[LUM#12] A giant leap forward for prosthetics
Can a prosthetic limb be made sensitive enough to allow its wearer to feel contact with the environment? That’s the bold challenge that a consortium of European researchers has successfully met, with their findings published in the journal *Nature Medicine*. At the heart of this innovation is the selective neural stimulation technology developed by Camin, a team based in Montpellier.
Imagine a leg prosthesis capable of restoring the wearer’s sense of touch. Rediscovering the warmth of sand in the summer, the softness of damp earth, or even the roughness of a worn-out rug—even though the receptors that generate these sensations were removed along with the amputated limb. Sometimes there is only a small step from science fiction to reality—a step that science has just taken by successfully implanting the first sensory-feedback neuroprostheses in a lower limb, having already achieved this in the upper limb.
EPIONE is the name of this project led by a European consortium of Italian, Swiss, German, and French researchers, each of whom has contributed their unique expertise. “The Germans, for example, are responsible for the electrodes, while our specialty is selective neural stimulation. “ You have to think of this prosthesis as a complete chain connecting sensors to a stimulator, which is itself connected to electrodes,” explains David Andreu, a researcher at the Montpellier Laboratory of Computer Science, Robotics, and Microelectronics (LIRMM) and a member of the Camin team.
A complete chain
At the heart of this system are the sensors installed on the prosthesis, which use their 36 contact points to gather information about the external environment. Is the ground hard, smooth, or warm? The information is then received by the stimulator implanted in the patient’s body. This stimulator, designed by the Montpellier team, is the cornerstone of this revolutionary prosthesis’s operation. It must both translate the information from the sensors into electrical signals and use these signals to stimulate, via the four electrodes implanted in the nerve, the area corresponding to the desired sensation. “This is the core of our expertise; our stimulator is capable of generating this by precisely stimulating a specific area.”
“To fully understand the mechanism of sensory feedback, we can compare the nerve to a thick cable containing a multitude of small wires: the axons,” continues David Andreu. “Depending on which axons are stimulated, specific sensations are generated. This is what allows the patient to truly integrate the prosthesis into their body schema. ”
Each electrode has 16 contact points with the nerve, thereby activating different sets of axons to transmit a whole range of sensations to the brain. The intensity with which the axons are stimulated then determines the intensity of the sensation felt.
A personalized map
The problem is that the relationship between axons and sensations varies from person to person, so the stimulator must be configured by mapping each patient’s responses after implantation. “For a given point, the patient must describe not only what they feel but also the threshold at which they begin to feel it, and the maximum threshold at which they experience pain… “In the early trials, it took a whole day to explore the full range of possibilities,” explains David Guiraud, a researcher atthe French National Institute for Research in Computer Science and Automation (INRIA) and project lead for this initiative within the Camin team.
It was a demanding trial for the patients, whose sensory responses were monitored for several hours a day throughout the six-month trial period. “Since this was an experimental trial, the stimulator was external, so the electrodes protruded from their bodies, which meant they had to stay in the hospital to avoid any risk of infection,” adds David Andreu.
All over the place
While these initial human trials conducted in Serbia were a success, carrying them out required a great deal of commitment not only from the patients, of course, but also from the researchers. “We were present at all the surgeries to test the electrodes and ensure they hadn’t been damaged by the surgeon during implantation,” recalls David Guiraud.“ We then monitored the electrodes remotely to adjust the stimulation.”
The two researchers have now left the Camin team to launch their own spin-off company called Neurinnov. And after working on sensory feedback for amputated limbs, the scientists have decided to focus on grasping to help quadriplegic patients regain control of their hands. “It’s the same thing but in reverse: we’re no longer trying to create sensory feedback from a prosthesis, but to restore motor function to a real hand,” explains David Andreu. A new challenge that the two scientists should tackle… with flying colors!
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