Dry and Dielectric Contact

Putting a dry metal disk on dry human skin results initially in a quick step admittance increase. It corresponds to the stratum comeum as a dielectric between the two plates 

Invasive electrodes usually have metal in direct contact with the tissue. The metal must then be biocompatible for the duration of tissue contact. The electrical properties will be dependent on the metal and the tissue. 

A special ionic contact medium, to skin or any tissue surface, is a gas plasma. Plasma is an ionized, and therefore conductive, gas. The ionization process is high electric field—induced, and the process is enhanced in inert gas atmospheres such as argon. 

In its simplest form, an arc may be drawn from a needle electrode to the skin surface. The arc will seek high conductive spots like the sweat duct orifices. The arc may be DC or AC driven, and in air about 2000 V/mm distance is necessary for ignition with a steady state current in the microampere range. 

A single spark is the usual contact medium with electrostatic discharges in daily life situations (Section 10.16). 

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