I believe the part I spent most of the time thinking about is how I was going to push and retract the tools. While there are a bunch of mechanisms to provide linear motion, most of them have the same issue: length. Things like pistons and rack and pinion mechanisms all need to have the same length (or slightly more) than the travel they can do. As the travel I'm using is around 6 cm (this is also the max length of each tool), this means I would need at least a bit more than that only for the dispensing mechanism. Now go ahead, measure 12 cm in your forearm. As this is a project planned for below-the-elbow amputees, I didn't want to limit it to people with extremely short stumps, so I knew I had to go with something different.  

For a good while the best I had was to use something like a birthday horn , which I'd activate with pneumatics or hydraulics.But I didn't want to risk pumping fluid into the machinery or having to mess around with a pump, plus materials and assembling would have been difficult.

Then one day and while browsing the Internet, I found it: rigid chain actuators. Here's the wikipedia article I found with a list of them.

https://en.wikipedia.org/wiki/Rigid_chain_actuator

After looking up and researching each one I decided to modify and adapt the 1901 chain rammer 

A simple mechanism: basically a rolled up toothed chain and a gear that pushes it through a path. I decided to go even further and make the gear perpendicular with the plain of the chain to allow a greater chain length.

To avoid the complications of little details in 3D printing, I forego the traditional concept of a linked chain and decided to just link the links together by pasting them to a flexible piece of plastic.

This is my current design:

Write to me if you have any questions about the mechanisms or if you have any better ideas!