MDR-xx Microdrive User Guide (Rev. 2.0) Axona Ltd Unit 4U St. Albans Enterprise Centre Long Spring, Porters Wood St. Albans, Herts, AL3 6EN United Kingdom http://www.axona.com
Table of Contents Axona MDR-xx microdrive geometry 1 MDR-xx microdrive with connector 2 Integral ground wire 3 Making a tetrode 4 Materials and equipment 4 Method 4 Attaching a cannula 6 Method 6 Wiring the microdrive 8 Materials and equipment 8 Method 8 Bubble test 10 Final cut 10 Reclaiming a used microdrive 11 dacqusb System Documentation - i -
Axona MDR-xx microdrive geometry The diagram below shows the overall construction of the MDR-xx series of microdrives. The drive works by means of an acrylic nut which moves vertically as the drive screw is rotated, because the nut itself is prevented from rotating by the cannula that sits parallel to the main screw. Each full rotation of screw moves the electrodes by 0.2 mm. The screw turner slots also allow quarter-turns to be made quite easily, which result in 0.05 mm electrode movements. The diagram shows the microdrive without any kind of connector attached. The diagram on page 2 shows how the connector fits in with this structure. dacqusb System Documentation - 1 -
MDR-xx microdrive with connector The connector provides the interface between the microdrive and the headstage buffer amplifier. A variety of connectors is available, but in most cases, these are configured so that short lengths of stiff insulated wire (shown in blue in the diagram below) run from the connector shell (shown in black) to near the top of the electrode cannula at the front of the microdrive. In later sections, we will describe how to attach electrodes to the bared ends of these wires. In some configurations, a Mill-Max connector is attached to the microdrive structure, and the electrodes are wire-wrapped directly onto the pins of the connector. dacqusb System Documentation - 2 -
Integral ground wire Some microdrives use connectors that include an integral ground wire (thick black insulated wire emanating from the connector shell at the back of the microdrive). If provided, this wire is used as follows: 1. Prior to implant, solder a short piece of insulated wire to a small steel screw (shown in green in the diagram below). Use wire that isn t too stiff, otherwise this will impede the movement of the microdrive. 2. Strip the end of the wire and tin it. 3. Strip the end of the ground wire coming out of the microdrive connector shell (shown in black) and tin it as well. 4. At implant, turn the screw prepared at step 1 into the skull, and then solder the free end of the wire attached to it to the free end of the microdrive ground wire. This connects the skull to the ground pin on the microdrive connector robustly, avoiding the need to have a separate ground wire. This makes for a much more robust grounding arrangement. The skull ground screw should be cemented in place at implant. dacqusb System Documentation - 3 -
Making a tetrode Materials and equipment Sticky tape (e.g., brown packing tape) cut into approx. 7 mm squares Reel of electrode wire (e.g. 25 µm platinum-iridium alloy) A pair of small and very sharp surgical scissors (note: a grindstone should be used regularly on the blade to keep the edge razor-sharp) A pair of fine-tipped watchmaker s forceps A rod, about 8 mm in diameter, oriented horizontally at least 30 cm above the tabletop or ground (we use a rod gripped in a bench-mounted vice) A weighting object with hook and magnet (e.g., a small stir-plate magnet fixed onto a 30g needle bent into a hook) A magnetic stir-plate Cigarette lighter or other gentle flame Method 1. Measure out a ~20 cm length of electrode wire (we do this by laying it out between two pre-drawn marks on the edge of the bench), cut with the scissors and lay one end on the tape so that it lies at an angle, with the wire crossing the edge of the tape at the edge s centre. Note crossing point Note angle 2. Pick up the other end of the wire with the forceps, bring it across and lay it down on the tape, at a different angle from the first end but also crossing in the middle of the edge. Thus, the two wire ends meet at the edge of the tape, and diverge thereafter: this maximizes the frictional hold of the tape on the wires and stops it slipping off when tensioned. Note crossing point Note angles 3. With the forceps, lift the far edge of the tape and fold it over so that the ends of the wire are now sandwiched between the two layers of tape, and held firm. Press down with a firm edge (the blunt end of a scalpel, or a fingernail) to make sure the tape is holding firmly onto the wires. dacqusb System Documentation - 4 -
4. Drape the wire over the suspended rod so that the taped end lies on one side and the fold on the other. 5. With the weighted hook (the illustration here shows a small stir-plate magnet clipped onto a bent syringe needle but any weighted hook of similar size and weight will do) catch both the loop formed by the fold in the wire and the loop formed by the taped join, and carefully hang the weight from these loops. Be careful not to jar the weight while doing this, as the wires will snap. Note: finding the correct weight is a matter of trial and error too heavy and the wires will snap or pull free of the tape, two light and the tetrode will not be sufficiently stiff. Turn on the stir plate, very carefully increasing the speed. Do not exceed the 2 setting. The twists will start at the bottom and work their way towards the top they should not be allowed to get too close to the rod, or the wires will not slide off the rod easily. If the tetrode is overwound it will start to form secondary coils (manifest as a wiggly appearance) or even snap: if not wound enough, the ends will unravel when cut free. Stop when the twists approach about 1-2 cm from the top of the rod. 6. Carefully cut the wires just above the point where they emerge from the tape. Be careful to support the weight with one hand or even disconnect it by sliding the hook out, while doing this, otherwise the sudden release of tension will cause the tetrode to recoil and develop kinks. 1-2 cm cut 7. Ease the tetrode off the rod. Cut through the loops at the top end of the tetrode, and tease them apart with the tip of a needle or forceps. Then, with the flame of a cigarette lighter or similar, carefully flame the distal 2/3 of the ends so that they glow briefly: this removes the insulation without burning the wire itself. dacqusb System Documentation - 5 -
Attaching a cannula Microdrives are supplied without electrode cannulae attached. The reason for this is that the appropriate cannula for a given implant varies according to the number and size of wires that will be implanted. Also, special cannula arrangements are sometimes used in which electrodes and a liquid delivery system are both moved down together by the drive. Furthermore, it is generally necessary to remove and replace the electrode cannula each time the microdrive is recovered at the end of an experiment for reuse in the next. Finally, the length of the cannula is determined by how deep the electrodes will penetrate. In this section, we show how to attach a typical cannula that would be suitable for straightforward implants of four tetrodes of 25 µm wire. Method 1. Using a diamond blade attached to a mini hand drill, cut a length of cannula of the required size and length. Using the sanding wheel or a file, smooth the ends and clear the inside of the cannula. For four tetrodes of 25 µm wire, use 21g cannula. For hippocampal implants, cut the cannula to 12 mm. 2. Cut a length of 30g cannula to 25 mm, and smooth the ends. The will acts as a holding cannula. Insert the 30g holding cannula into the 21g tetrode cannula. 3. Wind the microdrive so that the nut is positioned about 1/3 of the way from the top. Using a piece of Blu-Tack or similar sticky putty, fix the microdrive to a firm surface so that the connector is down and the surface to which the cannula will be attached is horizontal and facing up. Place a small piece of Blu-Tack at either end of the microdrive shaft, as far away from the centre as possible, taking care not to get Blu- Tack on the screw. Blu-Tack screw nut Blu-Tack 4. Lay the cannulae along the length of the microdrive, positioning the ends of the 30g cannula so that they are embedded in the Blu-Tack, and sliding the 21g cannula so that the upper end sits just proud of the nut and the lower end sits in the Blu-Tack. By adjusting the 30g cannula in the Blu-Tack, ensure that the cannulae are parallel with the screw in both dimensions, and centered on the nut. 21g cannula 30g cannula dacqusb System Documentation - 6 -
5. Mix up some dental acrylic so that it is thick and will flow slightly but not very much. With the tip of a 25g needle, apply the cement of the cannula where it lies across the nut, ensuring that it surrounds the cannula as much as possible (especially over the top of it) and blends with the cement surrounding the microdrive wires. When dry, carefully remove Blu-Tack and central holding cannula. If convenient, the sleeve (17g cannula, 7 mm in length) may be positioned at this time, and held in place with Blu-Tack. 17g sleeve Dental acrylic dacqusb System Documentation - 7 -
Wiring the microdrive Materials and equipment Two pairs of straight, fine-tipped forceps (these should be in good condition, so that the ends abut without gaps) Bottle of electrically conductive silver paint Bottle of colored but non-conductive (i.e., non-sparkly!) nail varnish Two 30g needles Blu-Tack or similar sticky putty Four tetrodes with insulation flamed off the ends Method 1. With a 30g needle, ream out the inside of the tetrode cannula to make sure it is clear of obstructions. Wind the microdrive down and then up to make sure it is working properly, and leave it with the nut close to the top, with only one thread of the screw visible. If you haven t already done so, slide the sleeve over the cannula and fix in place with a small blob of Blu-Tack. (Note: it is very difficult, though not impossible, to position the sleeve after the tetrode is in place: far better to remember to do this first!). 2. With the top facing towards you, fix the microdrive firmly to the work surface by embedding the feet and the connector in Blu- Tack (may also clamp in a vice). Slant the drive gently so that the long axis of the drive tilts down slightly. 3. With the forceps, carefully pick up one of the tetrodes at the top (where the wires fan out) and slide the tetrode into the cannula until the top of the tetrode (where the fanning begins) is just flush with the top of the cannula. Sometimes the tetrode may need to be pushed very gently to ease it into the cannula. Watch the tetrode carefully as you do this: if there is any hint of further resistance, stop pushing, withdraw the tetrode and ream out the cannula again if the tetrode bends at this juncture, it will be useless. dacqusb System Documentation - 8 -
4. With the forceps, bend the four wires sharply at the point where they emerge from the cannula this will stop them slipping back down into the cannula. Then take each wire in turn and wrap it very carefully as many times as possible (usually about three) around the bare end of each microdrive wire, taking care not to let the bare wires touch each other. Then slip the second tetrode into the cannula and repeat the procedure. Bend wires here 5. With the tip of a 30g needle, coat the wirewrapped wires with electrically conductive paint and leave to dry. When they are dry, with a different needle, coat all conductive areas (the wire-wrapped end plus any exposed de-insulated wire still showing) with nail varnish. When the nail varnish is dry (which may take some time), carefully fold the wires over towards the midline, exposing the second row of to-be-wire-wrapped wires. When both sides have been folded over, cover the whole area with a thin layer of varnish, being careful not to obstruct the entrance to the cannula. Electrical paint Nail varnish 6. Using the remaining two tetrodes, repeat the whole procedure for the second row of wires. When finished, coat all exposed wires with nail varnish. Photo of a completed microdrive dacqusb System Documentation - 9 -
Bubble test 1. Create tester by soldering guide cannula to the end of a wire and attached other end to the (-) lead of 9V battery. Attach a second wire to the (+) lead and stick the other end into a small beaker of saline 2. Clamp drive in helping hand and position tips of wires in the saline Place testing cannula over a gold pin of the drive and check for bubbles coming out of just one wire (no cross-connection). Do this for each pin and its corresponding wire (see diagram; ground = black) Final cut 1. Fill little beaker with saline and clamp drive in vice, wires pointed upwards 2. Try to get tetrodes to stick together with a little distilled water applied with needle or forceps 3. Use canned air to remove any dust or particles from gold-handled final cut scissors 4. Consider how long you need your wires to be to reach your area. Too short and you ll have problems with drive feet touching screws, too long and guide cannula won t be able to cover all the wire 5. Steady your arm (use arm rest, extra wiring boxes, etc) and cut wires to be a bit longer than you want 6. Repeat cutting process until all your wires are the same length (within the range you need) and all or most of the wire tips are shiny (indicates free of insulation and perfectly perpendicular) 7. Gold plate if desired dacqusb System Documentation - 10 -
Reclaiming a used microdrive When the microdrive has been removed from the animal at the end of an experiment, its feet will be embedded in dental acrylic. To use the drive again, this needs to be removed without destroying the integrity of the drive nut, which is also made of dental acrylic. The best way to do this is to wind the nut up to the top as far as it will go, to keep it out of the way, and then prop the drive upright in acetone, with the liquid just covering the dental cement but no more. Make sure the container is placed in a reasonably well ventilated area, or the build-up of acetone fumes will soften the nut as well. Leave the drive to soak in the acetone for a few hours or so, until the cement develops the consistency of almostset toffee, and then remove the acrylic with a small pair of wire cutters. It shouldn t take too much force to cut through the acrylic if it does, it has not softened enough, and the procedure needs to be repeated. The cannula will often loosen during this process, so expect to have to replace it (though this is not always necessary). Once the drive is free of dental acrylic, the nail varnish and electrical paint can be removed by swabbing the drive with cotton buds soaked in acetone. This is a time-consuming and uses a lot of cotton buds, but the drive can be completely cleaned this way and then used again. Be careful not to bend the wires on the drive more than necessary, as they will eventually fatigue and snap. dacqusb System Documentation - 11 -