Microchannels as Axonal Amplifiers

Abstract

An implantable neural interface capable of reliable long-term high-resolution recording from peripheral nerves has yet to be developed. Device design is challenging because extracellular axonal signals are very small, decay rapidly with distance from the axon, and in myelinated fibres are concentrated close to nodes of Ranvier, which are around 1 mum long and spaced several hundred micrometers apart. We present a finite element model examining the electrical behavior of axons in microchannels, and demonstrate that confining axons in such channels substantially amplifies the extracellular signal. For example, housing a 10-microm myelinated axon in a 1-cm-long channel with a 1000-microm(2) cross section is predicted to generate a peak extracellular voltage of over 10 mV. Furthermore, there is little radial signal decay within the channel, and a smooth axial variation of signal amplitude along the channel, irrespective of node location. Additional benefits include a greater extracellular voltage generated by large myelinated fibres compared to small unmyelinated axons, and the reduction of gain to unity at the end of the channel which ensures that there can be no crosstalk with electrodes in other channels nearby. A microchannel architecture seems well suited to the requirements of a peripheral nerve interface.

Neural growth into a microchannel network: Towards A regenerative neural interface

“…These results also support the theory of signal amplification of APs recorded within a microchannel with sufficiently reduced extracellular volume relative to the neurite diameter [12].…”

“…This implies that electrode noise holds the same relationship with channel size. Comparatively, curve fitting of the amplitude median values fits well with a log-log model with a slope (-0.756) similar to that derived from [12] (-0.783). However, the 2.5 μm wide channels show signal amplification lower than expected.…”

Flexible and stretchable micro-electrodes for in vitro and in vivo neural interfaces

“…R in ) with R out being the electrical resistance of the extracellular fluid and R in the resistance inside the cytoplasm. Given the low resistivity of the extracellular medium, recordable extracellular signals are small, i.e., in the 5-100 lV range [8]. Recording electrodes have typical impedance at 1 kHz of a few tens of kohms to 1 MX, typically characterizes recording electrodes.…”

A Microchannel Neuroprosthesis for Bladder Control After Spinal Cord Injury in Rat

“…The microchannels greatly increase extracellular resistance, thus amplifying recordable voltage potential. Inclusion of guard/reference electrodes at the ends of the microchannels suppresses noise interference (28,29). We have further developed this to produce implantable neuroprosthetic devices with encapsulated electrodes to record cutaneous (30) and bladder activity in acutely teased dorsal roots of rats (31).…”