A prosthesis with temperature sensing

A prosthesis with temperature sensing

When testing the prosthesis with an integrated thermal sensor. © EPFL/Caillet

So far, prostheses have replaced the mechanical function of lost limbs, but cannot yet transfer all sensations. A new type of sensor unit for hand prostheses could in the future enable amputees to detect temperatures with their artificial hand. In a pilot test, a test person was able to use the device not only to distinguish the temperature of objects, but also to perceive human physical contact more naturally. The sensors can be integrated into existing hand prostheses and do not require surgery. However, further tests and developments are required before it is ready for the market.

People who have lost a hand can regain some functions of the amputated body part thanks to technically sophisticated prostheses. Modern prostheses can be controlled using electrical signals from the muscles in the stump, so that the movements feel relatively natural after a certain training phase. Sensors in the fingers of the artificial hand also provide sensory feedback so that those affected can better regulate how hard they grip.

“Sensory feedback is crucial for amputees to naturally interact with the environment using the prosthesis. Many research teams have therefore already developed neuroprostheses that restore tactile sensation,” writes a team led by Jonathan Muheim from the Swiss Federal Institute of Technology in Lausanne in Switzerland. “But what was missing so far was the possibility of conveying a thermal sensation.”

Phantom feeling in lost fingers

Muheim and his colleagues have now developed a solution for this. In a previous study, they had already shown that stimulating certain points on the stump can give the feeling that the fingers that are no longer there are touching something warm or cold. They took advantage of this effect. Their device, called “MiniTouch,” measures the temperature with a sensor attached to the fingertips of the prosthesis and transmits the signals in real time to a unit that stimulates the appropriate location on the residual limb.

“This is a very simple idea that can be easily integrated into commercial prostheses,” says Muheim’s colleague Silvestro Micera. “Temperature is one of the final frontiers for restoring feeling in robotic hands. For the first time, we are truly close to giving amputees the full range of sensations.” Since stimulation occurs only on the surface of the skin, no surgery is required.

Promising results

The first test subject was a 57-year-old man whose hand was amputated 37 years ago and who has been using a prosthesis ever since. First, the researchers identified the spot on his stump that, when stimulated, caused him to feel phantom temperature sensations in his lost index finger. They adapted the MiniTouch device to this. The test subject was able to pull the corresponding sensors over his prosthesis like a thimble. The researchers used various tests to test the test subject’s ability to perceive temperatures using the device. In fact, he was able to reliably distinguish between three externally identical bottles of cold, hot and room temperature water. He was also able to recognize the material of small plates made of plastic, glass or copper with a similar level of accuracy as with his real hand.

“Thermal sensation is not only important for recognizing the temperature or material of an object, but also for a more natural feeling when touched by humans,” explain the researchers. In a further test, the test subject was blindfolded and asked to determine whether they were touching a human or an artificial hand. While his hit rate with the device switched off was only 60 percent, not much higher than the chance probability of 50 percent, with the help of MiniTouch he correctly recognized in 80 percent of cases whether it was a human or an artificial hand.

Further improvements planned

“Adding temperature information makes touch more human-like,” explains Muheim’s colleague Solaiman Shokur. “We believe that the ability to sense temperature will improve the body feeling of amputees – the feeling that this hand belongs to me.” When it comes to the perception of human touch, the researchers still see room for improvement. For example, they also want to integrate technologies that also make it possible to perceive the softness and texture of the skin. “Our goal is to develop a multimodal system that integrates touch, proprioception and temperature sensations,” says Shokur. “With this type of system, people will be able to say, ‘This is soft and hot’ or ‘This is hard and cold’.”

In the pilot test, all temperature information transmitted only related to the index finger. With future models, the researchers want to bring these feelings back to all fingers. They are also planning appropriate sensors for the back of the hand, as naturally perceived touch in this region can improve the feeling of human connection. Before the device is ready for practical use, it must also be tested for functionality and safety in larger studies.

Source: Jonathan Muheim (École Polytechnique Fédérale de Lausanne, Switzerland) et al., Med, doi: 10.1016/j.medj.2023.12.006

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