Electrode Contact

Impedance

Bioelectric signals are electric signals that naturally occur in your subject. These signals can offer substantial information about your subject! However, most bioelectric signals are very small, and usually require substantial amounts of amplification to sense and record. For this reason, needle electrodes, in contact with live tissue, usually work the best for most subjects. This is because the contact impedance between each needle electrode and the subject's tissue is very low.

But for human subjects, needle electrodes are normally not acceptible. The only other alternative are skin surface electrodes, electrodes actually placed on the surface of the skin, at sites that hopefully optimize the specific bioelectric signal you are after. It is a fact of life that human skin is a good resistor, and electrodes quickly pulled from a pouch and slapped onto the skin will often supply a very high contact impedance. Electrode deficiencies can contribute to this problem as well. And the higher the contact impedance, the more suseptible the sensed signal will be to external interference and cable movement induced noise. For very small bioelectric signals, the added noise can easily swamp out and obscure the signal you are after.

The layer of dead skin cells on the skin surface is usually the main culptit where high contact impedance is encountered. As mentioned above, dry or otherwise marginal electrodes can also insert an extremely high contact impedance into the electrode connection. Many researchers agree that a contact impedance below 5,000 Ohms is ideal, but impedances above 20,000 Ohms will probably yield very poor data.

Experience has shown that getting good, clean signals with skin surface electrodes is just not easy. For human subjects, you can start by using good quality Ag-AgCl, wet gel column electrodes where possible. UFI can supply such electrodes; order our part number 1081DSP. High quality Ag-AgCl electrodes are available from other sources as well. Other types of electrodes can be used, but may not supply as good a quality of data. In some cases (EEG Caps, for example), you may not even have a choice.

There are a number of other ways that UFI can help with this important physiometry problem. For example, our 1092 BioBrade(R) are small non-reactive abrasive pads. These can be used to lightly abrade each electrode site, in an effort to reduce the thickness of the dead skin layer, which can help minimize the Contact Impedance.

Another very helpful contribution is the ability to carefully MEASURE what the Contact Impedance between two applied electrodes actually is! UFI designed the original 1089 "Checktrode(R)" Contact Impedance tester over 30 years ago, to allow researchers to quickly evaluate this crucial aspect of any subject connection. In addition to our current 1089 MkIII model, we have recently introduced our new, lower cost 1089e. Also, for arrays of electrodes, such as EEG Caps, our 1089-ES and 1089-NP Checktrodes supply Lead Selection capability to individually test the contact impedance for all electrodes in an EEG electrode cap.

The problem of a high contact impedance can also be addressed by the the signal conditioner. We designed our unique Fetrode(R) technology to supply a much "cleaner" bioelectric signal from electrodes with high contact impedance. The Fetrode is actually a very high input impedance buffer placed directly at electrode sites. All UFI Biologs include the Fetrodes as standard for channels configured for bioelectric signals. This technology is available for inclusion in your equipment as well. Let us know if you are interested in this solution.

If you still have questions, or if you don't see what you need, drop us a line, and we will see how we can help you!