Large-area microelectrode arrays for recording of neural signals

To understand the neural code, that the retina uses to communicate the visual scene to the brain, large-area microelectrode arrays are needed to record retinal signals simultaneously from many recording sites. This will give a valuable insight into how large biological neural networks (such as the brain) process information, and may also be important in the development of a retinal prosthesis as a potential cure for some forms of blindness. We have used the transparent conductor indium tin oxide to fabricated electrode arrays with approximately 500 electrodes spaced at 60 /spl mu/m. The fabrication procedures include photolithography, electron-beam lithography, chemical etching and reactive-ion etching. These arrays have been tested electrically using impedance measurements over the range of frequencies important when recording extracellular action potentials (0.1-100kHz). The data has been compared to a circuit model of the electrode/electrolyte interface. One type of array (512 electrodes) behaves as theory would dictate and exhibits an impedance of 200 k/spl Omega/ at 1kHz. The other array (519 electrodes) has an impedance of 350 k/spl Omega/ at this frequency, which is higher than predicted by the models. This can perhaps be attributed to the difference in fabrication techniques. The 512-electrode array has been coupled to low-noise amplification circuitry and has recorded signals from a variety of retinal tissues. Example in vitro recordings are shown here.