With all of the mechanical advances in prosthetic limbs recently, few, if any, prosthetic limbs send sensory information back to the wearer, giving them a realistic sense of touch. That could soon, change, due to research being conducted at the Cleveland Veterans Affairs Medical Center and Case Western Reserve University, Cleveland, OH. Researchers there say that they have developed a new kind of nerve interface that can convey a sense of touch from 20 spots on a prosthetic hand by directly stimulating peripheral nerve bundles in the arms of patients.
The new technology allows force detectors on the digits of a prosthetic hand to convey touch information directly to three pea-sized nerve interfaces surgically implanted in a patient’s lower right arm. The user controls the hand through a standard technology called a myoelectric interface, which uses signals from the muscles in the lower arm to govern prosthetic hand movements. Two people have been fitted with the interface, and the implants have continued to work for more than 18 months.
At the heart of the technology is a custom version of an interface known as a cuff electrode. Three nerve bundles in the arm—radial, median, and ulnar—are held in the seven-millimeter cuffs, which gently flatten them, putting the normally round bundles in a more rectangular configuration to maximize surface area.
Then a total of 20 electrodes on the three cuffs deliver electrical signals to nerve fibers called axons from outside a protective sheath of living cells that surround those nerve fibers. This approach differs from other experimental technologies, which penetrate the sheath in order to directly touch the axons. Sheath-penetrating interfaces are thought to offer higher resolution, at least initially, but with a potentially higher risk of signal degradation or nerve damage over the long term. And so they have not been tested for longer than a few weeks.