The overall aim of this procedure is to assemble an ultra light motorized micro drive suitable for making stable extracellular recordings from behaving small animals. This is accomplished by first securely mounting the connector and motor to the chassis. The second step is to shape the shuttle to fit the chassis and attach the shuttle tubes.
Next, the electrode guide tubes are installed and the recording electrodes are loaded into the micro drive. Finally, transparent covers are added to protect the electrodes and wiring from accidental damage. Once properly assembled and implanted the micro drive described may be used to record extracellular activity over a period of up to 10 weeks.
Our design and assembly procedure have evolved over time to maximize ease of use and minimize cost, such that it is more practical for and accessible to a wider array of researchers than many previously published versions. A complete micro drive is shown here. It consists of several main components, a chassis that serves as the super structure for the drive a motor with a finely threaded output shaft, a threaded shuttle that carries the electrodes and provides a point of electrical connection and a noetics connector shown also at the shuffle tubes, electrodes, and electrode guide tubes.
The chassis electrode shuttle and electrode shuttle tubes are custom components that were designed with a 3D CAD software and were machined by a local precision machinist. All other components are commercially available. First use diagonal cutters to trim the contact pins on the bottom of the connector.
Trim the rear row of pins to 1.5 millimeters and the front row to one millimeter as shown on this schematic. Then carefully secure the chassis in a small vice with the open cavity down and use cyanoacrylate glue to attach four 15 millimeter lengths of polyamide tubing to the chassis so that they're flushed with the rear chassis surface and oriented parallel to each other as shown here. Once the cyanoacrylate glue is dry, trim the polyamide tubes to approximately six millimeters in length and remove the waist sections.
The remaining tube sections should be approximately three millimeters from the bottom edge of the chassis. Next, use epoxy on the chassis to construct a small pedestal for the connector. Place the connector on top and add additional epoxy around the connector.
The connector surface should be a few millimeters above the polyamide tubes and be parallel with the chassis surface. Once the epoxy has completely cured, remove the chassis from the vice and transfer to a jig with the front surface facing up. Then guide the motor wires through the hole in the chassis and slide the motor into the groove in the base.
After checking that the motor and drive shaft are parallel to the chassis and fits snugly against the bottom and side, epoxy the motor to the chassis. Once the epoxy has cured, rotate the chassis so that the connector is accessible To reduce mechanical stress on the motor and wires, add a small amount of fast set epoxy to the wires where they emerge from the chassis. Now trim strip and sold the wires to the appropriate contacts on the connector.
Test the motor to ensure smooth shaft rotation in both directions and then proceed to assembling the electrode shuttle. First, test the fit of the shuttle by carefully threading it onto the motor shaft and running it up and down the full length of the drive shaft. At low speed.
The shuttle should fit snugly in the chassis and should not become wedged in the drive, nor should the motor shaft cease turning. If the shuttle does not move smoothly down the shaft. Inspect the surface of the chassis and the drive screw for debris.
Remove any debris with fine forceps or compressed air, and then coat the threading in light mineral oil and recheck the fit. If the shuttle is too tight, use forceps and a fine grain wetstone to remove obstructing material from the side of the shuttle. Take care to remove the material evenly.
Finally, if the shuttle is loose as evidenced by tilting and chattering during travel glue, a small sliver of transparency film to the shuttle to fill the gap between it and the chassis surface. Once a proper fit has been achieved, remove the shuttle from the shaft and press a stainless steel tube into each of the four large holes in the shuttle, ensuring that the tubes are centered on the shuttle and that the ends of all four tubes are flush as seen here at a small drop of cyanoacrylate glue to the base of each tube, ensuring that the glue does not get on the edges of the shuttle or into the tubes. Once the glue is dry, thread the completed electrode shuttle back onto the drive shaft and bring it all the way to the base of the threading.
Cut 4 25 millimeter long sections of polyamide tubing for uses electrode guides. In addition, cut four electrodes into 40 millimeter sections for uses dummy electrodes during assembly. Slide each section of tube onto a dummy electrode such that the electrode extends around 10 millimeters beyond the end of the tube.
Now arrange each of the polyamide tubes in the chassis as shown here. Mix a small amount of quick cast and apply approximately two millimeters below the top end of the electrode and allow to dry. Next, remove the dummy electrodes and reposition the free ends of the electrode guides so that they form a tight bundle at the bottom of the drive.
Wrap a fine wire around the electrodes to hold them in place if need be. Then fix the new position with quick cast as shown here. Use a sharp scalp or to trim the electrode tube guides where they extend beyond the bottom of the micro drive.
These tubes will eventually guide the electrodes to the surface of the brain, so the chosen length depends on the anatomy of the implantation site. Then cut a 30 millimeter long section of insulated platinum wire and strip one millimeter of insulation from one end. Next, solder it to one of the signal pins on the connector.
Then bend the wire under the connector and push it through one of the wire guide tubes. Pull the wire taut and bring it to the guide slot in the top of the chassis with the electrode shuttle at the top of the shaft. Insert a 25 millimeter electrode into the polyamide tube and pull it up through the stainless steel tube.
In the shuttle. Position the tip of the electrode so that it is flush with the bottom end of the polyamide tube. Then trim the electrode one millimeter above the shuttle tube.
Use fine forceps to strip the insulation from the last two millimeters of the electrode and the last two millimeters of the platinum wire. Reposition the electrode such that the tip is aligned with the bottom of the guide tubes. Then insert the platinum wire into the shuttle tube and pin the two together by sliding a short one millimeter section of tungsten wire into the shuttle tube.
Check that there is a tight fit between the electrode and wire to provide both a mechanical and electrical connection. Then run the shuttle up and down the length of the drive shaft to check for continued smooth movement. After installing all of the electrodes in the same manner, cut a 30 millimeter long section of silver wire for uses a ground wire.
Strip one millimeter of insulation from one end and sold it to the ground pin to reduce friction of the electrodes against the guide tube walls. Place a drop of light mineral oil at the end of the guide tubes and allow capillary action to draw the oil into the tubes. Next, cut a 12 millimeter by 25 millimeter rectangle of transparency film and fold it into a wide U shape.
Use glue to attach this cover to the outer surfaces of the chassis. Then cut a 15 millimeter by six millimeter rectangle of transparency. Film and use cyanoacrylate glue to secure it around the connector, ensuring that the long edge of the piece of transparency film is flush with the top edge of the connector.
The final device should look like this. The following images show representative recordings from the robust nucleus of the arch AUM in the behaving zebra finch. The first image shows single unit activity recorded with a 10 mega platinum iridium electrode one week after implantation.
This next image shows a recording from the same electrode used for the previous image, but captured approximately nine weeks later. Note the continued high quality of the recording by using lower impedance electrodes. Stable multiunit activity can be recorded.
This final image shows multiunit activity recorded with a one mega oh platinum electrode. Once the assembly procedure is mastered, a micro drive can be fully assembled in about five hours, excluding epoxy drying time once initially built. However, the drive can be prepared for reuse in less than two hours.
