Optetrodes

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Picture of the "optetrode" drive [1]. Note that the tetrodes were accidentally bent on this particular drive. Courtesy of Aaron Andalman, Deisseroth Lab.

Generalities

Optical fibers can be coupled to existing multitrodes (multi-electrodes with short distances between wires to allow amplitude-based spike sorting). The most popular form of multitrode is the tetrode (4 wires bundled together or integrated into the same shaft). Existing tetrode drives can be adapted with minimal work to integrate one or several optical fibers. An easy solution consists in replacing one tetrode by an optical fiber; this way the fiber runs along the tetrode assembly with the possibility of moving it independently.

The "optetrode"

Principle

A compact "optetrode" design was published by the labs of K. Deisseroth and L. Frank [1]. This optetrode is compact, robust, lightweight (2 g, therefore adapted for small animals) and low-cost. The drive can be assembled in less than 3 hours once you have all the parts.

The optetrode drive is based on a simple co-axial mechanism. The drive design consists of a vented screw, a thumbnut and a plastic housing. The fiber-tetrode assembly runs concentrically through the vented screw, and the vented screw is machined such that the plastic housing prevents its rotation. Thus, rotation of the thumbnut translates (without rotation) both the screw and the fiber-tetrode assembly co-axially through the brain. This design yields a device with a modest weight (2 g on completion and ~2.5 g on implantation, including the weight of the acrylic affixing the device to the scull) and spatial dimensions (height, 22 mm).

The optetrode drive published by Anikeeva et al. [1] was equipped with 16 microwires (polymer-coated nickel-chromium alloy, 12-μm diameter) wound into four tetrode bundles (diameter, ~25 μm). To ensure consistent and sufficient light intensity near the recording sites, the authors rigidly attached tetrode bundles to the fiber shaft (diameter, ~200 μm) and cut them to extend 300–1,000 μm beyond the end of the optical fiber. The high fiber diameter to tetrode diameter ratio (200:25) insures negligible shadowing of the light by tetrode bundles. Thus, the fiber acts as both a light source and a structural support for the tetrodes during translation through the brain tissue.

Parts for optetrode v01, imperial units

Vertical cross section of the optetrode. The body of the device consists of a plastic housing and a thumbscrew, held tightly in place with two friction-fit plastic pins, mechanically driving a vented screw into which a protective tube containing the four tetrodes and the multimode fiber is glued. The head of the screw is epoxied to both the metal ferrule optical connector end of the fiber and an electronic interface board, which connects the tetrode microwires to an 18-pin electrical connector. Inset, horizontal cross section of the optical fiber with four tetrode bundles affixed. From [1].
Exploded view of the optetrode drive showing its different components. From top to bottom: vented screw with 2 flat surfaces, thumbnut, plastic housing with 2 interference Dowel pins. From [1].

Anikeeva et al. [1] used a screw with a 2-56 UNC thread (0.454 mm pitch i.e. 56 threads per inch, around 1.8 mm diameter) and a thumbnut with the corresponding internal thread.

Thumbnut

BEWARE: comparison of the above thumbnuts from McMaster and Farnell shows that the aluminium thumbnut from Farnell shows less variability in the same batch, in particular in the concave round part (which is crucial for the precision of the friction fit of the interference pins).

Vented Screw

Interference Pins

  • From McMaster: part #97155A206: Plastic Dowel Pin 1/16" Diameter, 1/4" Length. Material: acetal.

BEWARE: the McMaster reference given in the paper [1] is wrong; use the one given here.

Plastic Housing

  • The plastic housing was designed in SolidWorks 2007 (Dassault Systèmes SolidWorks). You can download the original SolidWorks file from the Deisseroth lab (dimensions in inches). The drive can be machined based on this file. It was originally machined in Delrin (polyoxymethylene) but other materials should work fine (PVC or even aluminium).

Protective Tubing

In order to fit inside the vented screw, the protective tubing needs to have an OD < 0.02". The original paper [1] used a PEEK tubing with an ID close to 0.01" to have a tight fit around the fiber and tetrodes but the corresponding Small Parts reference seems to be discontinued. The following references from Small Parts have not been tested but should work:

  • Translucent Amber PEEK Tubing, 24 AWG, 0.020" ID, 0.024" OD, 0.002" Wall. ASIN B00169NLGE. Link here.
  • White Translucent Miniature PTFE Tubing, 24 AWG, 0.020" ID, 0.024" OD, 0.002" Wall. ASIN B0013HMTI0. Link here.

Optical fiber + connector

Interface Board

The original paper describing the optetrode used the EIB-16 interface board from Neuralynx.

Parts for optetrode v02 (semi-metric equivalent)

A semi-metric equivalent of the optetrode drive was designed by Guillaume Dugué at the Ecole Normale Supérieure, Paris. The dimensions of the plastic housing are in mm but the 2-56 screw and thumbnut can still be used. The plastic housing can be machined with metric tools only. A 1.59 mm reamer will be necessary to obtain the right hole diameter in the plastic housing for inserting the 1/16 inches diameter Dowel pins.

Thumbnut

The 2-56 thumbnut was ordered from Farnell: (part #1874004 (aluminium). The CAD drawing is here. OEM: RAF Electronic Hardware.

Vented Screw

For this drive, plain 2-56 screws were purchased online from Modelfixing. Two flat surfaces were milled on each screw so that it fits in the 1 mm notch at the bottom of the plastic housing. In the lower, flattened part of the screws, holes were bored using a 0.5 mm bit (in the upper part, a larger hole e.g. 1 mm can be bored). This small diameter in the lower part of the screw allows milling nice large flat planes (1 mm spacing between the two planes to fit in the 1 mm notch of the housing).

Plastic Housing

Step-by-Step Fabrication Instructions

Taken from [1]

Tetrodes

  1. Use epoxy to glue ~16 mm long plastic tubing to the screw hole next to the ground pinhole of the EIB-16. Plastic tubing should rise ~0.3-0.5 mm above the EIB-16 in order to prevent glue from getting inside. Plastic tubing should protrude ~14 mm under the EIB-16. Let the epoxy dry for at least 15 min.
  2. Cut ~5 cm long ground wire. Burn or strip plastic coating off the wire ~1 cm at each end. Use a gold pin to attach the ground wire to the EIB. It may be necessary to widen the pinhole beforehand by pushing in a pin.
  3. Make 4 tetrodes following standard protocols reported in literature (e.g. Ref. 18)
  4. Affix the tetrodes to the EIB-16. Feed the 16 individual ends of the wires into the 16 pinholes of the EIB-16 following the bottom up direction. Fix the wires to the EIB-16 with gold pins. The EIB-16 should now have a guiding tube and 4 tetrodes hanging underneath.
  5. Use soldering iron to melt the gold pins into the pin-holes. Press hot iron to each pin for ~10-15 seconds. Also do this for the ground wire.
  6. Feed the tetrodes through the guiding tube

Drive

  1. Screw the vented screw into the thumbnut half way, so that the bottom half is protruding from the thumbnut by 2-3 mm.
  2. Insert the screw-thumbnut assembly into the plastic housing. It will only fit in the designed orientation in which flattened facets of the vented screw align with the flat sides of the tunnel of the plastic housing.
  3. Fix the thumbnut to the plastic housing with the interference dowel pins. From this point turn the thumbscrew counterclockwise to lower the screw through the housing. Lower the screw to near the end of its range of motion.

Assembly

  1. Use a tall holder (e.g. optics post) to suspend the EIB-16 with tetrodes above the mechanical drive affixed to a shorter holder.
  2. Lower the EIB-16 towards the drive, so that tetrodes go into the vented screw first, followed by the protective tubing until the EIB-16 is flush with the cap of the vented screw.
  3. Use a fiber-ferrule cut to the length appropriate for the experiment. The fiber length should be sufficient to protrude from the vented screw and reach the area of interest within the travelling distance of the vented screw.
  4. Lower the fiber-ferrule into the plastic tubing.
  5. Use Superglue or equivalent to attach the tetrodes to the fiber and wipe excess glue. At this point, the tetrodes should still be significantly longer than the fiber-ferrule; hence coating them in glue does not pose a problem. Use a small amount of glue to fix the fiber-ferrule to the guiding tube and EIB. It is important to ensure that some glue penetrates inside the vented screw; this will insure a good seal. Wait for glue to dry completely (~15 min).
  6. Use 2-part epoxy to attach the fiber-ferrule to the EIB-16.

Finishing

This part should be done one night before or the morning of surgery.

  1. Perform a final cut of the tetrodes <1 mm below the fiber. Measure impedance. It should be 4-8 MΩ.
  2. Electrochemically deposit colloidal gold onto tetrode tips until impedance drops to <400 kΩ (e.g. [2]).
  3. Use epoxy to cover the EIB-16 and all the exposed wires. This will protect the device from animal-related damage, while insuring structural integrity.

References

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  1. Error fetching PMID 22138641: [Anikeeva2011]
  2. Error fetching PMID 6621101: [McNaughton1983]
All Medline abstracts: PubMed HubMed