An active microelectrode array to detect extracellular nervous activity

Jobling, David Trevor (1980) An active microelectrode array to detect extracellular nervous activity. University of Southampton, Doctoral Thesis.

Abstract

The detection of extracellular potentials in nervous tissue using conventional methods has many limitations. In particular, it is extremely difficult to detect potentials simultaneously at more than two or three sites on one tissue preparation, and elaborate precautions are needed to reduce electrostatic interference. This thesis describes the development of an active array of nine microelectrodes with buffer amplifiers for detecting extracellular potentials. Integrated circuit technology has been used, enabling the microelectrodes and buffer amplifiers to be made on the same silicon substrate at different stages of one fabrication process. The characteristics of the interface of the evaporated gold microelectrodes and physiological saline were measured, and it was found that the interface impedance was higher than that reported for other gold electrodes. A variety of surface coatings to insulate and passivate the integrated circuit buffer amplifiers were investigated so that they could be operated submerged in saline. Connections to the buffer amplifiers were made using low resistance diffused layers coated with thermally grown silicon dioxide. Photoresist was used to insulate other parts of the circuit. The integrated buffer amplifiers were formed by using MS transistors in the source follower configuration. Each microelectrode was part of the gate electrode of an underlying transistor. The micrcroectrode array was used with off-chip voltage amplifiers and a multiplexer to permit a simultaneous display of the potentials at each electrode. In experiments with nervous tissue, results which would have been almost impossible to obtain with conventional electrodes were achieved. Experiments showed that the threshold voltages of the M S transistors in the array changed slightly during their operation in saline, but the devices performed satisfactorily throughout several hundred hours of continuous operation.

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