Two recent articles detail advancements in neuroprosthetic technology that enable individuals to perceive the shape and movement of objects through the ‘skin’ of a bionic hand.
You likely complete numerous tasks with your hands without having to look at them. However, if you wear gloves that dull your sense of touch, those straightforward tasks can turn frustrating. If you lose proprioception — the capacity to sense your body’s position and movement — you risk damaging objects or hurting yourself.
“Many people underestimate how much they depend on touch rather than sight – activities like typing, walking, or grabbing a delicate cup of water,” noted Charles Greenspon, PhD, a neuroscientist at the University of Chicago. “When you can’t feel anything, you must constantly monitor your hand to perform tasks, increasing the risk of spilling, crushing, or dropping items.”
Greenspon and his team of researchers have recently released papers in Nature Biomedical Engineering and Science showcasing significant progress in a technology aimed at tackling this issue: direct and accurately timed electrical stimulation of the brain to recreate touch sensations for prosthetic hands.
The Science of Restoring Sensation
The new studies build on years of collaboration between scientists and engineers at institutions including UChicago, the University of Pittsburgh, Northwestern University, Case Western Reserve University, and Blackrock Neurotech. Together, they are developing brain-computer interfaces (BCIs) and robotic prosthetic arms intended to restore both movement and sensation for those who have lost significant limb function.
The UChicago effort was led by neuroscientist Sliman Bensmaia, PhD, who sadly passed away unexpectedly in 2023.
The researchers’ method for restoring sensation in prosthetics involves implanting tiny electrode arrays in brain regions responsible for hand movement and sensation. One side allows participants to move a robotic arm just by thinking about it, while on the other side, sensors on the robotic limb induce electrical pulses known as intracortical microstimulation (ICMS) in the brain’s touch area.
For roughly a decade, Greenspon remarked, this stimulation could only provide a simplistic sense of contact in various sections of the hand.
“We could create the sensation of touching something, but it was mostly just a simple on/off signal that was often weak and hard to locate,” he explained.
The newly published findings represent a major step forward in overcoming these limitations.
Enhancing Understanding of Artificial Touch
In the first study featured in Nature Biomedical Engineering, Greenspon and his colleagues aimed to ensure that electrically induced touch sensations were stable, accurately located, and strong enough for practical use.
By sending brief pulses to individual electrodes in participants’ touch centers and having them indicate where and how strongly they felt each sensation, researchers created intricate “maps” of brain areas corresponding to specific parts of the hand. The experiments showed that when two nearby electrodes are activated together, participants experience a stronger, clearer touch, improving their capability to identify and assess pressure at the correct part of their hand.
The team also conducted thorough tests confirming that the same electrode consistently stimulates sensations in a specific area.
“If I activate an electrode on day one and a participant feels it in their thumb, we can test that electrode again on day 100, day 1,000, even years later, and they will still feel it around the same location,” said Greenspon, the lead author of this study.
From a practical view, any clinical device must be consistent enough for patients to rely on in their daily lives. An electrode that frequently shifts its “touch location” or provides inconsistent sensations would be frustrating and necessitate regular recalibrations. In contrast, the long-term stability shown in this study could lead prosthetic users to gain trust in their motor skills and sense of touch, similar to their natural limbs.
Creating Feelings of Movement and Shape
The accompanying Science paper took the next step to make artificial touch even more engaging and intuitive. Led by Dr. Giacomo Valle, a former postdoctoral researcher at UChicago now advancing bionics research at Chalmers University of Technology in Sweden, the project explored this further.
“When two nearby electrodes in the brain are stimulated, they don’t simply create neat patches that correspond one-to-one with the hand; instead, the sensory areas overlap,” Greenspon noted, sharing senior authorship of this paper with Bensmaia.
The researchers aimed to see if they could leverage this overlapping nature to create sensations that allow users to perceive the edges of an object or the movement of something sliding across their skin. After identifying pairs or groups of electrodes with overlapping “touch zones,” the scientists meticulously activated them in coordinated patterns to produce sensations that traveled across the sensory map.
Participants reported experiencing a smooth, gliding touch over their fingers, even though the stimuli were delivered in small, discrete steps. Researchers credit this achievement to the brain’s extraordinary capability to integrate sensory inputs and interpret them as cohesive, moving experiences by “filling in” perceptual gaps.
The sequential electrode activation approach also greatly enhanced participants’ abilities to sense complex tactile shapes and respond to the variations in the objects they touched. They could even identify letters of the alphabet electrically “drawn” on their fingertips and keep a bionic arm steady on a slipping steering wheel.
These advancements bring bionic feedback closer to the intricate, adaptable capabilities of natural touch, paving the way for prosthetics that empower users to confidently interact with everyday items and adapt to varying stimuli.
The Future of Neuroprosthetics
Researchers are optimistic that as electrode technology and surgical techniques advance, the sensitivity across the hand will become even more refined, resulting in more lifelike feedback.
“We aspire to integrate the findings from these two studies into our robotic systems, where we’ve already demonstrated that even basic stimulation strategies can enhance individuals’ abilities to control robotic arms using their thoughts,” co-author Robert Gaunt, PhD, an associate professor of physical medicine and rehabilitation and leader of the stimulation research at the University of Pittsburgh, said.
Greenspon stressed that the aim of this research is to improve independence and quality of life for individuals experiencing limb loss or paralysis.
“We care deeply about those in our lives who suffer injuries and lose limb function – this research is dedicated to them,” he remarked. “This is how we can restore the sense of touch to people. It represents the cutting edge of restorative neurotechnology, and we are working to broaden this approach to additional areas of the brain.”
This methodology also shows potential for individuals with other types of sensory impairment. In fact, the team has worked with surgeons at UChicago on the Bionic Breast Project, aiming to create an implantable device to restore touch after a mastectomy.
While there are numerous challenges ahead, these recent studies provide promising evidence that the journey toward restoring the sense of touch is becoming clearer. With every new discovery, researchers are moving closer to a future where prosthetic limbs are not merely functional tools but also a means to experience the world.
