Fabrication of High Contact-Density, Flat-Interface Nerve Electrodes for Recording and Stimulation Applications

Summary

This article provides a detailed description on the fabrication process of a high contact-density flat interface nerve electrode (FINE). This electrode is optimized for recording and stimulating neural activity selectively within peripheral nerves.

Abstract

Many attempts have been made to manufacture multi-contact nerve cuff electrodes that are safe, robust and reliable for long term neuroprosthetic applications. This protocol describes a fabrication technique of a modified cylindrical nerve cuff electrode to meet these criteria. Minimum computer-aided design and manufacturing (CAD and CAM) skills are necessary to consistently produce cuffs with high precision (contact placement 0.51 ± 0.04 mm) and various cuff sizes. The precision in spatially distributing the contacts and the ability to retain a predefined geometry accomplished with this design are two criteria essential to optimize the cuff's interface for selective recording and stimulation. The presented design also maximizes the flexibility in the longitudinal direction while maintaining sufficient rigidity in the transverse direction to reshape the nerve by using materials with different elasticities. The expansion of the cuff's cross sectional area as a result of increasing the pressure inside the cuff was observed to be 25% at 67 mm Hg. This test demonstrates the flexibility of the cuff and its response to nerve swelling post-implant. The stability of the contacts' interface and recording quality were also examined with contacts' impedance and signal-to-noise ratio metrics from a chronically implanted cuff (7.5 months), and observed to be 2.55 ± 0.25 kΩ and 5.10 ± 0.81 dB respectively.

Introduction

Interfacing with the peripheral nervous system (PNS) provides access to highly-processed neural command signals as they travel to different structures within the body. These signals are generated by axons confined within fascicles and surrounded by tightly-jointed perineurium cells. The magnitude of the measurable potentials resulting from the neural activities is affected by the impedance of the various layers within the nerve such as the highly resistive perineurium layer that surrounds the fascicles. Consequently, two interface approaches have been explored depending on the recording location with respect to the perineurium layer, namely intrafascicular and extrafa....

Protocol

1. Electrode Components Preparation

1. Gather four electrode components that require precision cut (laser-cut was used, Please refer to the Materials List) prior to the manufacturing process. These components are (Figure 1):
   Contacts array frame: This frame is made out of 125 µm thick Polyether ether ketone (PEEK) sheet. It covers the entire width of the cuff and holds the middle contacts and has serpentine-shaped edges (Figure 1B). The middle contacts are wrapped in the guide channels; hence the exposed width of the contacts is limited by the width of the channels and the spacing is determined by the spacing betw....

Results

Recording neural activity was performed with a customized pre-amplifier using super-β input instrumentation amplifier (700 Hz - 7 kHz bandwidth and total gain of 2,000). An example of the fabricated FINE electrode with the presented protocol is shown in Figure 3. Implanting the FINE around the nerve is done by suturing the two free edges together. A demonstration of the cuff's flexibility (Figure 3B) indicates that the cuff flattens the nerve while retaining flexibility in the longitudin.......

Discussion

The manufacturing method described in this article requires dexterous and fine movements in order to ensure the quality of the final cuff. The recording contacts must be placed precisely in the middle of the two reference electrodes. This placement has been shown to significantly reduce interferences from surrounding muscles electrical activity²⁷. Any imbalance in the relative position of the contact during the fabrication can degrade the rejection of common mode interfering signals generated outside the cuff........

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Platinum-Iridium foil	Alfa Aesar	41802	90% Platinum Iridium
DFT wires	Fort Wayne Metals	35N LT-DFT-28%Ag
Lead connector	Omnetics Connector Corporation	MCS-27-SS
Silicone sheet	Speciality Silicon Fabricator	0.005" x 12" x 12" Silicone Sheet	High durometer, vulcanized
Polyether ether ketone (PEEK) sheet	Peek-Optima	0.005 sheet LT3 grade
polyester stabelizing mesh	Surgicalmesh	PETKM2002
Silicon tubing (0.04" I.D. 0.085" O.D.)	Silcon Medical/NewAge Industries.	2810458
Outer shielding layer	Alfa Aesar, A Johnson Matthey	MFCD00003436 (11391)	Gold foil, 0.004" thick
Transparency sheet	APOLLO	APOCG7060
Ultrasonic bath cleaner	Terra Universal	2603-00A-220
Isotemp standard lab oven	Fisher Scientific	13247637G
Optical microscope	Fisher Scientific	15-000-101
Tweezers	Technik	18049USA (2A-SA)
Surgical blade handles	Aspen Surgical Products	371031
Base frame	McMaster-Carr	9785K411
Support beam	McMaster-Carr	9524K359
Two parts silicone	Nusil	MED 4765
Soldering Flux	SRA Soldering Products	FLS71
Tape	3M Healthcare	1535-0 (SKUMMM15350H)	Paper, hypoallergenic surgical tape
Spot welding machine	Unitek	125 Power Supply with 101F Welding Head
Laser cutting platform	Universal Laser Systems	PLS6.150D	150 watts laser