2.3 bio elctrodes.pptx

MODULE – 2 :ORGANISATION OF CELL
2.1 Nernst equation for membrane Resting Potential
Generation and Propagation of Action Potential
2.2 Conduction through nerve to neuromuscular junction
2.3 Bio Electrodes: Bio potential Electrodes-External
electrodes, Internal Electrodes. Biochemical Electrodes

2.3 Bio Electrodes: Bio potential Electrodes-External electrodes, Internal
Electrodes. Biochemical Electrodes
• Bioelectric signals have to be picked up from the surface of the body before they can
be put to the amplifier for subsequent record or display. This is done by use of
electrodes
• The potentials produced at different points are measured by placing electrodes at
various points on the body.
• They carry the currents produced due to potential differences to instrumentation
amplifiers, where the signals are amplified and further processed by signal processing
systems.
Bio electrodes can be classified as:
Surface electrodes : These electrodes pick up potentials from the surface of
the tissue.
Deep seated electrodes: These electrodes are inserted inside a live tissue or cell.
 When a measurement is made outside the body by placing surface electrodes it is
called in vitro measurement.
 When measurement is made by inserting a needle electrode inside the tissue, it is
called in vivo measurement.
https://drive.google.com/file/d/1NuDBFrvFj9OzGWgvkHwc4Lp7fTwpaDBP/view?usp=sharing
Video link on Bio potential Electrode

Due to the movement of electrodes, noise signals are generated. They are referred to as artifacts.
To avoid artifacts and establish a low impedance path, an electrolyte or a jelly is applied to the area
where the electrodes make contact.
An electrolyte is a medium containing ions that is electrically conducting through the movement of
ions, but not conducting electrons.
This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon
dissolving, the substance separates into cations and anions, which disperse uniformly through the
solvent
Key Properties of Bioelectrodes
Bioelectrodes should possess the following properties:
• They should be good conductors
• They should have low impedance
• They should not polarize when a current flows through them
• They should establish a good contact with the body and not cause motion
• Potentials generated at the metal electrolyte (jelly) surface should be low.
• They should not cause itching, swelling or discomfort to the patient for example the metal should not
be toxic.
• They should be mechanically rugged

Figure below shows how the electrodes make contact with the skin surface:
The metal of the electrode has a tendency to discharge ions into the electrolyte. This
voltage is called half-cell potential or offset potential.
Half-cell potential
The skin and other tissues of higher-order organisms, such as humans, are electrolytic
and so can be modeled as an Electrolytic Solution.
• Imagine a metallic electrode immersed in an electrolytic solution.
• Immediately after immersion, the electrode will begin to discharge some metallic ions
into the solution, while some of the ions in the solution start combining with the
metallic electrodes.
• A gradient charge build up, creating a potential difference, or electrode potential and
half cell potential.

A complex phenomenon is seen at the interface between the metallic electrode and the
electrolyte.
• Ions migrate towards one side of the region or another, forming two parallel layers of
ions of opposite charge.
• This region is called the electrode double layer and its ionic differences are the source
of the electrode or half-cell potential.

MODULE – 2 :ORGANISATION OF CELL
Materials used for Electrodes
The materials used to make Electrodes include:
• Aluminium (Al) - . Non-Polarizable Electrode
• Copper (Cu) - . Non-Polarizable Electrode
• Silver (Ag)
• Gold (Au)
• Platinum (Pt) - Polarizable Electrodes
Types of Electrodes used in Biomedical Measurements
1. Microelectrodes
2. Skin surface electrodes
3. Needle electrodes

Equivalent circuit for bio-potential electrode
Where Rd and Cd are components that represent the impedance associated with the
electrode-electrolyte interface and polarization at this interface.
Rs is the series resistance associated with electrode materials.
The battery Ehc represents the half-cell potential

Polarizable and Non-Polarizable Electrodes
1. Polarizable Electrodes
No actual charge crosses the electrode-electrolyte interface when a current is
applied. (e.g Platinum electrode) .The current across the interface is displacement
current and a electrode behaves like a capacitor
2. Non-Polarizable Electrode
Current passes freely across the electrode-electrolyte interface. (e.g. Ag/AgCl
Electrode) requiring no energy to make the transition.
Electrode interface impedance is represented as a resistor.

Equivalent circuit for bio-potential with two electrode

Bio Potential Electrodes
Bio-potential electrodes transduce ionic conduction to electronic conduction so that bio-
potential signals can be obtained.
They generally consist of metal contacts packaged so that they can be easily attached to
the skin or other body tissues
Classification of Electrodes
1. Micro Electrodes--- Bio electric potential near or within a single cell
Metal Type —Tip must be tungsten or stainless steel
Micro pipette--It is a glass micro pipet with size of 1 micron, It is filled with electrolyte

2.Skin surface electrode —Measure ECG,EEG,EMG
3. Needle electrode ---Penetrate the skin to record EEG
1. Micro Electrodes: To measure potential across the cell membrane.
• Microelectrodes are electrodes with tips having tips sufficiently small enough to
penetrate a single cell in order to obtain readings from within the cell.
• The tips must be small enough to permit penetration without damaging the minute
cell.
• The main functions of microelectrodes are potential recording and current
injection.
• when microelectrode is inside cell, reference electrode is outside the cell
• Microelectrodes are having high impedances in mega ohm range because of their
smaller size.

Two types of Microelectrode are
1. Metal Microelectrode
2. Non- Metallic (Micropipette)
Metal Microelectrode
The tungsten filament or stainless steel wire made into minute structure forms the
tip of the microelectrode.
This technique is electropointing. The insulating material covers the entire
electrode for safety purpose.
Few electrolytic processing is done to reduce the impedance. Measurement of
bioelectric potentials requires two electrodes. The resulting voltage potential is the
difference between the potential of microelectrode and reference electrode. The
total sum of the three potentials is as follows.

Body Surface Electrodes
• Surface electrodes are those which are placed in contact with the skin of the subject
in order to obtain bioelectric potentials from the surface.
• Body surface electrodes are of many sizes and types. In spite of the type, any surface
electrode can be used to sense ECG, EEG, EMG etc
a. Immersion electrodes
They are one of the first type of bioelectric measuring electrodes.
• Immersion electrodes were simply buckets of saline solution in which the subject
placed his hands and feet.
• So it was not a comfortable type of measurement and hence it was replaced with
plate electrodes.

b.Suction type
• Modification of metal-plate electrode that require no straps or adhesives for holding
it in place
• Frequently used in precordial (chest) leads
• Can be placed at a particular location
• Consists of a hollow metallic cylindrical electrode that makes contact with the skin at
its base.
• A Lead wire is attached to the metal cylinder
• A rubber suction bulb fits over its other base.
• Electrolyte gel is placed over the contacting surface.
• The bulb is squeezed and placed on the chest wall and then the bulb is released and
applies suction against the skin, holding the electrode assembly in place.
• Suction & pressure of the contact surface against the skin creates irritation
• Small contacting area with a large overall size

C.Metal-Plate Electrodes
• Historically, one of the most frequently used forms of bio-potential sensing
electrodes is the metal-plate electrode.
• 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.

d. Floating electrodes
Conductive paste reduces effect of electrode slippage and resulting motion artifact.
• In practice the electrode is filled with electrolyte gel and then attached to the skin
surface by means of a double-sided adhesive tape ring.
• The electrode element can be a disk made of a metal such as silver coated with AgCl.

3. Needle electrodes
Needle electrode records the peripheral nerve action potential. It
resembles a medicinal syringe.
At one end a short insulated wire is bent. The bent portion passes through
the lumen of the needle. This setup goes into the muscle.
Now the needle is withdrawn. The bent wire remains inside the muscle.
Two type of needle electrodes namely
Mono-polar Electrode: This type uses single reference electrode placed on
the skin.
Bi – polar Electrode: This type has one reference electrode and one active
electrode.

Applications of Needle Electrodes
Needle electrodes are mostly used in the measurement of EEG and EMG signals.

C. Needle Electrodes:
• To reduce the interface and noise (artifact) caused due to electrode movement,
during the measurement of EEG, EMG etc we can use small sub-dermal needle
electrodes which penetrate the scalp.
• Actually the needle electrodes are not inserted into the brain. They nearly penetrate
the skin. Generally they are simply inserted through a small section of the skin just
beneath the skin parallel to it.
• The needle electrodes for EMG measurement consist of fine insulated wires placed in
such a way that their tips are in contact with the muscle, nerve or other tissues from
which the measurement is made. The needle creates the hole necessary for insertion
and the wires forming the electrodes are carried inside it.

Coaxial needle electrode
• A shielded percutaneous electrode 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
resin.
• 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 a shield of a coaxial cable,
thereby extending the coaxial structure to its very lip.
Bipolar coaxial needle electrode
• Two wires are placed within the lumen of the needle
• Connected differentially to be sensitive only to the electrical activity in the
immediate vicinity of the needle.

4.Disposable electrodes:
Normally plate electrodes, floating electrodes etc can be used more than
one time.
• This requires the cleaning and cares after each use.
• We can use disposable electrodes which can be used only once and be
disposed after the use.
• These types of electrodes are now widely used.

5.Ear clip & Scalp electrodes:
These type of electrodes are widely used in the measurement of EEG exclusively.
• Scalp electrodes can provide EEG easily by placing it over bare head. A typical ear clip
electrode is shown here
• The most common method for EEG measurement is 10 – 20 electrode placement
system and here we use scalp electrode usually.
• They can avoid measurement errors and movement errors. During labour internal
monitoring may be needed and is usually in the form of an electrode placed under the
baby’s scalp.
• It is called fetal scalp electrode which is used to monitor baby’s heartbeat while still
in uterus

2.3 bio elctrodes.pptx

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