In this protocol we demonstrate how to fabricate and condition tetrodes for use with a micro-drive array, which was designed for chronic electrophysiological recordings in rats. In addition, we illustrate the final stages of micro-drive array construction, which includes installing ground wires and a protective cone.
1. Fabrication of tetrodes
2. Loading the tetrodes and ground wires into the micro-drive array
3. Setting the length of the tetrodes
4. Gold plating of tetrodes and quality check
Gold plating the nichrome tetrode wires is critical for long term stable recordings. It prevents corrosion and improves biocompatibility.
5. Attaching the protective cone
The protective cone’s purpose is to shield micro-drives and the exposed tetrode wire from the environment. It also provides support for handling during an experiment and reduces electrical noise. We use a 3-D printed plastic cone (Reference Part I: drive fabrication). However, it is possible to make the cone from other materials, like a bent piece of soda can.
6. Representative results
Figure 1: A) Schematic of tetrode twisting configuration. A horizontal bar suspends the unbonded electrode wires of the tetrode above the automated tetrode twisting device. B) Setup for tetrode impedance measurement and electrode plating. When checking for short circuits, the circuit is different: the black wire is attached to another electrode channel on the connector board. C) A close-up of a custom made micro-drive array holder tool. The cylindrical bar is an X-Acto knife handle. The connector is a 26-pin, double row, Mill-max connector. The two pieces are bonded together using dental acrylic. D) Configuration for short checking. Channel X and Channel Y are any two different channels of the four tetrode channels. E) Configuration for measuring electrode impedance. The current source is ‘off’. The impedance meter is set to ‘on’ and ‘test’. F) Configuration for gold plating of electrode tips. The current source is set to ‘on’, audio is ‘on’, mode is ‘unipolar’, DC Test is ‘on’, current is set to 2 microamps.
This protocol is the second installment of a two part protocol: “Micro-drive array for chronic in-vivo recording.” The combination of the micro-drive array and tetrode is a powerful tool for obtaining simultaneous recordings of many neurons in an awake, behaving animal preparation. Hopefully, we have provided you with all the necessary information for you will need to begin or improve electrophysiological recordings in your own laboratory.
The motorized tetrode twister device was custom designed and fabricated. We have provided a vendor and part number that will allow you to order a device with similar functionality. Alternatively, a motorized turning device may be constructed in-house simply by combining a motorized rotating bar magnet with a magnetized stirring rod that is attached to an alligator clip; this configuration would most likely require manual counting of rotations, manual reversal of rotation direction, and manual starting/stopping.
Some electrode interface boards do not have pin-and-hole mechanisms for connecting the electrode wire. Instead, you may have obtained an interface board that has metal pins that extend out from the connector board; such metal pins are similar to those often found on standard electronic components. We briefly outline a methodology for connecting the tetrode channels to metal pins. First, remove the insulation on the tips of the four electrode wires of each tetrode. This is accomplished by applying a flame to the tip of each electrode as briefly as possible using a handheld butane torch. Obtain shrink wrap that will fit easily over the metal pin, and will squeeze the pin tightly after the shrink wrap has been heated. Cut the shrink wrap tubing to the length of the metal pin. Use capillary action to fill the shrink wrap with silver paint. Insert the exposed tip of one electrode wire into the shrink wrap. Gently slide the shrink wrap over the metal pin such that the electrode wire is between the pin and the wall of the shrink wrap. Repeat these steps for the remaining three electrode channels of the tetrode. Finally, with a side-to-side motion of a heat gun, heat the four pins covered in shrink wrap (with the electrode wires inside the shrink wrap). At this point, you should have stable and reliable connections between the tetrode and connector board.
Throughout these fabrication procedures handle the electrode wires with care as very little force is required to damage them. There are many steps in the tetrode and micro-drive array procedure and each step is as crucial as the others for obtaining high quality data. The protocol described here has worked well for our laboratory considering the equipment available. The resources in your own laboratory may be different, therefore, consider each step carefully and do not hesitate to change the protocol to meet your own requirements and constraints in order to achieve optimal performance.
We would like to acknowledge Jun Yamamoto for his design and construction of the automatic, motorized tetrode twister device depicted in the video. In addition, we would like to acknowledge all past and present Wilson Lab members for their contributions to micro-drive and tetrode technology development in the lab.
Material Name | Typ | Company | Catalogue Number | Comment |
---|---|---|---|---|
Panavise | Tool | Panavise | Model 301 | Holds either micro-drive array or serrated scissors |
Modified Xcelite 378M Pliers | Tool | Newark | 96F8903 | For inserting connector board pins. To make, remove 2mm from one tip of the pliers. |
Slow Cure Glue | Material | Great Planes (Champaign, IL) | GPMR6015 | To glue the tetrode to the micro-drive |
Gold Plating Solution | Material | Neuralynx | Sifco Selectron Gold NC | Contains gold particles for electroplating |
Current Generator | Tool | World Precision Instruments | A365D-B | A.K.A. Stimulus Isolator. Used to electroplate gold solution onto electrode tips. |
Impedance Meter | Tool | BAK Electronics | IMP-1 / 6662 / 2788 | To check impedance of electrodes |
Hot Air Gun | Tool | Steinel (Germany) | HG 3002 LCD | To fuse electrodes together to make tetrode |
Accu-Tek Carbofib Tip Tweezers | Tool | Aven (Ann Arbor, MI) | 18768 (Pattern 304) | For handling tetrodes |
Alligator Clip | Tool | Fine Science Tools | 18050-35 | A.K.A. Bulldog serrefine. To clip the tetrodes together during twisting |
Micro Dissecting Scissors | Tool | Biomedical Research Instruments | 25-1000 | For general cutting of tetrode/electrode wire |
Iris Toughcut Serrated Scissors | Tool | Fine Science Tools | 14058-09 | For final cut of tetrodes |
Teflon Coated Stainless Steel Wire | Material | A-M Systems (Carlsborg, WA) | 790500 | Ground wire |
12.5 micron nichrome wire | Material | Kanthal Palm Coast | Rediohm-800 1/4 HARD PAC COATING | Polyimide coated micro-wire for making tetrodes. |
Tetrode Twister | Tool | Neuralynx | Tetrode Assembly Station | Increases the speed and consistency of tetrode fabrication. |
Silver Paint | Material | GC Electronics | 22-023 | For connecting the micro-electrode wires of the tetrode to the connector pins (see Discussion) |
X-Acto Knife Handle | Material | MSC Direct | X3001 | For constructing a micro-drive holding device. |
26 Pin, 2 Row Connector | Material | Mill-max | 852-10-026-30-001000 | For constructing a micro-drive holding device. |