The incorporation of nanocarbons as coatings of neural electrodes can improve their surface area. However, the microfabrication of electrodes requires the use of harmful chemicals and extreme heating. In a recently accepted article in the journal iScience, researchers adopted a scalable, easy, and safe reduction technique to obtain reduced graphene oxide (rGO) films by using vitamin C (VC) for reduction reaction.
Study: Reduced graphene oxide with vitamin C improves the performance and stability of multimodal neural microelectrodes. Image credit: Africa Studio / Shutterstock.com
VC-rGO coatings showed a conductivity of approximately 44 siemens per centimeter. RGO / gold (Au) microelectrodes showed an impedance approximately eight times lower and a capacity 400 times greater than pure Au, thus improving injection capacity and charge storage. The rGO / Au matrices allowed the voltammetric detection of dopamine (DA) in vitro and allowed the high-resolution microscale recording in vivo.
Coating materials for neural electrodes
Circuits, 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 modeling ability in a different structure using standard lithographic techniques.
In addition, simulating activity at the cellular level requires solving the corresponding temporal and spatial scales by miniaturizing the contacts of the electrodes. However, microscale electrodes based on inorganic materials suffer from high impedance and exhibit a modest charge injection capacity, which causes a degradation of the signal-to-noise ratio (SNR) of the recordings and restricts safe neuromodulation.
Nanoscale roughness and surface coating of metal or silicon (Si) electrodes are common strategies to overcome these limitations. These strategies improve analytical detection by providing additional adsorption sites and a large effective surface area. In addition, these modifications result in improved secure storage and delivery capacity.
Titanium nitride (TiN), carbon nanotubes (CNT), nanodiamonds, conductive polymers (CP) and hybrid materials are some of the common materials used to improve the surface of the electrode. However, these materials have limitations in their practical applicability.
Reduced graphene oxide (rGO) has a predominant capacitive nature, ease of processing, electrochemical stability, lower impedance, tuning, and high charge delivery, which aid in its application as a neural electrode coating material.
VC-rGO films in multimodal neural microelectrodes
In the present work, the researchers presented a new 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 reducing agents at room temperature. The researchers demonstrated an easy construction method to obtain rGO-coated neural microelectrode arrays, in which GO and VC were spray-sprayed onto micropatterned Si naked wafers.
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 the manufacture of 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 / injection capacity, compared to Au electrodes or other coating materials. reported earlier.
In addition to the improved surface area, rGO coating also increased the number of adsorption sites, which allowed in vitro detection of dopamine DA with a low detection limit and high sensitivity.
The viability of the rGO / Au μECoG matrix to control neural circuits in the microscale and high-resolution range was demonstrated by showing a high-density mapping of cortical responses induced in a rat’s somatosensory cortex to stimulate its mustache.
Conclusion
The present study proposed a new strategy to improve the stimulation, recording, and biochemical detection properties of neuronal microelectrodes using rGO coatings. RGO films took advantage of VC kinetics and completed the reduction in a biocompatible, safe, highly scalable, and non-destructive manner.
VC reduction is optimized to achieve compatibility with polymeric substrates in terms of its comfort and smoothness for its applications in implantable medical devices. In addition, rGO processing and the designed film deposition method allowed its easy integration into the microfabrication process to produce neural microelectrodes.
The demonstrated VC-rGO coatings were suitably conductive, stable, and significantly improved the electrochemical properties compared to the metal electrodes. The impedance of the rGO-coated electrode under uninterrupted charge injection in combination 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 an application. long-term.
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 and chronic recording studies.
In future work, the researchers plan to complete in vivo stimulation and recording experiments simultaneously using rGO / Au matrices to record electrophysiology and neural tissue stimulation with a direct electrical charge step.
Reference
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 neuronal registration, stimulation, and dopamine detection. iScience. https://www.sciencedirect.com/science/article/pii/S2589004222009245
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