Unit 1

MEDICAL INSTRUMENTATION / UNIT-1 / BIOPOTENTIAL ELECTRODES

M.S/DEPT OF EEE/ V.V.C.E.T Page 1

SYLLABUS

UNIT – I BIO-POTENTIAL ELECTRODES Electrode electrolyte interface, half-cell potential, polarisation and non- polarisable electrode, calomel electrode, needle and wire electrode, microelectrode-metal micropipete.

UNIT - II RECORDING SYSTEM

Low-Noise preamplifier, main amplifier and driver amplifier, inkjet recorder, thermal array recorder, photographic recorder, magnetic tape recorder, X-Y recorder, medical oscilloscope.

UNIT - III BIO-CHEMICAL MEASUREMENT

pH, pO2, pCO2, pHCO3, Electrophoresis, colorimeter, spectro photometer, flame photometer, auto analyzer.

UNIT - IV NON-ELECTRICAL PARAMETER MEASUREMENTS

Respiration, heart rate, temperature, pulse blood pressure, cardiac output, O2, CO2 measurements, manual and automatic counting of RBC, WBC and platelets.

UNIT - V COMPUTERS IN BIO-MEDICAL INSTRUMENTATION

ECG, EEG, EMG – machine description - methods of measurement – three equipmentfailures and trouble shooting, ECG Analysis. Basic ideas of CT scanner – MRI and ultrasonic scanner.



UNIT-1

BIO-POTENTIAL ELECTRODE

BASICS

Neurons

• cell body

• dendrites (input structure)

receive inputs from other neurons

perform spatio-temporal integration of inputs

relay them to the cell body

• axon (output structure)

a fiber that carries messages (spikes) from the cell to dendrites of other neurons



Synapse

• site of communication between two cells

• formed when an axon of a presynaptic cell “connects” with the dendrites of a

postsynaptic cell

• a synapse can be excitatory or inhibitory

• arrival of activity at an excitatory synapse depolarizes the local membrane potential

of the postsynaptic cell and makes the cell more prone to firing

• arrival of activity at an inhibitory synapse hyperpolarizes the local membrane

potential of the postsynaptic cell and makes it less prone to firing

• the greater the synaptic strength, the greater the depolarization or hyperpolarization

The Resting Potential

There is an electrical charge across the membrane.

This is the membrane potential.

The resting potential (when the cell is not firing) is a 70mV difference between the inside

and the outside

Ions and the Resting Potential

Ions are electrically-charged molecules e.g. sodium (Na+), potassium (K+), chloride

(Cl-).

The resting potential exists because ions are concentrated on different sides of the

membrane.

Na+ and Cl- outside the cell.

K+ and organic anions inside the cell



Maintaining the Resting Potential

Na+ ions are actively transported (this uses energy) to maintain the resting potential.

The sodium-potassium pump (a membrane protein) exchanges three Na+ ions for two K+

ions

Excitatory postsynaptic potentials (EPSPs)

Opening of ion channels which leads to depolarization makes an action potential more

likely, hence “excitatory PSPs”: EPSPs.

Inside of post-synaptic cell becomes less negative.

Na+ channels (NB remember the action potential)

Ca2+ . (Also activates structural intracellular changes -> learning.)



Inhibitory postsynaptic potentials (IPSPs)

Opening of ion channels which leads to hyperpolarization makes an action potential

less likely, hence “inhibitory PSPs”: IPSPs.

Inside of post-synaptic cell becomes more negative.

K+ (NB remember termination of the action potential)

Cl- (if already depolarized)

Action potentials: Rapid depolarization

When partial depolarization reaches the activation threshold, voltage-gated sodium

ion channels open.

Sodium ions rush in.

The membrane potential changes from -70mV to +40mV.

Action potentials: Repolarization

Sodium ion channels close and become refractory.

Depolarization triggers opening of voltage-gated potassium ion channels.

K+ ions rush out of the cell, repolarizing and then hyperpolarizing the membrane



Action potentials: Resuming the Resting Potential

Potassium channels close.

Repolarization resets sodium ion channels.

Ions diffuse away from the area.

Sodium-potassium transporter maintains polarization.

The membrane is now ready to “fire” again.



The Action Potential

The action potential is “all-or-none”.

It is always the same size.

Either it is not triggered at all - e.g. too little depolarization, or the membrane is

“refractory”;

Or it is triggered completely

Course of the Action Potential

• The action potential begins with a partial depolarization (e.g. from firing of another

neuron ) [A].

• When the excitation threshold is reached there is a sudden large depolarization [B].

• This is followed rapidly by repolarization [C] and a brief hyperpolarization [D].

• There is a refractory period immediately after the action potential where no

depolarization can occur [E]



Introduction to Bioinstrumentation

A basic block diagram for a computer-controlled instrumentation system is given below

Some Definitions:

Measurand : the physical property being investigated

Sensor : converts the measurand into an electrical signal

Analog processing : conditions the signal in the analog domain, typically amplification,

electrical isolation, filtering

A/D : analog to digital conversion at a specified sampling frequency and

precision

Digital processing : conditions the signal in the digital domain, typically, filtering,

compression, enhancements such as digital zoom and contrast enhancement

Display : organizes the information in a manner understandable to the user,

the user interface

Storage : e.g. disk or paper output

D/A : converts a digital signal (based on some reaction to the original

analog signal) back to the analog domain

Actuator : converts and electrical signal into a mechanical action.

Design factors to consider when designing instrumentation Measurand (signal) factors

- range

- desired accuracy

- differential or absolute

Transducer

- impedance

- sensitivity

- linearity

- reliability



- transient and frequency response

Environment factors (what is the environment? Laboratory? Hospital? Battlefield?)

specificity: how much does the system respond to the measurand of interest vs. other

signals (noise)?

- Temperature, humidity, pressure, acceleration, shock, electric fields, radiation,

magnetic fields. (these were all specified for the instrumentation on the Space Station)

Medical Factors

- invasiveness

- Non-invasive: does not puncture skin or penetrate body cavities (palpitation,

surface biopotential [ECG], low level surface energy [ultrasound, X-ray, MRI])

- Minimally invasive- skin penetration, no vital organs, arteries or thorax, abdomen,

neck, or cranium (needle electrodes, venipuncture

- Penetrating- penetrates internal body cavity (esophageal endoscopy, colonoscopy,

pap smear)

- Invasive- penetrates vital tissues (arterial catheters, pacemakers, EM flow probes)

- Highly invasive- penetrates vital organs (brain, liver, kidney, heart)

- negative effects on tissue (radiation, heat)

- patient cooperation (children or animals more difficult)

- electrical safety

Economic factors

- cost

- availability of parts (custom more expensive)

- consumable vs. reusable

- compatibility

Constraints

- low signal level

- variability among patients

- government regulations

- operational simplicity (physicians are generally not engineers!)



Biopotential Electrodes

- A transducer that convert the body ionic current in the body into the traditional electronic

current flowing in the electrode.

- Able to conduct small current across the interface between the body and the electronic

measuring circuit.

Electrode – Electrolyte Interface

General Ionic Equations

a) If electrode has same material as cation, then this material gets oxidized and enters the

electrolyte as a cation and electrons remain at the electrode and flow in the external circuit.

b) 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-)

Figure .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-.

• Electrons move in opposite direction to current flow

• Cations (C+ ) move in same direction as current flow

• Anions (A– ) move in opposite direction of current flow

• Chemical oxidation (current flow right) - reduction (current flow left)

reactions at the interface:

•

• No current at equilibrium



Let us consider what happens when we place a piece of metal into a solution containing ions of that metal. These ions are cations, and the solution, if it is to maintain neutrality of

charge, must have an equal number of anions. When the metal comes in contact with the solution, the reaction represented by (a) begins immediately.

Initially, the reaction goes predominantly either to the left or to the right, depending on the concentration of cations in solution and the equilibrium conditions for that particular

reaction. The local concentration of cations in the solution at the interface changes, which affects the anion concentration at this point as well. The net result is that neutrality of

charge is not maintained in this region. Thus the electrolyte surrounding the metal is at a different electric 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 solution, and the temperature, as well as other second-order factors. Knowledge of the

half-cell potential is important for understanding the behavior of bio-potential electrodes.

It is not possible to measure the half-cell potential of an electrode because—unless we use

a second electrode—we cannot provide a connection between the electrolyte and one terminal of the potential-measuring apparatus. Because this second electrode also has a

half-cell potential, we merely end up measuring the difference between the half-cell potential of the metal and that of the second electrode. There would of course be a very

large number of combinations of pairs of electrodes, so tabulations of such differential half-cell potentials would be very extensive. To avoid this problem, electrochemists have

adopted the standard convention that a particular electrode—the hydrogen electrode—is defined as having a half-cell potential of zero under conditions that are achievable in the

laboratory.

We can then measure the half-cell potentials of all other electrode materials with respect to this electrode

The hydrogen electrode is based on the reaction

Half-cell Potentials for Common Electrode Materials at 25 8C The metal undergoing the reaction shown has the sign and potential E0 when referenced to

the hydrogen electrode where H2 gas bubbled over a platinum electrode is the source of hydrogen molecules. The platinum also serves as a catalyst for the reaction on the lefthand

side of the equation and as an acceptor of the generated electrons



Half Cell Potential

• When metal (C) contacts electrolyte, oxidation (C C + + e –) or

reduction (A- A + e –) begins immediately.

• Local concentration of cations at the surface changes. • Charge builds up in the regions.

• Electrolyte surrounding the metal assumes a different electric potential from the rest of the solution.

• This potential difference is called the half-cell potential ( E0 ). • Separation of charge at the electrode-electrolyte interface results in a electric double

layer (bilayer). • Measuring the half-cell potential requires the use of a second reference electrode.

• By convention, the hydrogen electrode is chosen as the reference.

A characteristic potential difference established by the electrode and its surrounding

electrolyte which depends on the metal, concentration of ions in solution and temperature

(and some second order factors) .



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.

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.

Fig: measuring half cell potentials





POLARIZATION

If there is a current between the electrode and electrolyte, the observed half cell potential

is often altered due to polarization.

Note: Polarization and impedance of the electrode are two of the most important electrode

properties to consider.

The difference is due to polarization of the electrode. The difference between the observed

half-cell potential and the equilibrium zero-current halfcell potential is known as the overpotential.

Three basic mechanisms contributeto this phenomenon, and the overpotential can be

separated into three components: the ohmic, the concentration, and the activation overpotentials.

The ohmic overpotential is a direct result of the resistance of the electrolyte. When a

current passes between two electrodes immersed in an electrolyte, there is a voltage drop along the path of the current in the electrolyte as a result of its resistance. This drop in

voltage is proportional to the current and the resistivity of the electrolyte. The resistance between the electrodes can itself vary as a function of the current. Thus the ohmic

overpotential does notnecessarily have to be linearly related to the current. This is

especially true in electrolytes having low concentrations of ions. This situation, then, does not necessarily follow Ohm’s law.

The concentration overpotential results from changes in the distribution ofions in the

electrolyte in the vicinity of the electrode–electrolyte interface. Recallthat the equilibrium half-cell potential results from the distribution of ionicconcentration in the vicinity of the

electrode–electrolyte interfacewhen no current flows between the electrode and the electrolyte.Under these conditions, reactions (a) and (b) reach equilibrium, so the rates of

oxidation and reduction at the interface are equal. When a current is established, this equality no longer exists. Thus it is reasonable to expect the concentration of ions to

ACRp VVVV



change. This changeresults in a different half-cell potential at the electrode. The difference between this and the equilibrium half-cell potential is the concentration overpotential.

The third mechanism of polarization results in the activation overpotential.

The charge-transfer processes involved in the oxidation–reduction reaction(a) are not entirely reversible. In order for metal atoms to be oxidized to metal ions that are capable of

going into solution, the atoms must overcome anenergy barrier. This barrier, or activation energy, governs the kinetics of thereaction. The reverse reaction—in which a cation is

reduced, thereby plating out an atom of the metal on the electrode—also involves an activation energy,but it does not necessarily have to be the same as that required for the

oxidation reaction. When there is a current between the electrode and the electrolyte, either oxidation or reduction predominates, and hence the height of the energy barrier

depends on the direction of the current. This difference in energy appears as a difference in voltage between the electrode and the electrolyte, which is known as the activation

overpotential.

These three mechanisms of polarization are additive. Thus the net overpotential

of an electrode is given by

where Vp =total potential, or polarization potential, of the electrode

E0 = half-cell potential

Vr =ohmic overpotential Vc = concentration overpotential

Va = activation overpotential

When an ion-selective semipermeable membrane separates two aqueous ionic solutions of different concentration, an electric potential exists across this membrane. It

can be shown (Plonsey and Barr, 2007) that this potential is given by the Nernst equation

where a1 and a2 are the activities of the ions on each side of the membrane

When the electrode– electrolyte system no longer maintains this standard condition, half-cell potentials different from the standard half-cell potential are observed. The

differences in potential are determined primarily by temperature and ionicactivity in the

electrolyte. Ionic activity can be defined as the availability of an ionic species in solution to enter into a reaction.

The standard half-cell potential is determined at a standard temperature; the

electrode is placed in an electrolyte containing cations of the electrode material having



unity activity. As the activity changes from unity (as a result of changing concentration), the half-cell potential varies according to the Nernst equation:

where

E = half-cell potential E0= standard half-cell potential

n =valence of electrode material acn+ =activity of cation cn+

The more general form of this equation can be written for a general oxidation–reduction

reaction as

where n electrons are transferred. The general Nernst equation for this situation is

where the a’s represent the activities of the various constituents of the reaction.

An electrode–electrolyte interface is not required for a potential difference to exist. If two

electrolytic solutions are in contact and have different concentrations of ions with different ionic mobilities, a potential difference known as a liquid-junction potential, exists

between them. For solutions of the same composition but different activities, its magnitude is given by

where µ+and µ-are the mobilities of the positive and negative ions, and at and an are the activities of the two solutions. Though liquid-junction potentials are generally not so high as

electrode–electrolyte potentials, they can easily be of the order of tens of millivolts.



POLARIZABLE AND NONPOLARIZABLE ELECTRODES Perfectly Polarizable Electrodes (used for recording)

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 : Ag/AgCl Electrode

Perfectly Non-Polarizable Electrode(used for simulation)

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 :

Platinum electrode

The Classic Ag/AgCl Electrodes



•

Features:

–

Practical electrode, easy to fabricate.

–

Metal (Ag) electrode is coated with a layer of slightly soluble ionic compound of the metal

and a suitable anion (Cl).

•

Reaction 1: silver oxidizes at the Ag/AgCl interface

•

•

Reaction 2: silver cations combine with chloride anions

AgCl is only slightly soluble in water so most precipitates onto the electrode to form a

surface coating.

Under equilibrium conditions the ionic activities of the Ag+ and Cl- ions must be such that their product is the solubility product.

AgCl is only very slightly solublein water, so most of it precipitates out of solution onto the silver electrode and contributes to the silver chloride deposit. Silver chloride’s rate of

precipitation and of returning to solution is a constant Ks known as the solubility product.

We can determine the half-cell potential for the Ag/AgCl electrode by

By using , we can rewrite this as

or

The first and second terms on the right-hand side are constants; only the third is determined by ionic activity. In this case, it is the activity of the Cl- ion, which is relatively

large and not related to the oxidation of Ag, which is caused by the current through the electrode. The half-cell potential of this electrode is consequently quite stable when it is

placed in an electrolyte containing Cl- as the principal anion, provided the activity of the Cl-



remains stable. Because this is the case in the body, we shall see in later sections of this chapter that the Ag/AgCl electrode is relatively stable in biological applications.

Ag/AgCl Fabrication

• Electrolytic process

• Large Ag/AgCl electrode serves as the cathode.

• Smaller Ag electrode to be chloridized serves as the anode.

• A 1.5 volt battery is the energy source.

• A resistor limits the current.

• A milliammeter measures the plating current.

• Reaction has an initial surge of current.

• When current approaches a steady state (about 10 µA), the process is terminated.

Sintered Ag/Ag Electrode



Sintering Process

• A mixture of Ag and AgCl powder is pressed into a pellet around a silver lead wire.

• Baked at 400 ºC for several hours.

• Known for great endurance (surface does not flake off as in the electrolytically generated

electrodes).

• Silver powder is added to increase conductivity since AgCl is not a good conductor.

Calomel Electrode

• Calomel is mercurous chloride (Hg2Cl

2).

• Approaches perfectly non-polarizing behavior

• Used as a reference in pH measurements.

• Calomel paste is loaded into a porous glass plug at the end of a glass tube.

• Elemental Hg is placed on top with a lead wire.

• Tube is inserted into a saturated KCl solution in a second glass tube.

• A second porous glass plug forms a liquid-liquid interface with the analyte

being measured.

• refer Nernst equation



The basic approach is to place a solution of known pH on the inside of the membrane and the unknown solution on the outside. Hydrochloric acid is generally used as the solution of

known pH. A reference electrode, usually an Ag/AgCl or a saturated calomel electrode, is

placed in this solution. A second reference electrode is placed in the specimen chamber. A salt bridge is included within the reference to prevent the chemical constituents of the

specimen from affecting the voltage of the reference electrode. The potential developed across the membrane of the glass electrode is read by a pH meter. This pH meter must

have extremely high input impedance, because theinternal impedance of the pH electrode is in the 10 to 100 MV range.

Electrode Circuit Model

•

Ehc

is the half-cell potential

•

Cd is the capacitance of the electric double layer (polarizable electrode properties).

•

Rd is resistance to current flow across the electrode-electrolyte interface (non-polarizable

electrode properties).

•

Rs is the series resistance associated with the conductivity of the electrolyte.

•

At high frequencies: Rs

•

At low frequencies: Rd + R

s



Electrode-Skin Interface Model

fig: 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.

Motion artifact:

• Gel is disturbed, the charge distribution is perturbed changing the half-cell potentials at

the electrode and skin.

• Minimized by using non-polarizable electrode and mechanical abrasion of skin.

• Skin regenerates in 24 hours.

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

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 biopotential measurements

Motion artifact is minimal for non-polarizable electrodes



Body Surface Recording Electrodes

1.

Metal Plate Electrodes (historic) ,Suction Electrodes (historic interest), Floating Electrodes,

Flexible Electrodes

Metal plate electrodes

– Large surface: Ancient, therefore still used, ECG

– Metal disk with stainless steel; platinum or gold coated

– EMG, EEG

– smaller diameters

– motion artifacts

– Disposable foam-pad: Cheap!



fig: Body-surface biopotential electrodes (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 electrocardiograph monitoring apparatus

In its simplest form, it consists of a metallic conductor in contact with the skin. An electrolyte soaked pad or gel is used to establish and maintain the contact.

Figure. shows several forms of this electrode. A limb electrode for use with an

electrocardiograph is shown in Figure (a). It consists of a flat metal plate that has been bent into a cylindrical segment. A terminal is placed on its outside surface near one end;

this terminal is used to attach the lead wire to the electrocardiograph. The electrode is

traditionally made of German silver (a nickel–silver alloy). Before it is attached to the body with a rubber strap or tape, its concave surface is covered with electrolyte gel. Similarly

arranged flat metal disks are also used as electrodes. Although based upon preceding sections of this chapter, one would expect that better electrode designs could be used with

electrocardiographs today, these traditional electrodes are still occasionally used.

A more common variety of metal-plate electrode is the metal disk illustrated in Figure (b). This electrode, which has a lead wire soldered or welded to the back surface, can be made

of several different materials. Sometimes a layer of insulating material, such as epoxy or polyvinylchloride, protects the connection between lead wire and electrode. This structure

can beused as a chest electrode for recording the ECG or in cardiac monitoring for long-term recordings. In these applications the electrode is often fabricated from a disk of Ag

that may have an electrolytically deposited layer of AgCl on its contacting surface. It is coated with electrolyte gel and then pressed against the patient’s chest wall. It is

maintained in place by a strip of surgical tape or a plastic foam disk with a layer of

adhesive tack on one surface. This style of electrode is also popular for surface recordings of EMG or EEG.

In recording EMGs, investigators use stainless steel, platinum, or gold-plated disks to minimize the chance that the electrode will enter into chemical reactions with perspiration

or the gel. These materials produce polarizable electrodes, and motion artifact can be a problem with active patients. Electrodes used in monitoring EMGs or EEGs are generally

smaller in diameter than those used in recording ECGs. Disk-shaped electrodes such as these have also been fabricated from metal foils (primarily silver foil) and are applied as

single-use disposable electrodes. The thin, flexible foil allows the electrode to conform to the shape of the body surface. Also, because it is so thin, the cost can be kept relatively

low. Economics necessarily plays an important role in determining what materials and apparatus are used in hospital administration and patient care. In choosing suitable cardiac

electrodes for patient-monitoring applications, physicians are more and more turning to pregelled, disposable electrodes with the adhesive already in place. These devices are

ready to be applied to the patient and are disposed after use. This minimizes the amount of

personnel time associated with the use of these electrodes



INTERNAL ELECTRODES

Needle and wire electrodes for percutaneous measurement of biopotentials

(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 coiled fine-wire electrode in place.

The basic needle electrode consists of a solid needle, usually made of stainless steel, with a sharp point. The shank of the needle is insulated with a coating such as an insulating

varnish; only the tip is left exposed. A lead wire is attached to the other end of the needle, and the joint is encapsulated in a plastic hub to protect it. This electrode, frequently used in

electromyography, is shown in Figure (a). When it is placed in a particular muscle, it

obtains an EMG from that muscle acutely and can then be removed. A shielded percutaneous electrode can be fabricated in the form shown in Figure (b).

It consists of a small-gage hypodermic needle that has been modified by running an insulated fine wire down the center of its lumen and filling the remainder of the lumen with

an insulating material such as an epoxyresin. When the resin has set, the tip of the needle is filed to its original bevel, exposing an oblique cross section of the central wire, which

serves as the active electrode. The needle itself is connected to ground through the shield of a coaxial cable, thereby extending the coaxial structure to its very tip.

Multiple electrodes in a single needle can be formed as shown in Figure (c). Here two

wires are placed within the lumen of the needle and can be connected differentially so as to be sensitive to electrical activity only in the immediate vicinity of the electrode tip.

Figure shows different types of percutaneous needle and wire electrodes.

The needle electrodes just described are principally for acute measurements, because their

stiffness and size make them uncomfortable for long term implantation. When chronic recordings are required, percutaneous wire electrodes are more suitable. There are many

different types of wire electrodes and schemes for introducing themthrough the skin.

The principle can be illustrated, however, with the help of Figure (d). A fine wire—often made of stainless steel ranging in diameter from 25 to 125 mm—is insulated with an

insulating varnish to within a few millimeters of the tip. This non insulated tip is bent back on itself to form a J-shaped structure. The tip is introduced into the lumen of the needle, as

shown in Figure (d).



The needle is inserted through the skin into the muscle at the desired location, to the

desired depth. It is then slowly withdrawn, leaving the electrode in place, as shown in Figure (e). Note that the bent-over portion of wire serves as a barb holding the wire in

place in the muscle. To remove the wire, the technician applies a mild uniform force to straighten out the barb and pulls it out through the wire’s track.

Realizing that wire electrodes chronically implanted in active muscles undergo a great

amount of flexing as the muscle moves (which can cause the wire to slip as it passes through the skin and increase the irritation and risk of infection at this point, or even cause

the wire to break), they developed the helical electrode and lead wire shown in Figure (f). It, too, is made from a very fine insulated wire coiled into a tight helix of approximately

150 mm diameter that is placed in the lumen of the inserting needle. The uninsulated barb protrudes from the tip of the needle and is bent back along the needlebefore insertion. It

holds the wire in place in the tissue when the needle is removed from the muscle. Of course, the external end of the electrode now passes through the needle and the needle

must be removed—or at least protected—before the electrode is connected to the recording

apparatus.



Another group of percutaneous electrodes are those used for monitoring fetal

heartbeats. In this case it is desirable to get the electrocardiogram from the fetus during labor by direct connection to the presenting part (usually the head) through the uterine

cervix (the mouth of the uterus). The fetus lies in a bath of amniotic fluid that contains ions and is conductive, so surface electrodes generally do not provide an adequate ECG as a

result of the shorting effect of the amniotic fluid. Thus electrodes used to obtain the fetal ECG must penetrate the skin of the fetus.

Fetal ECG Electrodes

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

needles (a) Suction electrode.

(b) Cross-sectional view of suction electrode in place, showing penetration of probe

through epidermis. (c) Helical electrode, which is attached to fetal skin by corkscrew type action.

An example of a suction electrode that does this is shown in Figure (a).A sharp-pointed probe in the center of a suction cup can be applied to the fetal presenting part, as shown in

Figure (b).When suction is applied to the cup after it has been placed against the fetal skin, the surface of the skin is drawn into the cup and the central electrode pierces the stratum

corneum, contacting the deeper layers of the epidermis. On the back of the suction electrode is a reference electrode that contacts the fluid, and the signal seen between these

two electrodes is the voltagedrop across the resistance of the stratum corneum. Thus, although the amniotic fluid essentially places all the body surface of the fetus at a common

potential, the potentials beneath the stratum corneumcan be different, and fetal ECGs that

have peak amplitudes of the order of 50 to 700 mV can be reliably recorded.



Implantable electrodes for detecting biopotentials

(a) Wire-loop electrode. (b) platinum-sphere cortical-surface potential electrode.

(c) Multielement depth electrode Insulated multistranded stainless steel or platinum wire suitable for implantation has one end stripped so that an eyelet can be formed from the

strands of wire.

This is best done by individually taking each strand and forming the eyelet either by twisting the wires together one by one at the point at which the insulation stops or by spot-

welding each strand to the wire mass at this point. The eyelet can then be sutured to the point in the body at which electric contact is to be established. Silver should not be used for

this type of electrode due to the toxicity of this metal and its effects on surrounding tissue. Figure (b) shows another example of an implantable electrode for obtaining cortical-surface

potentials from the brain. Critchfield et al. (1971) applied this electrode for the radio

telemetry of subdural EEGs. The electrode consists of a 2 mm-diameter metallic sphere located at the tip of the cylindrical Teflon insulator through which the electrode lead wire

passes. The calvarium is exposed through an incision in the scalp, and a burr hole is drilled.

Multielement depth electrode array

Wire-loop electrode Cortical surface potential electrode

A small slit is made in the exposed dura, and the silver sphere is introduced through this opening so that it rests on the surface of the cerebral cortex. The assembly is then

cemented in place onto the calvarium by means of a dental acrylic material.



Microelectrodes

Why

Measure potential difference across cell membrane Requirements

– Small enough to be placed into cell – Strong enough to penetrate cell membrane

– Typical tip diameter: 0.05 – 10 microns Types

– Solid metal -> Tungsten microelectrodes

– Supported metal (metal contained within/outside glass needle) Glass micropipette -> with Ag-AgCl electrode metal

Intracellular Extracellular

Metal Microelectrodes

The metal needle is prepared in such a way as to produce a very fine tip. This is

usually done by electrolytic etching, using an electrochemical cell in which the metal needle is the anode. The electric current etches the needle as it is slowly withdrawn from the

electrolyte solution. Very fine tips can be formed in this way, but a great deal of patience and practice are required to gain the skill to make them. Suitable strong metals for these

microelectrodes are stainless steel,platinum–iridium alloy, and tungsten. The compound tungsten carbide is also used because of its great strength.



The etched metal needle is then supported in a larger metallic shaft that can be insulated. This shaft serves as a sturdy mechanical support for the microelectrode and as a

means of connecting it to its lead wire. The microelectrode and supporting shaft are usually insulated by a film of some polymeric materialor varnish. Only the extreme tip of the

electrode remains uninsulated. Metal Supported Microelectrodes

Figure shows examples of supported metal microelectrodes. The classic example of

this form is a glass tube drawn to a micropipette structure with its lumen filled with an appropriate metal. Often this type of microelectrode, as shown in Figure (a), is prepared

by first filling a glass tube with a metal that has a melting point near the softening point of the glass. The tube can then be heated to the softening point and pulled to form a narrow

Constriction. When it is broken at the constriction, two micropipettes filled with metal are formed. In this type of structure, the glass not only provides the mechanical support but

also serves as the insulation. The active tip is the only metallic area exposed in cross section where the pipette was broken away.

(a) Metal inside glass (b) Glass inside metal

Metals such as silver-solder alloy and platinum and silver alloys are used. Insome cases metals with low melting points, such as indium or Wood’s metal,are used.

New supported-metal electrode structures have been developed usingtechniques

employed in the semiconductor microelectronics industry. Figure (b) shows the cross section of the tip of a deposited-metal-film microelectrode. A solid glass rod or glass tube is

drawn to form the micropipette. A metal film is deposited uniformly on this surface to a thickness of the order of tenths of a micrometer. A polymeric insulation is then coated over

this, leaving just the tip, with the metal film exposed.



Glass Micropipette

Glass micropipette microelectrodes are fabricated from glass capillaries. The central

region of a piece of capillary tubing, as shown in Figure (a), is heated with a burner to the

softening point.

It is then rapidly stretched toproduce the constriction shown in Figure (b). Special devices, known as microelectrode pullers, that heat and stretch the glass capillary in a

uniform reproducible way to fabricate micropipettes are commercially available.

The two halves of the stretched capillary structure are broken apart at the constriction to produce a pipette structure that has a tip diameter of the order of 1 mm.

This pipette is fabricated into the electrode form shown in Figure (c).

It is filled with an electrolyte solution that is frequently 3M KCl. A cap containing a metal electrode is then sealed to the pipette, asshown. The metal electrode contacts the

electrolyte within the pipette. The electrode is frequently a silver wire prepared with an electrolytic AgCl surface. Platinum or stainless steel wires are also occasionally used

A glass micropipet electrode filled with an electrolytic solution

(a) Section of fine-bore glass capillary. (b) Capillary narrowed through heating and stretching.

(c) Final structure of glass-pipet microelectrode. Ag-AgCl wire+3M KCl has very low junction potential and hence very accurate for dc

measurements (e.g. action potential) Intracellular recording – typically for recording from cells, such as cardiac myocyte Need

high impedance amplifier…negative capacitance amplifier!



ELECTRICAL PROPERTIES OF MICROELECTRODES



ELECTRICAL PROPERTIES OF GLASS MICROPIPET



SIMULATING ELECTRODES

Practical Hints in Using Electrodes

Ensure that all parts of a metal electrode that will touch the electrolyte are made of the same metal.

o Dissimilar metals have different half-cell potentials making an electrically unstable, noisy junction.

o Do not let a solder junction touch the electrolyte. If the junction must touch the electrolyte, fabricate the junction by welding or mechanical clamping or

crimping.

For differential measurements, use the same material for each electrode.

o If the half-cell potentials are nearly equal, they will cancel and minimize the saturation effects of high-gain, dc coupled amplifiers.

Electrodes attached to the skin frequently fall off.

o Use very flexible lead wires arranged in a manner to minimize the force exerted on the electrode.

o Tape the flexible wire to the skin a short distance from the electrode, making this a stress-relief point.

A common failure point in the site at which the lead wire is attached to the

electrode. o Prove strain relief by creating a gradual mechanical transition between the wire

and the electrode.



o Use a tapered region of insulation that gradually increases in diameter from that of the wire towards that of the electrode as one gets closer and closer to

the electrode.

Match the lead-wire insulation to the specific application. o If the lead wires and their junctions to the electrode are soaked in extracellular

fluid or a cleaning solution for long periods of time, water and other solvents can penetrate the polymeric coating and reduce the effective resistance,

making the lead wire become part of the electrode.

o Such an electrode captures other signals introducing unwanted noise.

Match your amplifier design to the signal source. o Be sure that your amplifier circuit has input impedance that is much greater

than the source impedance of the electrodes.

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PART-A (2 MARKS)

1. Write a short note on neurons 2. Write a short note on synapse & its types

3. Write a short note on resting potential 4. Write a short note on action potential

5. Define EPSP & IPSP 6. Draw the block diagram of bioinstrumentation

7. Define biopotential electrodes 8. Give the ionic equations for electrode electrolyte interface

9. Give the equation for reaction of hydrogen electrode 10. Explain the oxidation & reduction reaction of electrode electrolyte interface

11. Define half cell potential 12. Give some of the examples of half cell potential

13. Define polarization & its types 14. Define overpotetials

15. Define ohmic over potential

16. Define concentration over potential 17. Define activation over potential

18. Give the equation for net over potentials of an electrode 19. Write a note on Nernst equation

20. Define liquid junction potential 21. Differentiate polarizable & non polarizable electrode

22. Write a note on Ag/Agcl electrodes with equations 23. Explain the sintering process

24. Give a note on calomel electrode 25. Draw a diagram for electrode skin interface model

26. Define motion artifact



27. Give the classification of body surface recording electrode 28. Write notes on needle & wire electrode

29. Write notes on microelectrode 30. Write a note on glass micropipette

31. Explain intracellular & extracellular recording

PART-B (16 MARKS)

1. Explain the electrode-electrolyte interface with neat diagram

2. Explain in detail about the polarizable & non polarizable electrode 3. Explain the calomel electrode with neat diagram

4. Explain the types of polarization 5. Explain the half cell potential with neat diagram

6. Explain the metal microelectrode & its electrical properties with neat diagram 7. Describe in detail about the glass micropipette & its electrical properties with neat

diagram

8. Write a brief note on needle & wire electrode 9. Give the practical hints in using electrodes

Date post:	29-Jul-2015
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Unit 1

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