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Sensor helps prosthetic arm move on thought

When former Marine Cpl. Claudia Mitchell thinks about bending her bionic arm, it bends.

A neural sensor on her chest, along with an advanced mechanical arm and hand, give her a far greater range of motion than current prosthetic arms with hooks.

“I don’t have to think of the entire sequence of movements,” she said, because with the technology, her thoughts control her nerves, and her nerves control the movement.

“The hand can close in different ways,” said Mitchell, who lost her arm in a motorcycle accident in May 2004, shortly after leaving the Marine Corps.

It’s also more comfortable. At times, she’s done things like crossing her arms that seemed so natural, and not realized she was doing it with the bionic arm.

Mitchell is helping test the arm, developed by the Johns Hopkins University Applied Physics Lab, which is leading an international team of researchers in work that could have a profound effect on amputees — including the service members who lose limbs in the current wars in Afghanistan and Iraq.

Mitchell and other amputees were part of a demonstration of cutting-edge technology in the field of arm prosthetics by the Defense Advanced Research Projects Agency on Wednesday on Capitol Hill. DARPA’s goal within the next two years is to perfect this arm with neural control.

“They train the arm to understand what it is that I want it to do,” Mitchell said.

In her case, peripheral nerves left just above where her arm was amputated were transferred to her chest. She wears electrodes on top of those nerves. When she thinks of the movement, the brain tells the nerve, and the electrode on top of the nerve tells the arm what to do. A device could also be implanted in the pectoral muscles of the chest.

The goal is even greater control, with the use of a microchip about half the size of a fingernail that would be placed on the surface of the brain, said Army Col. Geoffrey Ling, program manager of DARPA’s Revolutionizing Prosthetics initiative.

This would allow an amputee to play the piano and even write, by tapping into the nervous system, Ling said, adding that primates now testing prosthetic arms are feeding themselves.

The goal of this program is a prosthetic that will be able to function as well as a normal human arm, with strength and dexterity. The system will provide the ability to sense touch, temperature, vibration, the position of the arm and hand relative to other parts of the body, as well as provide a power source that will allow at least 24 hours of normal use.

The mechanical components will tolerate heat, cold, water, humidity, dust and other elements, and will last at least 10 years with normal use.

Retired Marine Capt. Jonathan Kuniholm is an engineer at Duke University doing research with “grasp control” software for the prosthetic arm. His interest is in the signals analysis and control of the hand — allowing the user to do things like pinching.

His work at Duke is part of the Johns Hopkins project, funded by DARPA. But he is also helping test the arms, having lost one of his own to a roadside bomb in Iraq on New Year’s Day 2005.

About 300 service members have lost arms, Ling said.

“Most have the hook,” he said. “It’s the best available.” With those devices, all the patient can do is open and close the hook. In contrast, the goal within a couple of years is to have an arm with 24 different directional movements.

In the meantime, an arm has been developed that is superior to the hook, which Ling describes as “strap and go.” It allows a person to feed and groom himself, and requires only a couple of hours of training.

Randy Campbell, who lost his arm in an accident in April 2004, is taking part in clinical trials of the arm, developed by DEKA Research, who also has a contract with DARPA. The company’s work with DARPA will wrap up this winter, and will move on to the next phases, which will include approval from the Food and Drug Administration.

This would give service members, as well as other amputees, more options than the hook. It uses low-fidelity neural signals complemented by more traditional control strategies such as surface electroencephalogram.

Campbell demonstrated by picking up a power drill and drilling a hole, then a screw, into a piece of wood. He also picked up Skittles with his fingers and ate one, but it took a few tries.

“It returns your life,” Ling said of the new artificial limb technologies that are emerging. “But we want to go beyond that.”