Sensitizing Prosthetics

Look at the typical prosthetic arm and it looks remarkably similar to one from the end of World War II. It straps across the back and uses a cable system to allow the user to control a metal hook by shrugging his or her other shoulder. Newer electric powered versions use external electrodes instead of a cable system to control them, look more life-like and allow for lifting heavier loads. But even they are limited to grasping and picking things up.

“The human hand can do a lot more than grasp and pick things up,” said Richard Weir, an associate professor in the University of Colorado Denver’s new bioengineering department.

Weir has spent more than a decade working to build a better prosthesis  — one that more closely resembles the complex system that allows the human hand to, for instance, play a Brahms concerto or spell out the alphabet in sign language. This spring, he will move one step closer to that goal, when below-elbow amputees at the Walter Reed Army Medical Center in Bethesda, Md. begin testing a next-generation implantable prosthesis control interface he helped design with partners Alfred E. Mann Foundation, Illinois Institute of Technology and Sigenics Inc.

“It could someday mean that rather than just open and close,  a person with a forearm amputation will be able to have individual finger and thumb control and wrist control,” said Weir.

While the typical prosthetic hand has 1 or 2 degrees of freedom, the human hand has 22. Some systems use surface electrodes that sit on the upper forearm to tap into the brain-to-muscle signals that remain even when the appendage is gone. But they only tap into two muscles. There are 18 muscles in the forearm involved in the control of the human hand and wrist.

The new system relies on rice-sized capsules called Implantable MyoElectric Sensors (IMES) that are injected into the residual muscles of the upper forearm and wirelessly telemeter signals from these muscles to an external prosthesis controller.

“It could totally blow the socks off what people with upper-limb amputations can currently do,” said Weir.

Previously, Weir helped to design a hand prototype with 18 degrees of freedom that could spell out the alphabet in sign language if the signals were available to control it. Now, he’s working on research that could someday enable this hand to do one more thing it can’t do now: Feel touch.

“We have a ways to go on that one,” he said.

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