UNIT 1--Biopotential Electrodes

• Origin of biopotential and its propogation Electrode- electrolyte interface, electrode skin interface, half cell potential, contact impedance, polarization effects of electrode- non polarizable electrodes, types of electrodes- surface, needle and micro electrodes and their equivalent circuit- recording problems- motion artifacts, measurement with two electrodes

Origin of biopotential and its propogation

• Biopotential: Electric signal generated by physiological process
• Transducer is used to convert ionic potential into electric signal
• How biopotential is produced?

Produced by electrochemical activity of type of cell called an excitable cell.

• When excitable cell is stimulated then it generates an action potential
• Types of excitable cell:
• Afferent neurons
• Efferent neurons
• Effector cells
• Interneurons

• Recordings of bioelectric potential:
• Electrocardiogram (ECG)
• Electroencephalogram (EEG)
• Electroneurogram (ENG)
• Electromyogram (EMG)
• Electroretinogram (ERG)

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Cell membrane

• The cell membrane, also called the plasma membrane, is found in all cells and separates the interior of the cell from the outside environment.
• The cell membrane consists of a lipid bilayer that is semipermeable.

• Cell membrane is very thin (7-15nm)
• Cell membrane acts as capacitor ie., in between two plates there will be flow of ions
• Impermeable : proteins and other organic anions
• Selectively permeable: Sodium, potassium and chlorine ions

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Cell membrane potential:

• Ion concentration difference across membrane creates a diffusion gradient
• Gradient causes ions to flow
• Ion flow creates electric field
• Electric field: opposes ion flow until an equilibrium is established

Similar to P-N junction:

• Ion flows due to diffusion
• Diffusion creates a potential difference
• Which inhibits further flow of charged ions.

Electrical states of excitable cells

Resting state

Acting state

Resting state

• Excitable cells maintain a steady electrical potential difference between the internal and external environment [-50 mV to -100mV]
• Slightly permeable: Sodium ions
• Freely permeable: Potassium and Chlorine ions

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• Inside the cell it is more negative than outside cell
• Cell in the resting state is called Polarized cell.

Acting state

• After the resting state membrane change its characteristics and allow sodium (Na+) ions to pass through it
• Therefore, the cell has slightly +ve potential on the inside
• This is known as Action potential.

• The process of changing from the resting state to the action potential is called Depolarization.

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Electrode

• Converts ionic potential into electronic potential.
• Inorder to process the signal in electronic circuits, it will be better to convert ionic conduction into electronic conduction
• Types of electrode:
• Polarized
• Non- polarized
• Micro electrode
• Needle electrode
• Body surface

Half cell potential

• Half-cell potential refers to the potential developed at the electrode of each half cell in an electrochemical cell. 

Nernst Equation

• The Nernst equation defines the relationship between cell potential to standard potential and to the activities of the electrically active (electroactive) species.
• It relates the effective concentrations (activities) of the components of a cell reaction to the standard cell potential

• Ecell = E0cell - (RT/nF)lnQ

Ecell = cell potential under non-standard conditions (V)�E0cell = cell potential under standard conditions�R = gas constant, which is 8.31 (volt-coulomb)/(mol-K)�T = temperature (kelvin), which is generally 298°K (77°F/25°C)�n = number of moles of electrons exchanged in the electrochemical reaction (mol)�F = Faraday's constant, 96500 coulombs/mol�Q = reaction quotient, which is the equilibrium expression with initial concentrations rather than equilibrium concentrations

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• Skin and other tissues of human are electrolytic (electrolytic solution)
• Ions migrate from one side of the region or another forming parallel layers of ions of opposite charge
• This region is called electrode double layer.
• Ionic differences are the source of the electrode or half-cell potential

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Electrode-Electrolyte interface

• Current crosses from left to right
• Electrode consists of metallic atoms C
• Electrolyte is an aqueous solution containing cations of the electrode metal C+ and anions A-.
• Oxidation and reduction reaction takes place.

Oxidation reaction

• Oxidation reaction causes atoms to lose electron
• Current flow from electrode to electrolyte

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Reduction reaction

• Reduction reaction causes atom to gain electron
• Current flow from electrolyte to electrode

Electrode-Skin interface

• Transparent electrolyte gel containing Cl- is used to maintain good contact between electrode and skin.
• A body surface electrode is placed against skin

Contact impedance

• Determines how much current can be injected into the ground for a given voltage
• Two double layers form between the electrode metal conductor
• Gel
• Cell membrane of active skin

How to reduce contact impedance?

• A layer of electrically conductive gel is applied between the skin and the electrode to reduce contact impedance

Types of electrode

Polarization electrode

• Polarization is the change of potential from a stabilized state
• No actual charge crosses the electrode- electrolyte interface when a current is applied
• Behaves like a capacitor

Non Polarization electrode

• Current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition
• There are no over-potentials
• Represented as resistor.

Microelectrode

• Microelectrodes are electrodes having tips sufficiently small to penetrate a single cell in order to obtain readings from within the cell.
• Microelectrodes are also known as intracellular electrode
• Tips must be small enough to permit penetration without damaging the minute cell

• Function: Potential recording and current injection
• Have high impedance in mega ohm range because of their small size
• Uses
• Recording of neural signals
• Recording of electrical simulation of neurons tissue
• Measure:
• Membrane potential
• Intracellular free ion concentrations and cell-to-cell communication

Types of microelectrode

• Metal microelectrode
• Micropipette

Metal microelectrode:

• Formed by electrolytically etching the tip of fine tungsten to the desired size and dimension
• Wire is coated almost to the tip with any type of insulating material
• Metal-ion interface takes place where the metal tip contacts the electrolyte.

• Few electrolytic processing is done to reduce the impedance
• Requires 2 electrodes to measure bioelectric potential
• Resulting potential is the difference between the potential of microelectrode and reference electrode.

E=EA+EB+EC

EA- metal electrode-electrolyte potential at microelectrode tip

EB- Reference electrode-electrolyte potential

EC- Variable cell membrane potential

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Micropipet

• Uses non metallic material to measure the potential from a single cell
• Consists of glass micropipette of diameter 1mm
• Micropipette filled with electrolyte solution
• Stem of micropipette has a thin flexible wire made out of chloride silver, stainless steel or tungsten.
• One end of the electrode is attached to the rigid support othe end rests on the cell

E=EA+EB+EC+ED

EA- Potential voltage between the metal wire and an electrolyte filled inside micropipette

EB- Potential between the reference electrode and extracellular fluid

EC- Variable cell membrane potential

ED- Potential at the tip due to electrolytes present inside the pipette and the cell

Body Surface Electrode

• A small device that is attached to the skin to measure or cause electrical activity in the tissue under it.
• Senses signal from heart, brain and nerves.
• Large surface electrode sense ECG signal.
• Small surface electrode sense EMG, EEG signal.

Types:

Metal Plate electrode

Suction cup or Welsh cup electrode

Adhesive type electrode

Multipoint type electrode

Floating type electrode

Ear clips and scalp electrode

Metal Plate Electrode:

• ECG measurement requires either rectangular or circular shaped plate
• Plate made of nickel, silver or german silver materials
• Has smaller contact area.
• Electrodes are pasted on the skin using electrolyte paste.
• Suitable for application on four limbs- Limb electrode

Disadvantage:

• Electrode clippage
• Plate displacement
• Very sensitive
• Leading to measurement errors

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Suction cup or Welsh cup electrode

• To measure ECG from various positions on the chest.
• Attached on flat surface of the body and on soft tissue regions
• Physically they are large but skin contacts only the electrode rim.
• High contact impedance
• They have a plastic syringe barrel, suction tube and cables.
• Due to infection and cleaning procedures these electrodes are not used.

Adhesive type electrode

• Disadvantage of surface electrode:

Pressure of surface electrode against the skin squeezes out the electrode paste

• To avoid this problem– adhesive electrodes are used
• Light weight metallic screen
• Have pad at behind for placing electrode paste.
• Adhesive backing hold the electrode on place and tight
• Avoids evaporation of electrolyte present in the electrode paste

Multipoint type electrode

• Used in ECG measurement
• More than 1000 active contact points
• Helps to establish low resistance contact with the human

Floating electrode

• Major disadvantage of metal plate or limb electrodes is the measurement errors
• Motion artefact occurs due to the motion at the interface between electrode and electrolyte
• Interface gets stabilized using floating electrode
• Floating electrode contact with human surface via electrolytic paste or jelly
• Mechanical stability is the major advantage

Ear clip or Scalp electrode

• In measurement of ECG, ear clip electrodes are used
• Scalp electrodes provide EEG signal easily when placed on bare head
• In 10-20 electrode system EEG measurement scalp electrodes are used
• Avoid measurement errors

Needle electrode

• When electrode gets closer to the bioelectric generator, it penetrates into the skin
• Therefore the electrode should be sharp for penetration to obtain and record the bioelectric events
• Needle electrode records the peripheral nerve action potential
• Resembles a medicinal syringe

• One end short insulated wire is bent
• Bent portion passes through the lumen of the needle
• Setup goes into the muscle
• Needle is withdrawn

• Bent wire remains inside the muscle

2 types of needle electrode

Monopolar electrode:

This type uses single reference electrode placed on the skin

Bi-polar electrode:

This type of electrode has on reference electrode and one active electrode

Applications:

Measurement of EEG and EMG signal

Recording problems

• Need of medical recording
• Better clinical decision making
• Proper treatment
• Save patient

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• Noise in biomedical recording:

Surface electrodes involves the measurement of small potential difference noise plays vital role

Output Voltage is not always accurate

• Biopotential= desired voltage + No. of unwanted voltages

Desired voltages- signal

Unwanted voltage- noise

• Electrode- Electrolyte noise:

Stability depends upon material and electrode preparation

Low noise recording of biopotential is done by silver- silver chloride electrode

• Noise at the electrolyte skin interface

EMG signals and other noise sources will always contaminate the recording

Increasing the diameter reduces the excess noise

Motion artifacts

• Motion artifact is a patient-based artifact that occurs with voluntary or involuntary patient movement during image acquisition.
• What causes motion artifacts?�Motion artifacts are related to cardiac motion which are caused by cardiac pulmonary or body motion and can cause blurring or double images. 

How do you reduce motion artifacts?

• Several methods of reducing motion artifacts are then suggested.
• These include: randomization of views, averaging views, matching repeat times to the respiratory period, hybrid imaging, ROPE and COPE.
• The latter two methods reorder the data acquisition to destroy the coherence of the motion.

Measurement with two electrodes

• Voltage measured is the difference between the potential of 2 electrodes
• DC voltage due to the difference in electrode potential is called as electrode offset
• 2 electrodes of same material may also produce small electrode offset voltage
• Chemical activity takes place within an electrode can cause voltage fluctuations to appear without any physiological input

• Such variations may appear as noise on bioelectric signal
• Noise can be reduced by proper choice of materials or by coating the electrodes to improve stability
• Electric events inside the human body cause potential differences on the skin
• Surface electrodes are mainly used.
• Converts the potential differences due to ion flow inside the human body