A MEMS-based flexible multichannel ECoG-electrode array.

We present a micromachined 252-channel ECoG (electrocorticogram)-electrode array, which is made of a thin polyimide foil substrate enclosing sputtered platinum electrode sites and conductor paths. The array subtends an area of approximately 35 mm by 60 mm and is designed to cover large parts of a hemisphere of a macaque monkey's cortex. Eight omnetics connectors are directly soldered to the foil. This leads to a compact assembly size which enables a chronic implantation of the array and allows free movements of the animal between the recording sessions. The electrode sites are 1 mm in diameter and were characterized by electrochemical impedance spectroscopy. At 1 kHz, the electrode impedances vary between 1.5 kOmega and 5 kOmega. The yield of functioning electrodes in three assembled devices is 99.5%. After implantation of a device with 100% working electrodes, standard electrocorticographic signals can be obtained from every electrode. The response to visual stimuli can be measured with electrodes lying on the visual cortex. After an implantation time of 4.5 months, all electrodes are still working and no decline in signal quality could be observed.