Among various studies pursuing multi-site recording from brain circuits, planar microelectrode arrays have been shown to be appropriate for recording responses of dissociated cultured neurons obtained from fetal and neonatal rat brains (Pine, 1980; Gross et al., 1982)*. However, it was recognized that the networks of dissociated cultured neurons might respond differently from the neural organization of intact brains.

Pine, J., J. Neurosci. Meth., 1980; 2, 19-31

Gross, G. W., Williams, A. N. and Lucas, J. M., J. Neurosci. Meth., 1982; 5: 13-22.

Application of planar microelectrode arrays to acute brain tissue was reported by Novak and Wheeler in 1988, where they showed that their array could both stimulate and record from multiple sites from hippocampal slice preparations. Although their work definitely showed that planar microelectrode arrays could be useful for studies of brain tissue, stimulation by their array electrodes seemed somewhat ineffective, and their recordings were limited to 2-4 hours.

Novak, J. L. and Wheeler, B. C., J. Neurosci. Meth., 1988; 23: 149-159

1. planar electrodes with a large area; and
2. the coating on the surface of the array.

An array of 64 planar microelectrodes was developed, each having a size of 50 x 50 mm, arranged in an 8 by 8 pattern (the MED probe, USP 5,810,725). The material of the insulating layer of the MED probe, which directly contacts the brain tissue, was carefully chosen. The coating material and method of preparation for the surface of the MED probe were also studied. These technologies, together with electronics designed exclusively for the MED probe (namely, the MED system, USP 5,563,067), enabled multi-channel recordings of field EPSPs induced by stimulation of array electrodes for more than six hours (Oka et al.).