tim's journal

High Impedance Electrode Techniques

This entry summarizes important points from the James Roman article discussing the NASA spray-on electrodes.

Introduction
James Roman in [1] describes, in particular, the electronic techniques used in NASA's spray-on dry electrode [2]. The last point in the introduction is most important, that the "acceptance of high impedance allows the use of small, or dry, electrodes." These electrodes were designed for electrocardiography and impedance pneumography.

The Requirement
The spray-on electrodes were tested at the USAF Aerospace Research Pilot School at Edwards Air Force Base in California. The requirements for the project were quite stringent, and serve as a good reference for dry EEG. They were as follows:

1. Neither the electrodes nor the electrode wire should be felt by the subject.
2. Electrodes must be resistant to motion artifact.
3. Skin irritation must not result from frequent application of the electrodes.
4. Shaving must not be a part of the procedure of applyign electrodes.
5. Application of an electrode should take less than 30 seconds.

The NASA Flight Research Center Electrode
Summarized earlier, the NASA spray-on electrode consists of a layer of conductive glue applied by a spray gun or aerosol package. The site is prepared in 3 seconds using an oscillating toothbrush saturated in electrode paste. A thin, non-shielded wire is captured in the glue during application. Finally, an insulating glue is sprayed over the electrode. The final electrode is approximately twenty-thousandths of an inch thick and three-fourths of an inch in diamater.

Impressively, these sensors were tested for over 700 hours in-flight, and 500 hours on the ground over hundreds of patients with no cases of skin irritation, folliculitis, or contact dermatitis. Roman claims that the tracings obtained were either equal or superior to those obtained with more conventional electrodes but does not provide hard analysis for this.

The electrodes are said the require ampliifiers with input impedance in excess of 2 MΩ. This seems quite low, though at the time it was considered quite high. At lower values of input impedance there was increased attenuation of lower-ferquency components. Roman claims that at 100,000 Ω distorition was "barely detectable without a comparison record". This comparison was made using signal averaging - though the number of signals used was not reported. Why would they have low frequency attenuation with insufficient input impedance? There must be a high-pass filter being implmented. And there is. There must be a capacitor between the electrode and the amplifier which acts as a high-pass filter. At low frequencies, the impedance of this capacitor is very large. If the input impedance is too low, most of the voltage drop occurs through this capacitor so we see that the low-frequencies will be attenuated. This is a good lesson.

Electrical Characteristics of Electrodes
The NASA dry-electrodes were speced to operate between 0.1 to 100 Hz. This is exactly what we are looking for in the dry EEG sensors. The model used for the skin-electrode combination was a parallel RC. Using this, Roman computed the change in gain over frequency for a given input impedance. He found that there were two plateaus. The model has two plateaus for the following reason: at low frequencies, the capacitor is effectively open, so the resistor R forms the voltage divider with the input impedance. At high frequencies, the capacitor acts like a short, and there is no voltage divider, so the gain is 1. These are the two plateaus. As long as the resistor R is small compared with the input impedance, the difference in level for the two plateaus will be minimal. This, in fact, is the traditional approach. One could argue that if you knew a priori what the values would be you could just correct it in software.

"The resistive component of an electrode is a function primarily of skin preparation and only secondarily of electrode size." The electrode paste under the dry electrode cuts the resistive component of the electrode impedance in half whether the thin film is dry or still wet. As the film is drying, resistance for the first hour is higher than right when the spray is applied. After one hour, this difference disappears. Electrodes with different types of conductive paste show similar capacitance but different resistance. With "vigorous preparation", Roman and his collaborators were able to obtain resistive components of less than 1 kΩ with electrodes 1/8 inch or less in diameter. They gave some important caveats. For instance, "variation between subjects for similar electrodes applied by similar techniques can easily be of the order of 500 percent." Also, as we already know, the location also is a factor in resistance.

For the spray electrodes, "parallel capacitance appears to be primarily a function of electrode size... and is liniearly related to electrode area." This is good to know. Skin preparation played a secondary role in determining capacitance.

Partially addressing a concern that I have had, Roman found that the distance between the electrodes had little effect on the parallel RC. He gives typical RC values for the electrodes in Table I, which I replicate below. Unfortunately, no details were given as to the dimensions of the electrodes.

The last section discusses motion artifacts. Roman admits that "it is not known how the circuit parameters associated with electrodes operate to produce motion artifact." I suspect that this has to do with the skin potential effect. As the skin gets squeezed down, ions flow and there is a momentary disruption and recovery which causes the artifact. They found that electrodes which are more rigid show less artifact. Direct tapping would cause an artifact, however, having a plastic shield avoided this. They did conclude that change in impedance due to skin or electrode distortion was not responsible since using different shunt resistances did not alter the ratio of artifact amplitude to signal amplitude. Thus the artifact must be some sort of low source impedance ac generator.

References
[1] J. Roman, Flight Research Program III - High-Impedance Electrode Techniques, Nasa Technical Note D-3414 Supplement, National Aeronautics and Space Administration, Washington, D.C., June 1966. Preprint of article published in Aerospace Medicine, August 1966.
[2] C. W. Patten, F. B. Ramme, J. Roman, Dry Electrodes for Physiological Monitoring, Nasa Technical Note NASA TN D-3414, National Aeronautics and Space Administration, Washington, D.C., May 1966.