Bending Properties of Materials for Peripheral Nerve Interfaces

Intrafascicular peripheral nerve interfaces (PNIs) with penetrating electrodes have the potential to chronically record from nerves on the scale of single axons. The small size and dynamic environment of peripheral nerves makes material selection important for these devices. In this work, we describe how the bending properties of common PNI electrode materials contribute to their effectiveness as self-inserting PNIs. First, tungsten, platinum-iridium, and carbon fiber wires are tested to assess their ability to survive repeated bending stresses when embedded in silicone. Next, carbon fiber wires are attached to a flexible circuit board encased in silicone to characterize how they survive stresses in prototype PNI devices. Finally, in order to validate experimental results, we use COMSOL to investigate the optimal thickness of the embedded silicone layer by simulating the stress distribution in carbon fiber wires on a flexible circuit board. Carbon fiber wires were shown to survive bending stresses better than tungsten and platinum-iridium wires. Physical testing and COMSOL modeling of carbon fiber prototype devices showed an optimal silicone thickness of 200 μm that prevents carbon fiber breakage but minimizes PNI device size. Overall, these results serve as a guide for selection of self-inserting PNI materials and development of carbon fiber PNIs.