by Ruggiero Corcella
Scuola Superiore Sant’Anna of Pisa and EPF of Lausanne successfully tested the prototype on a 57-year-old patient who lost his right hand. The new frontiers
In the long journey towards the creation of humanoid robots, their “covering” is one of the most challenging objectives to achieve. In fact, human skin is so complex that it is considered a real organ. Its complexity makes it almost impossible for engineering to reproduce: it detects temperature, pressure and consistency. Not to mention its self-repair ability.
Now researchers from Sant’Anna High School of Pisa and the École Polytechnique Fédérale de Lausanne have created a new sensorized hand prosthesis, capable of providing realistic and real-time thermal feedback. The study was published in the journal Med (Cell Press) .
How the bionic hand works
This innovative hand prosthesis senses temperature differences and restores some sensitivity to people who have suffered an amputation. Sensors on the prosthetic hand are connected to a system that thermally stimulates the surface of the stump. The stimulus is perceived by the nervous system which transmits the message to the brain. In fact, in our body there are sensory nerves through which thermal stimuli reach the central nervous system.
Amputee patients often suffer from “phantom” limb syndrome, meaning they still feel the presence of the limb even after removal. The subject perceives the position and tactile sensations coming from the amputated limb, thinking he can move it. This condition often causes pain and frustration.
The test on a patient
With the new device it will be possible to restore the natural sensation of touching objects and people by being able to perceive their temperature and humidity. The prototype was tested on Fabrizio, a 57-year-old man with a transradial amputation who was able to manually distinguish and sort objects at different temperatures and perceive body contact with other human beings.
«When one of the researchers positioned the sensor on his body, it was a very strong emotion for me – says Fabrizio -. I could feel the warmth of another person with my “ghost” hand. It was like reactivating a connection that I had lost.”
«Temperature is one of the last frontiers for restoring sensitivity to robotic hands. For the first time, we are really close to restoring the full range of sensations to amputees” he comments Silvestro Micerasenior author of the research.
Prosthetics capable of restoring a full range of sensations
Sensory perception is one of the most important aspects of allowing people with an amputation to interact with their surroundings. Building on previous findings about phantom thermal sensations, which stimulate specific points on the residual arm evoking perceptions in the missing hand, researchers have developed a new approach that allows amputees to sense and respond to temperature by transmitting thermal information from the fingertip of the prosthetic to the residual limb of the amputee. « In May 2023 we published a study that demonstrated how it was possible to make the phantom hand of an amputee experience thermal sensations – explains Micera -. By stimulating specific points on the remaining arm, we evoked perceptions in the missing hand. Now we have taken a step further, we have created a real prototype. The device developed in our Institute is called MiniTouch and allows amputees to perceive temperature differences, transmitting information from the fingertip of the prosthesis to the stump.”
«The noteworthy aspect is that we have inserted this technology into a commercial prosthesis, thanks to the use of consumer electronics and without the need for surgery – continues the expert -. It is extremely relevant to have sensory perception in the prosthesis, not only to avoid burning yourself with hot objects (for example a cup or a pot), but we have observed how important it is to be able to re-establish and perceive body contact with another person. In the laboratory, a patient was accidentally touched by a researcher and we were struck by his reaction of amazement at being able to relive the perception of human warmth from the prosthesis. Precisely this aspect is fundamental for us: it triggers benefits in psychological and mood terms for amputees.”
Touch more and more similar to the human one
“Adding temperature information makes touch more human-like,” explains the senior author Solaiman Shokur of the École Polytechnique Fédérale de Lausanne. «We think that the ability to perceive temperature will improve the embodiment of amputees, the feeling that “this hand is mine”».
«Until now, thermal sensations have been greatly neglected in neuroprosthetic research, even though their importance in daily life is increasingly evident. We think that amputees could benefit from recovering thermal sensations that go well beyond the detection of cold or hot objects,” says Jonathan Muheim, PhD student at EPFL and first author of the work together with Francesco Iberitedoctoral student at the BioRobotics Institute of the Scuola Superiore Sant’Anna.
The testing phase was carried out at Inail Prosthetic Center in Vigorso di Budrio (Bologna) with the collaboration of Dr. Emanuele Gruppioni (INAIL) and his team, who carried out the clinical tests with amputees. The device was integrated into the patient’s personal prosthesis and was attached to a location on the residual limb that elicited thermal sensations in the person’s phantom index finger.
«Traditional prostheses do not allow you to receive any type of sensory feedback, the most widespread ones in fact allow you to grasp and move objects effectively – underlines the engineer Gruppioni -. We are working to also restore tactile sensations and develop increasingly natural bionic interfaces. The richness and realism of the sensations provided by bionic interfaces to amputee patients is the true key to embodiment and therefore to the effectiveness of a prosthesis in replacing a natural limb in carrying out the activities of daily life. Scientific research, clinical studies with patients and technological development are the ingredients to achieve solutions that aim to recreate that perfection that to date only nature has managed to develop.”
The tests: 100% accuracy
The research team tested the amputee’s ability to distinguish between objects of different temperatures and materials. Specifically, the patient was able to discriminate between three visually indistinguishable bottles containing cold water, room temperature water and hot water with an accuracy of 100%, while, without the device, his accuracy stopped at 33%. Its ability to accurately and quickly classify metal cubes of different temperatures has also improved.
“When you reach a certain level of dexterity with your robotic hands, you need to have sensory feedback to be able to use the robotic hand to its full potential,” explains Shokur. Furthermore, the patient was better able to distinguish when he came into contact while blindfolded with human arms or with prosthetic arms: from 60% without the device to 80% with the device. «Our goal is to develop a multimodal system that integrates touch, perception and temperature» adds Shokur. “With this type of system, people will be able to say ‘this is soft and warm’, or ‘this is hard and cold’.”
The next steps: a device for home use
The technology developed by the team from Scuola Superiore Sant’Anna and École Polytechnique Fédérale has currently been tested in the laboratory. The next step will be to make the device ready for home use and to integrate thermal information from multiple points of an amputee’s phantom limb: for example, allowing people to differentiate thermal and tactile sensations on the finger and thumb could help them grasp a hot drink, while enabling sensations on the back of the hand could improve the feeling of human connection by allowing amputees to sense when another person touches their hand.
«This study – concludes Micera – paves the way for more natural hand prostheses that restore a complete range of sensations, offering amputees a richer and more natural perception. These interfaces will have to allow the wearer to distinguish smooth or rough, hot or cold objects, i.e. they will have to return a complete range of natural sensations.”
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February 9, 2024 (changed February 9, 2024 | 5:00 pm)
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