New Electronic Skin Restores Sense of Touch to Prosthetic Hands

For decades, medical technicians and engineers created more realistic prosthetics meant to restore some sense of normalcy to the lives of amputees. One of the most frequent issues for amputees is "phantom limb" syndrome, or feeling like a missing body part is still there. Engineers from Johns Hopkins University got several steps closer to bringing the feeling of those limbs back to patients, thanks to a new type of prosthetic. 

A new electronic skin can be layered on top of a prosthetic hand and bring a sense of touch through the fingers of a prosthetic and to the body. Patients who have already used the e-dermis as test subjects reported positive changes. 

"After many years, I felt my hand, as if a hollow shell got filled with life again," said the anonymous amputee who served as the team's principal volunteer tester.

The study was published in a recent edition of the journal Science Robotics, and it detailed how the team restored a sense of touch-based feelings back to amputees. Engineers made the new skin using a combination of fabric and rubber embedded with sensors. Those sensors act as nerve endings, allowing the e-dermis to recreate both touch and pain in response to stimuli. It sends those impulses back to the wearer's peripheral nerves (those nerves outside of a person's brain and spinal cord). 

Luke Osborn, a graduate student in Johns Hopkins' biomedical engineering department, said the sensors act like a secondary skin. Osborn noted the research was largely inspired by recent biological studies regarding how the body registers both touch and pain.

"This is interesting and new," Osborn said, "because now we can have a prosthetic hand that is already on the market and fit it with an e-dermis that can tell the wearer whether he or she is picking up something that is round or whether it has sharp points."

The perception of pain remained a critical point of study for the researchers. While most people don't enjoy pain, the feeling helps the human body avoid potentially harmful situations. Pain serves as one of the body's best protection mechanisms, Osborn explained.

"Pain is, of course, unpleasant, but it's also an essential, protective sense of touch that is lacking in the prostheses that are currently available to amputees," he said. "Advances in prosthesis designs and control mechanisms can aid an amputee's ability to regain lost function, but they often lack meaningful, tactile feedback or perception."

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The e-dermis stimulates the amputee's nerves in a non-invasive way via the skin, the researchers explained. The e-dermis allows a user to experience a continual stream of tactile perceptions. This ranges from a light touch to actual pain. The team developed a "neuromorphic model" that mimics the human nervous system's touch and pain receptors. This gave the e-dermis the ability to produce sensations just like the skin's nervous receptors would. 

The team used an EEG to track brain activity and determined that their test subjects could feel these sensations in a phantom hand. They then connected the e-dermis to the volunteer using transcutaneous electrical nerve stimulations (TENS). The volunteer experienced a natural, reflexive reaction to pain and non-pain. 

The paper's senior author Nitish Thakor serves as a professor of biomedical engineering and the director of the Biomedical Instrumentation and Neuroengineering Laboratory at Johns Hopkins. 

"For the first time, a prosthesis can provide a range of perceptions, from fine touch to noxious to an amputee, making it more like a human hand," said Thakor. The professor also co-founded Infinite Biomedical Technologies, the Baltimore-based company that provided the prosthetic hardware used in the study.

There is one limitation to the e-dermis. It does not sense temperature yet. The team focused its abilities on detecting curvature and sharpness. However, the researchers are ready to test just how far they can develop the technology with the hope of preparing it for global patient usage.