Touch is a powerful sensation. One that connects us to the world around us, allowing us to distinguish between a kitten rubbing against the back of our hand and a sharp nail poking out from an unfinished wall.

But tactile sensations are more than a way to identify objects. They allow us to fine-tune the movements of our muscles, as sensory information is passed to the brain from the muscles and skin. This added information makes the difference between cradling a grape between our thumb and finger, and making a lot of grape juice.

For amputees, sensory feedback from their prosthetic limbs has long been absent, even as the artificial limbs themselves have evolved over the past few decades into highly sophisticated robotic devices. But two research groups, whose work is published in the Oct. 8 issue of Science Translational Medicine, are attempting to give those artificial limbs a full range of sensations.

"The sense of touch is one of the ways we interact with objects around us," said Dustin Tyler, an associate professor of biomedical engineering at Case Western Reserve University and director of one of the research studies, in a press release. "Our goal is not just to restore function, but to build a reconnection to the world. This is long-lasting, chronic restoration of sensation over multiple points across the hand."

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Men Feel Almost 20 Points on Prosthetic Limbs

To provide amputees with a sense of feeling in their artificial limb, the group of researchers led by Tyler implanted electrode cuffs into the forearm of one male patient and the upper arm of another. These types of electrodes wrap around the bundle of nerves rather than penetrating the protective membrane, which has the potential to cause long-term nerve damage.

Sensors on the patients’ artificial hands fed information about the amount of pressure to the electrodes, enabling the men to feel up to 19 distinct points on their prosthetic limbs.

A study subject holds a cherry tomato. Photo courtesy of Russell Lee.

To generate more complex sensations, like the difference between sandpaper and a smooth surface, a computer converted the information from the sensors into varying electrical signals. These were picked up by the peripheral nerves, which carried the sensory information to the brain. Over time, the researchers were able to fine-tune the signals as the patients became attuned to them. 

"The sense of touch actually gets better," said Keith Vonderhuevel, of Sidney, Ohio, in a press release. He lost his hand in 2005 and had the system implanted in January 2013. "They change things on the computer to change the sensation. One time, it felt like water running across the back of my hand."

Because of the need for a computer to adjust the touch sensations coming from the sensors, this system currently only works in the lab, but the researchers expect to develop a home-based system within five years.

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Bone Implant Provides Added Strength

The system developed by the other group of researchers, who are based in Sweden, generates sensory input from the artificial limb in a similar way. However, the researchers were also able to attach the artificial arm directly to the skeleton, in a process called osseointegration, rather than using the socket attachment in commercially available prosthetics.

“We have used osseointegration to create a long-term stable fusion between man and machine, where we have integrated them at different levels,” said Max Ortiz Catalan, research scientist at Chalmers University of Technology in Gothenburg, Sweden, and leading author of the publication, in a press release. “The artificial arm is directly attached to the skeleton, thus providing mechanical stability.”

Max Ortiz Catalan (left) and associate professor Rickard Brånemark (right) with the first patient treated with the osseointegrated implant system. Photo courtesy of Catalan.

In addition, Catalan and his colleagues connected the prosthetic arm to the remnants of the nerves and muscles in the arm of the amputee, providing him with both a sense of touch and the ability to control the artificial limb.

This has allowed him to retain his job as a truck driver, using his prosthetic arm for tasks including operating machinery and unpacking crates of eggs. He also uses it for tying the laces on his children’s skates. 

"Reliable communication between the prosthesis and the body has been the missing link for the clinical implementation of neural control and sensory feedback, and this is now in place," said Catalan. 

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Touch Deepens Connection to Prosthetic

A sense of touch can provide amputees with greater control of their limb, especially the amount of pressure applied. However, even without that sensation, people are still able to control artificial limbs with a remarkable amount of dexterity, in large part because of the sensory feedback coming from the eyes and the muscles.

But adding touch sensation to a prosthetic can benefit amputees in other ways. One of these is the reduction of phantom pain, the intense sensation that the limb is still attached, even though it is no longer present. Patients in the new studies reported that their phantom pain lessened after they began to feel with the artificial limb. 

A sense of touch can also help amputees integrate psychologically with their prosthetic, allowing them to see it not as an external tool, but as part of their own body. This, in turn, could reduce the number of amputees who stop using their prosthetic after a short time, and enhance the quality of their everyday lives.

The thumbnail image is of a study subject plucking a grape from a bunch. Photo courtesy of Dale Omori.