WEEK FIVE (2)

Bismillahirrahmanirrahim. 

Continue from last post. There are a few designs of surface electrodes. There are Silver–Silver Chloride Electrodes, Gold Electrodes, Conductive Polymer Electrodes, Metal or Carbon Electrodes and Needle Electrodes.

Silver-Silver Chloride

The common surface electrode design used is the Silver-Silver Chloride design of electrode. This is the classic design that high-quality electrode design consists of highly conductive metal, silver, interfaced to its salt, silver chloride, and connected via an electrolytic gel to the human body. This design is known to produce the lowest and most stable junction potentials. Additionally, an electrolytic gel typically based on sodium or potassium chloride is applied to the electrode.

 

Reusable silver–silver chloride electrodes (a) are made of silver disks coated electrolytically by silver chloride, or, alternatively, particles of silver and silver chloride are sintered together to form the metallic structure of the electrode. The gel is typically soaked into a foam pad or is applied directly in a pocket produced by the electrode housing. The electrode is secured to the skin by means of nonallergenic adhesive tape.

Disposable electrodes (b) are made similarly, although the use of silver may be minimized because of the snap-on button itself may be silver coated and chlorided. To allow for a secure attachment, a large foam pad attaches the electrode body with adhesive coating on one side. Such electrodes are particularly suited for ambulatory or long term use.

Gold-plated electrodes

Gold-plated electrodes have the advantages of high conductivity and inertness desirable in reusable electrodes, are commonly used in EEG recordings. Small reusable electrodes are designed so that they can be securely attached to the scalp. The electrode body is also shaped to make a recessed space for electrolytic gel, which can be applied through a hole in the electrode body. The electrodes are attached in hair-free areas by use of a strong adhesive such as colloidon or securely attached with elastic bandages or wire mesh. Disadvantages of using gold electrodes over silver–silver chloride electrodes include greater expense, higher junction potentials, and greater susceptibility to motion artifacts. On the other hand, gold electrodes maintain low impedance, are inert and reusable, and are good for short-term recordings as long as a highly conductive gel is applied and they are attached securely.

Conductive Polymer Electrodes

This electrode does not need additional adhesive or electrolytic gel and hence can be immediately and conveniently used. The conductive polymeric electrode performs adequately as long as its relatively higher resistivity over metallic electrodes and greater likelihood of generating artifacts are acceptable. The higher resistivity of the polymer makes these electrodes unsuitable for low-noise measurement. The polymer does not attach as effectively to the skin as does the conventional adhesive on disposable ECG electrodes built with a foam base and, furthermore, the potentials generated at the electrode–skin interface are more readily disturbed by motion.

 

Metal or Carbon Electrodes

These electrodes have a much higher resistivity and are noisier and more susceptible to artifacts, but they are inexpensive, flexible, and reusable and are thus chosen for applications such as electric stimulation or impedance plethysmography. For these applications, gel is usually not applied and the electrodes are used in “dry” form for easy attachment and removal. These electrodes have the potential for producing very high levels of artifact and are bulky and awkward to use, but do offer the advantage of being reusable and tend to be inexpensive. Carbon or carbon-impregnated polymer electrodes are also used occasionally although they are mainly used as electrical stimulation electrodes.

Needle electrodes

Needle electrodes (e) comprise a small class of invasive electrodes, used when it is absolutely essential to record from the organ itself. The most common application is in recording from muscles or muscle fibers. A metallic, typically steel, wire is delivered via a needle inserted at the site of the muscle fiber. The wire is hooked and hence fastens to the muscle fiber, even as the needle is removed. Small signals such as motor unit potentials can be recorded in this manner. For research applications, similar needle or wire electrodes are sometimes connected directly to the heart muscle. Since such electrodes are noninvasive, their use is limited to only highly specialized and supervised clinical or research applications.

For my project, I use Silver-Silver Chloride disposable electrode even though the reusable one is maybe better in most aspect. 

Barakallah.