Micro-/nanoelectromechanical electromechanical system (M/NEMS) is the technology of microscopic devices, particularly those with moving parts. M/NEMS technologies have become ubiquitous in many areas such as health care, energy, and transportation, they remain largely limited to ambient conditions. Our research aims to expand the application of M/NEMS to harsh environments such as high temperature, high pressure, high radiation, and corrosive. Sensing within these harsh environments would enable real-time monitoring of sub-surface environments, combustion and critical components. This effort is largely based on silicon carbide, a wide bandgap, physicochemically stable and biocompatible semiconductor.
Recently Completed Projects
Nano-materials for High Temperature Energy Storage
Supercapacitors that can withstand harsh environments such as high temperature (i.e., >300 ° C) have received interest due to their relevance for space, military, and electric vehicle applications. This motivation has sparked the search for suitable active materials and electrolytes that can work stably and reliably at high temperatures. Silicon carbide is known to be stable in many harsh physicochemical environments, including high-temperature oxidizing environments. Yttria-stabilized Zirconia (YSZ) has a high ionic conductivity at T > 400°C and is hence a promising solid electrolyte for high temperature energy storage. We investigated the use of YSZ in conjunction with SiC nanowires for the development of high-temperature stable supercapacitors. Good cycling stability was demonstrated with a capacitance retention of over 60% after 10,000 cycles at the operation temperature of 450 °C.
Silicon Carbide ECoGs for Chronic Implants in Brain-Machine Interfaces
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. 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.