(M-497) Microstimulation of Amorphous Silicon Carbide and Polyimide Microelectrode Arrays Implanted in the Rat Motor Cortex.

Introduction:: Implantable microelectrode arrays (MEAs) have the potential to provide chronic stimulation of cortical neurons and are a valuable tool in the development of brain-machine interface devices [1]. However, device failure due to the foreign body response in the brain is a significant challenge that impedes their effectiveness. This response contributes to the interface's failure and restricts the recording capabilities of the arrays under chronic conditions. Within minutes of device implantation, astrocytes, microglia, and macrophages migrate to the implant site to initiate the foreign body response. This response is a common occurrence with all implanted devices, but MEAs with cross-sectional areas greater than 200 µm² have been shown to cause more significant inflammation and neural loss than devices with cross-sectional areas less than 160 µm² [1]. To overcome this challenge, we report on the design and development of ultra-thin amorphous silicon carbide (a-SiC) MEAs and polyimide (PI) MEAs and observe which devices provide a significant improvement in terms of neuronal survivability and high signal-to-noise ratio. We compare the local field potentials (LFPs) recorded by ultra-thin a-SiC devices to LFPs recorded with PI-only MEAs for up to 8 weeks (about two months) post-implantation.



Materials and Methods:: We designed MEAs with four penetrating shanks with dimensions 75 µm wide, 8 µm thick, and 3.5 mm long. Each MEA had 16 electrodes that we used to stimulate neurons and obtain neural recording signals. We compared two groups of MEAs with the same design: one group was made with only PI substrate layers and the second group was made with PI and a-SiC substrate layers. We used spin coating and thin-film photolithographic methods to create the device layers. Sputter deposition was used to add a low impedance coating of Iridium Oxide (IrOx) on the electrode sites to enhance their stimulation capabilities. Omnetics connections were added to the completed devices, and each device was placed in phosphate buffered saline (pH 7.1 - 7.3) to record baseline electrochemical measurements of performance. We then implanted two devices, one of each substrate type, into the motor cortex of five Long Evans rats and recorded the power spectral density (PSD) of LFPs in response to stimulation once per week for eight weeks. After completing the stimulation and recording study, we perfused the animals for tissue processing and histological assessments.


Results, Conclusions, and Discussions:: Results: We successfully fabricated and completed the electrochemical assessments of the impedance and charge storage capacity for all MEAs. The initial impedance and charge storage capacity values were not significantly different between PI only MEAs and a-SiC MEAs. For the PI-based MEAs we observed a decline in the total LFP band power whereas the a-SiC MEAs show stable total LFP band power over chronic stimulation. Overall, ultra-thin a-SiC devices more reliably recorded LFPs than PI-only MEAs.



Significance: The amorphous silicon carbide microelectrode array MEAs provide a considerable improvement in the stability of neural recording and can potentially lessen tissue injury and foreign body reactions. Our results suggest that the performance of polyimide only microelectrode degraded over time while the performance of amorphous microelectrode array remained stable. The use of ultra-thin amorphous silicon carbide in thin-film microelectrode arrays can significantly advance the development of reliable devices that are ready for clinical translation and large-scale manufacturing.



Conclusion: Our ultra-thin a-SiC MEAs significantly improved signal stability and signal-to-noise ratio compared to PI-only MEAs. By comparing the LFPs recorded by these devices, we can gain insight into which MEAs offer the greatest potential for long-term, chronic stimulation of cortical neurons. These results suggest that ultra-thin a-SiC MEAs may be a promising alternative to PI MEAs for chronic implantation in the brain. Further studies are needed to confirm these findings and to investigate the long-term safety and efficacy of ultra-thin a-SiC MEAs.




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[1] Jeakle EN, Abbott JR, Usoro JO, Wu Y, Haghighi P, Radhakrishna R, Sturgill BS, Nakajima S, Thai TTD, Pancrazio JJ, Cogan SF, Hernandez-Reynoso AG. Chronic Stability of Local Field Potentials Using Amorphous Silicon Carbide Microelectrode Arrays Implanted in the Rat Motor Cortex. Micromachines. 2023; 14(3):680. https://doi.org/10.3390/mi14030680.