Design and fabrication of novel multi-channel floating neural probes for intracortical chronic recording

This paper presents a novel design approach and production process flow for the fabrication of multi-channel silicon probes with highly flexible ribbon cables integrated monolithically on wafer level. The integrated flexible cables are based on the biocompatible polymer polyimide BPDA-PPD. Compared to state-of-the-art silicon probes, this novel design allows a floating chronic implantation of the probes into the cortex with reduced mechanical coupling to the skull. The electrode area is additionally coated with the electrical conductive and biocompatible polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to reduce the electrode interface impedance. In-vitro electrical impedance spectroscopy measurements demonstrate the functionality of these novel probes and verify a significant reduction of the electrode impedance using PEDOT:PSS coating. For precise and complete insertion of these neural probes in the cortex an insertion tool was developed, which can be attached to a variety of different electrode drives. Using this tool the silicon probes were successfully implanted into rat motor cortex. Prior to implantation the probes were steam autoclaved at 121 °C, 1 bar for 20.5 min for sterilization. Electrophysiological recordings as well as in-vivo electrical impedance spectroscopy supported the results of the in-vitro functional tests. After an initial increase of electrical impedance in the course of the first week after implantation the average impedance of the electrodes settled for the following weeks. Recordings of single- and multi-unit activity along with the local field potential were successful for more than 7 weeks after implantation and are still continuing. These current results indicate an appropriate chronic use of the presented neural probes.