- Bio Electrode Potential Definition: Bio electrode potential is the electrical signal generated on the body’s surface, recorded by electrodes to measure body functions.
- Electrode Types: There are two main types of electrodes: surface electrodes, which measure signals from the tissue surface, and needle electrodes, which measure signals from inside the cell.
- Half Cell Potential in Biomedical Instrumentation: This is the voltage at the electrode-electrolyte interface, crucial for understanding how electrodes interact with body fluids.
- Perfectly Polarized vs. Non-Polarized Electrodes: Polarized electrodes act like capacitors and do not transfer charge, while non-polarized electrodes allow free charge flow.
- Motion Artefact: This occurs when electrode movement disturbs charge distribution, affecting half-cell potential, and is minimized by using non-polarizable electrodes.
Electrode Potential
The human body generates electrical signals on its surface. Recording electrodes capture these bioelectric events. The signals are then amplified and displayed. Electrodes convert ionic conduction in the tissues to electronic conduction for measurement.
There are two main types of electrodes: surface electrodes and needle electrodes. Surface electrodes pick up potential difference from the tissue surface without causing damage. Needle electrodes measure the electric potential 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—these are Perfectly Polarized Electrodes. They behave like capacitors. Example: Platinum Electrode. Polarized electrodes are used for stimulating signals.
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 a semipermeable membrane separates two ionic solutions of different concentrations, an electric potential appears across the membrane. The Nernst Equation determines this half-cell potential.
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.
