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For 1st Woman With Bionic Arm, a New Life Is Within Reach
The particular achievement with Mitchell is that her prosthesis 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.
Nevertheless, Kuiken said: "This is very much a prototype device. We have a lot of smoke in this lab. We fry a lot of transistors."
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 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 -- or, more precisely, hijacked -- for that purpose.
They cut the nerves to two chest muscles, the pectoralis and serratus, at a point where those nerves have branched to go to different parts of the muscles, 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 bionic arm makes use of this feat of anatomical alchemy.
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 pectoral or 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.
Staff researcher Madonna Lebling contributed to this report.