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    Unit I Bio Potential Generation and Electrodes Types: Prepared by Dr.K.Helenprabha, Mr.P.Arul

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    Unit I Bio Potential Generation and Electrodes Types: Prepared by Dr.K.Helenprabha, Mr.P.Arul

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    priya dharshini
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    16 views52 pages

    Unit I Bio Potential Generation and Electrodes Types: Prepared by Dr.K.Helenprabha, Mr.P.Arul

    Uploaded by

    priya dharshini

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
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    of 52

    UNIT I Bio potential generation and

    electrodes types

    Prepared By
    Dr.K.HelenPrabha, HOD/ECE
    Mr.P.Arul, Assistant Professor
    Introduction
    • In order to measure and record potentials (currents) in the
    body, it is necessary to provide some interface between the
    body and the electronic measuring apparatus.
    • Current flows in the measuring circuit for at least a fraction
    of the period of time over which the measurement is
    made.
    • Bio potential electrodes is a transducer that convert the
    body ionic current in the body into the traditional
    electronic current flowing in the electrode.
    • Current is carried in the body by ions, whereas it is carried
    in the electrode and its lead wire by electrons.
    Electrode change an ionic current into an electronic current

    2
    electrical behaviour

    equivalent circuit
    ? ?
    3
    electrical behaviour

    equivalent circuit
    ?
    4
    electrical behaviour

    equivalent circuit

    5
    equivalent circuit
    electrode-electrolyte

    6
    more precise approximation of
    double layer – Randles circuit
    electrode-electrolyte

    Rct – active charge transfer resistance


    W – Warburg element reflecting diffusion
    with impedance ZW = AW/(jω)0.5
    AW – Warburg coefficient

    7
    Electrode-Electrolyte Interface
    Oxidation reaction causes atom to lose electron
    Reduction reaction causes atom to gain
    electron

    Oxidation is dominant when current flow from electrode to electrolyte, and


    reduction dominate when the current flow is the
    opposite.
    Oxidation Reduction

    anion cation

    Current flow Current flow


    C → C + nen+ −
    C  C + ne
    n+ − 3
    Half-Cell Potential
    Half-Cell potential is determined by
    -Metal involved
    -Concentration of its ion in solution
    -Temperature
    -And other second order factor

    Certain mechanism separate charges at the metal-electrolyte interface results in


    one type of charge is dominant on the surface of the metal and the opposite
    charge is concentrated at the immediately adjacent electrolyte.
    9
    Polarization
    Half cell potential is altered when there is current flowing in the electrode.

    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


    -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.

    - Concentration overpotential: Current changes the distribution of ions at the


    electrode-electrolyte interface

    - 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 =VR +VC +VA
    Note: Polarization and impedance of the electrode are two of the most important
    6
    electrode properties to consider
    Half Cell Potential and Nernst Equation
    When two ionic solutions of different concentration are separated by
    semipermeable membrane, an electric potential exists across the
    RT  a  membrane.
    E=− ln  1 
    nF  a2 

    a1 and a2 are the activity of the ions on each side of the membrane.
    Ionic activity is the availability of an ionic species in solution to enter
    into a reaction.
    Note: ionic activity most of the time equal the concentration of the ion
    For the general oxidation-reduction reaction
    A + B  C + D + ne−
    The Nernst equation for half cell potential is
    RT  a a 
     
    E=E +
    0
    ln   
    C D
    nF  aA aB 
    1
    1
    Polarizable and Nonpolarizable Electrodes

    Perfectly Polarizable Electrodes


    Electrodes in which no actual charge crosses the electrode-electrolyte
    interface when a current is applied. The current across the interface is
    a displacement current and the electrode behaves like a capacitor.
    Overpotential is due concentration. Example : Platinum electrode
    Perfectly Non-Polarizable Electrode
    Electrodes in which current passes freely across the electrode-
    electrolyte interface, requiring no energy to make the transition.
    These electrodes see no overpotentials. Example: Ag/AgCl Electrode

    Example: Ag-AgCl is used in recording while Pt is used in stimulation

    1
    2
    chemical reactions
    silver / silver chloride

    13
    The Silver/Silver Chloride Electrode
    Advantage of Ag/AgCl is that it is stable in liquid that has large
    quantity of Cl- such as the biological fluid.
    For biological fluid where Cl- ion is relatively high
    a − 1
    Cl

    E = EAg
    0
    +
    RT
    nF
     
    ln aAg +

    K s = aAg +  aCl − = 10−10


    is solubilityproduct

    0 + RT ln K s

    E = EAg  
    Performance of this Ag  Ag + +e− nF aCl − 
    electrode
    Ag + + Cl −  AgCl
    constant
    effective 9
    Electrode Behavior and Circuit Models
    Advantages: metal + - Electrolyte
    –Low Noise (vs. Metal Electrodes) esp. ECG + -
    –Biocompatible + -
    The characteristic of an electrode is + -
    -Sensitive to current density + -
    - waveform and frequency dependent + -

    Rd and Cd make up the


    impedance associated with
    electrode-electrolyte interface
    and polarization effects. Rs is
    associated with interface effects
    and due to resistance in the
    Dr. Shamekhi, Sahand University of electrolyte. 1
    Technology 5
    The Electrode-Skin Interface

    Transparent electrolyte gel containing Cl- is used to maintain good


    contact between the electrode and the skin.

    1
    6
    The Electrode-Skin Interface
    For 1 cm2, skin impedance
    reduces from approximately
    200K at 1Hz to 200 at 1MHz.

    A body-surface electrode is placed against skin, showing the total


    electrical equivalent circuit obtained in this situation. Each circuit
    element on the right is at approximately the same level at which the
    physical process that it represents would be in the left-hand diagram.
    Motion Artifact
    When polarizable electrode is in contact with an electrolyte, a double
    layer of charge forms at the interface. Movement of the electrode will
    disturb the distribution of the charge and results in a momentary
    change in the half cell potential until equilibrium is reached again.
    Motion artifact is less minimum for nonpolarizable electrodes.

    Signal due to motion has low frequency so it can be filtered out when
    measuring a biological signal of high frequency component such as
    EMG or axon action potential. However, for ECG, EEG and EOG whose
    frequencies are low it is recommended to use nonpolarizable
    electrode to avoid signals due to motion artifact.

    Must be considered:
    –good adhesive connection to skin
    –skin cleaning
    –floating electrode
    1
    8
    19
    21
    22
    23
    24
    26
    Disposable Foam-Pad Electrodes

    Disposable foam-pad electrodes, often used with electrocardiograph monitoring apparatus.

    2
    7
    28
    30
    32
    NEEDLE ELECTRODES
    Needle electrodes are generally used in clinical electro myography,
    neuro graphy and other electrophysiological investigations under the
    skin and in the deeper tissues.

    ➢Material used: Stainless steel which is preferred due to its


    mechanical solidity and low price.

    ➢These electrodes are generally designed to be fully auto clavable


    and should be thoroughly sterilized before use

    ➢Different types of needle electrodes are used for electro


    myographic work.
    35
    36
    Implantable electrodes

    (a) Wire-loop electrode. (b) Silver-sphere cortical-surface


    potential electrode. (c) Multielement depth electrode.
    mounted

    3
    7
    38
    Types of electrodes

    •Monopolar needle electrodes


    • Bipolar needle electrodes
    • Concentric (coaxial) core needle electrode
    •Multielement needle electrode

    39
    Internal Electrodes
    No electrolyte-skin interface
    No electrolyte gel is required

    Needle and wire electrodes for


    percutaneous measurement of
    biopotentials
    (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 coiled fine- wire
    electrode in place.
    For EMG Recording
    1
    MONOPOLAR NEEDLE ELECTRODE

    ➢Consists of Teflon coated stainless steel wire which is barely a tip

    ➢The coating recedes after being used for several times and the
    electrode must be discarded when this occurs. They are also color
    coded.
    BIOPOLAR (DOUBLE COAXIAL) NEEDLE ELECTRODE)

    The synthesis of a motor unit action potential,


    as recorded by bipolar needle electrodes

    Contains two insulated wires within a metal cannula (is a tube that can be
    inserted into the body, often for the delivery or removal of fluid).
    The two wires are bared at the tip and provide the contacts to the patient.
    The cannula acts as the ground
    These electrodes are electrically symmetrical and have no sense of polarity.
    CONCENTRIC(COAXIAL) CORE NEEDLE ELECTRODE

    Contains both the active and reference electrode within the same structure.
    Consists of an insulated wire contained within an hypodermic needle.
    The inner wire is exposed at the tip which forms one electrode.
    These needles have very stable electrical characteristics and are
    convenient to use.
    These electrodes are made by moulding fine platinum wire into hypodermic
    needle having outside diameter less than 0.6mm.
    One end is bevelled to expose he end of wire and provide easy penetration.
    The surface area of the exposed tip is less than 00005mm sq.
    44
    45
    46
    Microelectrodes

    •Rs :resistance of the metal


    •Cd: The metal is coated with an insulating material over all
    but its most distal tip
    Cd2: outside Cdi: inside
    •Metal-electrolyte interface, Rma, Cma, and Ema
    •Reference electrode: Cmb, Rmb, and Emb
    •Ri: electrolyte within the cell membrane
    •Re: extracellular fluid
    •Cw: lead wires Cap.
    •Emp: The cell membrane variable potential

    Dr. Shamekhi, Sahand University of Technology 4


    7
    48
    49
    50
    51
    53
    54
    Recording problems

    •Electrode-electrolyte noise
    •Noise at the electrolyte
    •skin interface
    •Motion artifact
    •Electric and magnetic field interference
    •Thermal noise
    •Amplifier noise
    •Noise from additional bioelectric events
    •Other noise sources
    55
    Silver –Silver Chloride electrodes
    Half cell potential is 2.5 mv only
    Reduces the noise voltage and Increases the
    stability electrochemically
    Stabilizes the half cell potential- no movement
    artifacts(variable electrochemical voltage)
    Reduce the low frequency electrode- electrolyte
    impedance

    56
    57
    Contd..
    Using large area electrode and bio
    electric recorder of high input
    impedance the distortion in the wave
    form is reduced

    58

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