Twister3: a simple and fast microwire twister

Abstract

Objective: Twisted wire probes (TWPs, e.g. stereotrodes and tetrodes) provide a cheap and reliable method for obtaining high quality, multiple single-unit neural recordings in freely moving animals. Despite their ubiquity, TWPs are constructed using a tedious procedure consisting of manually folding, turning, and fusing microwire. This imposes a significant labor burden on research personnel who use TWPs in their experiments.

Approach: To address this issue, we created Twister3, an open-source microwire twisting machine. This machine features a quick-draw wire feeder that eliminates manual wire folding, an auto-aligning motor attachment mechanism which results in consistently straight probes, and a high speed motor for rapid probe turning.

Main results: Twister3 greatly increases the speed and repeatability of constructing twisted microwire probes compared to existing options. Users with less than one hour of experience using the device were able to make ~70 tetrodes per hour, on average. It is cheap, well documented, and all associated designs and source code are open-source.

Significance: Twister3 significantly reduces the labor burden of creating high-quality TWPs so electrophysiologists can spend more of their time performing recordings rather than making probes. Therefore, this device is of interest to any lab performing TWP neural recordings, for example, using microdrives.

Figure 1.

(A) Overview of Twister3’s mechanical components. The motor and wire feeder assemblies are used to rapidly construct TWPs by drawing a wire bundle from the feeder, clipping it to the motor, and performing a twist. Additionally, the motor, wire guide, and stock spool assemblies are used to load wire onto the bobbins in the feeder after they are depleted. (B) Motor assembly. A NEMA-17 stepper motor is used to twist TWPs and reload bobbins. The wire clip, alignment jig, and magnet allow the wire bundle to be rapidly and reliably linked to the motor. The rotor base and adjustment plate allow one-time adjustment to achieve perfect alignment between the wire bundle and the motor axis. (C) (left) The 3D printed leaf spring showing deformation under tension. The shape of the spring permits approximately vertical deformation of the wire attachment point so that the center axis is maintained as the bundle is shorted due to twisting. (right) Spring tension as a function of vertical deformation. Best fit line indicates a spring constant of 32 nM mm⁻¹. The white dot indicates the spring deformation needed to oppose the wire-feeder’s stiction setting for TWPs made in our lab. (D) (left) Wire quick draw mechanism. (right) Isolated single bobbin indicating the wire tension, due to the leaf spring in (C), and counter stiction due to the adjustable torsional spring.

Figure 2.

(A) Control electronics block diagram. All user IO is provided via a combined rotary encoder and button. The Teensy’s NXP MK20DX256VLH7 microcontroller provides a programmable interrupt timer (PIT) to control step commands to the motor driver, independent of nominal operation. (B) Control box with callouts showing features, connections, and controls.

Figure 3.

Bobbin loading and TWP creation procedures. (A) Summary of the bobbin loading procedure. See text for details. (1) The leaf spring is removed and a bobbin is magnetically mated with the rotor. The wire guide is moved into close proximity to the wire groove on the bobbin. (2) Wire from the stock spool is threaded through the wire guide and around the bobbin. The control box is then used to load a desired amount of stock wire. (B) Summary of the TWP construction procedure. See text for details. (1) The leaf spring is attached to the base rotor and the wire guide is moved out of the way. (2) The loose wire is alligator clipped and drawn from the bobbins. (3) The user’s free hand is used pull up on the twisting attachment’s leaf spring until under slight tension. (4) The alligator clip is drawn down and flipped 180° to meet the magnet within the alignment jig (insets). This procedure improves the alligator clip’s grip on the bundle. A twist can then be performed. (5) After the twist, wires are fused starting from point at which they separate towards the bobbins using a hot air gun. (6) The bobbins are simultaneously rolled forward forward to release tension on the wire. (7) The loose wire is cut e above the point at which the wires are fused, and the alligator clip is removed from the magnet with a finished TWP. (8) The finished TWP is cut into a storage box.

Figure 4.

Tetrode characteristics for different twist pitches and wire types. (A) (right) Diagram of the mechanical test. Tetrodes were attached to a rigid column and the exposed portion cut to a length L. The rigid column was lowered in small increments using a micro-manipulator onto a precision scale and the resulting force was measured to find the buckling point. (middle) Feeder height versus twist pitch, probe length, diameter, and average stiffness across exposed lengths when using nichrome or tungsten microwire of different diameters. Aside from the feeder height, all other construction parameters were kept the same as the rat column of table 1. The circled row indicates the parameters used to create the tetrodes that produced the data in panels (D) and (E). (right) Micrographs of sections of probe shanks for each twist pitch and material presented in the table. Each arrow-line is a quarter pitch. (B) (left) Compressive force versus depth lowered (δL) onto a rigid surface. Each line is a single tetrode sample cut to one of 5 exposed lengths (L = 25, 20, 15, 10, and 5 mm). Data point symbols correspond to the nichrome portion of table in (A) for various probe lengths. The buckling force (value at which there is no increase in restorative force with δL) is length dependent. Different exposed probe lengths form clear groupings with the bucking force increasing as the probe length gets shorter. (right) Buckling forces for each sample on a log scale. The buckling force is clearly grouped for each probe length but is not affected by twist pitch. (C) Stiffness versus probe length for each of the nichrome samples tested in (A). Of the parameters tested, longer, wide-pitch TWPs tended to be stiffest. (D) Example hippocampal mouse CA1 recording during sleep. (top) Raw voltage trace from a single tetrode. Three sharp wave-ripple events are clearly visible in the trace. (bottom) Spike amplitudes for each combination of two wires on the tetrode. (E) Example hippocampal rat CA1 recording during sleep. (top) Raw voltage trace from a single electrode. Multiple sharp wave-ripple events are clearly visible in the trace. (bottom) Spike amplitudes for each combination of two wires on the tetrode. Recordings in (D) and (E) are skull referenced.

Figure 5.

Tetrode construction time of users with ~1 hour of experience with Twister3. (left) Total construction time for each tetrode (symbols), average time across tetrodes for each user (thin lines), and across all tetrodes and users (thick line). (right) Timing for each step of tetrode construction. The dotted line above the turn and fuse step is the constant motor-in-motion time. Twister3 parameters were the same as the ‘Mouse’ column of table 1.

Figure 6.

Wire feeder assembly. (1) Insert press-in components into the feeder base. This includes M3 nut (2x), M3 standoff (2x), and 3/16” diameter dowel pin (2x). This requires a mallet. (2) Mount the feeder base unto the C1545/M mounting clamp using M6 screws (2x). The top of the feeder should be flush with the mounting clamp. (3) Cut two 5 cm sections from the M3 threaded rod. Turn each section into the M3 nut which behind the feeder base. The position of the rod determines the stiction on each bobbin during wire draw. Lower positions provide less stiction. We have found that the second notch is a good position to start with. (4) Use the 60 mm M3 screw to mount the bobbin assembly to the standoff captive within the feeder base. Repeat for both sides. The thumb-screw head should be glued onto the M3 screw using epoxy prior to this step. (5) Thread a torsional spring onto the dowel pin. Squeeze it together and then set it between the threaded rod on one side and the shallow groove in each bobbin on the other. Repeat for each bobbin. (6) Install the wire shield above the bobbins using a single M6 screw.

Figure 7.

Other mechanical assemblies. (A) Motor assembly. (1) Remove the long, M3 step screws from the bottom of the stepper motor (4x). (2) Use 40 mm M3 screws (4x) to attach the motor mount to the bottom of the motor. (3) Fix the rotor base onto the shaft with the M3 set screw. (4) Press fit two magnets into the alignment plate in the same orientation. Make sure they are pushed down until recessed below the plastic surface so that they do not interfere with the flat mating surface of the piece. (5) Insert the alignment plate into the slot on the rotor base. (6) Attach two additional magnets on top of those you just inserted into alignment plate. Press the spring rotor onto these magnets to press fit them into the spring rotor base. This procedure ensures that magnets will be press fit into the spring rotor base with the correct polarity. Make sure the magnets are recessed below the bottom surface of the spring rotor so that it rests flat on top of the alignment plate. (7) Push the clip magnet into one of the slots on the spring rotor top. In the remaining slot, press the twist alignment jig into position over the magnet. (B) Wire clip assembly. (1) Put two pieces of heat shrink tubing over the wire clip jaws. (2) Shrink into position using the hot air gun. This prevents electrode wire from slipping during a draw. The clip can then be stuck under the wire alignment jig (figure 3). (C) Wire guide assembly. (1) Screw the wire-guide into a mini, 6 mm diameter optical post. (2) Push the optical post into a swivel post holder. (D) Stock spool assembly. (1) Push a bearing into each of the stock spool bearing cases. (2) Push each of the bearing cases into the stock spool of microwire. (3) Push a 40 mm, M3 screw through the two bearings and screw into a mini, 6 mm diameter optical post. (4) Push the optical post into a swivel post holder.

Figure 8.

Twister3 assembly. 1. Screw together the large, 1.5” diameter mounting posts and then screw this long post into either the left or right side of the optical bread board. Mount the stock spool assembly in on the opposite side of the optical breadboard using a M6 screw. Its exact position does not matter. 2. Mount the feeder assembly on the post using the post mounting clamp on its back. 3. Mount the rotor assembly directly in front of the post, as close as it will go, using 3 M6 screws. 4. Mount the wire guide assembly into a position that is in close proximity to the motor assembly using a single M6 screw. The tip of the wire guide should be able to extend into the center grove of a wire bobbin when it is mounted on the rotor bases for wire reloading. 5. With the wire-clip mechanism installed, slide the adjustment plate around until the motor axis of rotation (dotted black line) is precisely in line with the wire bundle (red lines). When properly aligned, the apex of the wire bundle will appear motionless during motor turning.