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Electrodes
The mechanism of electrical conductivity in the
body involves ions as charge carriers.
Picking up bioelectric signals involves converting
the ionic currents into electric currents
This conversion process is carried out by
electrodes that consist of electrical conductors in
contact with the body
At the interface between the electrode and the
electrolyte solution, an electrochemical reaction
needs to take place for a charge to be
transferred.
Early ElectrodesConductivity improved by immersing
limbs into buckets of saline solution
In 1924, Einthoven was awarded the Nobel Prize in Physiology or
Medicine for his development of the diagnostic applications of the
electrocardiogram, or ECG. To record the heart’s electrical activity, he
had determined that a patient must immerse his arms and his left leg
(creating a triangle, each side offering information about the voltage
generated by the beating of the heart) in tubs of saline solution. Wires in
the tubs picked up the heart’s electrical signals, which were then
transmitted over telephone wires from Leiden University Hospital to
Einthoven’s machine in a laboratory a mile away.
Willem Einthoven in 1906
Useful Electrical Signals
Bioelectric Signal Abbreviation Source
Electrocardiogram ECG Heart –seen from body surface
Cardiac Electrogram Heart – seen from within
Electromyogram EMG Muscle
Electroencephalogram EEG Brain
Electro-optigram EOG Eye dipole field
Electroretinogram ERG Eye retina
Action potential Nerve or muscle
Electrogastrogram EGG Stomach
Galvanic skin reflex GSR Skin
Many organs in the body generate some voltage. Most of the signals are
hard to measure. A few have proved to be useful for biological assessment.
Charge Transfer Mechanisms
Polarisable Electrodes (Noble Metal)
Polarizable electrodes pass a displacement current
between the electrode and the electrolytic solution by
changing the charge distribution in the solution near
the electrode. No actual current crosses the
electrode–electrolyte interface.
This limits the ability of the electrode to conduct DC or
low frequency signals
Movement of the electrode changes the charge
distribution generating a voltage (motion artefact)
Charge Transfer Mechanisms
Non Polarisable (Silver – Silver Chloride)
Nonpolarized electrodes allow current to pass freely
across the interface without changing the charge
distribution in the electrolytic solution.
Charge transfer is by conduction
Can conduct low frequency and DC signals
Less prone to motion artefacts
nonpolarizable electrodes are preferred for most
biomedical measurements
Body Surface Electrodes The earliest bioelectric potential measurements relied on immersion electrodes that
were simply buckets of saline solution into which the patient placed a hand and a
foot, as shown in Figure.
Plate electrodes, first introduced in 1917, were a great improvement on immersion
electrodes. They were originally separated from the skin by cotton pads soaked in
saline to emulate the immersion electrode mechanism.
Later, an electrolytic paste was used in place of the pad with the metal in contact with
the skin.
Electrodes that can be placed on the body surface for recording bioelectric signals.
The integrity of the skin is not compromised.
Can be used for short or long duration applications
Metal Plate Electrodes Metal plate electrode is used for applicaiton to limbs for
electrocardigraphy.
Consist of a flat metal plate (made of from German silver (a nickel-silver
alloy)) that has been bent into cylindrical segment.
A terminal is placed on its outised surface to attach lead wire to the
electrocardigraph.
Before it is attached to the the body with rubber a strap or tape, it is
covered with a thin film of conductive electrolyte gel between the plate and
the skin to establish contact.
A more common variety of metal plate electrode is the metal disk. It is
used as chest electrode for recording ECG.
Lead wire soldered or welded on the back surface.
It is often fabricated from a disk of Ag that may have electrolytically
deposited layer of AgCl on its contacting surface.
It is coated with electrolyte gel and then press against patient’s chest wall
and is sticked there by a strip of surgical tape or using adhesive material.
Stainless steel, platinum or gold plated disks minimize the the chance that
the electrode will enter into chemical reactions with perpiration ad gel.
These materials produce polarizable electrodes and motion artifat can be
a problem.
They are histroical electrodes but still in use.
They can be made from flexible foil so that conform to body surface
contour
These electrode types are primarily used for diagnostic recordings of ECG
and EEG signals
Disposable foam-pad electrodes, often used with ECG
monitoring apparatus
Relatively large disk of plastic foam with silver-plated disk serving as electrode,
coated with AgCl
Layer of electrolyte gel covers the disk
Electrode side of foam covered with adhesive material
They are preffered in hospitals due to being easy to apply and disposable.
Metallic suction electrode
• A metallic suction electrode
is often used as a
precordial (chest) electrode
on clinical
electrocardiographs.
• It requires no straps or
adhesives for holding it in
place.
• Electrolyte gel is placed on
the contacting surface of
electrode.
• This electrode can be used
only for short periods of
time.
Floating Electrodes Mechanical
technique to reduce
noise. Isolates the
electrode-electrolyte
interface from
motion artifacts.
Metal disk (actual
electrode) is
recessed.
Floating in the
electrolyte gel.
Not directly contact
with the skin.
Reduces motion
artifacts.
Flexible body-surface electrodes (a) Carbon-
filled silicone rubber electrode
Solid electrodes cannot conform to body-
surface topology resulting additional motion
artifacts.
Carbon particles filled in the silicone rubber
compound (conductive) in the form of a thin
strip or disk is used as the active element of
an electrode.
A pin connector is pushed into the lead
connector hole and electrode is usd like a
metal plate electrode.
Applications – monitoring premature infants
(2500gr) who are not suitable for using
standard electrodes.
Flexible thin-film neonatal electrode (after Neuman, 1973).
(c) Cross-sectional view of the thin-film electrode in (b)
The basic electrode consists of 13 μm-thick Mylar
film with Ag/AgCl film deposition.
Lead wire is sticked to this film with an adhesive.
Have the advantage of being flexible and
conforming to the shape of newborn’s chest.
No need to be removed as they are X-ray
transparent
Monitoring new-born infants
Drawback – High electric impedance
Examples
http://www.3m.com/product/information/Red-Dot-
Infant-Soft-Cloth-Monitoring-Electrode.html
Dry Electrodes
Electrodes that do not use externally applied
electrolyte gels are known as “dry”.
They can easily be applied and held in place
using a rubber band or tape.
They are made from graphite or metal-power
impregnated silicone rubber to produce a flexible
conductive material
The electrolyte layer is formed by a film of sweat
Electrically much noisier than signals obtained
from the wet types
Electroencephalograph
http://www.biosemi.com/pin_electrode.htm
Electrodes for Internal Use Detect biopotential within body, by introducing the electrode itself or
the lead wire through the skin.
Entirely internal electrodes - connection is to an implanted electronic
circuit such as a radiotelemetry transmitter.
Don’t require a gel layer because they are in intimate contact with
the organism’s own electrolyte solution.
To study particular biological phenomenon, specific internal
electrodes are designed.
Needle electrodes that are introduced through the skin
Cavity electrodes that can be implanted surgically or through a
catheter
Electrodes for Internal Use
Ther basic needle electrode consists of a solid needle, usually made
of stainless steel with a sharp point.
This electrode is frequently used in electromyography by placing it in
a particular muscle.
A shielded version can be fabricated. A fine wire at the center of the
needle is covered with epoxy and shield of needle acts like a
ground.
Electrodes for Internal Use
Multiple electrodes in a single needle - Bipolar coaxial electrode. (c)
When chronic recordings are required wire electrodes are more suitable.
(d) Fine-wire electrode connected to hypodermic needle, before being
inserted, (e) Cross-sectional view of skin and muscle, showing fine-wire
electrode in place, (f) Cross-sectional view of skin and muscle, showing
coiled fine-wire electrode in place.
Cochlear Electrode
Photo of electrode array
Diagram of the electrode
being inserted into cochlea
http://www.felixmay.com/ESB/images/
http://www.sciencecentric.com/news/08041747-research-may-
lead-to-improved-hearing-for-some.html
Arrays of surface or needle electrodes are used in a number of biomedical
prostheses, within the cochlea to bypass damaged sections of the ear.
Retinal Electrode Arrays
Silicon retina consists of an
array of photodiodes each
exciting an electrode
http://www.rachnaindia.com/fzone/fz/asr.htm
Electrodes are used as embedded in the
retina or directly in the visual cortex as the
final output of visual prostheses o
Pacemaker Electrodes
Current and potential
distribution around the
electrode
http://www.dur.ac.uk/ces/projects/atrial_receptors
http://www.nhlbi.nih.gov/health/health-topics/topics/pace/howdoes.html
Penetrating Electrode Arrays
Monitor or excite areas of the brain, the
optic nerve or the spinal cord
Electrode to stimulate
layer IV of the visual
cortex
Utah and Umich electrode arrays
http://www.sci.utah.edu/~gk/abstracts/bisti03/