New devices using pulses of electricity could help paralyzed people walk again

"The leg wasn't bouncing all over the table, but there were substantial twitches," says Matthew Schiefer, a neural engineer at Case Western Reserve University, Cleveland, Ohio.

Schiefer is describing an experiment in which pulses of electricity are used to control the muscles of an unconscious patient, as if they were a marionette. It represents the beginnings of a new generation of devices that he hopes will allow people with paralyzed legs to regain control of their muscles and so be able to stand, or even walk again.

His is one of a raft of gadgets being developed that plug into the network of nerves that normally relay commands from the spinal cord to the muscles, but fall silent when a spinal injury breaks the chain. New ways to connect wires to nerves allow artificial messages to be injected to selectively control muscles just as if the signal had originated in the brain. Limbs that might otherwise never again be controlled by their owners can be brought back to life.

The potential of this approach was demonstrated in 2006 when a different Case Western Reserve team enabled someone who was paralyzed from the waist down to watch their usually motionless knees straighten at the push of a button. With a little support they even stood for 2 minutes while signals injected into nerves in their thighs kept their knees straight.

But controlling one joint alone is not enough. Schiefer's latest experiment uses a new method to plug into a nerve to control the four muscles needed to stand up from a sitting position.

Motor nerves like this are in some ways like telephone cables; they're are made up of electrically isolated bundles of nerve fibers, each one of which connects to certain groups of muscle cells. In the 2006 trial, electrodes were simply placed on the nerve's surface using a spiral cuff, but this makes for a poor connection with fiber bundles close to the nerve's core. The new solution, known as the flat interface nerve electrode (FINE), is a cuff that squashes a nerve flat to bring fiber bundles closer to the surface -- and to the eight electrodes in the device's soft rubber lining.

It makes for a much better connection, says Dustin Tyler, who invented the FINE and heads research into its effectiveness. "We apply a little bit of pressure to reshape the cross-section without damaging the nerve," he notes.

Recent tests validated that approach. The cuff was temporarily implanted on the femoral nerves of seven patients undergoing routine thigh surgery. Pulses of current 250 microseconds long were used to selectively and independently activate the muscles that extend the knee and flex the hip joint when a person stands up. The pulses were not enough to bend the joints as much as they would when standing, but the results suggest that longer pulses should stimulate the muscles to provide enough force to support the body's weight (Journal of Neural Engineering). Longer trials are being planned, subject to approval from the U.S. Food and Drug Administration.

Future devices using FINE would likely be targeted at people paralyzed from the waist down. A computer interface to the implant could give them control of their legs. Further into the future, a brain interface might allow a person to control their implant with their thoughts.

The traffic through our nervous system is not just one-way, though, and for a device to restore function to paralyzed arms or legs it needs to be able to detect feedback from those limbs. The first commercial walking aid that plugs into nerves demonstrates just that ability, and goes on sale in Europe in a few months. Neurostep, from Neurostream Technologies of Saint-Augustin-de-Desmaures, Quebec, Canada, designed by Andy Hoffer at Simon Fraser University in Vancouver, controls ankle movement for people with foot-drop, a condition in which nerve damage makes one foot hang limply while stepping forward.

Neurostep connects using just four electrodes, placed around a nerve inside a cylindrical cuff similar to the spiral one used in Case Western's 2006 trial. But the device not only injects current into the nerve, it also reads signals sent back by the foot to communicate the pressure it feels. A control unit implanted in the thigh uses that pressure information to time its signal to flex the ankle in a way that achieves a normal gait.

Ultimately, though, realizing the ambition of neuroengineers to control every muscle a nerve connects to requires plugging in more literally, says Greg Clark at the University of Utah's Department of Bioengineering in Salt Lake City. Placing electrodes outside a nerve is like standing outside a stadium and trying to shout to someone watching a football match inside, he says.

With colleagues, Clark is testing a device called the Utah slanted array that bristles with up to 100 wires designed to be gently pushed into a nerve. "They get up close and personal with nerve fibers," says Clark.

The result is the most precise control yet of any of the limb-activating devices, he says. Though not yet approved for human trials, it has allowed previously paralyzed cats to stand, and has been used to control the movement of a monkey's fingers individually. More independent electrodes lead to more graceful movement and finer control, says Clark.

It won't be perfect. Nerves contain tens of thousands of axons, each capable of being controlled by the ultimate puppeteer: the brain. Learning to pull even a few of those strings, though, could restore partial function to a person's limb, restoring some control to an arm or leg that was previously paralyzed.

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Copyright © 2010 MacGregor Campbell, New Scientist Magazine