Development of Multi-Electrode Neural Probe on Optical Fiber Substrate for Brain-Machine Interfaces
Tisa, Tamanna Afrin
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Brain-machine interfaces (BMIs) aim to restore communication and control of prosthetic devices to individuals with neurological injury or disease, by recording the neural activity, and mapping or decoding it in to a motor command. One of the great challenges in this effort is to develop reliable neural probes that are capable of processing the activity of large ensembles of cortical neurons. In this dissertation, we reported a method for fabricating highly reliable neural probes with integrated, thin film conductor and dielectric coatings on the cylindrical surface of fine optical fibers for brain-machine interfacing. The use of optical fibers as probe substrates provide the strength and stiffness required for deep-brain applications, as well as the high intensity, multi-spectral light delivery with essentially no coupling loss is useful for optogenetics application in neuroscience. Early probes were fabricated on 65 µm optical fiber substrates with polyimide jackets. Electrodes were defined over this jacket, and high quality in-vivo recordings were acquired in area V1 of the Greater Northern Galago (Galago garnetti). Microscopic examination of the probes after extraction from the brain, showed that the jacket had cracked and delaminated; the glass itself may have cracked. Invariably this happened near the probe tip, suggesting that micro-cracking of the unprotected fiber end was the cause of the problem. So, a new jacket of cross-linked plasma-deposited styrene was developed. This layer was impervious to water vapor, as well as hot acids and bases. Single channel probes fabricated with this jacket survived a battery of reliability tests, including continuous soaking in PBS for 30 days, multiple insertions in agar gel and cannulas, disinfection, and marinating overnight in a whole mouse brain in the Dragoi lab. Moreover, test-to-test and lot-to-lot variation of the 2 kHz impedance was less than 1 % (3). High quality, in-vitro spike recordings were acquired in a living mouse brain slice at the Dragoi lab. Thus, reliability of the contact fabrication process has been established. In this dissertation, we also reported the extension of the technology that we developed for single channel prototypes to probes with a large number (>30) of micrometer-scale contacts that are needed to map laminar circuits in the brain. For fabricating those multi-electrode neural probes, significant advancement in alignment technology was required. The near-atomic straightness of fiber holder and accurate registration of the mask pattern with the V-grooves ensures that the printed pattern will be centered on the bottom of the fiber. The overlay of patterns on the fiber was ensured, a) longitudinally by using fiber stops, high precision ball bearings which were hold to lithographically defined pits at the tip-end of each V-groove, b) rotationally by using a high precision cubic bead glued to the end of the fiber as the reference. SEM images showed that longitudinal and lateral pattern overlay error was always below 2 µm without any outliers.