Session: Research Posters

Paper Number: 167243

Scalable Double-Etching Fabrication of Flexible Glassy Carbon Microsensors for Neurotransmitter Detection 

Flexible glassy carbon (GC) microelectrode arrays (MEAs) and GC fiber-like MEAs (GCF MEAs) have demonstrated great potential for in vivo multichannel neurotransmitter detection and electrophysiology recording while minimizing tissue damage due to their small size, flexibility, and biocompatibility.
However, hybrid MEAs, which combine GC electrodes with metal interconnects, may suffer from adhesion failure under prolonged electrical and mechanical stress. To address this challenge, we developed a scalable double-etching microfabrication process for fabricating both GC MEAs and full GC fibers (fGCFs) with GC electrodes and interconnects made entirely from a single homogeneous material on thin, flexible substrates. This method utilizes a 2 µm-thick low-stress silicon nitride layer (Si₃N₄) as the bottom insulator, eliminating the need for pattern-transfer steps and enabling precise miniaturization.
We confirmed the feasibility of this fabrication process by successfully producing GC MEAs and verifying the practical conductivity of 3 µm-wide, 2 µm-thick GC traces, as well as validating the GC microelectrode functionality using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Leveraging the miniaturization capability of this process, we batch-produced 10 μm × 10 µm fGCFs and fGCF arrays. The effectiveness of the fabrication process was further validated through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), confirming the uniformity of the Si₃N₄ insulation layer and ensuring the overall structural integrity of the devices. Raman spectroscopy of GC identified the two primary peaks typical for carbon materials at 1,323 cm−1 (D band) and 1,604 cm−1 (G band), with a D/G ratio of ~1.1, indicating high graphitization and enhanced electron transfer. Additionally, the presence of secondary peaks at 2,642 cm−1 (2D) and 2,916 cm−1 (D + D′) indicated high ordering and graphitization in the pyrolyzed GC. The defects and the high graphitization level promote adsorption and electron transfer in carbon materials. Electrochemical characterization performed using EIS and CV revealed high conductivity, a wide electrochemical window, and reliable detection of serotonin (5-HT) and dopamine (DA) using both fast scan cyclic voltammetry (FSCV) and square wave voltammetry (SWV). Additionally, finite element analysis optimized the fGCF structure to achieve self-penetration up to 3 mm into brain tissue without the need for external support. In vivo testing confirmed the insertion capability of fGCFs, which successfully self-penetrated brain tissue and were implanted into the dorsal striatum (DS) of anesthetized mice, following a straight trajectory predicted by finite element modeling. The electrodes were explanted without structural damage, confirming their robustness. fGCFs demonstrated the capability for in vivo tonic DA detection in the DS using SWV, with tonic DA levels validated through drug administration. Phasic DA release, stimulated by electrical activation of the medial forebrain bundle (MFB), was successfully detected using FSCV. Histological analysis showed minimal tissue damage and good electrode-tissue integration. 
These findings demonstrate the effectiveness of fGCFs for in vivo neurotransmitter monitoring, supported by a scalable fabrication process that addresses adhesion challenges, allows precise miniaturization, and enables large-scale production, advancing GC-based neurochemical sensors for next-generation neural interfaces.

Presenting Author: Elisa Castagnola Louisiana Tech University

Presenting Author Biography: Dr. Castagnola received her PhD in Robotics, Neurosciences and Nanotechnologies from the Italian Institute of Technology (IIT) in April 2011. She continued to conduct her postdoctoral research on neurotechnology at IIT, Departments of Robotics Brain and Cognitive Sciences, and Center for Translational Neurophysiology for Speech and Communication.
She moved to the United State in 2017, working as a senior researcher and adjunct assistant professor at the Department of Mechanical Engineering at San Diego State University, supported by the Center for Neurotechnology, a National Science Foundation Engineering Research Center.
At the end of 2018, she joined the Department of Bioengineering of the University of Pittsburgh in the Neural Tissue Engineering (NTE) Lab, working on the development and in-vivo validation of multimodal neural probes, while serving as instructor in the undergraduate and graduate Bioengineering programs.
Currently, she is an Assistant Professor at the Department of Biomedical Engineering at Louisiana Tech University. Her Neural Electrode Interface Technologies Lab focuses on (i) the development of flexible implantable neural probes for chronic multimodal electrophysiological and electrochemical sensing; (ii) the optimization of biocompatible materials and electrochemical techniques to enable neurotransmitter detection at different time scales, and (iii) in-vitro and in-vivo testing of the material stability and chronic performance of these devices.
Dr. Castagnola’s main interests focus on material science, microfabrication, electrochemistry, neurochemical and electrophysiological sensing, and neural stimulation.

Authors:

Elisa Castagnola Louisiana Tech University
Umisha Siwakoti Louisiana Tech University
May Yoon Pwint University of Pittsburgh
Austin M. Broussard Louisiana Tech University
Daniel R. Rivera Louisiana Tech University
X. Tracy Cui University of Pittsburgh