Over the years, various solutions have been developed to restore tactile information to people with limb amputation. However, this information is still far from those of the human hand in terms of naturalness and effectiveness. The solution to this important clinical and scientific problem is in a new research by a group of researchers of the Sant’Anna School of Advanced Studies, of the École Polytechnique Fédérale de Lausanne (EPFL), of the University of Freiburg in Germany and of the Policlinico Gemelli in Rome.
Prostheses that do not feel like a foreign body, but feel and move like a natural arm – that was the aim of the international research. The working group coordinated by Prof. Silvestro Micera, professor of Bioengineering at the BioRobotics Institute of the Scuola Superiore Sant’Anna, and Bertarelli Chair in Translational Neuroengineering at the EPFL, has in fact developed a code capable of transmit for the first time to the nerves of the amputated arm all the variety of perceptions that it would have received from the nerve fibers connected to the tactile receptors of one’s hand and then communicate the information useful for the movement in an extremely natural way.
Stimulating the nerves
For some years, researchers have equipped hand prostheses with technical sensors. With these, people whose arm has been amputated can recognize the shape and strength of gripped objects by electrostimulating the nerves in the stump. So far, however, the transmitted information was still far from a natural feeling and natural dexterity.
In the new study, the scientists developed a novel “stimulation code” that stimulates brain power pulses as though the skin’s natural sensors are producing them. As a result, the patient not only recognizes the gripping force and deformability of an object, but also grasps and moves it faster and more accurately. Through an interdisciplinary approach, which integrates practices deriving from neuroengineering, clinical neurology, and robotics, with computer simulations of the behavior of neurons, it was possible to ascertain that by stimulating the peripheral nerve with information very similar to those that natural finger sensors would provide under normal conditions, the patient is able to receive more natural and effective information.
“For the success of the procedure, we have developed implantable electrodes that are thinner than a human hair. They were stable in the nerves and did not move. Therefore, for months, current impulses could be encoded to the nerves, so the two patients gradually adopted the prosthesis as their own arm rather than as a technical foreign body,” Prof. Dr. Thomas Stieglitz from the University of Freiburg said. “If I can grasp raw eggs unerringly without fear, and also know with how muich strength I press it, then the question is where the line between man and machine is.”
Natural sensors and nerve fibers
“In this work, we did not start from the robotic hand, but from the source of tactile information, trying to reproduce the dynamics of the natural sensors and nerve fibers that spread from the fingers when a hand touches an object. In this way, we transmitted a signal to the patient’s nervous system that was immediately recognized as natural”, explains Giacomo Valle, a doctoral student at the Sant’Anna School of Advanced Studies and first author of the study.
This is a significant step forward towards an even more natural hand prosthesis because for the first time all aspects of tactile perception are taken into account. Furthermore, the code developed by the authors of the study can be applied to all prosthetic models, ensuring the sensitivity of perceptions and the effectiveness of movements. “Our results will allow prostheses to be both effective and usable in a natural way and not felt like a foreign body. This will significantly increase the clinical impact of these technologies,” says Silvestro Micera.
The use of the code allows greater sensitivity when the robotic hand comes into contact with an object, exceeding the “sensory” limits of traditional prostheses. “Our hand allows us to explore the environment around our person and interact with it,” commented Prof. Paolo Rossini, clinical principal investigator of the study. It allows us to hit hard or to stroke. It allows us to play a keyboard or lift a heavy barbell. All this variety of actions (and a thousand others) is also possible thanks to the sensory information that every movement and contact with an object sends to our nervous system. Losing sensory information is like living in a world without colors and without light / dark contrasts.”