Carbon-Based Fiber Materials as Implantable Depth Neural Electrodes

Abstract

Implantable brain electrophysiology electrodes are valuable tools in both fundamental and applied neuroscience due to their ability to record neural activity with high spatiotemporal resolution from shallow and deep brain regions. Their use has been hindered, however, by the challenges in achieving chronically stable operations. Furthermore, implantable depth neural electrodes can only carry out limited data sampling within predefined anatomical regions, making it challenging to perform large-area brain mapping. Minimizing inflammatory responses and associated gliosis formation, and improving the durability and stability of the electrode insulation layers are critical to achieve long-term stable neural recording and stimulation. Combining electrophysiological measurements with simultaneous whole-brain imaging techniques, such as magnetic resonance imaging (MRI), provides a useful solution to alleviate the challenge in scalability of implantable depth electrodes. In recent years, various carbon-based materials have been used to fabricate flexible neural depth electrodes with reduced inflammatory responses and MRI-compatible electrodes, which allows structural and functional MRI mapping of the whole brain without obstructing any brain regions around the electrodes. Here, we conducted a systematic comparative evaluation on the electrochemical properties, mechanical properties, and MRI compatibility of different kinds of carbon-based fiber materials, including carbon nanotube fibers, graphene fibers, and carbon fibers. We also developed a strategy to improve the stability of the electrode insulation without sacrificing the flexibility of the implantable depth electrodes by sandwiching an inorganic barrier layer inside the polymer insulation film. These studies provide us with important insights into choosing the most suitable materials for next-generation implantable depth electrodes with unique capabilities for applications in both fundamental and translational neuroscience research.

Stable, long-term single-neuronal recording from the rat spinal cord with flexible carbon nanotube fiber electrodes

“…But because the bending stiffness scales with electrode size to the fourth power [41,42], any further increase in the insulation thickness will increase the stiffness of the electrodes, which is associated with a higher level of inflammatory responses, glial encapsulation, and neuronal degeneration, thus impairing the long-term stability of the recordings. It was reported that by sandwiching nanometer-thick inorganic oxide layers from atomic layer deposition inside the polymer insulation, the stability of the CNTF electrode's electrochemical properties were significantly improved as shown by the in vitro accelerated aging tests [63]. The extra inorganic layers do not significantly compromise the mechanical compliance of the CNTF electrodes due to their nanometer thickness.…”

Implantable Neural Microelectrodes: How to Reduce Immune Response

“…Soft and stretchable substrates are better matched to the stiffness of the brain tissue. Common flexible neural electrode materials include carbon-based fibers, , conductive polymers (poly­(3,4-ethylene­dioxythiophene)-poly­(styrene­sulfonate) (PEDOT:PSS)), polydopamine, etc. The use of flexible polymer substrates to design microarrays is also a promising strategy for structuring electrode flexibility.…”

“…In recent years, various carbon-based conductive materials have been used to prepare flexible implantable electrodes to reduce the inflammatory response, such as carbon nanotube (CNT) fibers, carbon fibers, and graphene fibers. , Two-dimensional graphene with a single atomic layer is used as a conductive material for the preparation of neural electrodes with many advantages such as high mechanical compatibility for close contact with the brain tissue interface and good electrical conductivity and carrier mobility up to 100,000 cm 2 V –1 s –1 much higher than silicon. The characteristic roughness and high porosity of graphene can reduce the impedance of electrodes prepared from it and improve the charge injection capacity, thus enabling high-quality neural recordings.…”

2D Optimized Electrodes for Highly Sensitive and Biocompatible Neural Recording

“…Currently, researchers are employing coating strategies to modify the surfaces of electrodes. One approach involves the use of nanomaterials, such as Pt black and CNT coatings, − which increase the surface active sites of the electrodes, lowering the interfacial impedance and thus enhancing the sensitivity of neural signal recording. − However, nanomaterials as electrode surface modifications may degrade over time and leave residues, posing risks in terms of biocompatibility and safety. The second type involves the modification with a conductive polymer, such as PEDOT, PPy, and PDMS, which reduce impedance and enhance charge injection capacity. − Additionally, these modifications allow for better adaptation to the surface of biological tissues during implantation, reducing damage to the tissues. − However, deformation of the electrodes during implantation can lead to performance degradation, and poor environmental conditions and long-term use may lead to layering and failure of the coating due to the high environmental stability requirements of conductive polymer materials .…”