Non-invasive on-skin sensors for brain machine interfaces with epitaxial graphene

Abstract

Brain-machine interfaces are key components for the development of hands-free, brain -controlled devices. Electroencephalogram (EEG) electrodes are particularly attractive for harvesting the neural signals in a non-invasive fashion. Here, we explore the use of epitaxial graphene grown on silicon carbide on silicon for detecting the electroencephalogram signals with high sensitivity. This dry and non-invasive approach exhibits a markedly improved skin contact impedance when benchmarked to commercial dry electrodes, as well as superior robustness, allowing prolonged and repeated use also in a highly saline environment. In addition, we report the newly -observed phenomenon of surface conditioning of the epitaxial graphene electrodes. The prolonged contact of the epitaxial graphene with the skin electrolytes functionalize the grain boundaries of the graphene, leading to the formation of a thin surface film of water through physisorption and consequently reducing its contact impedance by more than 75%. This effect is primed in highly saline environments, and could be also further tailored as pre-conditioning to enhance the performance and reliability of the epitaxial graphene sensors.

Signal Quality Investigation of a New Wearable Frontal Lobe EEG Device

“…The skin-electrode impedance of traditional dry electrodes is usually high. In recent years, dry electrodes made of new materials have been proven to effectively reduce the impedance of skin electrodes [53,54]. Semi-dry electrodes are a combination of wet and dry electrodes, retaining the advantages of wet and dry electrodes, but they are still in the early research stage [55,56].…”

Thin-Film Electrodes Based on Two-Dimensional Nanomaterials for Neural Interfaces

“…This sensor type, although rigid, possesses among the best reliability data for long-term, extended, and repeated usage. It showed no degradation in performance after immersion in 3 M sodium chloride solution for 120 days …”

“…Once again, the investigation of graphene has produced to date substantially more literature as compared to other 2D materials. One aspect to note is that in most cases, the use of reduced graphene oxide, or a graphene with tailored functionalization, is generally preferred to the use of low-defect graphene, due to its high hydrophobicity in its pristine state …”

“…Silicon carbide (SiC) is a suitable substrate material for biological interfaces thanks to its inertness, corrosion resistance against biological fluids, light weight, and bio- and hemocompatibility. − Recently, we demonstrated epitaxial graphene grown on a highly doped conductive silicon/silicon carbide wafer (resistivity ∼10–20 Ω·cm) as a robust, long-term stable, noninvasive, on-skin neural sensor that outperformed commercial dry sensors …”

“…(iii) Equivalent circuit diagram of the on-skin EIS measurements, and (iv) the on-skin EIS setup on the forearm. Reproduced with permission from ref . Copyright 2021 IOP Science.…”