CHARACTERIZATION AND OPTIMIZATION OF MICROELECTRODE ARRAYS FOR IN-VIVO NERVE SIGNAL RECORDING AND STIMULATION

Revealing the complex signal-processing mechanisms and interconnection patterns of the nervous system has long been an intriguing puzzle. As a contribution to its understanding the optimization of the impedance behavior of implantable electrode arrays with via holes is discussed here. Peripheral axons will regenerate through these holes allowing fo r simultaneous nerve stimulation and signal recording. This approach i s part of the ESPRIT project INTER and may eventually lead to devices driving sensory motor prosthesis with closed loop control. In the firs t set of experiments, micromachined platinum electrode arrays were pre pared, characterized and optimized for nerve signal recording. The res ults of these studies are based on impedance spectroscopy and microsco pic techniques. Equivalent circuits were modeled describing formally t he electrical response behavior with ohmic resistances between 500 Ome ga and 10k Omega. To attain low impedances for all electrodes on the I NTER device, platinum from H2PtCl6 was electrodeposited, and sputter t echnology as well as electrochemical deposition from H2IrCl(6) solutio n were used to produce thin iridium films. For the former, a lift-off process was established at one of the institutes to generate electrode structures with a line width of 5 mu m. As a result in all three case s the electrodes showed almost constant impedances over the entire fre quency range (10 Hz-1 kHz), which is relevant for nerve signal recordi ng. In the second set of experiments, electrodes were optimized to all ow for nerve stimulation. For this purpose, the charge delivery capaci ty (CDC) had to be increased and the impedance had to be decreased. Ir idium oxide is the material of choice, because its CDC is much higher than the CDC of platinum at 75 mu C/cm(2) (Ziaie et al., 1991, IEEE Se nsors & Actuators Transducers, 6, 124-127). A significant increase of the electrochemically active surface of the electrode structures could be observed by measuring the surface roughness. In first experiments, an activated iridium oxide film was formed with cyclic voltammetry an d was evaluated using scanning force microscopy and impedance spectros copy. The evaluation of the cyclic voltammograms showed a CDC up to 40 0 mC/cm(2) for sputter deposited and oxidatively treated iridium films . Further investigations are directed towards increasing the stability of the iridium oxide electrodes with regard to long-term implants. Pa rallel experiments aim at the controlled axon adhesion without changin g the impedance behavior of the described electrodes. (C) 1997 Elsevie r Science Limited.