January 1997
If these instructions are found wanting, blame Mark Churchland
(and to some extent Sergei Rebrik).
Conceptually, making a tetrode is very simple. The steps are as follows:
In practice, making a tetrode is akin to building a ship in a bottle; it is very delicate work and it is remarkably easy to make a mistake and ruin the previous three hours worth of work. Before beginning work, it is probably best not to drink coffee, work out, take large doses of psychostimulants or do anything that will affect the steadiness of your hand. The whole process will take 2 - 6 hours depending on your skill and luck. Your mission, should you choose to accept it is to do the following:
(a) Take the end of the very thin wire and attach a clip to it. Be careful never to tug sharply on the wire, even if you don't break it you will cause it to kink. Lift up the spool, dangling the clip by the end. Feed it over the second notch of the tetrode hanger thingy. Let out wire until the clip is just touching the table on one side while the spool is sitting on the table on the other side. Attach a second clip to the wire on the spool side, just above the spool. Allow both clips to hang suspended by the wire between them. Cut the wire between the second clip and the spool. Now, clip both wires together just above the other two clips. Cut off the bottom two clips. The loop should hang approximately 35 cm from the top to bottom.
(b) Repeat step (a) for a second wire hung over the first notch of the tetrode hanger thingy. Make sure the two clips on the bottom of the two loops are hanging at the same height. You may have to re-clip one or both.
(c) Feed the tetrode winding tube (a pen shaft with four notches cut in it) through the tops of both loops and bring it downwards until the loops support its weight. Each wire should fit in a notch. This will hold the four ends in a square pattern.
Arrangement before winding
(d) . Use the tetrode winding device to wind the tetrode. Fast forward to the end of the tape. Reset the counter. Rewind the tape the number of rotations you wish to wind (the counter reads half the number of rotations; if you wish to wind the tetrode 400 times, you should rewind from 000 to 800). Raise the tetrode hanger 2 or 3 inches on a book or some such thing. Place the winding device underneath it so that the devices metal fork engages the tube. Remember that as the tetrode is wound, it will shorten slightly (a few cm). Be sure that when this happens it will not rise above the metal fork. Now press play. The tetrode will be wound, and the winding will stop when the end of the tape is reached. (It is not clear to me what the ideal number of twists per inch is, or even if there is an ideal number. 400 twists is a good place to start.)
The wire has a thin, plastic, insulating coating that can be melted. This allows the four wires to be permanently attached so that they do not come unwound.
(a) After winding, use the heat gun to melt the wires together. Be careful not to touch the wires with the gun or they may break. I use four or so up and down passes, with each pass taking about three seconds (for up and down). I try to make the passes at different orientations to ensure even melting, although this may be unnecessary.
(b) Attach a clip to the bottom of the hanging arrangement, just above where the four wires separate. Cut the four wires below this so that the wound wires are hanging by only one clip on their bottom. Allow the wires to unwind and the clip to spin. It will unwind quite rapidly. Stop it every two seconds or so, to see if the wires are still keen on unraveling. They should loose their urge to unwind after about four or five seconds of rapid unwinding. Much more than this (i.e. 15 sec) means you did not sufficiently heat the wires. You will not know until stage (3) if you have heated them too much.
(c) Attach a clip to the top of the wound wires, at the point where the four wires separate. This will help prevent things from tangling. Lift the wound wire from the top, above the top clip, and lay it on the table. Take the four wires emanating from the recently applied clip at the top, and attach another clip to each, about 2.5 cm from the previously mentioned clip at their join. Cut away the excess loops of wire.
(d) Cut the bottom end of the tetrode-to-be just above the clip. Insert this end into the highly poisonous fluid that is used for gold plating, and clip it there (really, you should use saline, though that will mean making another container). This fluid container is attached to an impedance meter. Also leaving this meter is a green wire that you should attach to one of the small black male plugs. This will allow you to see if the connections you make at the next stage are good. Be careful when doing this not to kink the wound part of the tetrode-to-be anywhere near the top. This is the end that will comprise part of the final tetrode. The bottom end will be severed later.
Layout after step (d)
(a) Mix the nickel print glue well. The nickel has a tendency to solidify at the bottom and will need to be mixed up with a knife or some such object. Poorly mixed glue will make it frustratingly difficult to make a good connection.
(b) The goal is for all four wires to be connected to their respective pins by a roughly equal amount of wire. This way, when the wires are inserted into the steel tube, there will be little slack sticking out. Take the bottom most wire and clip it with the forceps about 2-3 cm from the junction point. Cut the wire, leaving 5mm or so sticking out of the forceps.
Turn on the gas and light a flame at the end of the syringe. Adjust the flame until it is barely present. Turn off the light so that you can see yourself work. Use the bottom of the flame to burn the insulation off of the last 2 or 3 mm of the wire. Heat it until it glows but not curls. If it curls, it will be difficult to insert into the glue-filled tube in step (c). At this point I like to turn the forceps over so that the end of the wire is pointing away from me.
Shoving the wire in the plug
(c) Cut a number of 2-3 mm pieces from the end of the Teflon tubing. Pick one of these up by inserting one prong of the rubberized green forceps into it. Run the piece of tubing along the side of the glue brush so that it draws the glue up into itself. Wipe off any excess and then quickly shove in the end of the wire (if you take too long, it will be difficult to convince the wire to go through the glue). Around 1-2 mm of wire should enter the tube.
(d) Now remove the forceps from the wire and use the green forceps to push the tube onto the topmost pin. If the connection is good then the meter should read about 20 - 30 (corresponding to 2-3 mega-ohms). If the connection is not good (i.e. meter reads more than 30) then the tube should be removed, the wire removed from it, and the whole process repeated. Sometimes a little wiggling of the tube can cure a bad connection, but when this works, the connection will usually go bad again if the tube is wiggled further. I feel more confident if the connection is stable despite a little wiggling.
Repeat this process for the three remaining pins.
(a) First cut a length of steel tube. If making a tetrode for cortical recording, use a 6 cm length of 30 gauge tubing (Sergei says use larger). If making a tetrode for LGN recording use a 10 cm length of 31 gauge tubing. Cut it with the wire cutting tool (it has notches in its blades, and is less likely to crush the tube). Check to make sure the cut end is not overly crushed. You may have to make 3 or 4 cuts to get a good one. Deburr the end with a needle.
For LGN style tetrodes, also cut a 6 cm length of 23 gauge tubing (this will form a guiding tube) Use the grinder to sharpen one end into a syringe like tip. Use a spare length of 31 gauge tubing to deburr the inside (spend some time doing this). Thread the guiding tube over the supporting tube.
(b) Thread the braided tetrode wire into the supporting tube. Be careful not to kink the wire as you push it in. Once it appears out the other side pull it through, bringing the tube all the way to the plug end. Pull through as much of the slack as possible, but don't tug on it.
(c) Cut away all but the last 2 cm or so of the wire. Put the whole mess into the mold. You may want to support the tube with some object (such as a pen) to keep the wire from being bent. Check to see if the wires are touching the sides of the mold. If they are they may be torn when the tetrode is removed. You can rearrange them or tape them to the pins. Also make sure that the wires do not extend too far upwards, or they may protrude from the epoxy.
(d) Mix up some five minute epoxy and pour it in the mold. After it has set for fifteen minutes or so, put a drop of super glue on the end of the supporting tube where the wire exits (a syringe with a narrow gauge needle works well as an applicator). This will help support the wire, and will keep it from compressing into the tube. Be careful not to get any super glue past the first mm of the wire. If you are making an LGN style electrode, put very little glue on, as a large drop will prevent the supporting tube from sliding freely within the guide tube. You can just let a little glue touch the junction between the wire and tube, which should cause some glue to be drawn into the junction. From this point on be very careful not to bend the tip. It is very easy to bend the wire at the place where it enters the supporting tube. If this happens then, even if the wire can be bent back, the tetrode is probably ruined. The tip is likely to be too weak to penetrate. (For an LGN tetrode, this may be less critical, as the guiding tube makes the initial penetration, and to some degree will guide the wire in a straight path.)
Tetrode arranged in the mold
(e) Now cut away the end of the wire, leaving 4-5 mm for a cortical tetrode and 3-4 mm for an LGN tetrode. If the wire is too long, stiffness will be compromised. If it is too short it will not be able to go deep enough.
(f) Wait a full day before prying out the tetrode. Do this carefully and patiently; if it pops out of the mold suddenly the tip will almost certainly be bent.
(a) To aid penetration, the tip is sharpened slightly. Use the Dremel grinder, held with the grinding bit UP (not like in the picture) in the drill press attachment.
Carefully lower the tip onto the grinder. The tip should make contact with the grinder at an angle of around 45 degrees. Let the tip drag lightly on the grinder for around 3 seconds. Rotate the tetrode 90 degrees and repeat the process. Do this twice more until you are back where you started. You may want to look at the tip under a microscope.
(b) Put the tetrode in the manipulator and lower it into the small vial marked Poisonous. This contains gold and cyanide. Hook the red pole of the ohm-meter to the vial and the green (ground) to one of the four pins. You should be able to read the impedance (hopefully around 2-3 mega-ohms, less is fine too). Connect the output of the pulse stimulator to the EXT signal ports of the ohm-meter. Turn the pulse stimulator on. You should be sure the pulse stimulator is set at the right current. Use the pen-like mulitmeter in voltage mode. Adjust the stimulator until the voltage is 1V. As the internal resistance is 1 mega-ohm, this corresponds to a current of 1 micro-amp.
There is a switch on the ohm-meter with the settings TEST and EXT. Flick the switch to EXT and then quickly back again. This sends a current through the tetrode and will pull some gold onto its end. This should lower the impedance. You may have to wait ten seconds or more for the impedance to settle. The goal is 1 mega-ohm (10 on the meter). Continue giving pulses of current until the impedance comes to rest around 0.5-0.8 mega-ohm.
Repeat this for all four pins. Then check each pair of pins for a possible short. Remove the tetrode from the solution and use the green wire and the orange wire that is connected to the vial. The impedance between each pair should not be less than 3.5 mega-ohms (35 on the meter), and should ideally be considerably higher. If you do find a short, it was probably caused during the gold plating (the gold can form a bridge between the ends of two wires; the impedance will be zero). Grind the tip again as in step (a) and check for shorts. If the short has disappeared then replate the tip. Chances are good it will now be fine.
If you find a short with impedance significantly above zero, it is probably not as a result of a gold bridge. It is probably the fault of capacitance between the wires (the result of bad luck or of overheating at stage 2(a).)
You are now finished with the tetrode! Store it somewhere safely out of harms way.
Item |
Cat. No |
Company name |
Address |
Phone |
|
| 1 | fine wire | REDI-OHM 800, 0.0005 diam. | H.P.Reid CO., Inc. | One commerce Boulevard, PO Box 352440, Palm Coast, FL 32135-2440 | (904) 445-2000 |
| 2 | Male plug | H2096-ND | DIGI-KEY | http://www.digi-key.com/ | (800) 344-4539 |
| 3 | Teflon tubing | SWTT-24 | Small parts Inc. | PO Box 4650, Miami Lakes, FL 33014-0650 | (800) 220-4242 |
| 4 | Stainless steel tubing | HTX-30 (number = gauge) | Small parts Inc. | PO Box 4650, Miami Lakes, FL 33014-0650 | (800) 220-4242 |
| 5 | Nickel Print Paint | 22-207 | G-C Electronics Co | 1801 Morgan St, Rockford, IL 61102-2675 (*) | (815)968-9661 (*) |
| 6 | Gold-plating solution | 30302301 | SIFCO Selective plating | 300 S.Walnut Ave. Suite 401, San Dumas, CA 91773-3000 | (909) 592-8516 |
(*) Not sure that this is correct.