The method allows monitoring of large, distributed neuronal populations over days, weeks, or months, putting questions of how these populations change and support cognitive functions within reach. The implantation of multiple polymer devices and related implant construction combine to make a recording platform that's capable of high-channel count, multi-region, longterm, continuous, and stable recordings. Demonstrating the procedure will be Clay Smyth, a technician from my laboratory.
To prepare the polymer electrode arrays for insertion, insert a screw through aligned vertically oriented holes in the two insertion pieces to lock them together and hold the pieces in a vice. Attach double-sided tape to the top of the second piece, and attach the stabilizing piece to the end of the first piece. The stabilizing piece will be held in place by friction.
Manually align the electrode array and attach the insertion shuttle to the narrow end segment of the first piece. When the probe is aligned with the longitudinal axis of piece one, adhere the array connector to the polyamide double-sided tape on the flat portion of piece two. With plastic tipped forceps, connecting only the polyamide wing attached to the array ribbon, lift the insertion shuttle electrode array device tip from piece one to the exterior of the stabilizing piece.
Apply 10 microliters of a suitable adhesive to the end of the first piece. Using the plastic tipped forceps to contact only the polyamide wing attached to the array ribbon, use the square tab of the insertion shuttle to realign the device with the narrow segment of piece one. Manipulate the side of the silicon shuttle or the peg to make small alignment adjustments, taking care to avoid applying excessive force to the ribbon or shanks.
Then use the forceps to apply gentle downward pressure on both sides of the stabilizing piece and remove the stabilizing piece from the assembly without moving the array. Next, set the micro manipulator of the first piece to an extended position. The piston will slide to a terminal depth inside of piece one.
Load pieces one and two onto the retraction piston. Set the micro manipulator of the third piece to a retracted position. Fit the second piece within the top portion of piece three with the holes aligned and load piece three onto the insertion micro manipulator piston.
Screw the pieces in place on the underside of piece three, and load and screw pieces two and three together so that moving the insertion micro manipulator will move the whole insertion apparatus. Remove the screw that holds pieces one and two together. Then insert the screw into the lateral hole of piece one perpendicular to the piston track until the screw applies pressure on the piston to ensure that piece one moves in accordance with the retracting piston.
Piece one should move independently of piece two, allowing a separate retraction of the insertion shuttle from the apparatus. When the device construction is complete, viewing the insertion under a stereo microscope, quickly lower the stereotactic instrument at 25 micrometers per second. The device will not penetrate the brain immediately.
The degree of resistance and dimpling will depend on the target location and the device design. Once the device has penetrated the brain, switch the micro manipulator speed to 10 micrometers per second and lower the device to one to two millimeters above the target depth, visualizing the device wings and the point of insertion during lowering to avoid premature shuttle array detachment. When the device reaches between one and 0.5 millimeters above the target depth, slow the insertion to five micrometers per second.
And then, when the device is 500 micrometers away from the target, slow the insertion to one to two micrometers per second. When the target depth is reached, dry the attachment point on the base piece as necessary and then anchor both polyamide wings to the base piece attachment sites with a suitable adhesive. Prior to dissolution, the peg will appear as a globular mass sitting atop the array and the insertion shuttle interface.
To dissolve the peg, gently drip body temperature saline on the array at the point at which the peg is adhered to the shuttle. When the peg has fully dissolved, the boundaries of the array will be discernible from the shuttle and piece one. After the array is fixed in place, use the retraction micromanipulator to slowly withdraw the insertion shuttle, continuing to apply individual drops of saline onto the array as it is being retracted at the same retraction rates as the device was inserted, observing the interface between the array and the insertion shuttle during the retraction.
As it is retracted, the shuttle will visibly separate from the polymer array, which will appear translucent yellow between the shanks of the insertion shuttle. When the device has been fully retracted, move the array connector from piece two to a location that will not interfere with subsequent insertions. The polymer electrode array will be inserted in the brain and no longer connected to the stereotactic instrument.
Remove the insertion shuttle and other insertion hardware. If necessary, perform additional insertions. For construction of the implant, after the final array insertion, empty saline from the base piece, being careful not to disrupt the implanted arrays or ribbons.
Fill the craniectomies and the base piece with an appropriate artificial dural sealant. And allow the sealant to cure. Place the hardware connectors where they will not interfere, and if necessary, perform multiple insertions.
Appropriately orient and plug in the array connectors to the recording hardware, so the ribbons are in their final desired position. Finally, encase the ribbons from the point at which they leave the artificial dural sealant to the connector ends in a more viscous silicone gel. Following this protocol, a representative 1, 024 channel neural implant recording yielded 375 single units.
In this experiment, the recording longevity for a single unit was maintained for at least 160 days in data from 19 devices across three different rats. Of the 15 functional devices, a recording yield average of approximately one single unit was acquired per channel, and individual devices had yields of a few single units up to approximately two units per channel. During device insertion, it is important to make sure that the array shuttle is dry.
If it is not, there's a high likelihood that the array will detach from the shuttle during attempted insertion. The technique is compatible with other recording modalities, including micro wires, as well as with manipulations, including electrical simulation and optogenetics. This technique allows us to record from the same neurons across many days, making it possible to understand how the responses change over time.
