- 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
Bio electrode potential is the electrical signal generated in tissue and recorded at the body surface or inside a cell. Recording electrodes capture those bioelectric events, then the signals are amplified and displayed. The electrodes convert ionic conduction in the tissues to electronic conduction for measurement.
Two common electrode classes are surface electrodes and needle electrodes. Surface electrodes pick up potential difference from the tissue surface without cutting the skin. Needle electrodes can measure potential inside a cell when they are intracellular microelectrodes; clinical needle EMG usually records from tissue around muscle fibres.
Characteristics of Biopotential Electrodes
- Electric potential generated in the body is ionic potential.
- Transducers convert the ionic current in the body into an electronic current that flows through the electrode.
- The electrode conducts a small current across the interface between body and measuring circuit.
- A net current passes across the interface from the electrode to electrolyte.
At the electrode-electrolyte interface, current can flow from the metal toward the electrolyte. 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. At a metal-solution interface that voltage appears in two cases: ions leave the metal into the solution, or ions in solution combine with electrons in the metal and form metal atoms. When a metal electrode meets body fluid, the electrode can discharge ions into the solution while ions in the electrolyte combine with the electrode. The two processes set up a charge gradient.
When current is applied and net charge transfer across the metal-electrolyte interface stays near zero, the electrodes are treated as polarized. They behave like capacitors. Example: Platinum Electrode. Polarized electrodes are used for stimulating signals.
When charge can cross the metal-electrolyte interface with little extra voltage, the electrodes are treated as non-polarized. Here current flows freely across the interface and the extra energy needed is small. 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.
Dry outer skin has high impedance, so a metal plate alone makes a poor electrical contact. Electrode paste at the placement site lowers that impedance and completes the contact.
When a polarizable electrode meets the electrolyte, a double layer of charge forms at the interface. Electrode movement disturbs that charge distribution and changes the half-cell potential. That motion artefact is smaller for a non-polarizable electrode.
