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Tuesday, February 25, 2014
Real-Time Sensory Feedback Achieved in Bionic Limb

by Richard Robinson

It is still a long ways away, but a truly bionic hand may be one step closer, according to a new study that demonstrated that when tactile sensation is delivered from an artificial hand to intact peripheral nerves, motor function improves. During the course of this 30-day trial, the subject using the prosthetic reported that the new artificial hand began to feel like a part of his own body. A description of this approach was published in the Feb. 5 issue of Science Translational Medicine.


      The value of sensory feedback for control of an artificial limb has been understood for a long time, Paolo Maria Rossini, MD, PhD, co-leader of the study and director of the Institute of Neurology at Catholic University of The Sacred Heart in Rome, Italy, told Neurology Today. Current state-of-the-art hand prostheses rely on so-called “open-loop” control, in which the subject must use visual feedback to control the movement and position of the fingers of the hand. Visual feedback is useful, he said, but without tactile input, the hand may operate at only 25 percent efficiency in the natural environment. Many tasks, such as those requiring a delicate and graded grasp, may be impossible to perform. 

     The ideal prosthesis would instead employ “closed-loop” control, in which the hand’s actions generated sensory information to the brain, exploiting the pre-existing tight connection between tactile input and motor output. But that requires three key things, said study co-leader Silvestro Micera, PhD, director of the Translational Neural Engineering Laboratory at the Center for Neuroprosthetics, part of the Swiss Federal Institute of Technology in Lausanne, Switzerland.

FULLY ARTICULATED MECHANICAL HAND
Five years of developing those key features led to “LifeHand 2,” a fully articulated mechanical hand whose fingertips are covered in a pliable polymer. Pressure sensors are embedded in the tips of the index and little fingers. Wires lead from its base to an external computer, which in turn powers a set of multichannel electrodes. The electrodes are embedded in the remaining limb stump, where they contact the median and ulnar nerves. 

      The subject in this first-ever trial was a Danish man who, nine years earlier, had lost his left hand in a fireworks accident. The electrodes, thinner than a human hair, were implanted under general anesthesia, and two days later connected to the computer. Over several days, the researchers tested combinations of contacts and stimulation settings, eliminating those that registered as pain or temperature, while fine-tuning those that were sensed as pressure originating from the index and little finger.

      “At the end of this mapping procedure, we retained a number of contacts, from which the subject was receiving stable sensation, very precisely localized in the missing hand,” Dr. Rossini said. “Next, we connected the hand. At this point, we had a system which was theoretically able to dispatch sensory information from distinct parts of the fingers, that the subject could localize quite precisely.” 

      Regulatory permission for the implant was limited to only 30 days, so it is not yet known how far this device might go to restoring daily function. Dr. Rossini referred to the subject as a “hero,” willing to undergo two major surgeries despite knowing he had nothing to gain in the end. “He worked with us, to help make a small step towards a solution for millions of people suffering from limb amputation.”

      The team is already at work on a new iteration, with a goal of a fully wearable system than can be implanted for much longer periods. 

     For the extended discussion on the implications of this study for the future of bionic prosthetics, see the March 6 issue of Neurology Today. For previous coverage of similar devices, browse Neurology Now’s archives: http://bit.ly/1ljwxgE.

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