Transcript Biopotential electrodes

BIOPOTENTIAL ELECTRODES
Engr. Hinesh Kumar (Lecturer)
Electrodes for Biophysical Sensing
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Bioelectricity is a naturally occurring phenomenon
that arises from the fact that living organisms are
compared of ions in various different quantities.
Ionic conduction involves the migration of ionspositively and negatively charged molecules
throughout a region.
Electronic conduction involves the flow of electrons
under the influence of an electrical field.
Cont…
Electrical conduction in medical instrument
circuits is electronic.
 Electrical conduction in the body is ionic.
 Electrodes provide transduction between ionic
and electronic conduction.
 Biopotential electrodes function as sensor that
couple the ionic potentials generated inside the
body to an electronic instrument.
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Bioelectrodes
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Bioelectrodes are a class of sensors that transduce
ionic conduction to electronic conduction so that the
signal can be processed in electric circuits.
The usual purpose of bioelectrodes is to acquire
medically significant bioelectrica1 signals.
Such as Electrocardiographic(ECG),
Electroencephalographic (EEG), and
Electromyographic (EMG).
Bioelectrical signals are acquired from one of three
forms of electrode: Surface Electrodes, Indwelling
Electrodes, and Micro Electrodes
Electrode or Half Cell Potential
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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.
Cont…
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A complex phenomenon is seen at the interface
between the metallic electrode and the electrolyte.
Ions migrate toward 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 halfcell potential.
Metallic electrode
immersed in an
electrolytic solution
Electrode Offset Potential
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When two dissimilar metals immersed in a common
electrolytic solution, they both form the Half cell
potential.
The differential potential between these two Half cell
potential is called an electrode offset potential.
Electrode or Half Cell Potential
The Half-cell potential of the electrode depends on:
i.The
used metals,
ii.The electrolyte composition
iii.The temperature
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Different material exhibits different half-cell potentials
Polarizable and Non-Polarizable
Electrode
Perfectly Polarizable Electrodes
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Perfectly polarizable electrodes are those in which no
actual charge crosses the electrode–electrolyte interface
when a current is applied.
Of course, there has to be current across the interface and
the electrode behaves as though it were a capacitor
Perfectly Polarizable Electrodes or Perfectly Reversible
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Perfectly non-polarizable electrodes are those in which
current passes freely across the electrode–electrolyte
interface, requiring no energy to make the transition.
Thus, for perfectly non-polarizable electrodes there are no
over-potentials.
Electrode interface impedance is represented as a resistor.
Medical Surface Electrodes
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Surface electrodes are those that are placed in
contact with the skin of the subject.
Also in this category are certain needle electrodes
of a sire that prevents their being inserted in the
cell.
A conductive gel or paste is used to reduce the
impedance between the electrode and skin.
Cont…
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Human skin tends to have a very high impedance
compared with other voltage sources.
Typically, normal skin impedance, as seen by the
electrode.
For sweaty skin varies from 0.5 k Ω
For dry skin surfaces to more than 20 k Ω
Problem skin especially, dry, scaly, or diseased skin,
may reach impedances in the 500 k Ω range.
Types of Biopotential Electrodes
Bioelectrical signals are acquired from one of three
forms of electrode:
 Body
Surface Electrodes,
 Needle Electrodes
 Micro Electrodes
Body Surface Electrodes
There are four different types of body surface
recording electrodes;
Column Electrodes
2. Suction Electrodes
3. Floating Electrodes
4. Flexible Electrodes
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Column Electrodes
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The electrode consists of a silver-silver chloride metal
contact button at the top of a hollow column that is
filled with a conductive gel or paste.
This assembly is held in place by the adhesive coated
foam rubber disk
The use a gel filled or paste filled column that holds the
actual metallic electrode off the surface reduces
movement artifact.
Far this reason the column electrodes are preferred for
monitoring hospitalized patients.
Column Electrodes
Large surface: Earliest, and still used for ECG.
 Smaller diameters.
 Used for ECG, EMG and EEG.
 Susceptible to Motion artifacts.
 Disposable foam-pad.
 Very Cheap.
 Used for long term recording
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Figure (a): Metal-plate electrode used for application to limbs.
Figure (b): Metal-disk electrode applied with surgical tape.
Figure (c): Column electrodes, often used with ECG.
Suction Cup Electrodes
Straps or adhesives not required.
Often used for precordial (chest) ECG.
For short periods only.
Floating Electrodes
• Metal disk is recessed.
• Floating in the electrolyte gel.
• Not directly contact with the skin.
• Reduces motion artifacts.
Figure (a): Recessed electrode with top-hat structure.
Figure (b): Cross-sectional view of the reusable electrode in (a).
Flexible Electrodes
• Body surface are often irregular.
• Regularly shaped rigid electrodes
may not always work.
• Special case : infants.
• Material: polymer or nylon with silver,
carbon filled silicon rubber (Mylar
film).
Figure (a): Carbon-filled silicone rubber
electrode,
Figure (b): Flexible thin-film neonatal
electrode.
Figure (c): Cross-sectional view of the
thin-film electrode in (b).
Problems with Surface Electrode
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Several problems are associate with all types of
surface electrodes:
Adhesive will not stick for long on sweaty or clammy
skin surfaces.
Fleshy portion of chest and abdomen are selected as
electrode site.
After 8 hour change the electrode to avoid the
ischemia.
Movement Artifacts
Electrode position slips
Needle Electrode
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This type of electrode is inserted into the tissue
immediately beneath the skin by puncturing the skin at a
large oblique angle (i.e.,close to horizontal with respect
to the skin surface).
The needle electrode is only used for exceptionally poor
skin, especially an anesthetized patients, and in
veterinary situations.
Of course, infection is an issue in these cases, so needle
electrodes are either disposable (one time use) or are
resterilized in ethylene oxide gas.
Cont…
Needle and wire electrodes for
percutaneous measurement of
Biopotentials.
Figure:
(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 fine-wire electrode in
place,
(f) Cross-sectional view of skin and
muscle, showing coiled fine-wire
electrode in place.
Indwelling Electrode
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lndwelling electrodes are intended to be inserted into
the body.
These are not to be confused with needle electrodes,
which are intended for insertion into the layers beneath
the skin
The indwelling electrode is typically a tiny, exposed
metallic contact at the end of a long, insulated catheter.
In one application, the electrode is threaded through the
patient's veins (usually in the right arm) to the right side
of the heart to measure the intracardiac ECC waveform.
Cont…
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Certain low amplitude, high-frequency features
(such as the signal from bundle of His) become
visible only when an indwelling electrode is used.
Fetal ECG Electrodes
• Electrodes for detecting fetal electrocardiogram during
labor.
Figure (a): Suction electrode,
Figure (b): Cross-sectional view of suction electrode in place, showing penetration of
probe through epidermis,
Figure (c): Helical electrode, that is attached to fetal skin by corkscrew-type action.
EEG Electrodes
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The brain produce bioelectric signals that can be picked
up through surface electrodes attached to the scalp.
These electrodes will be connected to an EEG amplifier
that driver either an oscilloscope or strip chart recorder.
Typical Needle electrode is used.
The disc electrode have 1cm diameter concave disc
made either of silver and gold.
The disc electrode in a place by a thick paste that is
highly conductive, or by a headband in certain
monitoring applications.
Microelectrode
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The microelectrode is an ultrafine device that is used to
measure biopotentials at the cellular level.
In practice, the microelectrode penetrates a cell that is
immersed in an infinite fluid (such as physiological saline),
which is in turn connected to a reference electrode.
Although several types of microelectrodes exist, most of
them are of one of two basic forms: metallic-contact or
fluid-filled.
In both cases, an exposed contact surface is about 1to 2
um is in contact with cell.
Microelectrode used to measure the cellular potential
Microelectrodes
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There are three major types of microelectrode.
 Solid
Metal Electrodes (Tungsten Microelectrodes)
 Glass-Metal or Supported Metal Electrodes (metal
contained within/outside glass needle)
 Fluid-Filled Electrodes (with Ag-AgCl electrode metal)
Solid Metal Electrode
Glass-Metal Electrodes
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A very fine platinum or tungsten wire is slip-fit through a 1.5 to
2 mm glass pipette.
The electrode can then be connected to one input of the signals
amplifier.
There are two subcategories of glass-metal electrodes.
In the first type, the metallic tip is flush with the end of the
pipette taper.
In the second type, a thin layer of glass covers the metal point.
This glass layer is so thin that it requires measurement in
angstroms (1 angstrom = 1.0 × 10-10 meters) and it drastically
increases the impedance of the device.
Fluid-Filled Electrodes
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The fluid-filled glass microelectrode is shown in the fig.
In this type of electrode, the glass pipette is filled with a
solution of potassium chloride (KCI), and the large end is
capped with an silver-silver chloride (Ag-Ag Cl) plug.
The small end need not be capped because the 1μm opening
is small enough to contain the fluid.
The reference electrode is likewise filled with potassium
chloride (KCI), but is much larger than the microelectrode.
A platinum plug contains fluid on the interface end, while an
silver-silver chloride (Ag-Ag Cl) plug caps the other end.
Assignment
Biosensor..? Principle and how it works ?
Advantages and Disadvantages?