Artificial Arm Takes Strength from Composites

Developing a multifunctional, electronically-controlled artificial arm is extremely complex. Companies such as Motion Control, Salt Lake City, UT, which manufactures the Utah 2 Arm prosthetic, use the electrical signals produced when muscles are flexed (also known as electromyographic signals). This produces a level of control that requires far less effort, yet has more grace and dexterity than any previously available prosthetic for above-elbow applications. For the arm's exoskeletal structure the company needed a balance of strength and stiffness, not just for structural support, but to also protect the internal modules.

"The exoskeletal structure not only encloses the arm's electronic mechanisms and wires, it must also support ultimate loads of over 50 lbs," says Ed Iversen, director of research and development. Motion Control's engineers went to LNP Engineering Plastics, Exton, PA, which recommended Thermocomp QF, a glass-fiber reinforced nylon 6/10 composite rugged enough to minimize damage if patients should drop the arm or fall on it. The material was also selected for its low moisture absorption, which lends to dimensional stability. The ability of Thermocomp QF to be color matched was another important factor. "With an artificial arm, cosmetic appeal is an essential criteria," adds Iversen. "Some patients choose to wear a glove over the exoskeletal structure, but others like to leave it showing, so it needs to be cosmetically appealing."

The elbow joint in an above-elbow prosthesis also requires special consideration. "The humeral portion of the structure, in particular, needs to be extremely strong, because that is where you have the highest loads," says Iversen. Thermocomp RC, a carbon-fiber reinforced nylon 6/6 composite, was selected for this part of the arm.

The Utah 2 arm uses electrodes mounted in the socket that fits over the vestigial arm to sense the actions of the muscles in the upper arm. The patient's voluntary control of their biceps and triceps flexes and extends the prosthetic -- the amount of muscle flexure determines speed and force through the system controller.

--RM

For more information:

Circle 455 - LNP Engineering Plastics

Circle 456 - Motion Control