Most of the robotic arms now in use by some amputees are of limited practicality; they have only two to three degrees of freedom, allowing the user to make a single movement at a time. And they are controlled with conscious effort, meaning the user can do little else while moving the limb.
A new generation of much more sophisticated and lifelike prosthetic arms, sponsored by the Department of Defense’s Defense Advanced Research Projects Agency (DARPA), may be available within the next five to 10 years. Two different prototypes that move with the dexterity of a natural limb and can theoretically be controlled just as intuitively–with electrical signals recorded directly from the brain–are now beginning human tests.
Initial results of one of these studies–the first tests of a paralyzed human controlling a robotic arm with multiple degrees of freedom–will be presented at the Society for Neuroscience conference in November.
The new designs have about 20 degrees of independent motion, a significant leap over existing prostheses, and they can be operated via a variety of interfaces. One device, developed by DEKA Research and Development, can be consciously controlled using a system of levers in a shoe.
In a more invasive but also more intuitive approach, amputees undergo surgery to have the remaining nerves from their lost limbs moved to the muscles of the chest. Thinking about moving the arm contracts the chest muscles, which in turn moves the prosthesis. But this approach only works in those with enough remaining nerve capacity, and it provides a limited level of control. To take full advantage of the dexterity of these prostheses, and make them function like a real arm, scientists want to control them with brain signals.
“When you pick up an object, your brain knows automatically to rotate the wrist and move the fingers,” says Michael McLoughlin, who is overseeing the development of one of the prostheses at the Applied Physics Laboratory (APL) at Johns Hopkins University. “We want a dexterous limb and the ability to control it in a natural way, as well as some level of tactile feedback.”
Limited testing of neural implants in severely paralyzed patients has been underway for the last five years. About five people have been implanted with chips to date, and they have been able to control cursors on a computer screen, drive a wheelchair, and even open and close a gripper on a very simple robotic arm. More extensive testing in monkeys implanted with a cortical chip shows the animals can learn to control a relatively simple prosthetic arm in a useful way, using it to grab and eat a piece of marshmallow.
“The next big step is asking, how many dimensions can you control?” says John Donoghue, a neuroscientist at Brown University who develops brain-computer interfaces. “Reaching out for water and bringing it to the mouth takes about seven degrees of freedom. The whole arm has on order of 25 degrees of freedom.” Donoghue’s group, which has overseen previous tests of cortical implants in patients, now has two paralyzed volunteers testing the DEKA arm. Researchers at APL have developed a second prosthetic arm with an even greater repertoire of possible movements and have applied for permission to begin human tests. They aim to begin implanting spinal cord injury patients in 2011, in collaboration with scientists at the University of Pittsburgh and Caltech.