SPARC: A Carbon Fiber Nerve Electrode Appropriate for Chronic Recording

Abstract

State‐of‐the‐art intraneural peripheral nerve electrodes are large, silicon‐based structures that can cause substantial tissue response and are ill‐suited for recording from small autonomic nerves. The focus of this work is to adapt our minimally‐scarring carbon fiber brain electrodes to a chronic intraneural nerve array. The objective of this study is to increase the durability of the carbon fiber electrodes to withstand the surgical handling necessary for nerve implantation, while maintaining a cellular scale electrode that can insert through the outer epineurium layer of peripheral nerves. Toward that end, we embedded carbon fibers in silicone to increase robustness, sharpened the fibers to penetrate epineurium, and tested fibers coated with poly(3,4‐ethylene‐dioxythiophene):sodium p‐toluenesulfonate (PEDOT:pTS) in vivo.We tested the durability of carbon fibers by embedding uninsulated fibers in a body of silicone and inducing a 90‐degree bend to simulate the expected shear forces seen during surgery. We found that fibers of 175μm length were robust to thousands of bend cycles. After 3000 bends, 15 of 16 fibers remained intact. We blunt cut carbon fibers using a 532nm green laser and inserted fibers into silicone of stiffness similar to epineurium to determine a length for in vivo testing. We found that blunt fibers reliably insert into silicone without breaking when less than 200μm in length (N=8 fibers). We hypothesized that sharpened fibers would insert into tissue more easily, and therefore sharpened fibers with a 5mm butane flame when protruding from the surface of water. The dimensions of sharpened fibers were 224μm in length with 147μm of exposed carbon and a tip angle of 72 degrees (N=24 fibers).We inserted functionalized sharpened carbon fibers coated with PEDOT:pTS into peroneal and cervical vagus nerves of anesthetized rats (N=5 arrays, 71 fibers, 7 animals). In all experiments, we observed that the sharpened carbon fiber electrodes penetrated the epineurium for successful insertion. The average 1kHz impedance of coated fibers prior to and upon insertion was 38kΩ and 56kΩ, respectively. In both cases, the Z1kHz of roughly 90% of fibers measured below 100kΩ. We recorded spontaneous and evoked neural activity in each experiment and are analyzing the data for discernable single units. We also inserted sharpened fibers into anesthetized feline sacral dorsal root ganglia (N=3) and confirmed insertion with evoked neural recordings and 1kHz impedances (Z=59kΩ).This work shows that the durable nature of carbon fibers embedded in silicone allows them to withstand surgical handling and insert into various types of nerves. Our next step will be the fabrication of an array for chronic implantation.Support or Funding InformationNIH OT2OD024907, NINDS U01NS094375, NINDS UF1NS107659, NSF 1707316