What are Electrodes?
Electrodes are defined as a solid electric conductor through which an electric current enters or leaves an electrolytic cell. It converts ionic potentials to electronic potentials. Different types of electrodes used for biological measurements depend on the anatomical locations, from where the bioelectric signals are measured. Bioelectric electrodes acquire signals like ECG, EEG, EMG, etc.
There are three main types of electrodes:
- Needle electrodes
- Body Surface electrodes
Microelectrode measures the electric potential from within a single cell. It has very small diameter tips that can penetrate deep into the cell without damaging the human cell. The functions of microelectrodes are potential recording to inject medicines.
Generally,when microelectrode is inside cell, reference electrode is outside the cell. It has high impedances in range of mega ohm due to their small size. Two types of microelectrode are
- Metal Microelectrode
- Non- Metallic (Micropipette)
The tungsten filament or stainless steel wire made into minute structure forms the tip of the microelectrode. This technique is electropointing. The insulating material covers the entire electrode for safety purpose.
Few electrolytic processing is done to reduce the impedance. Measurement of bioelectric potentials requires two electrodes. The resulting voltage potential is the difference between the potential of microelectrode and reference electrode. The total sum of the three potentials is as follows.
EA – metal electrode-electrolyte potential at microelectrode tip.
EB – Reference electrode-electrolyte potential.
EC – Variable cell membrane potential.
Non-Metal Microelectrode (Micropipet)
This electrode uses Non – metallic material to measure the potential from a single cell. It consists of glass micropipette of diameter 1 micrometer. Micropipet filled with electrolyte solution that is compatible with cellular fluids is used. Stem of Micropipet has a thin flexible wire made out of chloride silver, stainless steel or tungsten. One end of the Micropipet attaches to the rigid support and other free end rests on the cell. The potential voltage generated is as follows.
EA – potential voltage between the metal wire and an electrolyte filled inside Micropipet.
EB – potential between the reference electrode and extracellular fluid.
EC – variable cell membrane potential.
ED – potential at the tip due to electrolytes present inside the pipet and the cell.
Depth and Needle Electrodes
When electrode gets closer to the bioelectric generator, it penetrates into the skin. Therefore, the electrode should be sharp for penetration to obtain and record the bioelectric events.
Depth electrode studies the electrical activity of the neurons in the surface of the brain. This type of electrode consists of bundle of Teflon insulated platinum and iridium alloy wires.
For easy insertion of the electrodes into the brain, the end of the supporting wire is round-shaped. The number of individual electrodes forms the electrode array or bundle. In the bundle of electrodes, the end of each individual wires has the individual electrode.
Applications of Depth Electrodes
The applications of depth electrodes include:
- To inject medicines into the brain.
- To measure oxygen tension.
Needle electrode records the peripheral nerve action potential. It resembles a medicinal syringe. At one end a short insulated wire is bent. The bent portion passes through the lumen of the needle. This setup goes into the muscle. Now the needle is withdrawn. The bent wire remains inside the muscle. Two type of needle electrodes namely
Mono-polar Electrode: This type uses single reference electrode placed on the skin.
Bi – polar Electrode: This type has one reference electrode and one active electrode.
Applications of Needle Electrodes
Needle electrodes are mostly used in the measurement of EEG and EMG signals.