Several technologies have been developed for interfacing with the brain such as microwires, electrode arrays, and electrocorticography (ECoG) arrays. While each of them has strengths and weaknesses, they all share a common disadvantage of limited device longevity due to a variety of failure modes; these include scar tissue formation and material failure, among others. A particularly pronounced problem is the failure of the insulating material at the insulator-conductor interfaces (e.g. recording sites and insulated conducting traces). Damage to these vital interfaces compromises device performance by altering the impedance of recording sites, or more deleterious, results in total device failure due to shorting between traces or between a trace and physiological fluid. To address these material issues, we have focused on the fabrication of silicon carbide (SiC) electrode arrays. As a surface coating, polycrystalline SiC has been shown to promote negligible immune glial response compared to bare silicon when implanted in the mouse brain. Additionally, due to its mechanical and chemical stability, SiC serves as stable platform and excellent diffusion barrier to molecules present in the physiological fluid. Moreover, and of particular interest to the neuroengineering community, the ability to deposit either insulating or conducting SiC films further enables SiC as a platform material for robust devices. Leveraging these unique properties, we have developed a fabrication process that integrates conducting and insulating SiC into 64-channel ECoG arrays. Recording sites 40 um in diameter are made of n- doped SiC while the insulating layers are either amorphous SiC or undoped polycrystalline SiC. To allow for low impedance interconnects, a metal stack of titanium/gold/titanium or a titanium/platinum is completely embedded in between layers of SiC. The result is an ECoG array that, to the physiological fluid, appears simply as a single SiC sheet wherein boundaries between conducting and insulating layers are seamless. The inner metal layer is well protected by SiC and therefore cannot be reached by molecules present in the physiological fluid. We believe this basic platform can be extended to a variety of electrophysiological devices, including penetrating probes of various geometries, and help mitigate the failure modes of the present technologies.
Project end date: 06/07/17