Last fall, I applied for a grant called the UConn IDEA Grant through my university to build a 3D printed prosthetic hand controlled with an EMG sensor. EMG stands for electromyography, which looks for the voltage in a muscle using a small adhesive pad. I started off by designing a hand in Inventor that doesn’t utilize the EMG sensor and just articulates like a normal hand.
Needless to say, my first versions were less than pretty.
Never mind pretty, most weren’t even functional. But going through the process of creating a hand from scratch and making quite a few models that don’t do what they should taught me a lot about how to make them work. Finally, I had created something that worked.
The two main things I learned from this version were that the fingers did not work well being made out of rigid material, and that my thumb design was quite useless. A real thumb moves using many compound angles, where mine moves in just one, and not a very useful one at that. I knew at this point a serious overhaul in the design was necessary.
The next step, however, was building the circuit that would run the hand. I ordered all the electronics, and prototyped them on a breadboard to make sure everything worked as expected.
After the electronics were all figured out, I had to find a way to contain and arrange them. Thus began my numerous attempts to create the “forearm” of the hand that would house all the electrical components.
Next, I began designing a hand that could be printed in all one piece. But in order to make this happen, I would have to print in NinjaFlex, a flexible 3D printer filament from Fenner Drives. Ninjaflex is incredibly hard to print due to its flexibility. Two things can help printing with NinjaFlex to keep it from messing up; the filament should be tightly constrained from the extruder stepper all the way down to the nozzle, and the print speed must be decreased significantly.
The stock extruder for a Lulzbot Taz 5 doesn’t support flexible material, but Lulzbot is an Open Source company, releasing all their files and build notes online. I went hunting through their forum for a thread that might contain some development notes about when Lulzbot was developing their flexible material extruder, and lo, I found a wealth of information. After printing out one of the prototypes posted in the thread, I replaced the one that came in my extruder, and I was printing NinjaFlex!
When I had designed the hand, I imported it into Cura for slicing.
This picture illustrates the channels cut throughout the hand to allow for cabling to move.
46 hours and 22 minutes later, my NinjaFlex hand was completed.
This print required quite a few hours of cleanup to get into a workable state, but eventually it was moving in the way it was designed.
Finally, all that was left to do was install the electronics in the slightly redesigned forearm piece, wire up the fingers, and finishing touches!
I took the hand to the 2015 World Maker Faire at the New York Hall of Science, and exhibited for the weekend! I met a tremendous number of cool and interested people, and made some fantastic connections for the future of this project.
All development files including STLs and Arduino code can be downloaded here.