In a First, a Prosthetic Limb Can Sense Temperature Like a Living Hand

Amputees’ hopes to experience the feeling of human touch using their prosthetics are becoming closer to reality. Now, new technology is allowing them to feel temperature—even in limbs that are no longer part of their bodies.

For the first time, a functional artificial limb has been fitted with fingertip sensors that allow an ordinary prosthetic hand to sense and respond to temperature just as a living hand does. The device provides a realistic sense of hot and cold in the missing “phantom” hand by delivering thermal information to nerve areas on the amputee’s residual limb that the brain believes are still connected to the missing hand. The MiniTouch, described in a study published Friday in Med, was created with affordable off-the-shelf electronics, requires no surgery and can be fitted to existing commercial prosthetic hands in a matter of hours.

“With these new technologies I can understand better what I am touching,” says Fabrizio Fidati, a 57-year-old man who had his arm amputated below the elbow 37 years ago and recently tested the device. “Certainly, my priority would be to use it in a kitchen to cook!”

The ability to sense hot and cold temperatures is an important advance for the functionality of prosthetic limbs, but it’s also much more. Temperature adds a human element to touch. Feeling warmth creates a stronger sense of embodiment, the belief that a prosthesis isn’t just an artificial tool but a part of a person’s body.

“We have a colleague who says touch without temperature is like vision without color,” says senior author Solaiman Shokur, a neuroengineer at the Swiss Federal Institute of Technology in Lausanne. Shokur notes researchers are improving prosthetics’ abilities to feel textures. They are also homing in on proprioception, the brain’s ability to know where a body part is and how it’s moving. Temperature, he notes, is an important third sensation. “The next step would be to put them all together, and that’s where you have the full palate of sensations” he says.

In May 2023, the same researchers published a study revealing that when they put thermal electrodes on the residual arms of amputees, then stimulated them by touching hot or cold objects with an attached sensor, subjects thought they were experiencing those hot or cold temperature touches in their missing hand. The small areas of skin that produce these phantom thermal sensations are different among amputees, suggesting that nerves cut off during amputation remain in various locations of the arm’s skin. When that patch of skin is stimulated by hot or cold objects, it sends signals through that pathway to the brain that used to come from the hand, causing the amputee to feel those sensations in the missing hand.

“It was the first time in 20 years that I could actually feel the warmth of another person with my phantom hand,” said Roberto Renda, from Rome, after a trial related to the 2023 study. “It’s like having a connection with someone. I would like to feel both of my kids’ hands when I walk down the street with them, holding their hands. That would be nice.”

These phantom thermal sensations, experienced by 17 of 27 people in the original study, were key to the viability of the MiniTouch system. MiniTouch is a thin, wearable sensor that’s easily slipped over an existing prosthetic finger and linked to those points on the residual limb’s skin that were known to produce sensations felt as if they were in the missing hand. While the 2023 study showed the sensor could help amputees detect temperature, the current study was the first to put it to use on a functional prosthetic.

Co-author Francesco Iberite, a biorobotics PhD candidate at Sant’Anna School of Advanced Studies in Pisa, Italy, recently ran the system through some real-world tests. “The experiments were always about the subject exploring the world around them in an active way, free to touch objects and interpret things without any training,” he explains, “and that’s so close to what happens with a natural hand.”

Fidati volunteered to test the system on his existing prosthesis. He was presented with three identical bottles that contained cold (53 degrees Fahrenheit), cool (75 degrees Fahrenheit), or hot (104 degrees Fahrenheit) water. Without the device, he could distinguish among the bottles just 33 percent of the time. Using the MiniTouch, Fabrizio’s accuracy was 100 percent.

Next Fidati was tasked with sorting through identical metal cubes that had been heated or chilled to different temperatures—and using the temperature-sensitive prosthetics significantly boosted his success rate. In fact, Fidati reported that sensations of hot and cold were actually more intense in his phantom hand than in his natural hand.

Finally, the team tested a blindfolded Fidati’s ability to tell whether he was touching another prosthetic hand or Iberite’s hand. The human hand’s warmth enabled Fidati to distinguish it more often, although his success still fell below his ability to do so with his uninjured arm. The authors suggest other input including textures and softness would likely improve this ability. This experiment illustrates how temperature, while important, is just one part of a sensory suite of human touch that can hopefully be built into prosthetics that more truly approximate a human hand.

Prosthetics have come a long way. The devices have been around for thousands of years, used for both cosmetic and functional purposes with varying degrees of success. In one ancient Egyptian burial chamber, the remains of a woman were found with a wooden and leather creation dubbed the “Cairo Toe,” an artistically crafted prosthetic that, 3,000 years ago, was painstakingly fitted to her foot. Circa 300 B.C.E, the “Capua Leg” was created out of wood and sheathed in bronze for an owner who lived north of present-day Naples, Italy. By the time of the Thirty Years’ War (1618-1648), people were being fitted with iron hands—in an attempt to provide function, cables, springs and other means were used to help limbs bend, or fingers grip objects, albeit imperfectly.

Today, tens of millions of people worldwide have had a limb amputated, and scientists and biomedical engineers are constantly improving prosthetics for these patients. Most projects to restore feeling to amputees involve implants in the body, which have advantages but also create some hurdles. Surgery is invasive, and both the process and the advanced devices it enables are expensive. Not so with the MiniTouch sensor, which can simply be integrated onto an existing prosthesis in a few hours and relies on widely available consumer-grade technologies. “The reason why cellphones are so cheap is because their technology can be replicated in scale. This is exactly the same for our sensor, which is the only custom part in the whole process,” says Iberite.

Lee Fisher, a biomedical engineer at the University of Pittsburgh not affiliated with the research, says efforts to add a sense of touch to prosthetics usually focus on tactile sensations like pressure. “It’s been less common with electrical stimulation to produce sensations that feel like temperature, and that’s super important, because a lot of the way we interact with the world is by that sense of temperature,” he says.

We can differentiate materials like metal and plastic, for example, less by how they feel than by their temperature. Humans, who lack specialized skin receptors for moisture, also tell when objects are wet largely by how the liquid conducts temperature away from their skin.

But sensing temperature goes beyond improving a prosthetic’s ability to perform. “As engineers we were coming from purely functional perspective at first,” says Shokur. Testimonies like Renda’s soon added another aspect to the research. “It’s a bit more intangible and harder to measure, but we are trying to study the social aspects of touch.”

Fisher, whose team recently used electrode implants near the spinal cord to restore feeling in the missing feet of prosthetic-wearing amputees, says that the emotional component of touch is strong. “It’s part of how we interact with other people, and it’s also part of how we recognize our limbs as our own,” he says. “These are ways to hopefully get toward prosthetic limbs feeling more like a part of the body than like a tool that somebody is wearing.”

To Francesca Rossi of Bologna, Italy, another amputee who took part in that 2023 study and sensed touch with temperature in her missing hand, that experience is already beginning to become a reality.

“Feeling the temperature variation is a different thing, something important … something beautiful,” she says. “It does not feel phantom anymore, because your limb is back.”

Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.