Head-fixing apparatus
Head-fixation is a commonly applied method in neuroimaging of mice. It allows for precise repositioning of the mice throughout various experiments and minimising motion artefacts. To be able to image the same neurons over time, the mice will be head-fixed in our experimental set-up.
The background
Head-fixing is a commonly used technique for which there are commercially available head-fixation systems, like the Neurotar clamp fixation system. However, these are often available at a high price and are designed for a variety of different experiments. We used this Neurotar setup as an inspiration for our initial design propositions, which included using screws to fix the headring to the base plate. However, their design lacked a simple mechanism to ensure a precise fixation. To address this, Neurotar has added magnetic coupling to their head-fixation setup to decrease the effort necessary to fix the mouse.
Our research led us to an open-source 3D printable head-fixation system protocol which provided detailed protocol and printable design sheets. In these protocols, lids were described with function to cover the mouse’s head until experimentation to better protect the open head window during downtime. We opted to include this in our design too, and the metal facility at the ESPCI was able to produce a prototype from resin for us to assess before the final metal piece would be manufactured.
Finally, we found that kinematic coupling using a three-groove-system in the headring can increase the precision of the head-position up to a micron-scale (Kim et al., 2022). With this idea, we included three grooves positioned in a triangular shape around the hole in the headring to further increase the stability of the fixation system. As described by Slocum et al. (1992), using three instead of four grooves provides better stability than using four or two grooves as there is exactly one point of stability when using three.
The design
The head-fixation system we designed consists of a headring that will be attached to the mouse's head during surgery using cement, and a base plate on which the mouse will be positioned. Using screws, magnets, and metal grooves, the mouse will be precisely fixed on a fixation bar attached to the base plate. Additionally, we designed a cover lid for the headring to protect the mouse’s head outside of the experiments.
We further included magnets to the headring and the fixation bar to decrease the effort of adjusting the head’s position. Moreover, we added one magnet to the cover lid to ensure it stays on the headring. Lastly, we included two holes to fit two metal rails.
The overall idea is to put the olfactometer onto a scaffold with two metal rails sticking out at the side of the odor output. That way, the base plate with the mouse can be easily aligned to the olfactometer and also increase the precision and stability of the set-up.
Click the link to access all STL files for the head fixing apparatus: Head Fixing Apparatus (Mouse) - Drive
The assembly
1. Fabrication of Parts
Utilise the provided files to manufacture all components required for the Head-Fixing apparatus. Ideally, opt for stainless steel material for enhanced durability during usage with mice. Alternatively, 3D printing or any preferred material can be employed for production.
2. Magnet Installation
Affix a 1mm disc magnet with a carrying weight of 31g into the designated holes within the base plate, headring, and lid. Use suitable adhesive to ensure proper bonding.
3. Fixation Bar Assembly
Place the fixation bar atop the four columns of the base plate. Secure the fixation bar by fastening four screws, each with a diameter of 2.5mm and a length of 10mm, into the designated holes.
4. Surgical Application
During surgery, adhere the headring onto the mouse's head using appropriate cement. Ensure the orientation of the bar is directed towards the rear of the mouse for optimal functionality.
5. Lid Placement
Place the lid onto the headring to shield the mouse's head during activities outside experimental sessions.
6. Experimental Setup
Before commencing experiments, remove the lid from the headring to expose the mouse's head. Position the mouse onto the base plate and adjust its placement so that the grooves, magnet, and screw hole of the fixation bar and headring align accurately.
7. Mouse Fixation
Secure the mouse in place by inserting a screw with a diameter of 2.5mm and a length of 10mm into the designated hole.
8. Post-Experiment Procedure
Upon completion of experimentation, remove the screw, attach the lid onto the headring, and carefully remove the mouse from the base plate.