Abstract
Several recent studies have reported improved histological and electrophysiological outcomes with soft neural interfaces that have elastic moduli ranging from 10 s of kPa to hundreds of MPa. However, many of these soft interfaces use custom fabrication processes. We test the hypothesis that a readily adoptable fabrication process for only coating the tips of microelectrodes with soft brain-like (elastic modulus of ~5 kPa) material improves the long-term electrical performance of neural interfaces. Conventional tungsten microelectrodes (n = 9 with soft coatings and n = 6 uncoated controls) and Pt/Ir microelectrodes (n = 16 with soft coatings) were implanted in six animals for durations ranging from 5 weeks to over 1 year in a subset of rats. Electrochemical impedance spectroscopy was used to assess the quality of the brain tissue–electrode interface under chronic conditions. Neural recordings were assessed for unit activity and signal quality. Electrodes with soft, silicone coatings showed relatively stable electrical impedance characteristics over 6 weeks to >1 year compared to the uncoated control electrodes. Single unit activity recorded by coated electrodes showed larger peak-to-peak amplitudes and increased number of detectable neurons compared to uncoated controls over 6–7 weeks. We demonstrate the feasibility of using a readily translatable process to create brain-like soft interfaces that can potentially overcome variable performance associated with chronic rigid neural interfaces.
Highlights
The viscoelastic properties of the resultant elastomer composite with carbon nanotubes were matched with the viscoelastic properties of rodent brain tissue that are in vivo determined in a prior study where we had shown that pristine cortical brain tissue had typical elastic moduli of around 3–10 kPa with shear moduli around 1.4–3 kPa [25]
We developed a novel, soft composite material composed of a siliconebased material with single-walled carbon nanotubes (CNT) incorporated in the silicone base
In this study we developed a soft brain-like, silicone/CNT based composite that made an excellent candidate for mimicking brain tissue material properties
Summary
Stability in the long-term performance of neural implants remains an elusive goal. It is widely shown that the signal reliability of chronic recordings from neural probes tend to fail within a few weeks to several months after implantation [1]. It has been shown that the mean failure time of implanted silicon microelectrode arrays (MEAs) in non-human primate is 334 days [2]. Especially under chronic conditions, is likely due to multiple abiotic (i.e., probe corrosion and insulation failure) and biotic factors (i.e., loss of neurons at the interface, gliosis and oxidative stress due to neuroinflammation, etc.) [3,4,5,6,7,8,9,10,11,12]
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