Notes:

The needle holder was originally printed with a Polyjet printer (Stratasys Objet Eden250 or equivalent) in VeroClear, HQ setting. It might be printable on a good FDM printer with soluble support, thin layers and some post-processing, but this has never been tested.

The other parts can be printed with a standard FDM printer (Ultimaker 2, Prusa i3 etc), but the racks and pistons need a 0.25mm nozzle to get sufficiently sharp teeth. The racks can also be laser-cut from 4mm acetal. See here for the T-48 parts and assembly instructions.

Assembly

Build five T-48 motors according to its assembly instructions. Four standard racks and one long rack with round eye. Print frame and three carriers.

Acrylic pins with diameter 4 mm are used as hinges in the frames/carriers/motors, while 3 mm pins are used in the needle holder link with the long rack.

Enlarge holes in frame/carriers/motors such that the pins fit nicely. Ideally, it should have a tight fit in the main structure and a loose fit in the rotating part. A small amount of friction is acceptable, it is more important to avoid backlash.

Print needle holder parts with a high-resolution printer and assemble. The combined ball/socket joint consists of three interlocked parts. See Figure 5 of this paper for details.

Use T-pieces to lock the four normal racks to the needle holder parts. For the needle, use MR conditional titanium 14G (2.1 mm) needle, or just a 2 mm metal rod with sharp tip if you don't intend to place it in a MRI.

If you need clarifications or have questions, just ask!