electrode_transducers.pptx

7/28/2019 Electrode_transducers.pptx

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Electrode Skin Interface

Apart from the electrode electrolyte interface, there isalso the skin interface.

when an electrode is placed on the skin surface, thereis some electrical resistance at the electrode-skininterface.

The skin consists of three layers, Epidermis, dermis

and subcutaneous layer.


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Stratum corneum(we see dead cells)

Stratum granulosum

Stratum (basale) germinativum(where new skin cells form)

Deep layers of skin consist ofvascular and nervous

components, as well as sweat glands, sweat ducts and hairfollicles. With the exception of sweat glands, no particular characteristics

affecting the electrode performance.


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Magnified section of skin, showing the various

layers


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Epidermis

We are most interested at epidermis, as that is themain contact with the electrode

Since the skins natural resistance is high comparedto the resistance of the fluids, the selected skin site isto be well prepared by cleaning withAlcohol orAcetoneand by applying a commercially available

conducting jelly (electrode paste).

This ensures a low value of electrode-skin interfaceresistance.


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Surface electrodes usually have resistances of 2000 to10000 ohms depending on their size, whereas, smallneedle electrodes have a much higher resistance.

A simple series equivalent circuit of an electrode-electrolyte interface is shown in figure.


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Here, Ehc is the half-cell potential; Rd and Cd represent the impedance associatedwith electrode-electrolyte interface; and RS is the total resistance in the circuit due toresistance in electrolyte and electrode lead wire.

However, when an electrode makes contact with the skinvia an electrolyte paste, the equivalent circuit modified asshown.

Now, Rs becomes the effective resistance of the paste between theelectrode and the skin.

The epidermis of the skin may be considered as a semipermeable membrane and the potential difference across itis represented by Ese.


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The epidermic layer has also an electrical impedance,which is represented by the parallel circuit Ce,Re.

The dermis and subcutaneous layer under it behave ingeneral as pure resistance Ru, as shown in figure.

Thus, it can be seen that to obtain a more stable

electrode, the effect of the epidermis (stratumcorneum) has to be reduced.


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This is achieved by many ways: by rubbing with a padsoaked in acetone or by puncturing the epidermis withdental burrs.

All these methods tend to short out Ese, Ce and Re,thus improving the stability of the signal.

Psychogenic electrodermal responses or the galvanicskin reflex (GSR), is the contribution of the sweatglands and sweat ducts.


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The fluid secreted by sweat glands contains Na+, K+,and Cl ions, the concentrations of which differ fromthose in the extracellular fluid.

Thus there is a potential difference between the lumenof the sweat duct and the dermis and subcutaneous layers.


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There also is a parallel RpCp combination in serieswith this potential that represents the wall of thesweat gland and duct, as shown by the broken lines.

These components are often neglected when weconsider biopotential electrodes unless the electrodes

are used to measure the electrodermal response orGSR.


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Motion artifact

If a pair of electrodes is in an electrolyte and onemoves while the other remains stationary, a potentialdifference appears between the two electrodes duringthis movement.

This potential is known as motion artifact and can be

a serious cause of interference in the measurement ofbiopotentials.


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Because motion artifact results primarily frommechanical disturbances of the distribution of chargeat the electrodeelectrolyte interface, it is reasonableto expect that motion artifact is minimal fornonpolarizable electrodes.

This artifact can be significantly reduced when thestratum corneum is removed bymechanical abrasion

with a fine abrasive paper.


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Remembering the dynamic nature of the epidermis,note also that the stratum corneum can regenerateitself in as short a time as 24 h, thereby renewing thesource of motion artifact.


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Stretching the skin changes this skin potential by 5 to10 mV, and this change appears as motion artifact.

Ten 0.5 mm skin punctures through the barrier layershort-circuits the skin potential and reduces thestretch artifact to less than 0.2 mV.


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Invasive and Non Invasive

Sensors that are used to measure electrical, chemical,physical activities from human body.


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Invasive

Ionization (radiation) X-ray, UV, -ray

Contact with bloodIntrusion into the body

Minimally invasive

Contact with blood

Intrusion into the body

Non-invasive

Surface or remote diagnosis / therapy


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Types of electrodes

A wide variety of electrodes can be used to measurebioelectric events, but nearly all can be classified asbelonging to one of three basic types:

1) Surface Electrodes: Used to measure ECG,EEG andEMG potentials on the surface of the skin.

2) Needle 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.

3) Micro Electrodes: Used to measure bioelectricpotentials near a single cell.


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METAL-PLATE ELECTRODES: SURFACE

In its simplest form, it consists of a metallic conductorin contact with the skin.

An electrolyte soaked pad or gel is used to establishand maintain the contact.

Metal-plate electrode used for application to limbs isshown.

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 theelectrocardiograph.


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The electrode is traditionally made ofGerman silver (anickelsilver alloy).

Before it is attached to the body with a rubber strap ortape, its concave surface is covered with electrolyte gel.

A limb electrode


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Metal-disk electrode

The structure shown can be used as a chest electrodefor recording the ECG or in cardiac monitoring forlong-term recordings.

In these applications the electrode is often fabricatedfrom a disk ofAg that may have an electrolyticallydeposited layer ofAgCl on its contacting surface. It iscoated with electrolyte gel and then pressed against

the patients chest wall.

Metal-disk electrode


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In recording EMGs, investigators use stainless steel,platinum, or gold-plated disks to minimize the chancethat the electrode will enter into chemical reactionswith perspiration or the gel.

Electrodes used in monitoring EMGs or EEGs aregenerallysmaller in diameter than those used inrecording ECGs.


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Disk-shaped electrodes such as these have also beenfabricated from metal foils (primarily silver foil) and

are applied as single-use disposable electrodes.

The thin, f lexible foil allows the electrode to conformto the shape of the body surface.

In choosing suitable cardiac electrodes for patient-monitoring applications, physicians are more andmore turning to pregelled, disposable electrodes with

the adhesive alreadyin place.

These devices are ready to be applied to the patientand are disposed after use.


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Disposable foam-pad electrodes

It consists of a relativelylarge disk of plastic foam materialwith a silver plated disk on one side attached to a silver-plated snap similar to that used on clothing in the center ofthe other side.

The silver-plated disk serves as the electrode and may becoated with an AgCl layer.

A layer of electrolyte gel covers the disk.

The electrode side of the foam is covered with an adhesivematerial that is compatible with the skin.


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A protective cover or strip of release paper is placedover this side of the electrode and foam, and thecomplete electrode is packaged in a foil envelope sothat the water component of the gel will not evaporateaway.


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SUCTION ELECTRODES

Amodification of the metal-plate electrode thatrequires no straps or adhesives for holding it in place isthe suction electrode.

Such electrodes are frequently used inelectrocardiography as the precordial (chest) leads,because they can be placed at particular locations andused to take a recording.


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They consist of a hollow metallic cylindrical electrodethat makes contact with the skin at its base.

An appropriate terminal for the lead wire is attached

to the metal cylinder, and a rubber suction bulb fitsover its other base.

Electrolyte gel is placed over the contacting surface ofthe electrode, the bulb is squeezed, and the electrodeis then placed on the chest wall.


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Used as a precordial electrode


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The bulb is released and applies suction against theskin, holding the electrode assembly in place.

This electrode can be used only for short periods oftime; the suction and the pressure of the contactsurface against the skin can cause irritation.


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Although the electrode itself is quite large, that theactual contacting area is relatively small.

This electrode thus tends to have a higher sourceimpedance than the relatively large-surface-area metalplate electrodes used for ECG limb electrodes.


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FLOATING ELECTRODES

We noted that one source ofmotion artifact inbiopotential electrodes is the disturbance of thedouble layer of charge at the electrodeelectrolyte

interface. The use of nonpolarizable electrodes, such as the

Ag/AgCl electrode, can greatly diminish this artifact.

But it still can be present, and efforts to stabilize the

interface mechanically can reduce it further. Floating electrodes offer a suitable technique to do so.


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Figure (a) depicts a floating electrode known as a top-hat electrode; its internal structure is illustrated incross section in Figure(b).

The principal feature of the electrode is that the actualelectrode element or metal diskis recessed in a cavityso that it does not come in contactwith the skin itself.

Instead, the element is surrounded by electrolyte gel inthe cavity.


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The cavity and hence the gel does not movewithrespect to the metal disk, so it does not produce anymechanical disturbance of the double layer of charge.

The electrode element can be a disk made of a metalsuch as silver coatedwithAgCl or sinteredAg/AgCl

pellet instead of a metal disk. These electrodes are found to be quite stable and are reusable afterappropriate cleaning between uses.


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A single-use, disposable modification of the floating

electrode is shown in cross section in Figure (c).

It has one added componenta disk of thin, open-cellfoam saturated with electrolyte gel.

The other surface of the foam that is placed against theskin is able to move with the skin, therebydiminishing

the motion artifact that sometimes results fromdifferential movement between the skin and theelectrolyte gel.


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The below figure shows one technique employed to provide

flexible electrodes.

Acarbon-filled silicone rubber compound in the form of athin strip or disk is used as the active element of an

electrode.

The carbon particles in the silicone make it an electricconductor.

Apin connector is pushed into the lead connector hole,and the electrode is used in the same way as a similar typeof metal-plate electrode.


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Carbon-filled silicone rubber electrode


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Flexible thin-film electrode

The basic electrode consists of a 13 mm-thick Mylarfilm on which an Ag and AgCl film have beendeposited, as shown in Figure.


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The flexible lead wire is attached to the Mylarsubstrate by means of a conducting adhesive, and asilver film approximately 1 micrometer thick is deposited

over this and the Mylar.

An AgCl layer is then grown on the surface of the silver

film via the electrolytic process.


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Advantage

In addition to the advantage of being flexible andconforming to the shape of the newborns chest, theseelectrodes have a layer ofsilver thin enough to be

essentiallyx-ray transparent, so theyneed not beremoved when chest x rays of the infant are taken.

Consequently, the infants skin is also protected fromthe irritation caused by removing and reapplying theadhesive tape that holds the electrode in place.


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INTERNAL ELECTRODES:NEEDLE

Electrodes can also be usedwithin the bodyto detectbio potentials.

They can take the form ofpercutaneous electrodes, inwhich the electrode itself or the lead wire crosses theskin, or they may be entirelyinternal electrodes, in

which the connection is to an implanted electroniccircuit such as a radio telemetry transmitter.


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No electrolyte gel is required to maintain this

interface, because extracellular fluid is present.

The basic needle electrode consists of a solid needle,usually made ofstainless steel, with a sharp point.

The shank of the needle is insulated with a coatingsuch 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 protectit.


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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 beremoved.

A shielded percutaneous electrode can be fabricated inthe form shown in Figure(b).

It consists of a small-gage hypodermic needle that hasbeen modified by running an insulated fine wire downthe center of its lumen and filling the remainder of thelumen with an insulating material such as an epoxyresin.


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The needle itself is connected to ground through theshield of a coaxial cable, thereby extending the coaxialstructure to its very tip.

Multiple electrodes in a single needle can be formed asshown in Figure(c).

Here two wires are placed within the lumen of theneedle and can be connected differentiallyso as to besensitive to electrical activity only in the immediatevicinity of the electrode tip.


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Percutaneous wire electrodes

The needle electrodesjust described are principally foracute measurements, because their stiffness and sizemake them uncomfortable for long term implantation.

When chronic recordings are required, percutaneouswire electrodes are more suitable.


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A fine wireoften made ofstainless steel ranging indiameter from 25 to 125 micromis insulated with aninsulating varnish to within a few millimeters of thetip.

This noninsulated tip is bent back on itself to form aJ-shaped structure.

The tip is introduced into the lumen of the needle, asshown in Figure(d).


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The needle is inserted through the skin into themuscle at the desired location, to the desired depth.

It is then slowly withdrawn, leaving the electrode inplace, as shown in Figure(e).

Note that the bent-over portion of wire serves as a barb holding thewire in place in the muscle.

To remove the wire, the technician applies a mild uniform force tostraighten out the barb and pulls it out through the wires track.


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Realizing that wire electrodes chronically implanted inactive muscles undergo a great amount of f lexing as

the muscle moves (which can cause the wire to slip as it passesthrough the skin and increase the irritation and risk ofinfection at this

point, or even cause the wire to break), they developed thehelical electrode and lead wire shown in Figure(f).


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It, too, is made from a very fine insulated wire coiledinto a tight helix of approximately150 microm

diameter that is placed in the lumen of the insertingneedle.

The uninsulated barb protrudes from the tip of theneedle and is bent back along the needle before

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 theneedle and the needle must be removedor at least protectedbeforethe electrode is connected to the recording apparatus.


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Intracutaneous needles

Another group of percutaneous electrodes are thoseused for monitoring fetal heartbeats.

(a) Suction electrode. (b) Cross-sectional viewof suction electrode in place, showing penetration ofprobe through epidermis.(c) Helical electrode that is attached to fetal skin by

corkscrew-type action.


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Implantable electrodes

Often when implantable wireless transmission is used,we want to implant electrodes within the bodyand notpenetrate the skin with any wires.

In this case the radio transmitter is implanted in thebody.

The simplest electrode for this application is shown inFigure(a).


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Wire-loop electrode


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Insulated multistranded stainless steel or platinumwire suitable for implantation has one end stripped sothat an eyelet can be formed from the strands of wire.

The eyelet can be sutured to the point in the body atwhich electric contact is to be established.

Platinum -sphere cortical-surface potential


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Platinum sphere cortical surface potential

electrode

Figure(b) shows another example of an implantableelectrode for obtaining cortical-surface potentials fromthe brain applied this electrode for the radiotelemetry

of subdural EEGs. .


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The electrode consists of a 2 mm-diameter metallicsphere located at the tip of the cylindrical Tefloninsulator through which the electrode lead wirepasses.

The calvarium is exposed through an incision in thescalp, and a burr hole is drilled.

A small slit is made in the exposed dura, and the silversphere is introduced through this opening so that itrests on the surface of the cerebral cortex.

The assembly is then cemented in place onto thecalvarium by means of a dental acrylic material.


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Multielement depth electrode

Deep cortical potentials can be recorded from multiplepoints using the technique as shown in Figure(c).


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This kind of electrode consists of a cluster of fineinsulated wires held together by avarnish binder.

Each wire has been cut transverselyto expose anuninsulated cross section that serves as the activeelectrode surface.

By staggering the ends of the wires as shown, we canproduce electrodes located at known differences indepth in an array.

The other ends of the electrodes can be attached to appropriateimplantable electronic devices or to a connector cemented on the skullto allow connection to an external recording apparatus.


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ELECTRODE ARRAYS

Although implantable electrode arrays can befabricated one at a time using clusters of fine insulatedwires, this technique is both time-consuming andexpensive.

Furthermore, when such clusters are madeindividually, each one will be somewhat different fromthe other.

A way to minimize these problems is to utilize microfabrication technology to fabricate identical two- andthree dimensional electrode arrays.


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One-dimensional plunge electrode array


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Two -dimensional array


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Three -dimensional array


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MICROELECTRODES

To be able to measure potential differences across thecell membrane we must have an electrode within thecell.

Such electrodes must be small with respect to the celldimensions to avoid causing serious cellular injury andtherebychanging the cells behavior.


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In addition to being small, the electrode used formeasuring intracellular potential must also be strongso that it canpenetrate the cell membrane and remain

mechanically stable.


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Electrodes that meet these requirements are known as

microelectrodes.

They have tip diameters ranging from approximately

0.05 to 10 mirom.

Microelectrodes can be formed from solid-metalneedles, from metal contained within or on the surface

of a glass needle, or from a glass micropipette having alumen filled with an electrolytic solution.


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METAL MICROELECTRODES

A fine needle of a strong metal that is insulatedwithan appropriate insulator up to its tip.

The structure of a metal microelectrode for intracellular recordings.


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The metal needle is prepared byelectrolytic etching,using an electrochemical cell in which the metalneedle is the anode.

The electric current etches the needle as it is slowlywithdrawn from the electrolyte solution.

Veryfine tips can be formed in this way, but a greatdeal ofpatience and practice are required to gain theskill to make them.


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Suitable strong metals for these microelectrodes are

stainless steel, platinumiridium alloy, and tungsten. The compound tungsten carbide is also used because

of its great strength.

The microelectrode and supporting shaft are usuallyinsulated by a film of some polymeric material orvarnish.

Only the extreme tip of the electrode remains uninsulated.


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SUPPORTED-METAL MICROELECTRODES

The properties oftwo different materials are used toadvantage in supported metal microelectrodes.

A strong insulating material that can be drawn to afine point makes up the basic support, and a metalwith good electrical conductivityconstitutes thecontacting portion of the electrode.


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Metal filled glass micropipette

The classic example of this form is a glass tube drawnto a micropipette structure with its lumen filled withan appropriate metal.


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Preparation

This is prepared by first filling a glass tube with ametal that has a melting point near the softening point of theglass.

The tube can then beheated to the softening point

and pulledto 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 mechanicalsupport but also serves as the insulation.

The active tip is the onlymetallic area exposed in crosssection where the pipette was broken away.


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(a) Section of fine-bore glass capillary.(b) Capillary narrowed through heating and stretching.(c) Final structure of glass-pipette microelectrode.


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.

Glass micropipette, coated with metal film

The figure shows the cross section of the tip of adeposited-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.


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Glass micropipette, coated with metal film


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MICROPIPETTE ELECTRODES

A glass micropipette electrode filled with an electrolyticsolution.

Electrolyte solution that is frequently3M KCl.

A cap containing a metal electrode is then sealed to the pipette.

The metal electrode contacts the electrolyte within the pipette.

The electrode is frequently a silver wire prepared with anelectrolytic AgCl surface.

Platinum or stainless steelwires are also occasionally used.

MICROELECTRODES BASED ON MICROELECTRONIC


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MICROELECTRODES BASED ON MICROELECTRONIC

TECHNOLOGY

The basic structure consists of narrow gold stripsdeposited on a silicon substrate the surface of whichhas been first insulated by growing an SiO2 film.

The gold strips are then further insulated bydepositing SiO2 over their surface.

The silicon substrate is next etched to a thin, narrow

structure that is just wide enough to accommodate thegold strips in the region of the tip.


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The silicon substrate is etched a millimeter or two backfrom the tip so that only the gold strips and their SiO2

insulation remain.

The insulation is etched awayfrom the verytip of thegold strips to expose the contacting surface of theelectrodes.


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Beam-lead multiple electrode


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Multielectrode silicon probe


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Multiple-chamber electrode


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Peripheral-nerve electrode


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ELECTRICAL PROPERTIES OF MICROELECTRODES

We must derive an electrical equivalent circuit fromphysical considerations.

The microelectrode contributes a series resistance Rsthat is due to the resistance of the metal itself.

A major contributor to this resistance is the metal inthe shank and tip portion of the microelectrode,because the ratio of length to cross-sectional area ismuch higher in this portion than it is for the shaft.


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The metal is coated with an insulating material over allbut its most distal tip, so a capacitance is set upbetween the metal and the extracellular fluid.

This is a distributed capacitance Cd that we canrepresent in lumped form by separating the shank andtip from the shaft.

In the shank region, we can consider the microelectrode to be a coaxialcylinder capacitor; the capacitance per unit length (F/m) is given by

Electrode with tip placed within a cell, showing origin


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p p , g g

of distributed capacitance


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Here the ratio of diameters would be practically unity, so we cansimplify the calculation by unwrapping the circumferential surface ofthe shaft and considering the system to be a parallel-plate capacitor ofarea equal to the circumferential surface area and of thickness equal to

t, the thickness of the insulation layer.

The capacitance per unit length (F/m) is given by

Note that this capacitance comes from only that portion of the electrode shaftthat is submerged in the extracellular fluid. Often only the shank issubmerged, so Cd2 is zero.


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Equivalent circuit


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The tip of the microelectrode is within a cell, so there is a seriesresistance Ri, associated with the electrolytewithin the cell membraneand another series resistance Re due to the extracellular fluid.

The cell membrane itself can be modeled simply as a variable potentialEmp, but in more detailed analyses an equivalent circuit of greatercomplexity is required.

Some of the distributed capacitance of the shank,Cd1, is between themicroelectrode and the extracellular fluid, as shown in the equivalentcircuit, whereas the remainder of it is between the microelectrode andthe intracellular fluid.


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Simplified equivalent circuit


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The above circuit neglects the impedance of the reference electrodeand the series-resistance contribution from the intracellular andextracellular fluid and lumps all the distributed capacitance together.

Under circumstances in which the input impedance of the amplifier

connected to this electrode is not sufficiently large, we see that thiscircuit can behave as a high-pass filter and significant waveformdistortion can result.

The effective impedance of metal microelectrodes is frequency

dependent and can be of the order of10 to 100 M.


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We can, however, lower this impedance byincreasingthe effective surface area of the tip of themicroelectrode through the application ofplatinum

black.

At lower frequencies, the impedance can be reducedby applying anAg/AgCl surface to the electrode tip.

Gl i i tt i l t d


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Glass micropipette microelectrode

The internal electrode in the micropipette gives themetal electrolyte interface components Rma, Cma,and Ema.

In series with this is a resistive element Rt

corresponding to the resistance of the electrolyte inthe shank and tip region of the microelectrode.

Connected to this is the distributed capacitance Cdcorresponding to the capacitance across the glass inthis region.

The distributed capacitance due to the shaft region has been neglected,because the glass wall of the electrode is much thicker in this region andthe capacitive contribution is quite small.

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

capacitance


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capacitance


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There are two potentials associated with the tip of the micropipettemicroelectrode.

The liquid-junction potential Ej corresponds to the

liquid junction set up between the electrolyte in themicropipette andthe intracellular fluid.

In addition, a potential known as the tip potential Et

arises because the thin glass wall surrounding the tipregion of the micropipette behaves like a glassmembrane and has an associated membrane potential.


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The equivalent circuit also includes resistancescorresponding to the intracellular Ri and extracellularRe fluids.

These are coupled to the microelectrode through thedistributive capacitance Cd, as is the case for the metalmicroelectrode.

The equivalent circuit for the reference electroderemains unchanged from that shown for the previouselectrode.

E i l i i


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Equivalent circuit


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Unlike the metal microelectrode, the micropipettesmajor impedance contribution is resistive.

This can be illustrated by approximating the

equivalent circuit.


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Here the overall series resistance of the electrode is

lumped together as Rt.

This resistance generally ranges in value from 1 to 100M.

The total distributed capacitance is lumped togetherto form Ct, which can be on the order of tens of picofarads.

All the associated dc potentials are lumped together inthe source Em, which is given by


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Note that the micropipette-type microelectrode behaves asa low-pass filter.

The high series resistance and distributed capacitancecause the electrode output to respond slowly to rapidchanges in cell-membrane potential.

To reducethis problem, positive-feedback, negative-capacitance amplifiers are used to reduce the effectivevalue of Ct.

TRANSDUCER


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TRANSDUCER

Generally, transducer is required to convertphysiological variables into electrical signals which areeasier to be processed.

The relationship between input and output variablecan be linear, logarithmic or square.

The transducer can be active or passive dependingupon conversion of non electrical variable intoelectrical signal.


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The active transducer directlyconverts input variableinto electrical signals while passive transducermodifies either excitation voltages or modulates the

carrier signals.

The passive transducers are externally poweredwhileactive transducers are self generating and require no

external power.

ACTIVE TRANSDUCERS


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PASSIVE TRANSDUCERS


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PASSIVE TRANSDUCERS

There are only three passive circuit elements that canbe used to change voltage at the output of the circuitaccording to the physical variable :

(1) resistors

(2) capacitors and

(3) inductors.

The passive transducer is part of a circuit normally anarrangement similar to awheatstone bridgewhich ispowered by an ac or dc excitation.


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INDUCTIVE PASSIVE TRANSDUCERS

Variable induction :

The property of inductance is varied in the circuit tochange the output voltage in accordance with the

input variable.

The inductance L = n2G

(n = number of turns in coil, G = form factor of coil and = permeability of core material inside the coil).

Induction Displacement Transducer


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Induction Displacement Transducer


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Variable reluctance :

In this, core remains stationaryinside the coil but the

air gap in the magnetic path of the core is varied tochange the net permeability, thereby varying theoutput signal as per the input variable (displacement).

i bl l d


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Variable Reluctance Transducer

Linear Variable differential transformer


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Linear Variable differential transformer

Applications


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Applications

1. LVDT is used as a catheter-tip blood pressuretransducer.

In this service, the core of the LVDT is affixed to asmall, circular, elastic diaphragm exposed to bloodpressure.

The mass of the core and the diaphragm are very smalland the system has a high stiffness.


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2. The movement of a bourdon tube and, thereby, thepressure inside the tube can be measured byconnecting the core of the LVDT to the tip of the

bourdon tube.

3. LVDT is used in Ballistocardiography for picking upthe movement of the ballistocardiograph platform (on

which a subject lies supine) due to the pumping actionof the heart.


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4.Vibrations of several body segments can bemonitored by using LVDT to study the effects ofvibrations on the human body placed on a vibration

platform.

PASSIVE CAPACITANCE TRANSDUCERS


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PASSIVE CAPACITANCE TRANSDUCERS

Variable capacitance :

The capacitance (C) of a capacitor having two parallel

plates of area A which are separated by a distance d is:

C = o r A/d

(0 = dielectric constant of free space and r = relative

dielectric constant).

Variation of Capacitance with Displacement


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Variation of Capacitance with Angular Displacement


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Variation of Capacitance with Angular Displacement

Applications


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Applications

1. The capacitance method has been applied to themeasurement of physiological events, particularlyblood pressure.

An elastic element exposed to blood pressureconstitutes one plate of the capacitor, the other plate isnearby and fixed.

To obtain a rapid response time, the elastic element ismade as small and as stiff as possible.


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2. The capacitance microphone is used for thedetection ofheart sounds.

3. A miniature capacitance microphone is used as a

high fidelitypulse pick-up. 4.An unusual application is that the dielectric property

of the living tissues itself as part of the capacitor.

This principle is used by placing electrodes on thechest and back and correlating the output of this, torecordvolume and cardiac output for each heart beat.

TEMPERATURE MEASUREMENT


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TEMPERATURE MEASUREMENT

It has been seen that a person in shock has reducedblood pressure in circulating system which results intolow body temperature.

Infection and illness are usually reflected by a highbody temperature.

Special heated incubators are used for maintaining thebody temperature of infants.

The temperature of the joint of an arthritic patient isclosely linked with the amount of local inflammation.


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The temperature can be measured by

(1) thermocouples

(2) thermistor and

(3) radiation and fiber optic detectors


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Resistance Temperature Detector (RTD)


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Electrical resistance of an electrical conductor is afunction of temperature


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Thin Film Gold Temperature Sensor


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Thin-Film Gold Temperature Sensor

Thermistors


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The resistance of thermistor can be given as

Thermistors can be formed into disks, beads, rods orany desired shapes.

Thermistor probes are available with resistance from afew hundred ohms to several megohms.

Most thermistor thermometers use the principle ofwheatstone bridge to obtain a voltage output whichvaries as per input temperature.


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The thermograph is an infrared thermometer

incorporated into a scanner which can be used to scanentire surface of body or some part of body like atelevision camera.

The infrared energy detected in scanning is used tomodulate the intensity of a light beam so that to getthe image on the photographic film in which thebrightness depends on the detected infrared radiation.

The image is called a thermogram.

Applications


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pp

1. They are used for continuous measurement ofskinand body core temperatures.

2. For the measurement ofblood f low.A heatedthermistor is mounted on the tip of a catheter or

hypodermic needle which is inserted in to the bloodvessel. (change in R)

3. The use of a thermistor for the respiration ratemeasurement is of special interest.

Sufficient amount of current passed through thethermistor to raise its quiescent temperature to approx

electrode_transducers.pptx

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