EA special device can enable people with forearm amputees to feel the temperature at the point where the hand is missing. A sensor, which could be attached to the finger of a hand prosthesis, for example, is connected to a special point on the arm stump, as a research team from Switzerland and Italy explains in the specialist magazine “Science”. “Temperature feedback is important for relaying information that goes beyond touch and leads to feelings of affection,” said Silvestro Micera of the École Polytechnique Fédérale de Lausanne (EPFL). “We are social beings and warmth is an important part of that.”
A special feature of the device is that it does not measure the actual temperature like a thermometer. Instead, the test persons feel the temperature – as with their healthy hand – depending on the material. For example, an object made of metal tends to feel warmer or colder than one made of plastic because it has a higher thermal conductivity. With the technology called “MiniTouch”, the sensor is connected to a miniature cooling/warming plate on a skin area on the residual limb.
The method, implemented in this form for the first time, uses the presence of so-called phantom zones, explained the neurotechnologist Rüdiger Rupp from Heidelberg University Hospital, who was not involved in the study himself. “Phantom zones are sections of skin on the remaining arm stump, touching which those affected perceive as touching the fingers that are no longer there.” If something warm or cold is placed on such areas, the amputee feels as if the corresponding finger is sensing the temperature. Whether there are such zones and where exactly they are varies from patient to patient.
The team led by Micera and EPFL scientist Solaiman Shokur initially tested 27 patients (24 to 65 years; four women) to see whether they had such areas on their residual limbs. At 17, it could be found in different forms, as the researchers report in “Science”. The “MiniTouch” system, in which the temperature sensor is non-invasively connected to the corresponding skin area, was then tested on nine amputees.
The subjects felt a temperature with the help of the sensor in their phantom hand, they were able to tell whether an object they touched was hot or cold, and at least in some of the tests whether it was made of copper, plastic or glass. The sensation of warmth on the phantom hand was similar to that on the intact hand, it said.
The participants could have distinguished a warm object from a cold object with a high degree of accuracy, said Rupp. On the other hand, it is difficult to differentiate between materials such as glass or plastic based solely on the temperature perception.
noticeable temperature changes
The technique is another step toward recreating the rich sensations offered by the natural hand, Micera said. Patient Francesca Rossi explained that when she touched the stump with her other hand, she felt a tingling sensation in her missing hand. Being able to feel temperature fluctuations in the phantom hand is important and nice.
“Sensitive feedback is generally important, because this is the only way for users to perceive the prostheses as part of their own body,” explained Rüdiger Rupp. An almost natural sense of touch can be restored with the direct stimulation of nerves that are still in the arm. Now, at least to a limited extent, this is also conceivable for temperature sensitivity – but so far the system has not been tested on any test person with an actual prosthesis.
There would also be other restrictions. The work shows that only a little more than half of the people with forearm amputations have suitable phantom zones for temperature perception, explained Rupp. “Unfortunately, the mechanisms by which these phantom zones develop are not exactly known, so it is not yet clear how many people with amputations the system can actually be used with.” The zones only develop over time after an amputation and unfortunately often go with it phantom pain, which further restricts the potential user group.
The research team now wants to fine-tune the temperature sensation achieved and integrate it into prostheses. “In terms of size, the system does not currently fit into a prosthesis, and given the technology used, the service life is not expected to be too long,” said Rupp. The stability of the temperature phantom zones over a longer period of time also needs to be clarified in more detail. “However, these aspects are crucial for everyday use.”
How warm or cold an object feels to us also depends on its thermal conductivity. The receptors in the skin do not record the actual temperature of the object touched, but the heat flow at the point – depending on the material, a rather slow or rather fast cooling or warming of the skin.
If a cool object is touched, energy is transported from the warm skin to the cooler material. How quickly heat is withdrawn from the skin depends on the thermal conductivity of the material. Metal, for example, cools the skin faster than wood due to its higher thermal conductivity. At low temperatures, a metal fence therefore appears colder than a wooden fence – even though both are at ambient temperature. For the same reason, a wooden floor feels warmer than a tiled floor at room temperature.
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