Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Stephen E. Saddow, Ph.D.

Co-Major Professor

Gokhan Mumcu, Ph.D.

Committee Member

Robert D. Frisina, Ph.D.

Committee Member

Arash Takshi, Ph.D.

Committee Member

Christopher L. Frewin, Ph.D.

Keywords

Brain machine interface, Magnetic resonance Compatibility, Michigan style neural probe, Monolithic silicon carbide

Abstract

One of the main issues with micron-sized intracortical neural interfaces (INIs) is their long-term reliability, with one major factor stemming from device material failure caused by the heterogeneous integration of multiple materials used to realize the implant. Single crystalline cubic silicon carbide (3C-SiC) is a semiconductor material that has been long recognized for its mechanical robustness and chemical inertness. It has the benefit of demonstrated biocompatibility, which makes it a promising candidate for chronically-stable, implantable INIs. In the first section of this dissertation, the fabrication and initial electrochemical characterization of a monolithic, Michigan-style 3C-SiC microelectrode array (MEA) probe is reported. The probe consists of a single 5 mm-long shank with 16 electrode sites. An ~8 μm-thick p-type 3C-SiC epilayer was grown on a silicon-on-insulator (SOI) wafer, which was followed by a ~2 μm-thick epilayer of heavily n-type (n+) doped 3C-SiC in order to form conductive traces and the electrode sites. Diodes formed between the p and n+ layers provided substrate isolation between the channels. A thin layer of amorphous silicon carbide (a-SiC) was deposited via plasma-enhanced chemical vapor deposition (PECVD) to insulate the surface of the probe from the external environment. Forming the probes on a SOI wafer enable the ease of probe removal from the handle wafer by simple immersion in HF, thus aiding in the manufacturability of the probes. Free-standing probes and planar, single-ended test microelectrodes, were fabricated from the same 3C-SiC epiwafers. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on test microelectrodes with an area of 491 μm^2 in phosphate buffered saline (PBS) solution. The measurements showed an impedance magnitude of 165 kΩ ± 14.7 kΩ (mean ± standard deviation) at 1 kHz, anodic charge storage capacity (CSC) of 15.4 ± 1.46 mC/cm^2, and a cathodic CSC of 15.2 ± 1.03 mC/cm^2. Current-voltage tests were conducted to characterize the p-n diode, n-p-n junction isolation, and related leakage currents. The turn-on voltage was determined to be on the order of ~1.4 V and the leakage current was less than 8 μArms. This all-SiC neural probe realizes nearly the monolithic integration of device components to provide a likely neurocompatible INI that should mitigate long-term reliability issues associated with chronic implantation.

The second section of this dissertation focuses on evaluation of magnetic resonance imaging (MRI) compatibility of the 3C-SiC based neural implants. This was performed using various finite element method (FEM) and Fourier-based method simulations, and multiple experiments under 7 T MRI excitation in a small-animal Bruker system at the Moffit Cancer Center. The MRI compatibility in the current work mainly focuses on static magnetic field (B0) perturbation induced by the magnetic susceptibility differences between the probe and tissue phantom. In addition, induced tissue heating and specific absorption rate (SAR) distribution caused by electromagnetic coupling between the radio frequency (RF) field inside the MRI and conductive parts of samples fabricated were evaluated. Hereby, a comparative study with respect to MRI induced B0 perturbation, which are revealed via image artifacts, and MRI induced heating was presented to compare the MRI compatibility of 3C-SiC neural implants to reference materials fabricated from platinum (Pt) and silicon (Si). The results of the Fourier-based simulations to predict image artifacts and the experimental results show that free-standing 3C-SiC probes released from SOI handle wafers are completely invisible inside the MRI at 7 T while the reference materials produced severe image artifacts. In addition, FEM simulations show an ~30% maximum SAR reduction for 3C-SiC material comparing to Pt. These results are validated via experimental measurements and appear to indicate that fully-functional all-SiC INI devices may be MRI compatible thus allowing for MRI as a diagnostic means to assess neural functionality without the need to first remove the INI.

Download

Share

COinS