An easy-to-assemble, robust, and lightweight drive implant for chronic tetrode recordings in freely moving animals

Abstract

Tetrode arrays are a standard method for neuronal recordings in behaving animals, especially for chronic recordings of many neurons in freely-moving animals.

Objective: We sought to simplify tetrode drive designs with the aim of enabling building and implanting a 16-tetrode drive in a single day.

Approach: Our design makes use of recently developed technologies to reduce the complexity of the drive while maintaining a low weight.

Main results: The design presents an improvement over existing implants in terms of robustness, weight, and ease of use. We describe two variants: a 16 tetrode implant weighing ∼2 g for mice, bats, tree shrews and similar animals, and a 64 tetrode implant weighing ∼16 g for rats and similar animals. These designs were co-developed and optimized alongside a new class of drive-mounted feature-rich amplifier boards with ultra-thin radio-frequency tethers, as described in an upcoming paper (Newman, Zhang et al in prep).

Significance: This design significantly improves the data yield of chronic electrophysiology experiments.

Figure 1.. Overview of the drive design.

A, Overview of the mouse drive implant for 16 individually movable tetrodes and 64 channels, shown with an eletrode interface board (EIB) and the miniature headstage [25]. B, Overview of the internal drive mechanism—linearly moving shuttles (green) are moved up and down in guide channels by captive screws. A straight guide tube array (orange) holds fused silica (Polymicro) shuttle tubes. C, Drive variant with 64 individually movable tetrodes for up to 256 channels, for use in rats, shrews, and similarly sized animals.

Figure 2.. Custom screw design and drive mechanism.

A, Key components that make up one of the linear adjustment (‘drive’) mechanisms. B, Overview of the internal drive mechanism—linearly moving shuttles (green) are actuated by captive screws. The screws move inside guide channels and are held at the bottom via their locating pins, and are held vertically at the top by gluing their retaining collars into concave pockets in the drive body. A straight guide tube array (orange) holds fused silica (Polymicro) shuttle tubes. At the topmost drive position, the shuttle tubes stay inserted in the guide tube array—this amount of insertion, plus the desired travel range, dictates the height of the guide tube array. The present design achieves ~4.5 mm of travel. C, Custom screw for the mouse drive. The central novel features are the retaining collar under the screw head, which acts as a thrust bearing, stopping the screw from moving up, but allowing rotation, and the locating pin at the bottom, which allows the screw to rotate but not move laterally. D, Screw variant for use in the 64 drive design for rats and similarly sized animals. ~10 mm of travel can be achieved.

Figure 3.. Tetrode depth measurements.

Measurements of tetrode depths over the entire adjustment ranges for the mouse (A) and rat (B) variants. This travel was measured only in the downward direction; some hysteresis will occur when reversing the drives. Hysteresis is measured as the amount of screw rotation after a direction reversal at which point the tetrode started moving again. Hysteresis is caused by different factors; see main text for a short discussion. See inserts for typical measured hysteresis. The mouse drive behaves almost completely linearly with a 1:1 correspondence of screw pitch (0.15 mm/turn) to travel range. The rat drive shows some scaling of the travel with a 0.205 mm/turn vs. the 0.2 mm pitch screw. This factor was measured for a tetrode at the periphery of a circular guide tube array, and other arrangements may result in slightly different factors. We recommend individual calibration if this level of dead-reckoning precision is desired. Reaching the design travel ranges of 4.5 mm and 10 mm requires starting at the absolute top position and moving until the shuttle touches the bottom position. Extra care is needed at these positions in order to not drive the shuttle into the end stops and strip the threads.

Figure 4.. Assembly preparation.

Main steps required for preparing drive assembly. A, Make guide tube array with the desired spacing and arrangement of polyimide guide tubes [18, 35], or use a 3D-printed or machined guide tube array. In contrast to some prior designs, the guide tube array is straight, and can therefore be prepared in one long bundle and cut to lengths later. A sufficient length of the guide tube array is crucial—(see figure 2) and main text for details. B, Glue guide tube array into drive body, taking extreme care not to let glue get into the guide tubes: Insert the guide tube first, then glue with epoxy at the lateral cutout in the drive body. Alternatively, apply a small amount of glue onto the sides of the array before sliding it into the drive body, or leave a small section sticking out of the drive body and glue there. C, cut shuttle tubes to size. Optionally: mark minimum insertion depth so it is apparent when the tubes are properly placed later during assembly. By using a length of 1 mm above the top of the shuttle (in top position) as reference, a correct length for all shuttle tubes can be determined. Now, by inserting these shuttle tubes to a depth where they extend 1 mm past the shuttle (figure 7 step 4), a proper minimum insertion depth can be assured.

Figure 5.. Assembly jigs.

A, Assembly jig for holding the drive body and EIB in fixed relative positions. The distance between EIB and drive body can be adjusted with a linear slide, driven by a screw. Both the EIB and the drive body can be independently rotated around their principal (vertical) axes by loosening lock screws and manual rotation, in order to facilitate access to all sides of the drive for drive mechanism assembly and tetrode loading. The whole linear slide mechanism which holds the drive and EIB can rotate back and forth (manual, with a friction adjustment) to provide access to the sides of the drive and EIB and to the bottom of the guide tube array (for instance for cutting tetrodes). The jigs are made from easily available stock components and 3D printed custom parts. B, Larger jig variant for use with the rat drive.

Figure 6.. Alignment of guide tube array in drive body.

Setting the guide tube array in the drive body with a small offset (2–5 mm) increases the bending radius of the shuttle tubes in lower drive positions. The effect of this offset is less pronounced on drives that do not make use of the full drive range. A separate benefit of leaving a small offset is that it makes even short guide tube arrays more visible during surgery and makes it possible to fit the array into smaller craniotomies.

Figure 7.. Step-by-step assembly process.

This overview starts with a drive body in which the guide tube array is already inserted and glued, and ends at a loaded drive. See [18, 35] for instructions of assembling guide tube arrays. This overview omits any additional wires such as ground and/or reference, electrical stimulation, LEDs etc Some steps can be grouped, such as inserting and seating the screws and shuttles, while others are easier if performed one drive mechanism at a time, such as loading and pinning tetrodes. See [26] and [18] for general procedures for tetrode assembly and loading. The procedure for the 64 drive variant is identical other than for sizing.

Figure 8.. Cement application for screw retention.

Step-by-step description of the process for applying light-cured cement to the screws. The main goals are to completely fill the cavity, with minimal air bubbles, and to fully cover the retaining collar of the screw. Usually some cement spills out of the cavity and extends up to the screw head; this should not cause any trouble as long as the half-moon section of the screw and a minimal section below stay accessible. The last steps of evenly covering the retaining collar of the screws can usually be accomplished by slowly turning the screw (in the direction that lifts the shuttle / pushes the screw into the cavity). This should pull the uncured cement around the screw, covering it. This gluing step can be done in groups of more than one screw if desired, but this can risk premature curing of the cement, especially if bright lights are used.

Figure 9.. Rotating EIB before lowering to bundle up tetrode wires.

Rotating the EIB by 180° or 360° while lowering it tucks the tetrodes in under the EIB, making it easier to avoid accidentally gluing tetrodes to the rim of the EIB. Mouse: distance 10-12 mm, tetrodes 15-20 mm, Rat: distance 20-30 mm, tetrodes 30-40 mm. Test these distances with a few tetrodes first before loading entire drive.

Figure 10.. Shielding the drive and connecting the ground wire.

In almost all applications the drive should be shielded. A, Schematic of the recommended shielding scheme. Here, GND and REF are shown connected. This is not always the ideal configuration, and some use cases require separate GND and REF connections, but works well for most applications. The ground wire (blue) is soldered to GDN, or GND and REF, and threaded through the loop at the side of the drive, making sure to place it between the screws in a position that allows screw function. The wire is then laid along the side of the drive. B, Recommended schema for connecting GND and REF with one wire. C, The shield can also be connected to GND by painting a trace up to a GND via on the EIB with conductive pain or epoxy. D, The ground wire can be laid flat along the side of the drive, threaded through the second loop, and then used to solder to the wire for the ground screw connection. The drive can then be painted with conductive paint to form a shield, or aluminum foil can be glued to the drive and connected to GND with conductive paint.

Figure 11.. Custom screw driver for tetrode adjustment.

Close-up of the custom screw driver, made from a stainless steel tube with an ID that snugly fits the screw head. A handle can be made by applying heat-shrink tubing to the cannula, or by 3D printing the handle from files available on the git repository.