The idea of having the first amplifier stage integrated with the electrode probably goes back as far as the introduction of the transistor. Over the past years, many researchers have described the realization of prototypes, and in most cases these prototypes indeed offered the advantages in signal quality that were promised by interference theory. However, there has never been the breakthrough to a real commercially available product.

When we started with the development of active electrodes, we initially approached the problem very much like other groups before us. That is, we concentrated on solving electronic problems. This resulted in a series of 100 prototypes, which worked very nice. Only then started the most difficult part of the development path: turning this prototype into a product that people actually wanted to buy.

In order to persuade the rather conventional EEG world to switch to active electrodes, the advantage of eliminating interference and skin preparation was not enough. A first problem was that people were not prepared to pay much more for the active electrodes than they were used to pay for the current passive electrodes. This meant that we had to reduce the production costs. Recent progress in SMD production technologies, allowed us to use cheap mass-production techniques instead of the initially used thick-film technology. A second important point was that many potential users wanted to explore the new features of the active electrodes especially in high density (64 to 128) EEG measurements. This meant that the active electrodes had to be made suitable for use in an electrode cap.

The breakthrough in making the active electrode prototype suitable for use in caps came when we teamed up with Dr. Peter Praamstra, (Behavioral Brain Sciences Center), University of Birmingham, GB. Based on his experience, the active electrode with pinned housing was developed.

In the setup that evolved from the cooperation with Dr. Praamstra, the headcap consists of a cap with plastic, holed buttons. The cap itself does not contain electrodes. The cap is placed on the subject's head, and the cap buttons are filled with electrode gel with a syringe. Because of the active electrode setup high electrode impedance's can be tolerated, so no skin preparation is required. Thus, the usual, unpleasant scratching of the scalp after the paste is applied can be omitted. After the buttons are filled with paste, up to 128 active electrodes are plugged in the cap one by one. The procedure is fast and reliable, and allows high density EEG measurement with a minimum of preparation time and subject discomfort.

	BioSemi's active version of the pin electrodes for use in high density EEG caps