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Chapter: Medical Electronics - Electro-Physiology and Bio-Potential Recording

Bio Potential Electrodes

If electrode has same material as cation, then this material gets oxidized and enters the electrolyte as a cation a nd electrons remain at the electrode and flow in the external circuit.

BIO POTENTIAL ELECTRODES

Electrode – Electrolyte Interface     

General Ionic Equations


C<->Cn+ +ne-      

Am-<->A +me -    

 

•        If electrode has same material as cation, then this material gets oxidized and enters the electrolyte as a cation a nd electrons remain at the electrode and flow in the external circuit.

•        If anion can be oxidized at the electrode to form a neutral atom, one or two electrons are given to the electrode

 

The dominating reaction can be inferred from the following :

 

·        Current flow from electrode to electrolyte : Oxidation (Loss of e-)

 

·        Current flow from electrolyte to electrode : Reduction (Gain of e-)



Half Cell Potential

 

·        A characteristic potential difference established by the electrode and its surrounding electrolyte which depends on the metal, concentration of ions in solution and temperature.

Half cell potential cannot be measured without a second electrode.

 

·        The half cell potential of the standard hydrogen electrode has been arbitrarily set to zero. Other half cell potentials are expressed as a potential difference with this electrode.

 

Reason for Half Cell Potential : Charge Separation at Interface

 

·        Oxidation or reduction reactions at the electrode-electrolyte interface lead to a double-charge layer, similar t o that which exists along electrically active biological cell membranes.

 

Measuring Half Cell Potential


Polarization

 

·   If there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization.

 

Nernst Equation

 

·        When two aqueous ionic solutions of different concentration are separated by an ion-selective semi-permeable membrane, an electric potential exists across the membrane.

 

The Nernst equation for half cell potential is E=E0 + R T/n[acyad/ aAαaBβ]

 

where

E0 : Standard Half Cell Potential 

E : Half Cell Potential

a : Ionic Activity (generally same as concentration)

n : Number of valence electrons involved

 

Polarizable and Non-Polarizab le Electrodes

 

Perfectly Polarizable Electrodes: These are 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. Example : Ag/AgCl Electrode

 

Perfectly Non-Polarizable Electrode: These are electrodes where current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition.

 

Over potentials. Example : Platinum electrode

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



Cd   : capacitance of electrode-eletrolyte interface

Rd   : resistance of electrode-eletrolyte interface

Rs     : resistance of electrode lead wire

Ecell  : cell potential for electrode

 

Electrode Skin Interface

 

Motion Artifact

 

·        When the electrode moves with respect to the electrolyte, the distribution of the double layer of charge on polarizable electrode interface changes. This changes the half cell potential temporarily.

 

·        If a pair of electrodes is in an electrolyte and one moves with respect to the other, a potential difference appears across the electrodes known as the motion artifact. This is a source of noise and interference in biopotential measurements.Motion artifact is minimal for non-polarizable electrodes

 

Body Surface Recording Electrodes


Commonly Used Biopotential Electrodes

Metal Plate Electrodes are

 

1.       Suction Electrodes

2.     Floating Electrodes

3.     Flexible Electrodes

 

Metal plate electrodes

 

–  Large surface: Ancient, therefore still used, ECG

 

–  Metal disk with stainless steel; platinum or gold coated

–  EMG, EEG

–  smaller diameters

–  motion artifacts

–  Disposable foam-pad: Cheap!

 



Suction electrodes

 

-           No straps or adhesives required

-           precordial (chest) ECG

-           can only be used for short periods

 


 


Floating electrodes


 

-         metal disk is recessed

-           swimming in the electrolyte gel

-           not in contact with the skin

-           reduces motion artifact

 

Flexible electrodes

 

-           Body contours 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)


(a)Carbon-filled silicon e rubber electrode. 

(b)Flexible thin-film neonatal electrode.

(c) Cross-sectional view of the thin-film electrode in (b).

 

Electrodes in Biopotential Measurements

 

         to make the electrode cheaper

 

           more suitable for lower noise measurement for EEG

 

           circumvent patents that are based on plastic/foam electrode body

 

           attractive to consumers for use with their ECG machines at home

 

           reduce artifact (minimize the motion of skin/electrode) in ambulatory recording

 

In a research laboratory, scientists want to record from single cells in a culture dish. They want to record action potentials from single, isolated heart cells. What kind of electrode would they need to use (describe material and design)? Give a simplified schematic (circuit model of the electrode) described in the notes given to you.

 

Neural electrodes/microelectrodes

 

It is used to measure potential within asingla cell.It is small in diameter and during insertion of microelectrode into cell will not damage to human cell.

 

·        It is classified into

1.     Metallic

 

2.     Non metallic(Micropipet)

 

Metallic Electrode

 

·        It is formed by electrolytically etching the tip of fine tungsten filament stainless wire into a minute structure.

 

·        Potential within the cell can be measured by using two electrodes

 

1. Micro electrode, 2. Reference electrode.

Non Metallic (Micropipet)

 

·        It is used to measure the potential within the single cell using non metallic material is used.

 

·        It is filled within an electrolyte ,that is compatible with the cellular fluids.



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