In a collaboration between Ecole Polytechnique Fédérale De Lausanne (EPFL), Sant’Anna School of Advanced Studies (SSSA), and Centro Protesi Inail, a research development allowed people with amputations to feel the sensation of warmth in their missing hands. The study tested the effects of temperature feedback directly to the skin on participants’ residual arms, and 17 of 27 of them felt temperature change in their phantom limbs with the technology.
One participant, Francesca Rossi, reported feeling reconnected to her missing hand.
“Temperature feedback is a nice sensation because you feel the limb, the phantom limb, entirely. It does not feel phantom anymore because your limb is back,” Rossi said. “When I touch the stump with my hand, I feel tingling in my missing hand, my phantom hand. But feeling the temperature variation is a different thing, something important… something beautiful.”
Researchers and coleaders of the related study, Silvestro Micera, PhD, a professor at EPFL and SSSA, and Solaiman Shokur, PhD, EPFL senior scientist neuroengineer, have focused on incorporating new sensory feedback into prosthetic limbs for providing more realistic touch to people with amputations, and their latest study focused on temperature.
If something hot or cold is placed on the forearm of a person without an amputation, that person will feel the object’s temperature locally, directly on his or her forearm. But in people with amputations, that temperature sensation on the residual arm can be felt in the missing hand.
By providing temperature feedback noninvasively, via thermal electrodes (thermodes) placed against the skin on the residual arm, some participants reported feeling temperature in their phantom limbs. They could feel if an object is hot or cold and if they were touching copper, plastic, or glass.
“Of particular importance is that phantom thermal sensations are perceived by the patient as similar to the thermal sensations experienced by their intact hand,” said Shokur.
The projection of temperature sensations into the phantom limb led to the development of new bionic technology that equips prosthetic devices with noninvasive temperature feedback, allowing prosthesis users to discern what they’re touching.
“Temperature feedback is essential for relaying information that goes beyond touch, it leads to feelings of affection. We are social beings and warmth is an important part of that,” says Micera. “For the first time, after many years of research in my laboratory showing that touch and position information can be successfully delivered, we envisage the possibility of restoring all of the rich sensations that one’s natural hand can provide.”
For the study, Shokur and Micera developed the MiniTouch, a device that provides thermal feedback and specifically built for integration into wearable devices like prostheses. The MiniTouch consists of a thin, wearable sensor that can be placed over a participant’s prosthetic finger. The finger sensor detects thermal information about the object being touched, more specifically, the object’s heat conductivity. If the object is metallic, it will naturally conduct more heat or cold than, for instance, a plastic one. A thermode that is in contact with the skin on the person’s residual arm heats up or cools down, relaying the temperature profile of the object being touched by the finger sensor.
The scientists found that small areas of skin on the residual arm project to specific parts of the phantom hand, like the thumb, or the tip of an index finger. As expected, they discovered that the mapping of temperature sensations between the residual arm and the entire projected phantom one is unique to each patient.
In previous research, Micera and colleagues discovered that people with amputations began to embody their prosthetic hands if provided with sensory feedback directly into their intact nervous system. The added sensation of temperature feedback is another step toward building bionic prostheses. Fine-tuning temperature sensations and integrating them into a wearable device that can be mapped out to each patient are part of the next steps.
Editor’s note: This story was adapted from materials provided by Ecole Polytechnique Fédérale De Lausanne.
To watch a video with the researchers and Rossi, visit the Ecole Polytechnique Fédérale De Lausanne website.
The study, “Restoration of natural thermal sensation in upper-limb amputees,” was published in Science.