Dennis Aabo Sørensen lost his left hand in a fireworks accident during a family holiday when he was in his mid-twenties. Last year, the 36-year-old Danish man got a chance to test out a new prosthetic hand that connected to his nervous system and allowed him to grip and manipulate objects. Even more remarkably, he actually felt what he was touching for the first time in the 9 years since his accident, according to a report published today in Science Translational Medicine.
The technology itself builds on decades of research, but the study may be the clearest demonstration yet of the importance of building sensory feedback into prosthetic devices to make them better able to perform the motions of everyday life. And in that sense it points to where the field of neural prosthetics is heading.
Electrodes implanted in the stump of his amputated hand allowed Sørensen to make basic grasping motions with the hand. Scientists have made great progress with neural prostheses in recent years, and other teams have demonstrated similarly impressive feats with amputees and paralyzed people. What's unusual about this new prosthesis is that it adds a sense of touch.
Here's how it works: Sensors in the artificial tendons that control the fingers track tension as the hand moves; they send that information to a nearby computer, which translates them into signals a person's nervous system can understand. Then – and all this happens in real time as the hand is in use – the computer fires those translated signals to electrodes implanted in nerves in the subject's upper arm, giving him a rudimentary sense of touch. Using this system allowed Sørensen to determine the shape of different objects and whether they were hard or soft – and adjust his grip accordingly. Imagine how useful this feedback would be to an amputee in real life: shaking hands, picking up fruit at the grocery store, performing tasks around the house. To prove that Sørensen was using tactile feedback from the hand, researchers blindfolded him and had him wear earphones while they tested his ability to feel and manipulate balls, cylinders, and other shapes.
The sensory capabilities of the hand were developed by Silvestro Micera and colleagues at the Scuola Superiore Sant’Anna in Italy and the Ecole Polytechnique Federale de Lausanne in Switzerland.
Prosthetic limbs that deliver tactile feedback directly to the nervous system have been developed previously (as early as 1974), says Dustin Tyler, a biomedical engineer at Case Western Reserve University. Where the European study breaks new ground is in showing just how much the sensory feedback improves the patient's performance. For those of us fortunate enough to have all our limbs, this feedback happens so automatically that we take it for granted. But without it, the nervous system can't make the on-the-fly adjustments necessary to produce smooth, natural movements.
"Going forward, sensory feedback is probably the most important thing," Tyler said. "It's what changes a prosthesis from a tool to a hand."
