Planar electrodes are suitable for monitoring fluctuation in the extracellular field potential produced by small groups of neurons when it is difficult to record and stimulate single neurons. In addition to the microwell-based approach of physically restraining connectivity of neurons with planar electrodes [23,65], an interesting line of development exploits the ability of microchannels to conduct electrical signals to achieve single cell patch recording. PDMS electrodes with modified surface chemistry and a subterranean microfluidic channel are capable of forming high-resistance seals with cell membranes, and have been used as a basis for planar patch clamp systems [66]. Recently, this approach was combined with MEA to create a system that allows simultaneous probing of individual neurons and high-resolution MEA recording at multiple sites in synaptically connected neuronal networks [67]. Further miniaturization of channel arrays will allow more precise control of the extracellular space as well as membrane potential at the sub-compartment level [68]. Moreover, advances in microfabrication technologies have enabled the creation of electrode arrays that can be used to simultaneously observe the firing of multiple cells, but the problems of contamination and cells escaping from the position of each electrode remain and often occur in long-term cultivation.
