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The peripheral nervous system (PNS) is an attractive target for modulation of afferent input (e.g., nociceptive input signaling tissue damage) to the central nervous system. To advance mechanistic understanding of PNS neural encoding and modulation requires single-unit recordings from individual peripheral neurons or axons. This is challenged by multiple connective tissue layers surrounding peripheral nerve fibers that prevent electrical recordings by existing electrodes or electrode arrays. In this study, we developed a novel microelectrode array (MEA) via silicon-based microfabrication that consists of 5 parallel hydrophilic gold electrodes surrounded by silanized hydrophobic surfaces. This novel hydrophilic/hydrophobic surface pattern guides the peripheral nerve filaments to self-align towards the hydrophilic electrodes, which dramatically reduces the technical challenges in conducting single-unit recordings. We validated our MEA by recording simultaneous single-unit action potentials from individual axons in mouse sciatic nerves, including both myelinated A-fibers and unmyelinated C-fibers. We confirmed that our recordings were single units from individual axons by increasing nerve trunk electrical stimulus intensity, which did not alter the spike shape or amplitude. By reducing the technical challenges, our novel MEA will likely allow peripheral single-unit recordings to be adopted by a larger research community and thus expedite our mechanistic understanding of peripheral neural encoding and modulation.

Implantable microfabricated neural electrodes have numerous neuroscientific research and clinical applications. However, these devices are prone to failure after several months in vivo. One mechanism is failure of passivation layers followed by corrosion of metal traces in the saline environment. It has been suggested that mechanical stress accelerates passivation layer failure and that stress is concentrated whenever passivation layers have a non-planar topography. Therefore, we developed a simple process for recessing metal traces within the substrate so that overlying passivation layers are planar. The process requires no extra masks and no post-passivation planarization steps.

Objective. Chronically-implanted neural microelectrodes are powerful tools for neuroscience research and emerging clinical applications, but their usefulness is limited by their tendency to fail after months in vivo. One failure mode is the degradation of insulation materials that protect the conductive traces from the saline environment. Approach. Studies have shown that material degradation is accelerated by mechanical stresses, which tend to concentrate on raised topographies such as conducting traces. Therefore, to avoid raised topographies, we developed a fabrication technique that recesses (buries) the traces in dry-etched, self-aligned trenches. Main results. The fabrication technique produced flatness within approximately 15 nm. Finite element modeling showed that the recessed geometry would be expected to reduce intrinsic stress concentrations in the insulation layers. Finally, in vitro electrochemical tests confirmed that recessed traces had robust recording and stimulation capabilities that were comparable to an established non-recessed device design. Significance. Our recessed trace fabrication technique requires no extra masks, is easy to integrate with existing processes, and is likely to improve the long-term performance of implantable neural devices.