Neuroscience
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Fabrication of High Contact-Density, Flat-Interface Nerve Electrodes for Recording and Stimulation Applications
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Summary October 4th, 2016
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.
Transcript
The overall goal of this protocol is to fabricate flat-interface cuff electrodes to use in neural interface applications. The use of flat-interface cuff electrodes can help solve problems in the field of neuroprosthetics, such as how to record the intent to move a limb from the nerves. Even though the cuff electrodes have submillimeter features, they can still be made by hand reliably.
And the design is flexible. It can be easily customized to many different neural interface applications. Start with a CAD software to produce a true scale diagram of the electrode.
This diagram will determine the dimensions of the electrode and the placement sites of the various electrode components. Similarly, create an additional CAD file of the contacts and electrode components, in order for them to be produced using laser cutting machinery. In preparation, place the center contacts under the microscope and bond them to the lead using a spot welder.
Hold down the contact to make an initial bend at an approximately 45 degree angle, and then fold it around the contact's frame. Alternate the contact leads on each side of the array frame. Next, place a transparency sheet over the guiding diagram and tape it to the baseplate.
Now, to start the build, cut a five cm square piece out from a five mil thick silicon sheet. Place it on the baseplate without trapping air bubbles. Then, prepare two grams of uncured silicon and de-gas it in a vacuum chamber for three minutes.
Release and reapply the vacuum periodically so trapped air can escape. Now, apply a thin line of uncured silicon at the locations of the spacer segments according to the guide. Then, adhere the prepared spacer components onto their designated regions by pressing them onto the silicon sheet.
Now, place the silicon in a preheated 130 degree Celsius isotemp oven for 30 minutes followed by cooling for 10 minutes. Now, place the reference contacts in their designated locations, with the weld points up and the contact leads routed towards the midline of the cuff to exit at the far end. After ensuring correct positioning, press the contacts down onto the silicon layer and deposit uncured silicon into the thru holes.
Next, temporarily secure the leads with tape, and cure the silicon at 130 degrees Celsius for 90 minutes, or overnight at room temperature. Once the reference contacts are secured with silicon, use the same technique to attach the center contact array. Importantly, the center contacts must be placed precisely to get a high-quality recording.
Proper routing of the leads to the designated exit sites is also critical. Cut a one cm by five cm transparency sheet, and use it to cover the center contact arrays. Then tape it down to the baseplate.
Place the small fixture bar across the center of the electrode and clamp it down to the baseplate to secure the middle contacts in place while the silicon cures. Once cured, removed the small fixture bar. Then, remove all the tape securing the leads for the references and metal contacts, and gently remove the transparent sheet to expose the metal contact arrays.
Now, cut a square piece of transparency that has the same width as the electrode and is five cm in length. Then, cut a square piece of silicon sheet that can cover the entire electrode surface. Now, lay this silicon over the piece of transparency, and stretch it flat to remove any irregularities and to prevent air bubbles from being trapped.
Next, cut four pieces of silicon tubing, each five cm long, and place them at the exit sites following the diagram. Leave a two mm gap between the edge of the electrode and the tube. While holding down each pair of tubes with tweezers, tape down the tubes starting at one mm away from the tube end.
Now, arrange the leads of the middle contacts and the references into bundles. Then, pass them through the corresponding tube near the exit sites. Do this for all the tubes.
Next, from a vacuumed mixing container, or a syringe, slowly deposit a generous amount of uncured silicon over the entire electrode body. Now, place the silicon and transparency structure onto the uncured silicon. Align the transparency piece with the electrode while keeping the silicon sheet adhered to it.
Then, tape down the transparency and apply light pressure to remove any trapped air bubbles. Finally, place the large fixture bar across the center of the electrode and over the transparency segment. Then clamp it down to the baseplate with moderate pressure and fully cure the silicon.
To add a shielding layer, remove the large fixture bar, and remove the transparency piece with tweezers. Then place the shielding sheet in the center of each face of the electrode, and gently press it into the electrode. Then deposit some uncured silicon into the thru holes and partially cure it for 30 minutes at 130 degrees Celsius.
Once cooled to room temperature, place adhesive tape over the outer ends of the electrode and over the closing flanges to keep silicon out of these segments. Then, repeat the process of adding a layer of silicon to cover the entire electrode body. Follow this by placing silicon and transparency sheets over the electrode, and then secure them with a large fixture bar and cure the silicon.
After curing, to complete the process, remove the excess silicon and the tape. Then, cut windows through the silicon to expose the spacer segments, and use tweezers to remove the spacer segments. Next, under a microscope, remove the tape and silicon over the tubes until it is level with the electrode body.
Then, carefully cut around the perimeter of the electrode down to the baseplate. Now, taking caution to not damage the electrode, follow the guide diagram to shape the lead's exit sites. Cut out a triangle between each tube pair that goes completely through the electrode layers and onto the outer side.
Now, cut windows into the silicon that covers the shielding layer. Then, glide the polypropylene suture between the electrode base and the adjacent transparent layer and use it to delaminate the nearly finished cuff electrode. Finally, cut windows out of the silicon to expose the center contacts and reference contacts.
The fabricated FINE electrode was used to record the activity of the sciatic nerve in a dog over 7.5 months. A customized pre-amplifier and a super-beta input instrumentation amplifier were implemented. The electrode was implanted around the nerve by suturing the two free edges together.
The cuff flattens the nerve while retaining flexibility in the longitudinal direction. The electrode response to increasing pressure levels inside the cuff was measured. At the diastolic pressure level of 67 mm mercury, the cuff was expanded to 1.25 its initial cross-section area.
This expansion can be employed to compensate for nerve swelling after the implant procedure. Several parameters were periodically measured during the chronic implant duration. Signal to noise stayed steady for the entire 7.5 months.
Contact impedance, measured at 1 kHz, also remained stable. Finally, there were never more than two inactive contacts which were observed to have occurred due to bad connections across the animal's skin. After watching this video, you should have a good understanding of how to fabricate similar multi-channel nerve electrodes, and be able to adjust the design to meet your end application requirements.
The use of these electrodes has paved the way for researchers in the field of neuroprosthetics to record voluntary signals from nerves.
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