Prosthesis with bionic touch to feel cold and heat: "It's like having a connection with someone"

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Recovering all the senses is one of the great desires of people who lose limb. A fact that is more acute in the case of the hands. To feel beyond the simple touch, to achieve that transmission of sensations of the rubbing of one hand with another: a bionic touch. "It's like having a connection with someone. I would like to feel the hands of my two children when I walk down the street with them, holding their hands. That would be nice."

Roberto Renda is an amputee from Rome, Italy. He recently participated in a study to test the effects of temperature feedback directly on the skin of his residual arm. He is one of 17 patients who have felt their phantom hand experience temperature changes thanks to new technology from the Ecole Polytechnique Fédérale de Lausanne (EPFL).

"What really impressed me was when Francesco [first author of the work] put the fingerprint sensor on his arm instead of the materials. He could feel the temperature of his arm. It was the first time in 20 years that I could feel the warmth of another person with my phantom hand. I felt like someone was touching my missing hand," Roberto explains.

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Today the journal Science publishes the results of the work of Francesco Iberite, Jonathan Muheim, Silvestro Micera and Solaiman Shokur who have worked to incorporate new sensory stimuli into prostheses to provide a more realistic touch to amputees, and whose latest study focuses on temperature. They have been working since 2014 on different features that are adding to the benefits of the prostheses. "It was surprising to see the reaction of the participants when we put the sensor on our skin during the experiments: for them the sensation of heat was vivid, real, and when they realized that what they felt was contact with another person, the emotion was seen in their eyes," says Iberite.

How does the prosthesis work?

If something hot or cold is placed on an intact person's forearm, they will feel the temperature of the object locally, directly on their forearm. But in amputees, that feeling of temperature in the residual arm can be perceived in the phantom hand they are missing. Shokur, a senior scientific neuroengineer at EPFL who co-led the study, said: "It is important that phantom thermal sensations are perceived by the patient as similar to those experienced by their intact hand.

The bionic thermal sensor at the tip of a prosthetic finger and its corresponding thermal image in the background as seen by a thermal camera. Alain Herzog, CC BY SAEPFL

By providing temperature information non-invasively, using thermal electrodes (also known as thermodes) placed on the skin of the residual arm, amputees claim to feel the temperature in their phantom limb. They can experience whether an object is cold or hot and distinguish whether they touch copper, plastic, or glass. This research is the result of a collaboration between EPFL, the Sant'Anna School of Advanced Studies (SSSA) and the Centro Protesi Inail. The technology was successfully tested in 17 of 27 patients.

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When I touch the residual limb with my hand, I feel a tingling in my missing hand, the phantom hand. But feeling the temperature variation is something else, something important... something beautiful," says Francesca Rossi, a patient. The projection of thermal sensations into the phantom limb has led to the development of a new bionic technology that endows prostheses with non-invasive thermal feedback that allows amputees to discern what they are touching.

Micera, from the Bertarelli Foundation Chair of Translational Neuroengineering at EPFL and professor at the SSSA, emphasizes that "temperature feedback is essential to transmit information that goes beyond touch, it leads to feelings of affection. We are social beings and warmth is an important part of that."

How did this breakthrough come about?

A few years ago, Micera and Shokur learned of the existence of a system capable of providing temperature information through the skin of healthy subjects, also developed at EPFL and derived by Metaphysiks.

This company has been developing a neurohaptic technology, MetaTouch, which connects the body with digital resources. MetaTouch combines the sense of touch and temperature to multiply the sensory effect of physical products. "This breakthrough highlights the power of haptics to improve medical conditions and quality of life for people with disabilities," said Simon Gallo, co-founder and chief technology officer of Metaphysiks.

"Until now, prosthetics have been designed mainly for simple movements, to help you in normal life. But the integration of these sensations of cold and heat, in my opinion, also serves to improve social interactions," says Fabrizio Fidati, a patient.

EPFL neuroengineers used this technology to provide thermal feedback directly to the user's skin. With this device they discovered phantom thermal sensations and later tested it on 27 amputees. "For the first time, after many years of research in my laboratory demonstrating that information about touch and position can be successfully transmitted, we contemplated the possibility of restoring all the richness of sensations that the natural hand can provide," says Micera.

How is the current prototype: Minitouch?

For the study, Shokur and Micera developed the MiniTouch, a device that provides thermal feedback and built specifically for integration into wearable devices such as prosthetics. This consists of a thin, wearable sensor that can be placed on the prosthetic finger of an amputee. The sensor detects thermal information about the object being touched and, more specifically, its thermal conductivity. If the object is metallic, it will naturally conduct more heat or cold than, say, a plastic one. A termodo, which is in contact with the skin of the amputee's residual arm, is heated or cooled, transmitting to the fingerprint sensor the temperature profile of the object being touched.

"When we presented the possibility of recovering the sensation of temperature in the phantom limb or of feeling the contact with different materials, we obtained many positive reactions. And in the end we managed to recruit more than 25 volunteers in less than two years," explains Federico Morosato, responsible for organizing the clinical aspect of the trials at the Protesi Inail Center.

The scientists found that small areas of skin from the residual arm project to specific parts of the phantom hand, such as the thumb or tip of the index finger. Unsurprisingly, they found that mapping the temperature sensations between the residual arm and the entirety of the projected phantom hand is unique to each patient.

"We focus on thermal sensations because we believe it is one of the keys to making the experience more realistic: every object we touch has a temperature; If we don't feel it, something is missing. A very interesting aspect of our technique is its intuitiveness: in a short time, even someone totally new to this type of stimulation effectively. During the experiment, the subjects began not only to distinguish hot and cold objects, but also to recognize different materials taking advantage of their temperature differences, which demonstrates the potential of this result," says Iberite.

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