Waiting For an Arm and a Leg

Fully Wired Limbs

In time, the brain will need to start hearing back from the prosthetics it´s sending signals to. It ordinarily receives a flurry of sensory feedback from a human leg-the terrain being navigated, the pitch of the incline-and can signal adjustments that need to be made. Next-generation prosthetics will be loaded with far more sensors than the Rheo and C-Leg have. They will pick up many, if not all, the cues that biological joints receive and be able to track their own pitch, forward thrust and spatial orientation.

But embedding prosthetics with sensors is useful only if those sensors can communicate with the wearer's brain and spinal cord. The most promising middleman for such chatter is an invention called artificial nerve cells, or BIONs–"bionic neurons." Developed by researchers at the Alfred E. Mann Institute at the University of Southern California, these implantable devices (centimeter-long glass capsules equipped with electrodes) have already been successfully injected in or near patients´ muscles and nerves to treat paralysis and to stimulate the limbs of stroke and arthritis patients. The researchers are now working on BION2, which will amplify existing nerve signals-sending instructions to the muscles and pr0cessing feedback from the limb-and might improve communication between a prosthetic limb, such as a hand with moving digits, and its wearer´s brain.

When these sensors determine that a leg needs extra power-for, perhaps, an uphill climb-they might operate in tandem with computer-controlled motors, also integrated into the prosthetic, to give the wearer a boost. Herr and his team have fashioned an "active ankle" that can behave like a spring or add or dissipate energy. His prototype has a motor in the ankle and a series of springs. The wearer would have wireless sensors implanted in his remaining leg muscle that would communicate with the ankle´s internal computer, augmenting power when needed. Herr has tried the device himself and describes it as equivalent to hopping onto a powered airport-terminal walkway. "If it were a product," he says, "I would call it the Catapult."

As all these technologies are refined, the artificial limbs will begin to look more and more like human limbs from the outside-and possibly even from the inside, as the human design is geared so well to its many tasks. Multi-jointed fingers will replace hands that have only three unjointed fingers, to achieve more naturalistic dexterity. Mechanical knees and ankles will become progressively more streamlined and efficient. Ultimately, even the means by which the joints are activated will become more humanlike.

Roy Kornbluh, a senior research engineer at SRI International in Menlo Park, California, has been experimenting with an electroactive polymer, also known as artificial muscle, that expands when voltage is applied and contracts when it´s turned off. "The more voltage, the more it stretches," Kornbluh says. The process mimics human muscle, which changes shape in response to chemical signals.