- Electrode Definition: An electrode is defined as a solid electric conductor that allows electric current to enter or leave an electrolytic cell.
- Types of Electrodes: There are three main types of electrodes: Microelectrodes, Needle electrodes, and Body Surface electrodes.
- Microelectrodes: Microelectrodes measure electric potential within a single cell without damaging it, with types including Metal Microelectrodes and Non-Metallic Microelectrodes.
- Needle Electrodes: Needle electrodes record peripheral nerve action potentials and come in Mono-polar and Bi-polar types.
- Biomedical Instrumentation: The types of electrodes in biomedical instrumentation include those used for measuring ECG, EEG, and EMG signals.
What are Electrodes?
Electrodes are solid electric conductors that let electric current enter or leave an electrolytic cell, converting ionic potentials to electronic potentials. In biomedical work the same idea is used to pick up body signals. Different electrode designs are used depending on where the bioelectric signals are taken from. These electrodes can acquire signals like ECG, EEG and EMG.
Three common biomedical electrode classes are:
- Microelectrodes
- Needle electrodes
- Body Surface electrodes
Microelectrodes
Microelectrodes measure electric potential from inside a single cell. Their tips are small enough to enter a cell; some membrane injury can still occur. Research labs also use them to inject drugs or current into a cell.
Generally, when the microelectrode is inside a cell, the reference electrode is outside the cell. Impedance is high, often in the megaohm range, because the tip is so small. Two types of microelectrode are
- Metal Microelectrode
- Non- Metallic (Micropipette)
Metal Microelectrode
The tip of a metal microelectrode is made from a tungsten filament or stainless steel wire formed into a tiny structure using a technique called electropointing. Insulating material covers the shaft, leaving the tip exposed.
A little electrolytic processing is done to reduce the impedance. Measurement of bioelectric potentials requires two electrodes. The resulting voltage is the difference between the microelectrode potential and the reference electrode potential. The total of the three potentials is as follows.
Where,
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 a glass micropipette of diameter 1 micrometer. The micropipet is filled with an electrolyte solution that is compatible with cellular fluids. The stem of the micropipet has a thin flexible wire made of chloride silver, stainless steel or tungsten. One end of the micropipet attaches to the rigid support and the other free end rests on the cell. The potential 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 the electrode must sit closer to the bioelectric generator, it penetrates the skin. The electrode is therefore sharp so it can enter tissue and record the event.
Depth Electrodes
Depth electrodes record electrical activity from neurons in deep brain structures, not only on the surface. These electrodes consist of a 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 Electrodes
Needle electrodes record peripheral nerve and muscle action potentials and can look like medicinal syringes. One hooked-wire design uses a short insulated wire bent at one end and passed through the needle. After insertion into muscle, the needle can be withdrawn while the wire stays inside. Two common needle types are monopolar and bipolar.
Mono-polar Electrode: This type uses a 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 used for EMG. Needle or sphenoidal contacts are also used in some EEG studies, but scalp surface electrodes remain the usual EEG pickup.
