Incorporating nanocarbons as coatings for neural electrodes can increase their surface area. However, microfabricating the electrodes requires the use of harmful chemicals and extreme heating. In an article recently accepted by the magazine iScienceresearchers have adopted a scalable, convenient and safe reduction technique to obtain reduced graphene oxide (rGO) films by using vitamin C (VC) for the reduction reaction.
Study: Vitamin C reduced graphene oxide improves the performance and stability of multimodal neural microelectrodes† Image Credit: Africa Studio/Shutterstock.com
The VC-rGO coatings showed a conductivity of approximately 44 siemens per centimeter. The rGO/gold (Au) microelectrodes exhibited approximately eight times lower impedance and 400 times higher capacitance than pure Au, improving injection capacity and charge storage. rGO/Au arrays enabled the voltammetric detection of dopamine (DA) in vitro and enabled high-resolution in vivo micro-scale recording.
Coating materials for neural electrodes
The circuitry, underlying brain function and disease depend on the ability to modulate and record neural activities with highly reliable electrodes. These electrodes are composed of doped inorganic materials with high conductivity, biocompatibility, electrochemical stability, and easy patterning ability in other structure by standard lithographic techniques.
Moreover, simulating the activity at the cellular level requires solving the corresponding temporal and spatial scales by miniaturizing the electrode contacts. However, the inorganic materials-based microscale electrodes have a high impedance and a modest charge-injection capacity, resulting in degradation of the signal-to-noise ratio (SNR) of recordings and limiting safe neuromodulation.
Nanoscale roughening and surface coating of the metal or silicon (Si) electrodes are common strategies to overcome such limitations. These strategies improve analytical detection by providing additional adsorption sites and a high effective surface area. In addition, these adjustments lead to increased capacity for safe storage and payload delivery.
Titanium nitride (TiN), carbon nanotubes (CNTs), nanodiamonds, conductive polymers (CPs), and hybrid materials are some commonly used materials used to improve the electrode surface. However, these materials have limitations in their practical applicability.
Reduced graphene oxide (rGO) has a predominant capacitive character, ease of processing, electrochemical stability, lower impedance, tunability and high charge release, aiding its application as a neural electrode coating material.
VC-rGO movies in multimodal neural microelectrodes
In the current work, the researchers presented a novel method to produce rGO coatings for application in neural microelectrode arrays. These coatings were safe and compatible with commonly used electrode materials, with easy integration into the conventional microelectrode process.
This method took advantage of the slow reaction kinetics of VCs to use them as ambient temperature reducing agents. The researchers demonstrated an easy construction method to obtain neural microelectrode arrays coated with rGO, in which GO and VC were sprayed onto bare Si wafers with a micro-pattern of Au.
The electrochemical and electronic properties of the coating were maximized by optimizing the heating time and VC concentration. In addition, a one-step coating process was also demonstrated in fabricating parylene-C encapsulated rGO/Au microelectrode arrays for cortical stimulation, microelectrocorticography (μECoG) recordings and neurochemical detection.
The electrochemical properties of the rGO/Au microelectrode were characterized in vitro† The results indicated that rGO coatings showed better stability and improved electrode surface area, resulting in reduced impedance and increased charge storage/charge injection capacity, compared to previously reported Au electrodes or other coating materials.
In addition to the improved surface, the rGO coating also increased the number of adsorption sites, allowing in vitro dopamine DA detection with low detection limit and high sensitivity.
The feasibility of the rGO/Au μECoG array was demonstrated to monitor neural circuits in the microscale range and at high resolution by showing high density mapping of induced cortical responses in a rat’s somatosensory cortex by stimulating its whisker.
The present study proposed a new strategy to improve the stimulation, recording and biochemical sensing properties of neural microelectrodes using rGO coatings. The rGO movies took advantage of the kinetics of VC and completed the reduction in a biocompatible, safe, highly scalable and non-destructive manner.
The reduction of VC has been optimized to achieve compatibility with polymeric substrates in terms of comfort and softness for their applications in implantable medical devices. Moreover, the processing of rGO and the designed film deposition method allowed their easy integration into the microfabrication process to produce neural microelectrodes.
The VC-rGO coatings demonstrated were sufficiently conductive, stable and significantly improved the electrochemical properties compared to metal electrodes. The impedance of the rGO-coated electrode under continuous charge injection combined with the conductivity of the VC-rGO DC in electrolytic and atmospheric environments indicated the potential stability and versatility of the VC-rGO film for long-term application.
In addition, the excellent charge transfer and charge storage properties of rGO-coated electrodes demonstrated their ability to serve as promising candidates for in vivo stimulation studies and chronic recording.
In future work, the researchers expect to complete simultaneous in vivo recording and stimulation experiments by using rGO/Au arrays for recording electrophysiology and stimulation of neural tissue with a direct passage of electrical charge.
Brendan B. Murphy, Nicholas V. Apollo, Placid Unegbu, Tessa Posey, Nancy Rodriguez-Perez (2022). Vitamin C-reduced graphene oxide coatings improve the performance and stability of multimodal microelectrodes for neural recording, stimulation and dopamine sensing. iScience† https://www.sciencedirect.com/science/article/pii/S2589004222009245
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