CMOS-compatible, Flexible, Intracortical Neural Probes
Flexible intracortical neural probes elicit a lower foreign body response than rigid implants. However, by incorporating complementary metal-oxide-semiconductor (CMOS) circuitry, silicon-based neural probes can offer an improved scalability and more functionalities than any other available technology. Here, we report on the development of a novel hybrid neural probe that combines the flexibility of polymeric substrates with the functionalities of active CMOS-based probes. This is achieved by interfacing CMOS-based probe tips of only a few millimeters in length with flexible polyimide ribbon cables. The slender probe design enables the complete implantation of the probe tip deeply into brain tissue using stiff insertion shuttles with a thickness as low as 50 µm, as verified in brain models. The multilayer platinum metallization of the cables is patterned using a novel combination of ion beam and oxygen plasma etching. It allowed to reliably pattern cables with signal lines of 2 μm in width and 3 μm in spacing. The additional oxygen plasma treatment had to be implemented to re-establish the high resistances between adjacent metal lines and recover the chemical composition of the polyimide surface, which has been deteriorated by the ion beam etching. We assembled neural probes of this design from probe tips as short as 1.5 mm and down to 100 µm in width. As an example, 100-µm-wide probes feature electrode arrays with 72 recording sites and multiplexing to 16 parallel output lines. Probes were successfully implanted to a depth of 7 mm using insertion shuttles and withstood forces of 63 mN. The synergetic approach of this new generation of neural probes surpasses the limitation of each individual probe technology and should be considered in future developments.