BIO POTENTIAL ELECTRODES
Electrode – Electrolyte Interface
General Ionic Equations
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
· If there is a current between the electrode and electrolyte, the observed half cell potential is often altered due to polarization.
· 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]
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
· 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!
- No straps or adhesives required
- precordial (chest) ECG
- can only be used for short periods
- metal disk is recessed
- swimming in the electrolyte gel
- not in contact with the skin
- reduces motion artifact
- 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.
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
2. Non metallic(Micropipet)
· 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.