The human body generates electrical signals on the body surface. Recording electrodes picks the bioelectric events produced in the body. The picked up signals are given to the amplifier and then to display. Electrodes transfer ionic conduction in the tissue to electronic conduction to measure the values. Generally, there are two types of electrodes namely surface electrodes and needle electrodes. A surface electrode picks the potential difference from the tissue surface without damaging the live tissue. Deep electrode indicates the electric potential value from inside the cell.
Characteristics of Biopotential Electrodes
- Electric potential generated in the body are ionic potential.
- Transducers convert the ionic current in the body into an electronic current that flows through the electrode.
- It conducts small current across the interface between body and measuring circuit.
- A net volume of current passes across the interface from the electrode to electrolyte.
At the electrode-electrolyte interface, current flows from left to right. The electrode has metal atoms C. The electrolyte has cations of electrode metal C+ and anions A-. During oxidation, atoms lose electrons. During reduction, atoms gain electrons.
Half Cell Potential
Half-cell potential is the voltage developed at the electrode-electrolyte interface. In a metal – solution interface, electrode potential arises at two conditions i) when ions travel from metal into the solution ii) when ions in solution combine with electrons in the metal they form the atom of metal. Hence, when metal electrode and body fluid interacts electrode discharges ions into solution. At the same instance ions in the electrolyte combine with the electrode. This results in the generation of charge gradient.
When current is applied there is no net transfer of charge across the metal electrolyte, interface electrodes are Perfectly Polarized Electrode. Electrodes behave like capacitors. Example: Platinum Electrode. For stimulating, signals polarised electrodes are used.
When the exchange of charge occurs across metal-electrolyte interface without hindrance electrode are Perfectly Non Polarized Electrode. Here current flows freely across the interface and energy is not required for it. Example: Ag/AgCl electrode. For recording applications, Non-Polarized electrodes are used.
When semipermeable membrane separates two different ionic solutions of two different concentration, an electric potential appears across the membrane. Nernst Equation determines the half-cell potential developed as given below.
Where a1 and a2 are the ion activities on each side of the membrane. Ionic activity defines the condition at which ionic species in solution enters into a reaction.
The outer skin of the body is dry and non-conductive. Therefore, the skin will not establish electrical contact with electrodes. To avoid this problem, use electrode paste at the place of electrode placement.
When polarizable electrode meets the electrolyte, it forms a double layer of charge at the interface. The electrode movement disturbs the charge distribution resulting in a change of half-cell potential. This is Motion Artefact. It is minimum for non-polarizable electrode.