Scalp and Special Electrodes

Abstract

In this chapter, the physical characteristics and the functions of the electrodes are reported. In the first part, the differences between the various types of electrodes, the biochemical mechanisms of electrical conduction and the electrode chloridation methods are described. The second part is dedicated to the description of standard EEG electrodes (intrinsic properties, positioning, caps, adhesive pastes). Finally, a description of characteristics and indications of special electrodes is also reported.

Appendix: Electro-Physical Characteristics of Electrodes

• M. Mistretta 

1. Genoa, Italy

   M. Mistretta

Silver (Ag): it is a transition, pliable, biocompatible metal for non-invasive use only. It is characterised by a higher electrical conductivity than the other metals and by a medium/low level of electrode potential. It is subjected to a fast oxidation in contact with ozone and therefore the air. It is often used as a basic material in the production of surface electrodes, and in order to reduce to the minimum the inherent noise, it is essential to use the 999% pure one. The pure silver is a non-magnetic metal.

Silver/silver chloride (Ag/AgCl): it is a mixed material with pure silver base and an external layer of silver chloride. The external match between silver metal and silver chloride salt guarantees unique features for the electrode. This mixed material is characterised by a fixed and specific electrode potential and by a low electrode/skin contact impedance. Therefore, it is perfect for recordings of low levels of AC and DC potentials and shows lower noises in low frequency in comparison with gold and pure silver. The layer of chloride silver that covers silver can be achieved through a chemical or electrolytic process. Both treatments make the electrode to get a characteristic dark grey colouring. Being of a least thickness, this external coat can deteriorate in the long term if high-saline content pastes and gel are used and if they are not removed at the end of use conveniently. Moreover, it is necessary to pay a particular attention to external mechanical abrasions that could take the treatment away. In case of a continuous employment, a treatment of re-chlorinating is needed when you notice the surface of the electrode has lost its original colour.

Sintered silver/silver chloride (Sint Ag/AgCl): it has the same electrical features of Ag/AgCl as it is formed by a mixture of Ag and AgCl very thin powders that are compacted through the sintering process. Therefore, you get an electrode of uniform thickness characterised by a greater stability of electrical features in the long term compared to an average Ag/AgCl. With regard to maintenance, this type of electrode does not need to be re-chlorinated but only to remove the remains of gel and conductive paste carefully.

Gold (Au): it is a transition, pliable, biocompatible metal used also for invasive employment. It is characterised by an excellent electrical conductivity, by a medium/low electrode potential and by the fact it is resistant to most chemical compounds. These features make it perfect for an employment with strong saline-based electrolytes and adhesives or diluents of a various nature. It is often employed as an external coat in the production of cup electrodes to be used in long terms monitoring or for sleep studies, as adhesives and diluents strong like collodion and acetone are applied with the electrode. Owing to a high biocompatibility, it is often used for invasive electrodes for recording of both potentials and electric stimulation.

Tin (Sn): it is a post-transition, pliable and resistant to metal corrosion. It is characterised by a good electrical conductivity, low level of electrode potential and a low level of innate noise. It is the material with the best value/quality relationship in the production of electrodes, and historically it is used in the production of head caps with prewired electrodes.

Carbon/graphite (C): it is a non-metal material with a good electrical conductivity; it has a very low level of electrode potential and shows a low level of noise similar to Ag/AgCL. Moreover, it has the quality to be radium-opaque and non-magnetic. Often, the graphite powder is inserted into plastics and paints to be employed in the production of electrodes. This solution makes the electrical features of this material worse but makes it easily usable in manufacturing systems.

Steel (alloys AISI 304,316): biocompatible, of high hardness and elastic alloy, characterised by a remarkable resistance to corrosion in water and in the air. It gives a medium electrical conductivity. Owing to mechanical and ductile features, it is used in the production of medical needles. Given its wide employment in the medical field, it guarantees the best quality/price relationship.

Platinum (Pt): it is a transition, pliable metal; it has a strong resistance to corrosion and is biocompatible. Usually it is used in alloy with iridium to improve its mechanical resistance. It is recognised for its good electrical conductivity and is employed for cortical electrodes and needles. It is a non-magnetic material.

Titanium (Gr. 5): it is a light and resistant metal, both mechanically and chemically. It is a biocompatible material and customarily is employed in prostheses. As far as the electrical characteristics, they are similar to tins. It is a non-magnetic material and for this feature it is used for MRI compatible products, especially needle electrodes, owing to its physical characteristics like steel. Given its high level of biocompatibility and resistance, it is also used for long-duration applied electrodes and implantable electrodes.