Amputees Feel Warmth in Missing Hands Thanks to Wearable Device

Researchers from the Swiss Federal Institute of Technology Lausanne (EPFL) have developed technology that enabled amputees to feel changes in temperature in their missing, “phantom”, hands. The study is published in Science.

Developing sensory feedback for prosthetics

The human hand is incredibly sensitive, with our fingertips capable of discerning differences in textures and communicating details of objects as small as half the width of a human hair. Through our hands, we can detect many different sensations such as pressure, pain, vibration and temperature that are crucial for us to interact safely and effectively with our environment.

It is therefore important to develop technology for hand amputees and prosthesis users to restore sensation – for example, by using artificial sensors. Some success has been reported involving the restoration of sensations of texture in study participants’ missing, or “phantom”, limbs through direct neural stimulation, though little has been reported for the temperature.

EPFL researchers Professor Silvestro Micera and Dr. Solaiman Shokur have been developing methods to incorporate non-invasive sensory feedback into prosthetic limbs. In the current study, Micera, Shokur and colleagues focused on providing temperature sensation and made a surprising discovery about temperature feedback.

Temperature sensation in phantom limbs

The study began with a neuro-haptic device placed on the skin called the MetaTouch, originally designed to connect the body with digital worlds and combine touch and temperature feedback to augment physical products for well-being. They used the MetaTouch to provide thermal feedback directly to the participants’ skin.

In intact individuals, when hot or cold objects are placed on the forearm, they will feel a local sensation of temperature directly where the object was placed. However, this is not the same for amputees, as with this technology, they discovered that the temperature felt on the residual arm may instead be felt in the phantom, missing hand.

The researchers tested the technology on a group of 27 amputees, discovering that 17 participants reported feeling the temperature in their residual limbs. The researchers then developed a device called the MiniTouch that provides thermal feedback and is specifically built for integration into wearable devices like prosthetics.

The MiniTouch is a thin, wearable sensor that can be placed over an amputee’s prosthetic finger. The finger sensor detects thermal information about the object being touched, particularly its heat conductivity. For example, metallic objects naturally conduct more heat better than plastic objects. Non-invasive thermal electrodes (thermodes) in contact with the skin on the amputee’s residual arm then heat up or cool down, relaying the temperature of the object the sensor is touching into the phantom limb.

They also found that small regions of skin on the residual arm could project sensations to specific parts of the phantom hand, such as the thumb or tips of the index finger, creating “thermal phantom hand maps”. As predicted by the researchers, these maps were unique to each participant.

“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,” said 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.”

A step toward bionic prostheses

“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,” said Francesca Rossi, an amputee from Bologna, Italy and one of 17 study participants to have felt temperature changes in her phantom 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 added.

The added sensation of temperature feedback is yet another step towards building bionic prosthetics for the human body and allowing amputees to discern touch. The next steps in this space are to fine-tune temperature sensations and integrate these into a wearable device suitable for mapping to the individual patient.

Reference: Iberite F, Muheim J, Akouissi O, et al. Restoration of natural thermal sensation in upper-limb amputees. Science. 2023. doi: 10.1126/science.adf6121

This article is a rework of a press release issued by the Swiss Federal Institute of Technology Lausanne. Material has been edited for length and content.