[LUM#12] A giant leap for prosthetics

[LUM Magazine] - Published on June 1, 2020 in Science-Society

Making a prosthetic limb sensitive, capable of allowing its wearer to feel contact with their environment? This is the crazy challenge taken up by a consortium of European researchers, whose findings have been published in the journal Nature Medicine. At the heart of this innovation is selective neural stimulation technology developed by Camin, a team based in Montpellier.

Imagine a prosthetic leg capable of restoring the wearer's sense of touch. Rediscovering the warmth of sand in summer, the softness of damp earth, or the roughness of a worn carpet, even though the receptors that generate these sensations were removed along with the amputated limb. Sometimes, science fiction is just a step away from reality, and science has just taken that step by successfully implanting the first sensory feedback neuroprostheses on a lower limb, having already achieved this on an upper limb.

EPIONE is the name of this project led by a European consortium of Italian, Swiss, German, and French researchers, each contributing their expertise. "The Germans, for example, are responsible for the electrodes, while our specialty is selective neural stimulation. This prosthesis should be seen 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 member of the Camin team.

A complete chain

At the heart of this chain are sensors installed on the prosthesis, whose role is to collect information about the external environment via their 36 contact points. Is the ground hard, soft, 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. 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 can generate this by precisely exciting a specific area."

"To fully understand the sensory feedback mechanism, we can compare the nerve to a large 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 image."
Each electrode has 16 points of contact with the nerve, activating different sets of axons to send a whole range of sensations to the brain. The intensity with which the axons are stimulated then varies the intensity of the sensation felt.

A map unique to each individual

The problem is that the correspondence between axons and sensation varies from one individual to another, so it is necessary to configure the stimulator by mapping each patient after implantation. "For a given point, the patient must characterize what they feel, but also the threshold at which they feel it, the maximum threshold at which they feel pain... The first trials 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 Control (INRIA) and head of this project in the Camin team.

This was a demanding experiment for the patients, whose sensations were tested for several hours a day during the six months of the trials. "As this was an experiment, the stimulator was external and the electrodes protruded from their bodies, which meant they had to stay in the hospital to avoid any risk of infection," adds David Andreu.

Feet and hands

Although these initial human trials in Serbia were successful, they required a great deal of commitment not only from the patients, of course, but also from the researchers. "We attended all the operations to test the electrodes and ensure that they had not 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. After working on sensory feedback for amputees, the scientists decided to focus on grip, with the aim of restoring hand control to quadriplegic patients. "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 take on... with flying colors!

Keywords: Innovation, LUM#12, LUM Magazine, Medicine, Health