Patterned Vertical Carbon Nanotube Dry Electrodes for Impedimetric Sensing and Stimulation

Abstract

Traditional wet or gel impedimetric electrodes for neuro-physiological signal (e.g., electroencephalography and electrocardiography) monitoring are usable for a short duration, as the performance of electrodes deteriorates rapidly when exposed to the environment. Dry impedimetric electrode is a promising alternative tool for long duration monitoring, however suffers from high interfacing impedance. This paper describes a novel dry interfacing electrode utilizing patterned vertical carbon nanotube (pvCNT) for impedimetric sensing. The electrodes were fabricated on circular stainless steel foil substrates (thickness = 2 mil) that are laser cut to circular discs (O = 10 mm). Pattern on the substrate was developed with a custom shadow mask while sputter coating the substrate with Al2O3 and iron. Electrically conductive multiwalled CNTs were then grown vertically in pillar formation ( $100~\mu \text{m}^{}$ each side of square footprint) with various interpillar spacing (50, 100, and $200~\mu \text{m}$ for various masks). The heights of the CNT pillars were between 1 and 1.5 mm. The impedances of the electrodes were 1.92, 3.11, and $8.15~\Omega $ for 50-, 100-, and 200- $\mu \text{m}$ spacing, respectively. A comparative in vitro study with commercial wet and gel electrodes showed pvCNT electrode has lower interfacial impedance, comparable signal capture quality, and ability to be used for stimulation. Long duration study showed minimal impedance degradation for pvCNT electrodes over a week. The results demonstrate pvCNT is a promising dry electrode for impedimetric sensing and stimulation of neurophysiological signals over a prolonged duration.