Bioengineering
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Open-source Toolkit: Benchtop Carbon Fiber Microelectrode Array for Nerve Recording
Chapters
Summary October 29th, 2021
Here, we describe fabrication methodology for customizable carbon fiber electrode arrays for recording in vivo in nerve and brain.
Transcript
Carbon fiber electrodes, are smaller than neurons and do minimal damage. In addition, they can be manufactured with minimal specialized equipment on the bench top. The main advantage of this technique is in its simplicity.
It allows users to build and customize neural arrays, with no cleanroom experience. Manipulating the carbon fibers and placing the silver epoxy is tricky and critical. These two parts take a lot of practice and fine motor control.
The first day is always the worst, but with practice, after about a week, you will build a functional neural array. To begin, set a soldering iron to 315 degrees Celsius. Apply flux to all soldering pads.
Form small mounds of solder on the back pads of the flex array, then solder the pins on either side of the connector. Once secure, gently push the soldering iron tip, between the front pins, to solder the remaining connections in the back. Apply an additional flux layer, if soldering takes a long time, then solder the front row of pins to the board.
Clean away excess flux with 100%isopropyl alcohol, and a short bristle brush. Next, slowly push the laid set epoxy with a syringe, placed bevel side down on the pins, to encapsulate the soldered connection. Lay a small line of the epoxy across the board's backside, and pull it onto the connector's edges to secure it.
Make capillaries with a glass puller and filament. Cut a pulled glass capillary, so that its tip fits between the traces of the array. Then, scoop approximately a one to one ratio of silver epoxy into a plastic dish, with the wooden ends of two cotton tipped applicators and mix it.
Discard the applicators after mixing. Cut two to four millimeters, from the end of a carbon fiber bundle, onto a printer paper with a razor blade. Pull a laminated paper, gently, over the top of the bundle to separate the fibers and the bundle.
Next, take a little epoxy onto the pulled capillary's end. And gently apply it between every other trace, on the end of the board, filling the gap. Place one carbon fiber in each epoxy trace, with teflon coated tweezers.
Then, adjust the carbon fibers with a clean pulled capillary, to make perpendicular to the end of the flex array board and bury them beneath the epoxy. Place the arrays on a wooden block, with fibered ends overhanging the edge of the block. Bake the wooden block and arrays at 140 degrees Celsius for 20 minutes, to cure the silver epoxy and lock the fibers into place.
If silver epoxy shorts two or more of the fibers together, it can be removed using a clean glass capillary, and gently scraping it off the board. Store the finished boards in a box with a raised platform, to suspend the fibered ends of the board, to prevent fiber breakage. Apply a tiny droplet of UV epoxy on the exposed traces with a clean capillary, and continue adding droplets, until the traces are completely covered.
Cure the UV epoxy under a UV pen for two minutes and repeat it for the other side of the board. Cut the fibers to one millimeter with a stereoscope reticle and surgical scissors. To check electrical connections, set the potentiostat to zero volts, for five seconds, and stabilize the recorded signal.
Run a one kilohertz impedance scan for each fiber with a potentiostat. Record the measurements via the potentiostat associated software. Then dip the fibers in deionized water in a beaker, three times, to rinse them.
Gently scrape away Parylene C from the ground, and reference wires on the board with tweezers. Next, cut two five centimeter lengths of insulated silver wire with a razor blade. De-insulate two to three millimeters of wire, from one end and around 10 millimeters from the opposite end.
Next, heat the soldering iron to 315 degrees Celsius, and apply a small flux to the wires. Insert two to three millimeters of one wire, into each electrophysiology wire on the board, and apply solder to the top of the wires. After allowing the probe to cool, flip it over to apply a little solder to the backside of the wire.
Snip off any exposed wire sticking out of the back solder mound. Place the arrays in the storage box, bending the wires back, away from the fiber and secure the wires on the adhesive tape, to prevent potential fiber wire interactions. SEM images of the tips were used to determine the exposed carbon length and tip geometry.
Scissor cut fibers have inconsistent tip geometries, with Parylene C folding over the end. The NDYAG laser cut fibers remain consistent, in the recording site area, shape and impedance. Blow torched fibers lead to the largest electrode size, and shape variability and a sharpened tip.
On average, 140 micrometers of carbon were re-exposed. UV laser cut fibers were similar to blow torched fibers, showing 120 micro meters of carbon, exposed from the tip. The resulting impedances were within range, for electrophysiological recording.
NDYAG laser cut fibers had the smallest surface area but the highest impedances. Followed by blow torched and UV laser cut fibers. However, in all cases, the PEDOT:pTS coated fibers, fell under the 110 kilo ohm threshold.
Acute recordings from four UV laser treatment fibers, of two millimeter length, simultaneously implanted in rat motor cortex, showed three units across all fibers, suggesting that the treatment of the fibers with the inexpensive UV laser, is similar to other cutting methods. When attempting this protocol, give yourself a large, clean space when populating carbon fibers. It is easy to accidentally sweep all of your fibers off the table, because there's not enough maneuvering room.
These arrays are now suitable for signal unit recordings from the brain. These building techniques allowed Dr.Barnes'group, at the University of Michigan, to report and Dr.Chiel's group at Case Western Reserve University, to report intracellular
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