The sense of touch plays a vital role in our daily lives. But most of us took it for granted until the pandemic happened. Suddenly we couldn’t hug our friends and family members. While the sense of touch is essential to our mental health to feel connected, it also gives us information about the things around us, thanks to the thousands of nerve fibers on our skin and millions of neurons in the brain.
According to neuroscientist Sliman Bensmaia, the nerve receptors detect cues about texture, motion, shape, temperature, pressure, and more. Those cues allow our central nervous system to interpret whether something is dry or wet, rough or smooth, still or moving.
Although neuroscience has always been central to research on touch, experts in other fields, like materials science, math, and mechanical engineering, are looking into translating the science into practical applications. Researchers in these fields hope that their collective work will lead to new technologies that can mimic tactile sensations.
Engineers and scientists are learning more about how our nervous system reacts to touch stimuli, as well as how our skin communicates with diverse materials. There is progress and challenges, such as devising ways for people to send and receive stimulated touch sensations. Success in these studies could mean that people who experienced limb loss might regain a sense of touch through their prostheses.
Detecting external signals through vibrations
One such development is the skin patch developed by physical chemist John Rogers and his colleagues at Northwestern University in 2019. Inside the skin patch are disk-like actuators that respond to signals by vibrating. The prototype already shows great potential to restore the sense of touch to prosthesis users.
The latest iteration of the skin patch is said to be lighter and thinner. It can now also provide the user with comprehensive information, and up to 6 patches can operate simultaneously on various body parts. Besides vibrating in response to signals, the team has also developed skin patches to detect pressure on the fingertips of a prosthetic arm.
Meanwhile, other researchers are planning to add physical feedback to prosthetic limbs. A 2019 report mentioned that supplementing feedback for motion and pressure encouraged people with artificial legs to walk more confidently. They also reported diminished phantom limb pain.
Haptic feedback to control prosthesis force
Meanwhile, at Johns Hopkins University, mechanical engineer Jeremy D. Brown is studying haptic feedback so prosthesis users can do things like judge an avocado ripeness by squeezing it.
Brown hopes to find a way to improve the way prosthesis users regulate the strength in their prosthetic limbs. Currently, two types of prostheses let upper-limb amputees perform specific arm movements.
The first is the “body-controlled” prosthetic hand, which can close or open when other muscle groups are moved. The second is the myoelectric prosthetic limb, which is controlled by residual limb muscles. The latter generally doesn’t provide tactile feedback, but it gives a better range of motion when compared with the former.
Brown’s team tested two techniques to add touch feedback. One, by applying force near the elbow. Two, by providing vibrations to the wrist. Non-amputee volunteers tested each set up.
They found that, compared to no feedback at all, the two approaches were more helpful. And when compared with the other, both worked equally well. The reason for this, Brown said, is that the human brain creates a map. The brain automatically matches the force according to the severity of each feedback type. The findings suggest that there are ways to enhance muscle-controlled bionic limbs.
However, the researchers found another issue: the human brain may not respond to all kinds of touch feedback. Neuroscientist Bensmaia’s group worked with collaborators in Sweden who embedded bionic hands with sensors. The signals from the thumb sensors traveled to the electrode inserted around the user’s ulnar nerve—the nerve that controls the fingers’ fine movements.
The bionic hands were tested by three upper-limb amputees. When the prosthetic thumb was moved, the volunteers reported a feeling touch from another area on the hand.
According to Bensmaia, researchers can’t predict which nerve fibers will be affected. Even after the volunteers used their prostheses for more than one year, their brains weren’t able to create a map; the mismatch in sensation didn’t improve.
In other studies, the same volunteers who used the bionic hands reported better force control and precision when clutching objects than prostheses that didn’t directly stimulate the nerve. Furthermore, people reported feeling like the prosthetic hand was a part of their bodies with the nerve stimulation.
What these studies show is that haptic technology in prostheses currently requires a lot of technological refinement. However, it can give us a new way to explore the world around us.What do you think of this new development?