Bionic touch prosthesis to feel hot and cold: "It's like having a connection with someone"

Recovering all the senses is one of the great desires of people who lose a limb. A fact that becomes more acute in the case of the hands. Feel beyond the simple touch, achieve that transmission of sensations from one hand rubbing against 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 go 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 the 17 patients who have felt how their phantom hand experienced temperature changes thanks to new technology from the Federal Polytechnic School of Lausanne (EPFL).

“What really impressed me was when Francesco [first author of the paper] put the finger sensor on his arm instead of the materials. I 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.. It felt like someone was touching my missing hand,” Roberto explains.

Today the journal Science publishes the results of the work of Francesco Iberite, Jonathan Muheim, Silvestro Micera and Solaiman Shokur who have worked hard to incorporate new sensory stimuli into prostheses to provide amputees with a more realistic touch, and whose latest study focuses on temperature. They have been working since 2014 on different characteristics that add 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 you could see it in his eyes”, emphasizes Iberite.

How does the prosthesis work?

If something hot or cold is placed on the forearm of an intact person, they will feel the temperature of the object locally, directly on their forearm.. But in amputees, that residual arm temperature sensation can be felt in their missing phantom hand.. On this, Shokur, senior scientific neuroengineer at EPFL who co-led the study, notes that “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 SA EPFL

By providing temperature information non-invasively, using heating 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 experiment if an object is hot or cold and tell if they are touching copper, plastic, or glass.. This research is the result of a collaboration between EPFL, the Sant'Anna School of Advanced Studies (SSSA) and the Protesi Inail Center. The technology was successfully tested in 17 of 27 patients.

“When I touch the stump 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 onto the phantom limb has led to the development of new bionic technology that provides prostheses with non-invasive thermal feedback that allows amputees to discern what they are touching.

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

How did this breakthrough come about?

A few years ago, Micera and Shokur found out about the existence of a system capable of providing information on temperature 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 CTO of Metaphysiks.

“Until now, prostheses have been designed primarily for easy movement, to help you in normal life.. But the integration of these sensations of cold and heat, in my opinion, also serve to improve social interactions”, says Fabrizio Fidati, a patient.

EPFL neuroengineers used this technology to deliver 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 touch and position information can be successfully transmitted, we contemplated restoring the full richness of sensation 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, for example, a plastic one.. A thermode, which is in contact with the skin of the amputee's residual arm, heats or cools, transmitting to the finger 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 contact with different materials, we got a lot of 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 discovered that small patches of skin from the residual arm project to specific parts of the phantom hand, such as the thumb or the tip of the index finger.. Unsurprisingly, they found that the mapping of 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 that 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, we are missing something. A very interesting aspect of our technique is its intuitiveness: in a short time, even someone totally new to this type of stimulation will effectively. During the experiment, the subjects began not only to distinguish hot and cold objects, but also to recognize different materials by taking advantage of their temperature differences, which demonstrates the potential of this result,” Iberite added.