Silicon Carbide and MRI: Towards Developing a MRI Safe Neural Interface

Abstract

An essential method to investigate neuromodulation effects of an invasive neural interface (INI) is magnetic resonance imaging (MRI). Presently, MRI imaging of patients with neural implants is highly restricted in high field MRI (e.g., 3 T and higher) due to patient safety concerns. This results in lower resolution MRI images and, consequently, degrades the efficacy of MRI imaging for diagnostic purposes in these patients. Cubic silicon carbide (3C-SiC) is a biocompatible wide-band-gap semiconductor with a high thermal conductivity and magnetic susceptibility compatible with brain tissue. It also has modifiable electrical conductivity through doping level control. These properties can improve the MRI compliance of 3C-SiC INIs, specifically in high field MRI scanning. In this work, the MRI compliance of epitaxial SiC films grown on various Si wafers, used to implement a monolithic neural implant (all-SiC), was studied. Via finite element method (FEM) and Fourier-based simulations, the specific absorption rate (SAR), induced heating, and image artifacts caused by the portion of the implant within a brain tissue phantom located in a 7 T small animal MRI machine were estimated and measured. The specific goal was to compare implant materials; thus, the effect of leads outside the tissue was not considered. The results of the simulations were validated via phantom experiments in the same 7 T MRI system. The simulation and experimental results revealed that free-standing 3C-SiC films had little to no image artifacts compared to silicon and platinum reference materials inside the MRI at 7 T. In addition, FEM simulations predicted an ~30% SAR reduction for 3C-SiC compared to Pt. These initial simulations and experiments indicate an all-SiC INI may effectively reduce MRI induced heating and image artifacts in high field MRI. In order to evaluate the MRI safety of a closed-loop, fully functional all-SiC INI as per ISO/TS 10974:2018 standard, additional research and development is being conducted and will be reported at a later date.

Concept of a fully-implantable system to monitor tumor recurrence

“…The concept of implantable monitoring, as it can only detect cancer in its immediate vicinity, will not obviate the need for MRI examinations that can also discover distant recurrence. On the design side, it is, therefore, crucial to ensure that potential sources of interference such as metallic parts are minimized or that new MRI-compatible materials 56 are incorporated. Moreover, it is essential to evaluate various established metal artifact reduction techniques in subsequent development phases and formulate strategies for obtaining MRI scans devoid of artifacts.…”

Silicon Carbide Technology for Advanced Human Healthcare Applications

“…Figure 17 shows one example of device failure observed in vivo which is representative of typical implant issues encountered with Si-based implants. Fortunately, numerous instances of SiC being used as a robust material for implantable neural implant (INI) applications have been reported [50][51][52][53][54]. In the USF SiC group, we have been working on this challenge for nearly two decades and have developed several possible solutions to address long-term in vivo INI issues: an all-SiC monolithic MEA constructed using 4H-SiC [51], an alternate device based on 3C-SiC on either bulk Si or SOI substrates [52], and, most recently, an ultra-thin interface using a-SiC as the base and capping insulation, which sandwiches a carbon electrode created using pyrolyzed photoresistant film (PPF) [53].…”

“…In the USF SiC group, we have been working on this challenge for nearly two decades and have developed several possible solutions to address long-term in vivo INI issues: an all-SiC monolithic MEA constructed using 4H-SiC [ 51 ], an alternate device based on 3C-SiC on either bulk Si or SOI substrates [ 52 ], and, most recently, an ultra-thin interface using a -SiC as the base and capping insulation, which sandwiches a carbon electrode created using pyrolyzed photo-resistant film (PPF) [ 53 ]. Not only has excellent electrical performance been demonstrated in PBS, but in the case of the 3C-SiC all-SiC interface on SOI, excellent MRI compatibility has been observed in a 7T animal-bore MRI system at the Moffitt Cancer Center [ 54 ]. Details of this research is included in [ 1 ].…”

“…( a ) CV performance comparison of an all-SiC (4H-SiC polytype) MEA showing superior performance to Pt electrodes of the same form-fit [ 51 ]. ( b ) Free-standing all-SiC (3C-SiC on SOI) 16-channel functional INI used to assess the MRI compatibility [ 54 ]. ( c ) Sagittal (left) and coronal (right) MRI images @7T showing no image artifacts and nearly transparent performance of 3C-SiC in a brain-tissue phantom.…”

A Flexible a-SiC-Based Neural Interface Utilizing Pyrolyzed-Photoresist Film (C) Active Sites

“…Furthermore, a -SiC has demonstrated low leakage current (≤10 pA) after the application of ±5 V potential bias [ 15 ]. Unlike many other insulating materials used in mINP, a -SiC possesses nearly complete chemical inertness, protecting it from oxidative species and ionic diffusion [ 15 , 16 , 17 , 18 ].…”