BIOMEDICAL INSTRUMENTATION

UNIT-1
BIOPOTENTIAL ELECTRODES
ORIGIN OF BIOPOTENTIAL AND ITS PROPAGATION:
• Cells transport ions across their membrane leading to ion concentration differences and
therefore charge differences - hence generating a voltage.
• Most cell groups in the tissues of the human body do not produce electric voltages
synchronously, but more or less randomly. Thus, most tissues have a resultant voltage
of zero as the various random voltages cancel out.
• When many cells produce voltages synchronously the resultant voltage is high enough
to be measurable e.g., EMG - muscle fibre contraction, most cells of the fibre perform
the same electric activity synchronously and a measurable electric voltage appears.
• Many organs in the human body, such as the heart, brain, muscles, and eyes, manifest
their function through electric activity.
• The heart, for example, produces a signal called the electrocardiogram or ECG. The
brain produces a signal called an electroencephalogram or EEG.
• The activity of muscles, such as contraction and relaxation, produces an
electromyogram or EMG.
• Eye movement results in a signal called an electrooculogram or EOG, and the retina
within the eyes produces the electroretinogram or ERG.
•

SAMPLE WAVEFORMS ECG, EEG, EMG, EOG

• The origins of these biopotentials can be traced to the electric activity at the cellular
level. The electric potential across a cell membrane is the result of different ionic
concentrations that exist inside and outside the cell.
• The electrochemical concentration gradient across a semipermeable membrane results
in the Nernst potential.
• The cell membrane separates high concentrations of potassium ion and low
concentrations of sodium ions (along with other ions such as calcium in less significant
proportions) inside a cell and just the opposite outside a cell. This difference in ionic
concentration across the cell membrane produces the resting potential.
• Some of the cells in the body are excitable and produce what is called an action
potential, which results from a rapid flux of ions across the cell membrane in
response to an electric stimulation or transient change in the electric gradient of the cell.
• The electric excitation of cells generates currents in the surrounding volume conductor
manifesting itself as potentials on the body.

• The continuum of electrophysiological signals from the (a) heart cells, (b) myocardium
(the heart muscle), and (c) the body surface.
• Each cell in the heart produces a characteristic action potential. The activity of cells in
the sinoatrial node of the heart produces an excitation that propagates from the atria
to the ventricles through well-defined pathways and eventually throughout the heart;
 this electric excitation produces a synchronous contraction of the heart muscle. The
associated biopotential is the ECG.
• Electric excitation of a neuron produces an action potential that travels down its
dendrites and axon; activity of a massive number of neurons and their interactions
within the cortical mantle results in the EEG signal.
• Excitation of neurons transmitted via a nerve to a neuromuscular junction produces
stimulation of muscle fibres.
• Constitutive elements of muscle fibres are the single motor units, and their electric
activity is called a single motor unit potential. The electric activity of large numbers
of single motor unit potentials from groups of muscle fibers manifests on the body
surface as the EMG.
• Contraction and relaxation of muscles is accompanied by proportionate EMG signals.
Biopotentials from organs are diverse. The most noteworthy features of biopotentials
are: -

• Small amplitudes (10mV to 10 mV),

• Low frequency range of signals (dc to several hundred hertz)
The most noteworthy problems of such acquisitions are
• Presence of biological interference (from skin, electrodes, motion, etc.),
• Noise from environmental sources (power line, radio frequency, electromagnetic, etc.).

ELECTRODE -ELECTROLYTE INTERFACE:

• Bioelectric events have to be picked up from the surface of the body before they can be
put into the amplifier for subsequent record or display.
• This is done by using electrodes. Electrodes make a transfer from the ionic conduction
in the tissue to the electronic conduction which is necessary for making measurements.
• Electrodes are also required when physiological parameters are measured by the
impedance method and when irritable tissues are to be stimulated in electrotherapy.
Two types of electrodes are used in practice-surface electrodes and the deep-seated
electrodes.
• Electrodes play an important part in the satisfactory recording of bioelectric signals and
their choice requires careful consideration.
• They should be comfortable for the patients to wear over long periods and should not
produce any artefacts. Another desirable factor is the convenience of application of the
electrodes.
• The most commonly used electrodes in patient monitoring and related studies are
surface electrodes. The notable examples are when they are used for recording ECG,
EEG and respiratory activity by impedance pneumography.

• At the metal-electrolyte transition, there is a tendency for each electrode to discharge

ions into the solution and for ions in the electrolyte to combine with each electrode.
The net result is the creation of a charge gradient (difference of potential) at each
electrode, the spatial arrangement of which is called the electrical double layer.

• The double layer is known to be present in the region immediately adjacent to the
electrode and can be represented, in its simplest form, as two parallel sheets of charge
of opposite sign separated by a thin film of dielectric.

• Therefore, the metal-electrolyte interface appears to consist of a voltage source in series

with a parallel combination of a capacitance and reaction resistance.

• The voltage developed is called the half-cell potential. To a first-order approximation,

the half-cell potential is equal to the electrode potential of the metal, if the electrodes
were used in a chemical measuring application.
ELECTRODE-SKIN INTERFACE:

• When biopotentials are recorded from the surface of the skin, we must consider an
additional interface, the interface between the electrode-electrolyte and skin, in order
to understand the behaviour of the electrodes.
• In coupling an electrode to the skin, generally use transparent gel containing Cl- as the
principle anion to maintain good contact.
• The interface between this gel and the electrode is called electrode-electrolyte
interface.
• Initially, have to understand about the structure of skin. The outermost layer of skin is
epidermis.
• This epidermis consists of three sublayers
• Stratum germinativum
• Stratum granulosum
• Stratum corneum.
• The deeper layers of the skin contain the vascular and nervous components of the skin
as well as sweat glands, sweat ducts, and hair follicles.
• To represent the electric connection between an electrode and the skin through the
agency of electrolyte gel.

ELECTRODE-SKIN INTERFACE
 • The series resistance Rs is now the effective resistance associated with the interface
effects of the gel between the electrode and the skin.
• We can consider epidermis or stratum corneum, as a membrane that is semipermeable
to ions, so if there is a difference in ionic concentration across this membrane, there is
a potential difference Ese, which is given by the Nernst equation.
• The epidermal layer is also found to have an electric impedance that behaves as a
parallel RC circuit.
• For 1 cm2, skin impedance reduces from approximately 200kΩ at 1Hz to 200Ω at
1MHz.
• The dermis and the subcutaneous layer under it behave in general as pure resistances.
• They generate negligible dc potentials.
• A factor that is sometimes important in examination, for example, psychogenic
electrodermal response or the galvanic skin responses (GSR), is the contribution of
the sweat glands and sweat ducts.
• The fluid secreted by sweat glands contains Na+, K+ and Cl- ions, the concentrations of
which differ from those in the extracellular fluid.
• Thus, there is a potential difference between the lumen of the sweat duct and dermis
and subcutaneous layers.

POLARIZABLE ELECTRODE:
• When the polarizable electrode is in contact with an electrolyte, a double layer of charge
forms at the interface.
• If the electrode is moved with respect to the electrolyte, this movement mechanically
disturbs the distribution of charge at the interface and results in a momentary change of
the half-cell potential until equilibrium can be re-established.
• If a pair of electrodes is in an electrolyte and one moves while the other remains
stationary, a potential difference appears between the two electrodes during this
movement. This potential is known as motion artifact.

NON-POLARIZABLE ELECTRODE:
• Because, motion artifacts result primarily from mechanical disturbances of the
distribution of charge at the electrode-electrolyte interface, it is reasonable to expect the
motion artifacts is minimal for non-polarizable electrodes.
• Observation of the motion-artifacts signals reveals that a major component of this noise
is at low frequencies.
• Different biopotential signal occupies different portion of frequency spectrum, that is
mentioned below.
• That low-frequency artifacts do not affect signals such as the EMG or axon action
potential (AAP) nearly it does the ECG, EEG and EOG.
•
 • In the former case, filtering can be effectively used to minimize the contribution of
motion artifacts on the overall signal.
• But, in the latter case, such filtering also distorts the signal.
• Consequently, it is important in these applications to use non-polarizable electrode to
minimize motion artifacts stemming from the electrode-electrolyte interface.
• The choice of a gel material is important that remembering the dynamic nature of the
epidermis.
• Stretching the skin changes this skin potential by 5 to 10 mV , and this changes appears
as motion artifacts.

HALF-CELL POTENTIAL:

Definition:
• 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 builds 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 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 half-cell potential.
ELECTRICAL DOUBLE LAYER

HALF CELL POTENTIAL

• 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 potentials is called an
electrode offset potential.
 POLARIZATION EFFECTS OF ELECTRODE:

• Half-cell potential is altered when there is current flowing in the electrode due to
electrode polarization.
• Overpotential is the difference between the observed half-cell potential with current
flow and the equilibrium zero-current half-cell potential.
• Mechanism Contributed to overpotential:
(i) Ohmic Overpotential
(ii) Concentration overpotential
(iii) Activation overpotential

(i) Ohmic overpotential:

• Voltage drop along the path of the current, and current changes resistance of electrolyte
and thus, a voltage drop does not follow ohm’s law.
(ii)Concentration overpotential:
• Current changes the distribution of ions at the electrode-electrolyte interface
(iii)Activation overpotential:
• Current changes the rate of oxidation and reduction. Since the activation energy
barriers for oxidation and reduction are different, the net activation energy depends on
the direction of current and this difference appear as voltage.

VP = E0 + VR + VC + VP

VP = Total potential or polarization potential of the electrode

E0 = Half-cell potential
VR= Ohmic overpotential
VC = Concentration potential
VP = Activation potential

TYPES OF ELECTRODES:

Bioelectrical signals are acquired from one of three forms of electrode: There are three types
of electrodes namely,

• Body Surface electrodes.

• Needle electrodes.
• Microelectrodes.

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.
There are four different types of body surface recording electrodes;

1. Column Electrodes
2. Suction Electrodes
3. Floating Electrodes
4. Flexible Electrodes

(i)Column Electrodes:
• 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.
• For this reason, the column electrodes are preferred for monitoring hospitalized
patients.

Function of 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
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:

Several problems are associate with all types of surface electrodes:

1. Adhesive will not stick for long on sweaty or clammy skin surfaces.
2. Fleshy portion of chest and abdomen are selected as electrode site.
3. After 8 hours change the electrode to avoid the ischemia.
4. Movement Artifacts
5. Electrode position slips
Needle Electrode:

• 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
patient, 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.
• 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.

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:
• The brain produces 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 has 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:
• 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.
Types of microelectrodes:
(i) Metal microelectrodes (Tungsten Microelectrodes)
(ii)Supported Metal Electrodes (metal contained within/outside glass needle)
(iii) Micropipette electrodes (with Ag-AgCl electrode metal).
(i)Metal microelectrodes:

(ii)Supported metal micro-electrode:

• 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 signal’s 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.
(iii)Micropipette electrode:

The fluid-filled glass microelectrode is shown in the

 In this type of electrode, the glass pipette is filled with a solution of potassium chloride
(KCI), and the large end is capped with a 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 a silver-silver chloride (Ag-
Ag Cl) plug caps the other end.