Microdrive Fabrication and Assembly

To develop the microdrive presented, manufacturing processes criteria such as the shape of the components, type of the required materials as well as resolution and cost were considered. For the micropositioning mechanism, high resolution is a critical requirement, hence CNC manufacturing was used; through the milling process, the bushing and the piston were manufactured: the bushing was made of brass, with overall dimensions of 6 mm in internal diameter, 7 mm in external diameter, 6 mm in height and a weight of 0.29 g; while the piston was made of acrylic and has overall sizes of 2 mm in the inner diameter, 6 mm in the outer diameter, 6 mm in height and a weight of 0.069 g. The baseplate (weight: 0.52 g), the housing element (weight: 1.42 g), the covers (electrode cover weight: 0.42 g and EIB cover weight: 0.35094 g), and the nut (weight: 0.045 g) were manufactured using an additive manufacturing process (material jetting) because of the complexity of their shapes; the equipment employed was a Connex 3 by Stratasys, and the material used was VeroClear resin (RGD810). This technology has the advantage of reducing manufacturing costs when the production batch increases. The other components are commercial. The compression spring used (by Industrial Springs Corp.) is made of stainless steel and has a mean diameter of 2 mm, a free length of 5 mm and a weight of 0.01 g. The polyimide tube (Polyimide 0.0135 ID by Neuralynx) has a length of 18 mm and weighs only 0.002 g, while the guide cannula (20G by Lanceta HG) has a length of 13.5 mm and weighs 0.37 g. The magnet (R063-063 by Amazing Magnets LLC) weights 0.023 g and has overall sizes of 1.59 mm diameter and 1.59 mm length. The Hall-effect sensor (A1324LLHLT-T by Allegro MicroSystems) has overall sizes of 2 × 3 × 1 mm and weighs 0.015 g. In order to avoid rust problems, stainless steel screws were used which, according to their application on the microdrive design, can be classified into three types: power screw, housing fixing screws and skull fixing screws. The first one (#21475 by MetricScrews) is M1-0.25 × 10 mm and weighs 0.045 g. The second one is composed of three different screws: four pieces of M1-0.25 × 3 mm (#20694 by MetricScrews) with a weight of 0.07760 g, two pieces of M1-0.25 × 8 mm (#21474 by MetricScrews) screws with a weight of 0.068 g and one 0–80 × 1/8 (by Plastics One Inc.) screw with a weight of 0.058 g. Finally, the skull fixing screws consist of four 0–80 × 1/8” screws with a weight of 0.23332 g. For the automated microdrive, a piezoelectric SQL-RV-1.8 actuator by New Scale Technologies is used, such an actuator has overall sizes of 2.8 × 2.8 × 6 mm and a weight of 0.16 g.

The complete assembly can be divided into three stages: the first one is related with the assembly of the micro positioning system; the second one involves the insertion and connection of the electrodes, and the last one is concerned with the placement of the protection covers with the appropriate fixing screws. The complete process can be seen in Figure 5. As a first step, the electrode shuttle system is assembled by fixing the polyimide tube and cannula (Figure 5A) to the piston (Figure 5B), the polyimide tube projects 2 mm from the lower face of the baseplate. Then, the magnet is placed in the piston hole (Figure 5C) and the compression spring is placed concentrically to the piston rod (Figure 5D); after that, all these parts are put concentrically in the bushing (Figure 5E), which in turn is placed in the housing element (Figure 5F). After this, the nut-screw or the piezoelectric actuator can be placed and fixed with a screw. This stage takes about 10 min. The second stage consists in introducing the electrodes through the polyimide tube (Figure 5G) and the conduits (Figure 5H). After this, the electrodes are cut 7.5 mm from the lower face of the baseplate for the NAcSh, and 6 mm for the NAcc in order to reach the region of interest (see Supplementary Figure S1); next, the EIB is placed in the housing element; and finally, the electrodes are connected to the EIB by means of gold-plated pins (Figure 5I). Figure 5J shows the assembly with the piezoelectric actuator. It is important to handle the electrodes carefully, making sure they do not bend. This task takes around 30 min. The last stage consists in protecting the EIB and the exposed part of the electrodes by fixing the covers with the housing element (Figure 5K). At the end of this stage, the final assembly time is less than 70 min. Including the weight of the baseplate, the total weight of the reimplantable microdrive is about 4.08 g when the nut-screw is used, and 4.15 g if the piezoelectric motor is implemented. In both configurations, its overall dimensions are 27.73 × 15.95 × 21.50 mm. Finally, before the implantation, the electrodes can be gold plated and then the device is ready. Supplementary Table S2 depicts the cost of all components to build one microdrive. In the case of the automated microdrive, the total cost of the parts is 280.2 USD. Considering that the actuator and the EIB are reusable, the total cost of each microdrive is approximately 53 USD (and it could be cheaper by producing a larger batch). All STL files are provided in the Supplementary Data Sheet S1.

Reimplantable microdrive assembly. (A) Polyimide tube in the guide cannula. (B) Guide cannula fixed to the piston. (C) A magnet placed in the piston. (D) Compression spring placed concentrically to the piston rod. (E) Piston placed concentrically and aligned to the bushing. (F) Electrode shuttle system assembled with the housing element. (G) Electrodes introduced through the polyimide tube. (H) Electrodes introduced through the conduits. (I) Nut-screw and EIB placed in the housing element. (J) Piezoelectric actuator placed in the housing element for the automated configuration. (K) Final assembly with the two covers and the housing fixing screws. (L) Illustrative picture of a rat implanted with a reimplantable microdrive.

The impedance of the electrodes was controlled by electroplating the tip of the wires. Electroplating prevents impedance damping and improves the signal-to-noise ratio (Ferguson et al., 2009). Before implantation (and reimplantation), the 16 tungsten wires with 35 μm in diameter were electro-plated with a gold plating solution and the recommended Neuralynx stepped protocol (Supplementary Table S1) using a nanoZ^TM multi-electrode impedance tester (Plexon, Dallas, TX, United States). Impedances were reduced to 70 kΩ.