Medical Electronics short notes

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 (and some second order factors) .

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 to 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

 

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-Polarizable 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. These electrodes see no

Over potentials. Example : Platinum electrode

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

Equivalent Circuit

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

Why

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.

What

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

1. Metal Plate Electrodes (historic)
2. Suction Electrodes

(historic interest)

1. Floating Electrodes
2. Flexible Electrodes

Commonly Used Biopotential 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!

(a) Metal-plate electrode used for application to limbs.
(b) Metal-disk electrode applied with surgical tape.
(c)Disposable foam-pad electrodes, often used with ECG

Commonly Used Biopotential Electrodes

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