Glassy Carbon Microneedle Electrode Arrays for Conformable Electrical Interfacing of the Skin

In the field of bioelectronic wearable systems for healthcare, there is a growing demand for reliable, non-invasive technologies capable of continuous monitoring of electrophysiological signals and precise electrical stimulation through the skin. Traditional wet skin interfaces, requiring gels or liquids to maintain low electrode impedance, often result in variable signal quality and user discomfort. Recent advancements focus on dry skin interfaces using microneedle electrodes that penetrate the epidermis, reducing skin-electrode impedance and eliminating the need for conductive gels. Glassy carbon has emerged as a promising material for these interfaces due to its high hardness, wide electrochemical stability window, and ease of patternability through the carbonization of polymer precursors like SU-8 photoresist. However, integrating rigid microneedles into a thin, flexible polymer substrate to enhance skin conformability remains a challenge. This study presents the design, fabrication, and characterization of glassy carbon microneedle electrode arrays (GC-MEAs) on flexible substrates for skin electrical interfacing. We optimized the fabrication process by combining photolithography, pyrolysis, and transfer printing techniques, enabling the scalable production of large, customizable GC-MEAs with lengths ranging from 150 to 400 μm and base diameters from 50 to 150 μm on flexible polyimide printed circuit boards. Extensive characterization tests demonstrated the suitability of glassy carbon for a reliable dry skin interface, with a hardness of 3.5 GPa and a conductivity of 10^3 S/m. Numerous experiments, including hematoxylin-eosin (H&E) staining and impedance spectroscopy, confirmed effective skin penetration using various explanted animal skin models. The successful integration of glassy carbon microneedles on flexible substrates opens up a wide range of potential applications for health wearable systems. Future work will focus on collecting valuable electrophysiological data (EMG, ECG, EEG) from both animals and humans using this technology, as well as on modulating the nervous system via transdermal electrical stimulation.