

at 25oC.



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Polarization • The above discussion was presented for the case where no electric current

flows between the electrode and the electrolyte (across the interface) • If there is current flow across the interface, then the observed half-cell

potential is altered • The difference is due to what is called the polarization of the electrode • The difference between the observed half-cell potential and the

equilibrium zero-current half-cell potential is known as overpotential Three basic mechanisms contribute to the overpotential: 1. The Ohmic overpotential

• Resistance of electrolyte • Voltage drop current and resistivity of electrolyte

2. The concentration overpotential

• Due to current flow the rates of reactions at the interface change a new distribution of charge at the interface

3. The activation overpotential

• Due to current flow either oxidation or reduction reactions predominate

• Due to different energy barriers The above overpotentials can be summarized in the following relationship:

Vp = Vr + Vc + Va Vp = Polarization potential Vr = Ohmic overpotential Vc = Concentration overpotential Va = Activation overpotential



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• Now let’s look at a situation where two electrolytic (ionic) solutions are separated by a semipermeable membrane

Semipermeable membrane ionic sol. 1 ionic sol. 2 • In this case the half-cell potential between the two solutions is:

Ehc = - [RT/nF] . ln (a1/a2) where a1 and a2 are the activities of each ion on sides 1 & 2 of the semipermeable (ion selective) membrane. R, T , n and F were previously defined in Nernst equation. • In dilute solutions, activity ≅ concentration • In concentrated solutions, activity < concentration due to intermolecular

effects [In case you have forgotten your Chemistry, Ionic activity is defined as: availability of ions in a solution to enter into a reaction] Under non-standard conditions, the half-cell potential is different from what was observed in Table 5.1 [Standard condition: Standard temperature = 25oC, standard ionic activity = unity] and is calculated from the formula below:

Ehc = Eohc

+ [RT/nF] . ln (a Cn+) Ehc = half-cell potential under non-standard conditions Eo

hc = half-cell potential under standard conditions a = ionic activity of cation Cn+



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• For a general oxidation-reduction reaction αΑ + βB γC + δD + ne-

Ehc = Eohc

+ [RT/nF] . ln [products/reactants]

Ehc = Eohc

+ [RT/nF] . ln (aCγ . aD

δ/aAα aB

β) where a’s represent activity of various constituents of the reaction • What happens if two electrolytic solutions are in contact and have

different ionic concentrations with different ionic mobilities? • A potential difference exists! This is called a liquid-junction potential.

For solutions of the same composition but different activities

µ+ - µ- Ej = [RT/nF] . ln (a’/ a”)

µ+ - µ- µ+ and µ- = mobilities of positive and negative charges. Ej can be in the order of millivolts! So for biopotential measurements we need uniform electrolytes. Example 5.1 from Webster



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Polarizable and Nonpolarizable Electrodes • Theoretically, two types of electrodes are possible:

1. Perfectly polarizable 2. Perfectly nonpolarizable

1. In perfectly polarizable electrodes, no actual charge crosses the electrode-

electrolyte interface, when a current is applied • Current flow across the interface is by displacement current. The

interface acts like a capacitor, therefore there is overpotentials 2. In perfectly nonpolarizable electrodes, current passes freely across the

interface requiring no energy to make the transition, therefore no overpotentials

• Neither type can be fabricated, but some practical electrodes come close • Electrodes made of noble metals (i.e., Au, Pt, Ir, etc.) come closest to

behaving as perfectly polarizable (because their material is relatively inert, therefore they do not oxidize and dissolve

• Ag-AgCl electrodes come closest to a nonpolarizable electrode Ag-AgCl electrode • Ag-AgCl electrode belongs to the class of electrodes, which consist of a

metal coated with a layer of slightly soluble ionic compound of that metal with a suitable anion.

• AgCl is only slightly soluble in water and remains stable



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• The behaviour of Ag-AgCl electrode is governed by two chemical equations:

Ag ↔ Ag+ + e- this takes place at the silver surface

Ag+ + Cl- ↔ AgCl↓ AgCl precipitates out of solution onto the silver electrode and contributes to AgCl coating • From chemistry we remember a constant called the solubility product

Ks, which was mainly defined as the rate of precipitation and returning to solution

• Under equilibrium conditions:

aAg+ . aCl

- = Ks Ks for AgCl = 10-10. For biological fluids, concentration of Cl- is relatively high, therefore aCl

- is very close to unity. Therefore when an Ag-AgCl electrode is in contact with biological fluid, aAg

+ is very close to Ks for AgCl (which is a constant) • Previously we showed that:

Ehc = Eohc

+ [RT/nF] . ln (a Cn+) For an Ag-AgCl electrode:

EAg = EoAg

+ [RT/nF] . ln (aAg+)

But, aAg+ . aCl

- = Ks aAg

+ = Ks/ aCl-

EAg = EoAg

+ [RT/nF] . ln (Ks/aCl-)

EAg = EoAg

+ [RT/nF] . ln (Ks) - [RT/nF] . ln (aCl-)

Note: Eo

Ag is constant, [RT/nF] . ln (Ks) is also constant. aCl- is

relatively large and is not related to the oxidation of Ag+, which is caused by current flow through the electrode. aCl

- remains stable when the electrode is placed in biological fluid. Therefore, the half-cell potential of the Ag-AgCl electrode remains relatively stable and this is desirable for a biopotential



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measurement as we will shortly see in the electrical model of the bioelectrode. How to Fabricate Ag-AgCl Electrodes • There are several fabrication procedures. Two are popular in making

bioelectrodes: • The electrolytic process • The sintering process

The electrolytic process • An electrochemical cell used to grow an AgCl film on the surface of an

Ag electrode is shown below:

• When the switch is closed, chemical reactions begin to occur and the

current jumps to a maximum • As the thickness of deposited AgCl increases, the rate of reaction

decreases and the current drops • After a few minutes the current reaches a relatively low value of the order

of 10 µA for most biological electrodes Example 5.2 from Webster



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The Sintering Process A sintered Ag-AgCl electrode is shown below:

• The electrode consists of an Ag lead wire surrounded by a sintered Ag-

AgCl cylinder • It is formed by placing a clean wire in a die that is then filled with a

mixture of powdered components Ag & AgCl • The die is compressed in a press to form the powdered components into a

pellet • The pellet is then removed from the die and baked at 400oC in an oven

for several hours



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Performance of Ag-AgCl Electrodes • Ag-AgCl electrodes exhibit a nonpolarizable behaviour • Ag-AgCl electrodes also exhibit less electrical noise from the equivalent

Ag electrodes as shown below

Spontaneous noise seen from pairs of electrodes immersed in a

physiological saline solution (a) From spherical metallic Ag electrodes coated with AgCl film (b) From the two electrodes when a AgCl has been removed using emery

paper (c) From the electrodes when a new AgCl layer has been deposited Calomel electrodes are discussed in detail when we study the pH electrodes in Clinical Laboratory Instrumentation (Chapter 10).



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Electrode Behavior and Circuit Models • The electrical characteristics of bioelectrodes have been extensively

studied • Without going into much detail and to make a long story short, it has

been shown by Geddes and others that an equivalent circuit explains the electrode behaviour quite satisfactorily

• Previously we described the electrode-electrolyte interface and the

creation of a charge double layer at this interface. This is shown below: C C+ A- C+ C C+ A- C+ e- C C+ A- A- C C+ A- C C+ A- e- C C+ A- C C+ A- C C+ A-

Electrode Electrolyte Interface

Rd Rs Ehc

Cd Rd = resistive component associated with the electrode-electrolyte interface and polarizable effects Rs = associated with resistance in the electrolyte Cd = capacitance associated with the charge double layer Ehc = half-cell potential associated with the charge double layer



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• This equivalent circuit demonstrates that the electrode impedance is frequency dependent

• At high frequencies, where 1/ωCd << Rd the impedance is a constant

at Rs • At low frequencies, where 1/ωCd >> Rd the impedance is again a

constant, but its value is large and is = Rs +Rd • At frequencies between these extremes, the electrode impedance is

frequency dependent A typical frequency response for a bioelectrode is shown below:

Example 5.4 in Webster



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The Electrode-skin Interface and Motion Artifact • When recording biopotentials from the surface of the skin, we must

consider an additional interface: the interface between the electrode-electrolyte and the skin

• A magnified cross-section of skin, showing its various layers is shown

below:



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• Now when we place a body-surface electrode on top of an electrolyte gel layer against the skin, we end up with an electrical equivalent circuit shown below:

The motion artifact in Ag-AgCl electrodes is shown in the figure below:

Heavy lines under curves indicate periods of agitation of saline solution

(a) Metallic Ag electrodes in agitated physiological saline solutions (b) The same electrodes with an AgCl surface film in agitated physiological

saline solution (c) Output from an amplifier used for recording when electrodes are replaced

by a 1.5kΩ resistor.



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Body-surface Recording Electrodes

There are a variety of electrodes for recording biopotential from the body surface. These are: Metal-plate Electrodes, Suction Electrodes, Floating Electrodes, Flexible Electrodes, and Dry Electrodes. Metal-plate Electrodes • These are used for application to limbs. The figure below shows a variety

of such electrodes.

(a) Metal-plate electrode (b) Metal-disk electrode (c) Disposable foam-pad electrodes



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Suction Electrodes • This is a modification of the metal-plate electrode that requires no

adhesives for holding it in place. These electrodes are mainly used as chest (precordial) electrodes in Electrocardiography. The figure below shows a metallic suction electrode.

Floating Electrodes These types of electrodes are developed to diminish the motion artifacts at the interface between the electrode and the skin. The figure below shows some examples of floating metal body-surface electrodes.



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Flexible Electrodes The body surface is irregularly shaped and can change its local curvature with movement. Solid electrodes (the ones discussed so far) can not conform to these surface changes and lead into generation of additional motion artifacts. Flexible electrodes have been developed to avoid such problems. The figure below shows some types of these electrodes.

(a) Carbon-filled silicone rubber electrode (b) Flexible thin-film electrode (c) Cross-sectional view of thin-film electrode



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Dry Electrodes All surface electrodes described so far require an electrolyte gel to establish and maintain contact between the electrode and the skin. Dry electrodes have been developed to eliminate the electrolyte layer and could be applied directly to skin. The figure below shows a dry electrode and its amplifier circuit.



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Internal Electrodes

• These electrodes differ from body surface electrodes in that they will eliminate the limitations associated with the electrode–electrolyte interface.

• The electrode itself or the lead wire crosses the skin and may be entirely internal.

• No electrolyte gel is required because the electrode comes into direct contact with the extracellular fluid in the body.

• There are many different designs of these electrodes. The figure below shows different types of percutaneous needle or wire electrodes.

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

place

Bio Potential

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