Bretl’s group develops impact resistant fingers for prosthetic hand


This prosthetic hand takes a licking and keeps on ticking!

Developed by Aerospace Engineering at Illinois Associate Prof. Tim Bretl and his students, Kyung Yun Choi (MS 17) and Aadeel Akhtar (PhD 16), the fingers of this flexible model hand are smashed, twisted and bent in every direction. Surviving the torment, the digits bend back into shape without so much as an “Ow."

The group chose to use a flexible, 3D-printed bone made of polyurethane material rather than the rigid materials used to make most prosthetics. They then routed the material with pressure sensor wires, wrapped the hand in a silicone skin, and inserted pre-stressed spring steel, equipping the thumb with a motor.

Tim Bretl with his graduate student, Kyung Yun Choi
Tim Bretl with his graduate student, Kyung Yun Choi
The result? A prosthetic that can withstand having its hand smacked! And a shout out from IEEE Spectrum online magazine.

Myoelectic prosthetic hands offer many advantages over the 20-year-old technology of body-powered prosthetic hands that simply open and close, Choi said. Controlled by the user's electrocardiography (EMG), the electric-powered hands can pinch, key and power grasp, among hand configurations, and they provide sensory feedback to users by electrotactile stimulation.

However, the new technology's high cost – about $30,000 per device – and its susceptibility to accidental impact had made its use less desirable. Choi sought to change this.
"I mainly use 3-D printing and a silicone molding technique to build the hand at a low-cost ($553). Also, 3-D printing enabled me to easily fabricate the hand using flexible polyurethane material," Choi said. "By replaycing the weakest point of the prosthetic finger with a compliant joint that I designed, I could make the finger resist high impact, like hammering, in any direction.

"Unlike the commercial prosthetic hand and 3-D printed hand, our hand uses compliant materials like polyurethane and silicone, and this also gives better texture like real human skin."

Choi currently is in Shenzhen, China, to mass-produe and commercialize the prosthetic hand design. "Our challenge in China is to find factories that can produce the product with unconventional material that has high compliance and proper stiffness as well, and electronic components like motors with good quality," Choi said. "Also, if necessary, modifying the design to meet the manufacturing process limits will be another challenge."