UnBIOMEDICAL INSTRUMENTATION UNIT 1 NOTES

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
BM3491 Biomedical Instrumentation
UNIT-I ELECTRODE CONFIGURATIONS
1.1 Origin of bio potential and its propagation:
1.1.1 Introduction:
Most of the physiological processes were accompanied with electrical
changes. This discovery formed the basis of the explanation of the action of living
tissues in terms of bioelectric potentials. Bioelectric potentials are generated at a
cellular level and the source of these potentials is ionic in nature. A cell consists of an
ionic conductor separated from the outside environment by a semipermeable
membrane which acts as a selective ionic filter to the ions. This means that some ions
can pass through the membrane freely where as others cannot do so. All living matter
is composed of cells of different types. Surrounding the cells of the body are body
fluids, which are ionic and which provide a conducting medium for electric potentials.
The principal ions involved with the phenomena of producing cell potentials are sodium
(Na+ ), potassium (K+ ) and chloride (Cl– ).
1.1.2 Resting and Action Potentials:
i. Certain types of cells within the body, such as nerve and muscle cells, are
encased in a semipermeable membrane that permits some substances to pass
through the membrane while others are kept out.
ii. Surrounding the cells of the body are the body fluids. These fluids are
conductive solutions containing charged atoms known as ions. The principal
ions are sodium (Na+), potassium (K+), and chloride (C-).
iii. The membrane of excitable cells readily permits entry of potassium and chloride
ions but effectively blocks the entry of sodium ions. Since the various ions seek
a balance between the inside of the cell and the outside, both according to
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concentration and electric charge, the inability of the sodium to penetrate the
membrane results in two conditions.
iv. First, the concentration of sodium ions inside the cell becomes much lower than
in the intercellular fluid outside. Since the sodium ions are positive, this would
tend to make the outside of the cell more positive than the inside.
v. Second, in an attempt to balance the electric charge, additional potassium ions,
which are also positive, enter the cell, causing a higher concentration of
potassium on the inside than on the outside. This charge balance cannot be
achieved, however, because of the concentration imbalance of potassium ions.
Equilibrium is reached with a potential difference across the membrane,
negative on the inside and positive on the outside. This membrane potential is
called the resting potential of the cell and is maintained until some kind of
disturbance upsets the equilibrium.
vi. Since measurement of the membrane potential is generally made from inside
the cell with respect to the body fluids, the resting potential of a cell is given as
negative. Research investigators have reported measuring membrane
potentials in various cells ranging from - 60 to - 100 mV. A cell in the resting
state is said to be polarized cell.
vii. When a section of the cell membrane is excited by the flow of ionic current or
by some form of externally applied energy, the membrane changes its
characteristics and begins to allow some of the sodium ions to enter. This
movement of sodium ions into the cell constitutes an ionic current flow that
further reduces the barrier of the membrane to sodium ions. The net result is

an avalanche effect in which sodium ions literally rush into the cell to try to reach
a balance with the ions outside.
viii. At the same time potassium ions, which were in higher concentration inside the
cell during the resting state, try to leave the cell but are unable to move as
rapidly as the sodium ions. As a result, the cell has a slightly positive potential
on the inside due to the imbalance of potassium ions. This potential is known
as the action potential and is approximately + 20 mV.
ix. A cell that has been excited and that displays an action potential is said to be
depolarized; the process of changing from the resting state to the action potential
is called depolarization.
x. Once the rush of sodium ions through the cell membrane has stopped (a new state
of equilibrium is reached), the ionic currents that lowered the barrier to sodium ions
are no longer present and the membrane reverts back to its original, selectively
permeable condition, wherein the passage of sodium ions from the outside to the
inside of the cell is again blocked. Were this the only effect, however, it would take
a long time for a resting potential to develop again.
xi. But such is not the case. By an active process, called a sodium pump, the sodium
ions are quickly transported to the outside of the cell, and the cell again becomes

polarized and assumes its resting potential. This process is called repolarization.
The rate of pumping is directly proportional to the sodium concentration in the cell.
Fig. Waveform of the action potential
xii. Figure shows a typical action-potential waveform, beginning at the resting
potential, depolarizing, and returning to the resting potential after repolarization.
The time scale for the action potential depends on the type of cell producing the
potential. In nerve and muscle cells, repolarization occurs so rapidly following
depolarization that the action potential appears as a spike of as little as 1 m.sec.
total duration. Heart muscle, on the other hand, repolarizes much more slowly,
with the action potential for heart muscle usually lasting from 150 to 300 m.sec.
xiii. Regardless of the method by which a cell is excited or the intensity of the
stimulus (provided it is sufficient to activate the cell), the action potential is
always the same for any given cell. This is known as the all-or-nothing law.

xiv. Following the generation of an action potential, there is a brief period of time
during which the cell cannot respond to any new stimulus. This period, called
the absolute refractory period, lasts about 1 msec in nerve cells.
xv. Following the absolute refractory period, there occurs a relative refractory
period, during which another action potential can be triggered, but a much
stronger stimulation is required. In nerve cells, the relative refractory period
lasts several milliseconds. These refractory periods are believed to be the result
of after-potential.
1.1.3 Propagation of Action Potentials:
i. When a cell is excited and generates an action potential ionic currents begin to
flow.
ii. This process can, in turn, excite neighbouring cells or adjacent areas of the
same cell.
iii. In the case of a nerve cell with a long fiber, the action potential is generated
over a very small segment of the fiber's length but is propagated in both
directions from the original point of excitation. In nature, nerve cells are excited
only near their ** input end'*
iv. As the action potential travels down the fiber, it cannot reexcite the portion of
the fiber immediately upstream, because of the refractory period that follows
the action potential.
v. The rate at which an action potential moves down a fiber or is propagated from
cell to cell is called the propagation rate. In nerve fibers the propagation rate
is also called the nerve conduction rate, or conduction velocity. This velocity
varies widely, depending on the type and diameter of the nerve fiber. The usual
velocity range in nerves is from 20 to 140 meters per second (m/sec).
vi. Propagation through heart muscle is slower, with an average rate from 0.2 to
0.4 m/sec. Special time-delay fibers between the atria and ventricles of the
heart cause action potentials to propagate at an even slower rate, 0.03 to 0.05
m/sec.
1.1.4 The Bioelectric Potentials:

i. To measure bioelectric potentials, a transducer capable of converting ionic
potentials and currents into electric potentials and currents is required.
ii. Such a transducer consists of two electrodes, which measure the ionic potential
difference between their respective points of application.
iii. Measurement of individual action potentials can be made in some types of cells,
such measurements are difficult because they require precise placement of an
electrode inside a cell.
iv. The more common form of measured biopotentials is the combined effect of a
large number of action potentials as they appear at the surface of the body, or
at one or more electrodes inserted into a muscle, nerve, or some part of the
brain.
v. The exact method by which these potentials reach the surface of the body is
not known.
vi. According to theory, the surface pattern is a summation of the potentials
developed by the electric fields set up by the ionic currents that generate the
individual action potentials.
vii. This theory, although plausible, fails to explain a number of the characteristics
indicated by the observed surface patterns.
viii. They can be measured as specific bioelectric signal patterns that have been
studied extensively and can be defined quite well. The designation of the wave-
form itself generally ends in the suffix gram, whereas the name of the in-
strument used to measure the potentials and graphically reproduce the wave-
form ends in the suffix graph. For example, the electrocardiogram (the name of
the waveform resulting from the heart's electrical activity) is measured on an
electrocardiograph (the instrument).
ix. The source of bioelectric signals are cells which undergo change of state from
resting potential to action potential under certain conditions. The change of
potential in many cells generate an electric field which fluctuates and, in this
process, it is to emit bioelectric signal. ECG and EEG are obtained from the bio
signals from heart and brain respectively.
x. The Bioelectric Potentials are
 The Electrocardiogram(ECG)

 The Electroencephalogram(EEG)
 The Electromyogram(EMG)
 The Electroretinogram(ERG)
 The Electro-oculogram(EOG)
 The Electrogastrogram(EGG)
******

1.2 Electrode Configuration
i. Devices that convert ionic potentials into electronic potentials are called
electrodes.
ii. The interface of metallic ions in solution with their associated metals results in
an electrical potential that is called the electrode potential.
iii. This potential is a result of the difference in diffusion rates of ions into and out
of the metal. Equilibrium is produced by the formation of a layer of charge at
the interface. This charge is really a double layer, with the layer nearest the
metal being of one polarity and the layer next to the solution being of opposite
polarity.
iv. Non-metallic materials, such as hydrogen, also have electrode potentials when
interfaced with their associated ions in solution.
v. It is impossible to determine the absolute electrode potential of a single
electrode, for measurement of the potential across the electrode and its ionic
solution would require placing another metallic interface in the solution.
vi. Therefore, all electrode potentials are given as relative values and must be
stated in terms of some reference. By international agreement, the normal
hydrogen electrode was chosen as the reference standard and arbitrarily
assigned an electrode potential of zero volt.
vii. All the electrode potentials listed in Table are given with respect to the hydrogen
electrode. They represent the potentials that would be obtained across the
stated electrode and a hydrogen electrode if both were placed in a suitable ionic
solution.

viii. Another source of an electrode potential is the unequal exchange of ions across
a membrane that is semipermeable to a given ion when the membrane
separates Liquid solutions with different concentrations of that ion.
ix. An equation relating the potential across the membrane and the two
concentrations of the ion is called the Nernst equation and can be stated as
follows:
E = −
R = gas constant (8.315 x 107
ergs/mole/degree Kelvin)
T = absolute temperature, degrees Kelvin
n = valence of the ion (the number of electrons added or removed to ionize the atom)
F = Faraday constant (96,500 coulombs)
C1, C2 = two concentrations of the ion on the two sides of the membrane
f1, f2 = respective activity coefficients of the ion on the two sides of the membrane
In electrodes used for the measurement of bioelectric potentials, the electrode
potential occurs at the interface of a metal and an electrolyte, whereas in biochemical
transducers both membrane barriers and metalelectrolyte interfaces are used.
1.1.2 Biopotential Electrodes:
A wide variety of electrodes can be used to measure bioelectric events, but nearly all
can be classified as belonging to one of three basic types:
1 . Microelectrodes: Electrodes used to measure bioelectric potentials near or within
a single cell.
2. Skin surface electrodes: Electrodes used to measure ECG, EEG, and EMG
potentials from the surface of the skin.
3. Needle electrodes: Electrodes used to penetrate the skin to record EEG potentials
from a local region of the brain or EMG potentials from a specific group of muscles.
All three types of biopotential electrodes have the metal-electrolyte interface
described in the previous section. In each case, an electrode potential is developed
across the interface, proportional to the exchange of ions between the metal and the
electrolytes of the body. The double layer of charge at the interface acts as a capacitor.
Thus, the equivalent circuit of biopotential electrode in contact with the body consists

of a voltage in series with a resistance-capacitance network of the type shown in
Figure.
Since measurement of bioelectric potentials requires two electrodes, the
voltage measured is really the difference between the instantaneous potentials of the
two electrodes, as shown in Figure. If the two electrodes are of the same type, the
difference is usually small and depends essentially on the actual difference of ionic
potential between the two points of the body from which measurements are being
taken. If the two electrodes are different, however, they may produce a significant dc
voltage that can cause current to flow through both electrodes as well as through the
input circuit of the amplifier to which they are connected. The dc voltage due to the
difference in electrode potentials is called the electrode offset voltage. The resulting
current is often mistaken for a true physiological event. Even two electrodes of the
same material may produce a small electrode offset voltage.
In addition to the electrode offset voltage, experiments have shown that the
chemical activity that takes place within an electrode can cause voltage fluctuations to
appear without any physiological input. Such variations may appear as noise on a
bioelectric signal. This noise can be reduced by proper choice of materials or, in most
cases, by special treatment, such as coating the electrodes by some electrolytic
method to improve stability. It has been found that, electrochemically, the silver-silver
chloride electrode very stable. This type of electrode is prepared by electrolytically
coating a piece of pure silver with silver chloride. The coating is normally done by
placing a cleaned piece of silver into a bromide-free sodium chloride solution. A
second piece of silver is also placed in the solution, and the two are connected to a
voltage source such that the electrode to be chlorided is made positive with respect to
the other. The silver ions combine with the chloride ions from the salt to produce
neutral silver chloride molecules that coat the silver electrode. Some variations in the
process are used to produce electrodes with specific characteristics.
Fig. 1.1.1 Equivalent circuit of biopotential electrode interface.

The resistance-capacitance networks shown in Figures 1.1.1 and 1.1.2
represent the impedance of the electrodes (one of their most important characteristics)
as fixed values of resistance and capacitance. Unfortunately, the impedance is not
constant. The impedance is frequency-dependent because of the effect of the
capacitance. Furthermore, both the electrode potential and the impedance are varied
by an effect called polarization. Polarization is the result of direct current passing
through the metalelectrolyte interface. The effect is much like that of charging a battery
with the polarity of the charge opposing the flow of current that generates the charge.
Some electrodes are designed to avoid or reduce polarization. If the amplifier to which
the electrodes are connected has an extremely high input impedance, the effect of
polarization or any other change in electrode impedance is minimized.
Size and type of electrode are also important in determining the electrode
impedance. Larger electrodes tend to have lower impedances. Surface electrodes
generally have impedances of 2 to 10 kΩ, whereas small needle electrodes and
microelectrodes have much higher impedances. For best results in reading or
recording the potentials measured by the electrodes, the input impedance of the
amplifier must be several times that of the electrodes.
Fig. 1.1.2 Measurement of biopotentials with two electrodes—equivalent circuit.

1.1.3 Skin Interface Impedance:
i. The bioelectrical events are usually recorded by means of metallic electrodes
placed on the surface of the body. The electrical activity generated by various
muscles and nerves within the body is conducted to the electrode sites
through the body tissues, reaches the electrodes through the skin electrode
transition and is then conducted by direct wire connection to the input circuit
of the recording machine.
ii. The impedance at the electrode-skin junction comes in the overall circuitry of
the recording machine and, therefore, has significant effect on the final record.
Skin electrode impedance is known as the contact impedance and is of a value
much greater than the electrical impedance of the body tissue as measured
beneath the skin.
iii. The outer horny layer of the skin is responsible for the bulk of the skin contact
impedance and, therefore, a careful skin preparation is essential in order to
obtain best results.
1.1.4 Polarization Effects of Electrodes:
i. If a low voltage is applied to two electrodes placed in a solution, the electrical
double layers are disturbed. Depending on the metals constituting the
electrodes, a steady flow of current may or may not take place.
ii. In some metal/liquid interfaces, the electrical double layer gets temporarily
disturbed by the externally applied voltage, and therefore, a very small current
flows after the first surge, thus indicating a high resistance. This type of
electrode will not permit the measurement of steady or slowly varying potentials
in the tissues. They are said to, be polarized or nonreversible.
iii. Thus, the phenomenon of polarization affects the electro-chemical double layer
on the electrode surface and manifests itself in changing the value of the
impedance and voltage source representing the transition layer. Parsons
(1964) stated that electrodes in which no net transfer of charge takes place
across the metal-electrolyte interface can be termed as perfectly polarized.
Those in which unhindered exchange of charge is possible are called non-
polarizable or reversible electrodes. The ionic double layer in metals of these

electrodes is such that they allow considerable current to flow when a small
voltage is applied, thus offering a low resistance.
iv. Although polarizable electrodes are becoming less common, they are still in
use. They usually employ stainless steel and are used for resting ECGs or other
situations where there is small likelihood that the electrodes would be exposed
to a large pulse of energy (such as a defibrillation discharge) in which case they
would retain a residual charge, become polarized, and will no longer transmit
the relatively small bioelectric signals, thus becoming useless.
v. Non-polarizing electrodes on the other hand, are designed to rapidly dissipate
any charge imbalance induced by powerful electrical discharges such as a
defibrillation procedure. Rapid depolarization enables the immediate
reappearance of bioelectric signals on the monitor after defibrillation. For this
reason, non-polarizing electrodes have become the electrodes of choice for
monitoring in the intensive care units and stress testing procedures. Historically,
these electrodes employ a conducting metal with a silver/silver-chloride
(Ag/AgCl) surface in contact with the conducting gel.
vi. The choice of metals for electrodes is not determined only by their susceptibility
to polarization, but other factors such as mechanical properties, skin irritation
or skin staining, etc. have also to be taken into consideration.
1.1.5 Non-polarizable electrodes:
A non-polarizable electrode is an electrode that does not undergo a significant
change in potential when a current is applied. In other words, it does not contribute to
the polarization effects commonly observed in some other types of electrodes.
Polarization typically refers to the change in electrode potential caused by the buildup
of reaction products or changes in the local environment during electrochemical
processes.
Non-polarizable electrodes are often used in electrochemical measurements
where the focus is on studying the behavior of the solution or the material being
analysed, rather than the electrode itself. These electrodes are designed to minimize
any changes in potential and provide a stable reference point for the measurement.
Common examples of non-polarizable electrodes include:

1. Saturated Calomel Electrode (SCE): It is a reference electrode widely used
in electrochemical experiments. The SCE is based on a mercury/mercurous
chloride (Hg/Hg2Cl2) system and is considered non-polarizable under certain
conditions.
2. Silver/Silver Chloride Electrode (Ag/AgCl): Another common reference
electrode that is non-polarizable under certain conditions. It consists of a silver
wire coated with silver chloride.
These electrodes are chosen for their stability and minimal interference with the
electrochemical processes occurring in the system under investigation. Non-
polarizable electrodes play a crucial role in accurate and reliable electrochemical
measurements in various fields, including analytical chemistry, corrosion studies, and
bio electrochemistry.
Fig. Reference electrodes: (A) silver-silver chloride electrode and (B) saturated-calomel electrode
i. The characteristic of an ideal polarizable electrode is that no faraday current
flow when the electrode potential is varied. This type of electrodes usually can
be used as Working or Counter electrodes.
ii. The feature of a non-polarizable electrode is once the electrode potential be
changed, the Faraday current flows out. In generally, this type of electrodes
can be used as Reference electrodes.
iii. An ideal polarizable electrode can be represented by a capacitor (condenser)
in equivalent circuit as shown in Fig. However, the extremely weak current

flows at an actual polarizable electrode indeed. Therefore, a high value
resistance (Rhi) is required to parallel with the capacitor to represent the
actual polarizable electrode in equivalent circuit.
iv. If a redox species coexists with the polarizable electrode, the redox reaction
of species is occurred on the electrode surface and the Faraday current flows
under the certain potential. In this case, a potential depending variable
resistance (RF) should be added to the equivalent circuit in parallel.
Furthermore, the effect of the species diffusion should be included, and a
Warburg Impedance element is connected to Faraday resistance (RF) in
equivalent circuit.
*******

1
1.3 Unipolar and Bipolar electrodes
Biopotential electrodes are used to measure the electrical activity generated by
biological tissues, such as the heart, muscles, or the brain. Unipolar and bipolar refer
to the configuration of these electrodes in terms of the number of active contacts and
the way they are arranged.
Unipolar Biopotential Electrodes:
Single Active Electrode: In a unipolar configuration, there is typically one active
electrode that records the electrical signal, while the reference electrode is located at
a different position. The reference electrode is often placed at a neutral site, such as
a distant location on the body or a common reference point.
We use this type of connection if we want to monitor the progress of the signal
under a certain electrode. This electrode is an active electrode. The reference
electrode will either be common to all connected together reference inputs of all
amplifiers and placed outside the active electrodes, or we will artificially create some
electrically neutral point by connecting all the active electrodes through resistors of the
same size to one point, where the arithmetic average of the potentials from all active
electrodes. The potential difference between the sensed location where the active
electrode is located and this neutral point is recorded. In the case of the ECG, this
neutral point is called the Wilson clamp. This principle is also used in other
examinations.

Example: In electrocardiography (ECG or EKG), one common unipolar lead is the
augmented limb lead aVR, where the active electrode is placed on the right arm, and
the reference is a combination of the left arm and left leg electrodes.
Placement of electrodes with unipolar precordial leads used to measure the ECG
Bipolar Biopotential Electrodes:
Two Active Electrodes: In a bipolar configuration, there are two active electrodes
placed at specific locations. The electrical potential is measured between these two
active electrodes.
Example: In ECG, leads I, II, and III are examples of bipolar leads. Lead I measures
the potential between the right arm and left arm, lead II measures the potential
between the right arm and left leg, and lead III measures the potential between the left
arm and left leg.
The choice between unipolar and bipolar configurations depends on the specific
requirements of the measurement and the information needed. Unipolar leads are
often used to measure the potential at a single point with respect to a distant reference,
while bipolar leads measure the potential between two nearby points. Different leads
or configurations provide unique perspectives on the electrical activity of the body.
It's worth noting that there are also multichannel configurations that use combinations
of electrodes to capture more complex information about the electrical signals in
biopotential measurements.

Figure shows electrical connections between the patient’s limbs and the
electrocardiograph for recording electrocardiograms from the so-called standard
bipolar limb leads. The term “bipolar” means that the electrocardiogram is recorded
from two electrodes located on different sides of the heart, in this case, on the limbs.
Thus, a “lead” is not a single wire connecting from the body but a combination of two
wires and their electrodes to make a complete circuit between the body and the
electrocardiograph. The electrocardiograph in each instance is represented by an
electrical meter in the diagram, although the actual electrocardiograph is a high-speed
recording meter with a moving paper.
***********

1
1.4 Classifications of electrodes
A wide variety of electrodes can be used to measure bioelectric events, but nearly all
can be classified as belonging to one of three basic types:
1. Microelectrodes: Electrodes used to measure bioelectric potentials near or
within a single cell.
2. Skin surface electrodes: Electrodes used to measure ECG, EEG, and EMG
potentials from the surface of the skin.
3. Needle electrodes: Electrodes used to penetrate the skin to record EEG
potentials from a local region of the brain or EMG potentials from a specific
group of muscles.
All three types of biopotential electrodes have the metal-electrolyte interface described
in the previous section. In each case, an electrode potential is developed across the
interface, proportional to the exchange of ions between the metal and the electrolytes
of the body. The double layer of charge at the interface acts as a capacitor. Thus, the
equivalent circuit of biopotential electrode in contact with the body consists of a voltage
in series with a resistance-capacitance network of the type shown in Figure.
Since measurement of bioelectric potentials requires two electrodes, the voltage
measured is really the difference between the instantaneous potentials of the two
electrodes, as shown in Figure. If the two electrodes are of the same type, the
difference is usually small and depends essentially on the actual difference of ionic
potential between the two points of the body from which measurements are being
taken. If the two electrodes are different, however, they may produce a significant dc
voltage that can cause current to flow through both electrodes as well as through the

input circuit of the amplifier to which they are connected. The dc voltage due to the
difference in electrode potentials is called the electrode offset voltage. The resulting
current is often mistaken for a true physiological event. Even two electrodes of the
same material may produce a small electrode offset voltage.
In addition to the electrode offset voltage, experiments have shown that the
chemical activity that takes place within an electrode can cause voltage fluctuations to
appear without any physiological input. Such variations may appear as noise on a
bioelectric signal. This noise can be reduced by proper choice of materials or, in most
cases, by special treatment, such as coating the electrodes by some electrolytic
method to improve stability. It has been found that, electrochemically, the silver-silver
chloride electrode very stable. This type of electrode is prepared by electrolytically
coating a piece of pure silver with silver chloride. The coating is normally done by
placing a cleaned piece of silver into a bromide-free sodium chloride solution. A
second piece of silver is also placed in the solution, and the two are connected to a
voltage source such that the electrode to be chlorided is made positive with respect to
the other. The silver ions combine with the chloride ions from the salt to produce
neutral silver chloride molecules that coat the silver electrode. Some variations in the
process are used to produce electrodes with specific characteristics.
1.4.2 Microelectrodes:
1. Microelectrodes are electrodes with tips sufficiently small to penetrate a single
cell in order to obtain readings from within the cell.
2. The tip must be small enough to permit penetration without damaging the cell.
3. This action is usually complicated by the difficulty of accurately positioning an
electrode with respect to a cell.
4. Microelectrodes are generally of two types: metal and micropipet.
5. Metal microelectrodes are formed by electrolytically etching the tip of a fine
tungsten or stainless-steel wire to the desired size. Then the wire is coated
almost to the tip with an insulating material. Some electrolytic processing can
also be performed on the tip to lower the impedance. The metal-ion interface
takes place where the metal tip contacts the electrolytes either inside or
outside the cell.

Microelectrodes—metal microelectrodes
6. The micropipet type of microelectrode is a glass micropipet with the tip drawn
out to the desired size [usually about 1 micron (now more commonly called
micrometer, µm) in diameter]. The micropipet is filled with an electrolyte
compatible with the cellular fluids. This type of microelectrode has a dual
interface. One interface consists of a metal wire in contact with the electrolyte
solution inside the micropipet, while the other is the interface between the
electrolyte inside the pipet and the fluids inside or immediately outside the cell.
Microelectrodes—micropipette or micro capillaries electrode
7. A commercial type of microelectrode is shown in Figure. In this electrode a thin
film of precious metal is bonded to the outside of a drawn glass microelectrode.
The manufacturer claims such advantages as lower impedance than the
micropipet electrode, infinite shelf life, repeatable and reproducible

performance, and easy cleaning and maintenance. The metalelectrolyte
interface is between the metal film and the el performance, and easy cleaning
and maintenance. The metal electrolyte interface is between the metal film and
the electrolyte of the cell.
8. Microelectrodes, because of their small surface areas, have impedances well
up into the megohms. For this reason, amplifiers with extremely high
impedances are required to avoid loading the circuit and to minimize the effects
of small changes in interface impedance.
1.4.3. Body Surface Electrodes:
1. Electrodes used to obtain bioelectric potentials from the surface of the body are
found in many sizes and forms.
2. Although any type of surface electrode can be used to sense EGG, EEG, or
EMG potentials, the larger electrodes are usually associated with EGG, since
localization of the measurement is not important, whereas smaller electrodes
are used in EEG and EMG measurements.
3. The earliest bioelectric potential measurements used immersion electrodes,
which were simply buckets of saline solution into which the subject placed his
hands and feet, one bucket for each extremity. As might be expected, this type
of electrode (Figure 4.4) presented many difficulties, such as restricted position
of the subject and danger of electrolyte spillage.
4. A great improvement over the immersion electrodes were the plate electrodes,
first introduced about 1917. Originally, these electrodes were separated from
the subject's skin by cotton or felt pads soaked in a strong saline solution. Later
a conductive jelly or paste (an electrolyte) replaced the soaked pads and metal
was allowed to contact the skin through a thin coat of jelly. Plate electrodes of
this type are still in use today.
Metal plate electrodes

5. Another fairly old type of electrode still in use is the suction-cup electrode
shown in Figure. In this type, only the rim actually contacts the skin.
Suction-cup electrode
6. One of the difficulties in using plate electrodes is the possibility of electrode
slippage or movement. This also occurs with the suction-cup electrode after a
sufficient length of time. A number of attempts were made to overcome this
problem, including the use of adhesive backing and a surface resembling a
nutmeg grater that penetrates the skin to lower the contact impedance and
reduce the likelihood of slippage.
7. All the preceding electrodes suffer from a common problem. They are all
sensitive to movement, some to a greater degree than others. Even the
slightest movement changes the thickness of the thin film of electrolyte between
metal and skin and thus causes changes in the electrode potential and
impedance. In many cases, the potential changes are so severe that they
completely block the bioelectric potentials the electrodes attempt to measure.
The adhesive tape and **nutmeg grater" electrodes reduce this movement
artefact by limiting electrode movement and reducing interface impedance, but
neither is satisfactorily insensitive to movement.
8. Later, a new type of electrode, the floating electrode, was introduced in
varying forms by several manufacturers. The principle of this electrode is to
practically eliminate movement artifact by avoiding any direct contact of the
metal with the skin. The only conductive path between metal and skin is the
electrolyte paste or jelly, which forms an electrolyte bridge. Even with the
electrode surface held at a right angle with the skin surface, performance is not

impaired as long as the electrolyte bridge maintains contact with both the skin
and the metal.
Floating type biopotential electrodes
9. Floating electrodes are generally attached to the skin by means of two-sided
adhesive collars (or rings), which adhere to both the plastic surface of the
electrode and the skin. Figure shows an electrode in position for biopotential
measurement.
10.Special problems encountered in the monitoring of the ECG of astronauts
during long periods of time, and under conditions of perspiration and
considerable movement, led to the development of spray-on electrodes, in
which a small spot of conductive adhesive is sprayed or painted over the skin,
which had previously been treated with an electrolyte coating.
11.Various types of disposable electrodes have been introduced in recent years
to eliminate the requirement for cleaning and care after each use. An example

is shown in Figure. Primarily intended for ECG monitoring, these electrodes can
also be used for EEC and EMG as well. In general, disposable electrodes are
of the floating type with simple snap connectors by which the leads, which are
reusable, are attached. Although some disposable electrodes can be reused
several times, their cost is usually low enough that cleaning for reuse is not
warranted. They come pregelled, ready for immediate use.
Disposable electrodes
12.Special types of surface electrodes have been developed for other applications.
For example, a special ear-clip electrode (Figure 4.11) was developed for use
as a reference electrode for EEG measurements. Scalp surface electrodes for
EEG are usually small disks about 7 mm in diameter or small solder pellets that
are placed on the cleaned scalp, using an electrolyte paste. This type of
electrode is shown in Figure.
Ear-clip electrode EEG Scalp Surface Electrode

1.4.4 Needle Electrodes:
To reduce interface impedance and, consequently, movement artifacts,
some electroencephalographers use small subdermal needles to penetrate the scalp
for EEG measurements. These needle electrodes, shown in Figure, are not inserted
into the brain; they merely penetrate the skin. Generally, they are simply inserted
through a small section of the skin just beneath the surface and parallel to it.
Subdermal needle electrodes
Needle electrodes for EMG- consist merely of fine insulated wires, placed so
that their tips, which are bare, are in contact with the nerve, muscle, or other tissue
from which the measurement is made. The remainder of the wire is covered with some
form of insulation to prevent shorting. Wire electrodes of copper or platinum are often
used for EMG pickup from specific muscles. The wires are either surgically implanted
or introduced by means of a hypodermic needle that is later withdrawn, leaving the
wire electrode in place.
A single wire inside the needle serves as a unipolar electrode which measures the
potentials at the point of contact with respect to some indifferent reference. If two wires
are placed inside the needle, the measurement is called bipolar and provides a very
localized measurement between the two wire tips.
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