Electrodes: What Are They? (And The Types of Electrodes Used)

What are Electrodes?

Electrodes are defined as solid electric conductors that let electric current enter or leave an electrolytic cell, converting ionic potentials to electronic potentials. Different types of electrodes are used for biological measurements depending on where the bioelectric signals are taken from. These electrodes can acquire signals like ECG, EEG, and EMG.
There are three main types of electrodes:

1. Microelectrodes
2. Needle electrodes
3. Body Surface electrodes

Microelectrodes

Microelectrodes measure electric potential from inside a single cell. They have tiny tips that can go deep into the cell without causing damage. Microelectrodes are used for recording potentials and injecting 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)

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 whole electrode for safety.

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.

Where,
E_A – metal electrode-electrolyte potential at microelectrode tip.
EB – Reference electrode-electrolyte potential.
E_C – 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.

E_A – potential voltage between the metal wire and an electrolyte filled inside Micropipet.
E_B – potential between the reference electrode and extracellular fluid.
E_C – variable cell membrane potential.
E_D – 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 Electrodes

Depth electrodes study the electrical activity of neurons on the brain’s 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:

1. To inject medicines into the brain.
2. To measure oxygen tension.

Needle Electrodes

Needle electrodes record peripheral nerve action potentials and look like medicinal syringes. A short insulated wire is bent at one end and passes through the needle. This setup goes into the muscle, and when the needle is withdrawn, the wire stays inside. There are two types of needle electrodes: Mono-polar and Bi-polar.

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.