PhD Thesis Defense: Yieljae Shin

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"Strategies for scaling up thin-film microfabricated electrode arrays"

Abstract

Recording from large populations of neurons at cellular resolution is central to neuroscience and to brain–computer interfaces. Polymer-based thin-film electrode arrays are well suited to this goal, because they can be microfabricated at scale with cellular-size electrodes and are thin and flexible enough to approach the mechanics of neural tissue. Scaling these arrays from tens to thousands of channels, however, is not simply a matter of adding electrodes: each increase in channel count exposes a limit at a different level of the device. This thesis treats scaling as a multi-level problem and develops one strategy for each of four levels, volumetric, interconnect, electrochemical and mechanical, on a common platform.

At the volumetric level, planar fabrication confines electrodes to two dimensions. The rolling-of-soft-electronics (ROSE) approach converts a planar array of flexible shanks into a monolithic three-dimensional probe in a deterministic way, preserving its electrical performance and enabling high-yield volumetric recordings in rodents and a nonhuman primate. At the interconnect level, a passive array requires a dedicated trace and connector pad for every electrode, so the interconnect footprint grows with channel count. For NeuroCROWN, an actively multiplexed probe built on foundry complementary metal–oxide–semiconductor (CMOS) circuitry, this thesis developed the post-fabrication and transfer-printing process that turns a rigid chip into a flexible, implantable shank, which recorded depth-dependent auditory-evoked potentials in vivo. At the electrochemical level, coating many small electrodes uniformly becomes increasingly difficult. Potentiostatic electroplating of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) holds every electrode at the same potential and produces array-wide uniform, low-impedance coatings. At the mechanical level, shanks must be thin to limit tissue damage, but thinning a shank makes it harder to insert. Validated on single polyimide shanks, local sharpening and tungsten reinforcement of the tip roughly halved the force needed to puncture a brain phantom without stiffening the rest of the shank.

Together, these strategies show that the limits that arise at each level of scaling can be addressed with processes compatible with conventional microfabrication, providing a path toward very-large-scale, flexible three-dimensional neural interfaces.

Thesis Committee

• Hui Fang (chair)
• William Scheideler
• Alexander Boys
• Jonathan Viventi (Duke University)

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu .