Sensors Could Make It Easier to Use a Prosthetic Limb

New methods may diminish the cognitive burden that can come with a prosthetic.

Posted Feb 02, 2020

LightField Studios/Shutterstock
Source: LightField Studios/Shutterstock

By Polina Porotsky

Prosthetics have advanced tremendously over time, but the devices can still pose a challenge to those who use them. Artificial limbs can't communicate with the brain seamlessly and unconsciously like natural limbs do. As a result, simple day-to-day tasks can require a lot of thought and energy. Someone with a leg prosthesis, for example, may have to think about the mechanics of walking with every step they take.

“When people with prosthetics are climbing the stairs, they can be hyper-concentrated because it’s so difficult,” says Stanisla Raspopovic, assistant professor of neuroengineering at ETH Zurich in Switzerland who studies prosthetics. “They have to make sure that all of their weight is on the prosthetic, otherwise they’ll fall.” 
 
But Raspopovic and others in the field of neuroprosthetics are tackling this challenge. His team recently developed a device that allows patients with an amputation above the knee to experience sensation via their prosthetic leg. The results of a study that they conducted to test the device were published in the journal Science Translational Medicine.

To increase unity between the body and the prosthetic, the scientists established an artificial extension of the patient’s real nervous system. They implanted tiny, hair-sized electrodes in three patients around the remains of the tibial nerve that normally runs down the back of the leg. They stimulated these implanted electrodes with electrical currents of different intensities, durations, and frequencies, which stood in for the many signals that the nerves would receive during regular foot movement. The patients described the location, intensity, and other aspects of what they felt through a graphical user interface.

These individual reports were used to simulate a natural connection between each person’s leg and both the prosthetic’s insole—which was equipped with sensors—and an encoder in the prosthetic’s knee that conveyed how bent or straight the knee was. (Components used in the device were provided by companies with which some of the study authors are commercially involved.)

To make sure the device was effective, the researchers disconnected the prosthesis from the participants and blind-folded and sound-isolated them. The patients then had to describe where the experimenter was touching the prosthetic insole, which they did with a roughly 90% success rate. When asked to determine at which of four possible angles the prosthetic knee was positioned, they did so with success rates ranging from 69–84%.

"I could tell when they touched the [big toe], the heel, or anywhere else on the foot. I could even tell how much the knee was flexed," says participant Djurica Resanovic in a video released with the paper

All three participants showed improvements in mobility, as measured during experimental tasks that involved walking in a straight line, going up and down stairs, and completing an obstacle course. “It’s much easier to walk when you feel where your foot is, where your knee is, so you don’t need any practice,” Raspopovic says. 

But was the cognitive burden of using a prosthesis alleviated? To find out, the researchers asked participants to silently count specific target sounds while they were sitting down (a low-effort control condition) or while walking with the prosthetic feedback either turned on or off. The research team used a mobile electroencephalography (EEG) system to measure electrical activity in each person’s brain as they completed the task; particular signals served as indicators of attention to the target sounds. 

The participants only showed the expected EEG characteristics when sitting or walking with the system turned on—not when walking with it turned off. That indicates that the new prosthetic may have allowed them to “allocate their attentional resources more efficiently (meaning that subjects have cognitive ease) to a task” while walking, the researchers write. 

Neuroprosthetics are not the only solution being developed to improve the lives of people with artificial limbs, says Goeran Fiedler, assistant professor of prosthetics at the University of Pittsburgh, who was not involved in the study. One alternative is osseointegration, in which the prosthetic is attached directly to the bone, which provides stability and helps the device feel integrated with the body. He points out some of the limitations of neuroprosthetics, such as the invasive surgery required to implant the electrodes. It remains to be seen which methods will become the new standard. 

“When you have a piece of hardware implanted in your body, there will be a process of adaptation: the downside is that there could be an infection, but the upside is that the nerves could get used to the stimulus and adapt to the response successfully,” Goeran says. 

While the trial for each volunteer lasted just three months, after the experiments had ended, the participants seemed to remain excited about the experience. Says Resanovic: “After all of these years, I could feel my leg and my foot again, as if it were my own." 

Polina Porotsky is a former Psychology Today Editorial Intern.