New Generations of "Bionic Arms"

Greater number of war survivors impulse the research on the issue

By on 15 Sep 2006, 11:21 GMT
Claudia Mitchell, from Ellicott City, US, is the first woman to carry a thought controlled bionic arm and the fourth person to receive it. Mitchell lost her left arm in a motorcycle accident. Now, if she wants to peel a banana, all she has to do is place her prosthetic left arm next to the banana and think about grabbing it. The mechanical hand closes around the fruit and she's ready to peel.

The bionic arm is capable of doing 4 of the 22 discrete movements of a normal arm, but researchers are working to improve this number.

Her device was designed by physicians and engineers at the Rehabilitation Institute of Chicago, it works by detecting impulses in a chest muscle that has been rewired from the stumps of nerves that once went to her now-missing limb.

The Institute is part of a multi-lab effort, funded with $54 million from the Defense Advanced Research Projects Agency (DARPA), to create more useful and natural artificial limbs for amputees. It is dedicated to helping people with all levels and types of physical disabilities regain or improve their physical functions and empowering them to participate more fully in family, social, vocational and leisure time pursuits.

Pentagon pushes to create a lifelike prosthetic arm and hand - a mission made even more urgent as more soldiers come back from Iraq and Afghanistan without limbs. As of July, 411 members of the military serving in Iraq, and 37 in Afghanistan, have suffered wounds requiring amputation of at least one limb.

The first person to benefit from a "bionic-arm" was Jesse Sullivan, a Tennessee power company worker who lost both of his arms above the shoulder in a work related accident in 2001. Dr. Todd Kuiken at RIC developed a procedure to graft Mr. Sullivan's amputated nerve endings from his shoulder onto the pectoral muscle. As those nerves grew to innervate the chest muscles, the electrodes located over the graft began to pick up electrical signals reflecting the impulses and transmitting those to the mechanical prosthesis.

Mitchell benefits from the new second generation of thought-powered "bionic-arm," the most advanced prosthetic in the world today, requiring its wearer only to think about what he wants his arm to do and his prosthesis will do the job. Soon, she will benefit from the third generation prosthesis, still under development, that will allow her to also "feel" with an artificial hand. She is ready for it now.

Last summer, surgeons took the first step by rewiring the skin above her left breast so that when the area is stimulated by impulses from the bionic arm, the skin sends a message to the region of her brain that feels the "hand".

Future arms will also be able to perform more complicated motions. She recently spent time at the Chicago hospital trying out a prototype with six motors, her current prosthesis having just three. It will theoretically allow her to reach for things over her head.

But even the first-generation device "has changed my life dramatically," she said. "I use it to help with cooking, for holding a laundry basket, for folding clothes -- all kinds of daily tasks."

For Todd A. Kuiken, 46, a physician and biomedical engineer, this is the latest step in his 20-year effort to make a better artificial arm.

The particular achievement of Mitchell's prosthesis is that it works with her breast intact. With previous versions, surgeons removed some chest tissue, so that electrodes in the arm could better detect twitches in the rewired chest muscles. But that would have been particularly disfiguring.

The bionic arm makes use of several features of the human body that would be impossible to create from scratch. Luckily, a person still has them even after suffering an injury as grievous as the loss of an arm at the shoulder.

One feature is the "motor cortex" of the brain, where cells that control voluntary muscles reside. The millions of nerve cells that "drive" the arm and hand remain after amputation. When an amputee pretends to move his missing hand, those cells fire and send impulses down the spinal cord and out to the nerves that terminate at the stump.

Those nerves are huge electrical conduits filled with tens of thousands of fibers carrying a wide assortment of information. Some are motor nerves telling muscles to move. Some are sensory nerves, carrying impulses back from the hand to the brain, where the information will be interpreted as touch, temperature, pressure and pain.

In preparation for the bionic arm, Kuiken and his surgical colleagues first re-create a biological control panel for a hand on the amputee's chest. They use muscle and skin that can be sacrificed for that purpose.

They cut the nerves to one chest muscle, the pectoralis serratus, at a point where those nerves have split to go to different zones of the muscle, but far "upstream" from the point where the nerves divide into tiny fibers that attach to individual bundles of muscle fiber.

They then sew the stumps of the large nerves that once went to the arm and hand to the cut ends of the chest-muscle nerves. In the same operation, the nerves carrying sensation from the skin over the pectoral muscle are also sewn into the arm nerves.

Over several months, the arm nerves grow down the sheaths of the motor fibers and attach to the muscles. (Interestingly, the amputee assists this process by mentally "exercising" the missing hand, which helps promote a firm nerve-muscle connection.) Simultaneously, the sensory nerves grow down the sensory sheaths and into the skin.

If all goes well, a person is left with chest muscles that twitch in different places in response to such thoughts as "bend the wrist back," "move the thumb" and "clench the fingers." The person also ends up with a patch of skin about the width of a baseball that, when stroked, warmed or pricked, feels like a hand rather than part of the chest.

The prosthesis is strapped onto the shoulder stump and torso in a way that positions electrodes over the regions of the chest muscles that are responding to different "hand instructions." Those electrodes, in turn, are wired to a computer and then on to motors in the forearm and hand of the device.

When the amputee tells the fingers to close, the designated part of the pectoralis serratus muscle twitches and the electrode over it detects the signal, activating the appropriate motor.

In the future, electrodes in the hand will send touch signals up the arm to the chest skin, which will send them on to the brain, where they will be perceived as sensation.

Kuiken is pioneering ways of reattaching severed nerves to other parts of the body and using them to generate faint electrical signals - in the same way that an EKG picks up the beating of the heart - which can then be used to control an arm.

Similarly, sensors that detect temperature, pressure and vibration can be used to stimulate a patch of skin where sensory nerves have been reattached, tricking the brain into feeling what the artificial arm feels.

This third generation of "bionic arm" incorporating the sense of touch and feeling will be ready by next year.

The goal - within four years - is that an injured soldier could once again play a guitar or thread a needle with an artificial limb that will be "controlled, feel, look and perform" like a real arm, according to the DARPA.

Today, a soldier who loses a leg can get a prosthetic replacement with a computer-controlled knee joint and eventually run a marathon or requalify as a paratrooper.

The advent of highly protective body armor has meant that about 90% of men and women injured in combat are surviving, compared with a 70% rate in past wars.

"That's what's driving the research," says Richard Weir, a biomedical engineering professor at Northwestern who's playing a central role in the project. "They are surviving injuries they wouldn't have survived in the past," but often at the cost of losing a limb.

"What we have to offer (today to the injured soldiers) is rudimentary," says DARPA's Dr. Ling, a physician who treats amputees.

In addition to the four-year goal, DARPA wants to create a much more advanced artificial limb within two years, pulling together research already under way independently around the world. The focus is on near-human strength and a natural-looking cosmetic covering, using nerves and muscle for control.

"The biggest challenge will be in creating realism," says Ed Colgate, a mechanical engineering professor at Northwestern.

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