bio electrodes

7/15/2019 bio electrodes

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BIO POTENTIAL ELECTRODES



ELECTRODES

• What is an electrode?

Device that converts ‘ionic potentials’ into

‘electronic potentials’

They are employed to pick up signals from our

body.

They play the role of ‘transducers’



Electrode-Skin Interface Model

Figure: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.

Sweat glandsand ducts

Electrode

Epidermis

Dermis andsubcutaneous layer Ru

Re

E se

E he

R s

Rd C d

E P

R P C P C e

Gel



Types of electrodes

• MICROELECTRODES:

Used to measure near or within the cell

•

SKIN SURFACE ELECTRODES:Used to measure from the surface of the skin

• NEEDLE ELECTRODES:

Used to penetrate the skin



MICRO ELECTRODE

• They are normally used to measure the

potential within a single cell.

• The microelectrodes are very small in diameter

and so it will not damage the human cell.

• Microelectrodes are classified into Metallic &

Non-Metallic



MICROELECTRODE



• The metal microelectrodes are formed by

electrolytically etching the tip of fine tungsten

filament into a minute structure.

‘ELECTROPOINTING’.

• Bioelectric potentials requires two electrodes,

the voltage measured is difference between

potential of microelectrode & reference

electrode



N = NucleusC = Cytoplasm

Metal rod

Tissue fluid

Membrane

potential

N

C

B

A

Reference

electrode

Insulation

C dCellmembrane

++ +

++

+++

++

++++++++

++

++ -

- - -

--

---

--- ------

-

--

-

B

A

Rmb Rma

E mb E ma

E mp Ri Re

C mbC ma

C di

C d2

Rs C w

C d1

Metal-

electrolyte

interface

Reference

electrode

model

Lead wirecapacitance

Shaft

capacitance

Electroderesistance

Tissue fluid

resistance

Shank

capacitance

Cell

membrane

Cytoplasm

resistance

To amplifier



SIMPLIFIED EQUIVALENT CKT

E mp

Membraneandaction potential

C ma

Rma

C d + C w

E ma - E mb

E

0

A

B



• Area of reference electrode is many times

greater than metal’s electrode tip(crosssection is small) ,its impedance is negligible

• Impedance of microelectrode ∞ 1

area of tip

• Electrode output is coupled with amplifier

• Input impedance is not high ‘HIGH PASS

FILTER’



MICROPIPET



• It consists of glass micropipet whose

diameter= 1 micrometer

• Pipet is filled with electrolyte 3M of KCl

(usually).

• A thin flexible metal wire is inserted to stem of

micropipet

• ClAg,Stainless steel or tungsten



SIMPLIFIED EQUIVALENT CIRCUIT



• When micropipet is coupled with amplifier

then membrane potential is coupled with High

series resistance & moderate shunt

capacitance

• Impedance of electrode places a limit on

response time of circuit & it behaves like

“LOW PASS FILTER”



SURFACE ELECTRODES

• The surface electrodes are used to measure the potential available from

the surface of the skin and used to sense the potential from heart, brain

and nerves.

• The smaller area surface electrodes are used to measure EEG,EMG

potentials and the larger area surface electrodes are used to measureECGpotentials.

• Depends on construction, the surface electrodes are classified into

1. Metal plate electrode

2. Suction cup electrode

3. Adhesive tape electrode

4. Multipoint electrode

5. Floating electrode



METAL PLATE ELECTRODE

• It is made up of Ag-AgCl (Silver-Silver Chloride). It is used to

pick up ECG from the limb lead positions. It is fixed to the

skin surface by means of conductive gel &rubber belt.

• Resistance value range 2 to 10 K ohms



METAL DISC ELECTRODE:

It is made up of Ag-AgCl. It is used to pick up

EEG from the scalp. It is fixed to the scalp by

means of adhesive tape.



Disposable foam-pad electrode:

It is made up of Ag-AgCl. It is used to pick up ECG or EEG for

those patients with contagious skin diseases. It is fixed to the

skin surface by means of adhesive tapes attached to the

electrode



Suction Electrodes

•

Well suited for attachment to flat

surface body & to

regions underlying

tissue is soft.

•A paste is

introduced into the

cup, & only rim is incontact with the skin.



FLOATING ELECTRODE

•Principle of this electrode is to eliminate movement artifact by

avoiding direct contact of metal with skin

•Conductive path is electrolyte paste, which forms electrolyte bridge

•Generally attached to skin by two adhesive rings, which adhere to

both the plastic surface of electrode & skin



MULTIPOINT ELECTRODE

• Practical electrode used for ECG easurement.

• It contains ‘active points’

•

Low resistance is established



NEEDLE ELECTRODE

• Used to record peripheral nerve action

potential (electroneurography)

• Needle resembles medicine dropper

• Consists of fine insulated wires with bare tips,

which is in contact with muscle or tissue

• Reminder of wire is insulated

• Wire electrodes of copper or platinum are

often used for EMG pick up



• Metal-electrolyte interface takes place

between the un-insulated tip of wire &

electrolytes of the body.

• ‘ unipolar electrode’ measures the potential at

the point of contact with respect to reference.

• ‘bipolar electrode’ one wire is active electrode

& other acts as reference



Simplified equivalent circuit of a Needle type EMG electrode

pair and equivalent circuit of the input stage of an amplifier



DEPTH ELECTRODE

•Used to study the electrical activity of neuronsin superficial layers of brain.

• It consists of teflon insulated platinum(90%)-

iridum (10%)alloy wires, bounded to centralsupporting stainless steel wire which acts as

indifferent electrode

•

Electrode rests on subcortial nerve cells• Area=0.5mm2

• Used to inject medicine into brain

• Used to measure oxygen tension

bio electrodes

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