Abstract
Neural electrodes hold tremendous potential for improving understanding of brain function and restoring lost neurological functions. Multi-walled carbon nanotube (MWCNT) and dexamethasone (Dex)-doped poly(3,4-ethylenedioxythiophene) (PEDOT) coatings have shown promise to improve chronic neural electrode performance. Here, we employ electrochemical techniques to characterize the coating in vivo. Coated and uncoated electrode arrays were implanted into rat visual cortex and subjected to daily cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for 11 days. Coated electrodes experienced a significant decrease in 1 kHz impedance within the first two days of implantation followed by an increase between days 4 and 7. Equivalent circuit analysis showed that the impedance increase is the result of surface capacitance reduction, likely due to protein and cellular processes encapsulating the porous coating. Coating’s charge storage capacity remained consistently higher than uncoated electrodes, demonstrating its in vivo electrochemical stability. To decouple the PEDOT/MWCNT material property changes from the tissue response, in vitro characterization was conducted by soaking the coated electrodes in PBS for 11 days. Some coated electrodes exhibited steady impedance while others exhibiting large increases associated with large decreases in charge storage capacity suggesting delamination in PBS. This was not observed in vivo, as scanning electron microscopy of explants verified the integrity of the coating with no sign of delamination or cracking. Despite the impedance increase, coated electrodes successfully recorded neural activity throughout the implantation period.
Highlights
Neural prostheses have seen effective use in a variety of applications, including auditory prostheses, visual prostheses, brain-computer interface, and even opto-electrical applications [1,2,3,4,5]
Dexamethasone (Dex) and Multi-walled carbon nanotube (MWCNT)-doped PEDOT coatings were characterized with regard to morphology and impedance (Figure 1)
We demonstrate that the rapid sub-acute increases in the 1 kHz impedance of MWCNT-doped
Summary
Neural prostheses have seen effective use in a variety of applications, including auditory prostheses, visual prostheses, brain-computer interface, and even opto-electrical applications [1,2,3,4,5]. Several examples employ arrays of penetrating microelectrodes that are implanted into cortex to record neural activity with single cell resolution [1,6,7,8]. When chronically implanted, these electrodes typically exhibit a large degree of variability of recording performance metrics such as impedance [9,10], single-unit yield [10,11,12,13,14], and signal-to-noise ratio [10,11,13,14]. Several interrelated inflammation mechanisms including the development of an encapsulating glial scar and the progressive degeneration and death of local neurons have been theorized to play important roles in recording quality deterioration [14,17,20,21,22,23,24,25,26,27] (see [19] for review)
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