Why prosthetics don’t have to look like the real thing

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i amNo photo shoot Kelly Knox—a British model and disability-rights advocate—poses with a prosthetic limb unlike any other. Ms. Knox was born missing her left arm from the elbow down. From its arm closure, dozens of vertebrae-like structures extend to form a long, slender tentacle.

The organ is certainly interesting. It’s masterful, too. Ms. Knox can control its movements through pressure sensors in her shoes, which connect wirelessly to the organ. Artificial tendons can be tight or loose, allowing tentacles to reach and grasp. The organ—detached from Ms Knox and titled VINE 2.0—will be featured in “BODIES…The Exhibition Luxor, a hotel and casino in Las Vegas, later this year.

The organ was designed by Sophie de Oliveira Barata, an artist, and Dani Claude, a designer and engineer. Although it’s a work of art, it’s inspired by some startling recent developments in neuroscience that indicate there’s no reason—at least from first neurological principles—why artificial organs have to look or behave like the biological organs they replace.

The developments concern a phenomenon called “sculpture,” which is of interest to both neuroscientists and manufacturers of prosthetic limbs. Embodiment is the means by which a brain identifies with the body it is situated in and demarcates what counts as “self” and what counts as “other.”

Sometimes, the embodiment can be wrong. Between 60% and 80% of the world’s amputees, according to various papers – experience the continued presence of their missing organs, often for years after their first removal. This “phantom limb syndrome” (Pls) show that the brain’s perception of body boundaries does not always match physical reality.

Prosthetic makers have long hoped to take advantage of embroidery to make their designs feel like extensions of the body rather than being attached to foreign objects. treatment for Plswhich uses mirrors to create the illusion that a missing limb is still present, suggests that the brain can be persuaded to embody an artificial limb if it matches the form and function of the transplanted limb.

But scientists’ understanding of embodiment is beginning to change. In a series of experiments in 2018 and 2020, Tamar Makin, a neuroscientist who runs the Plasticity Lab at the University of Cambridge, used functional magnetic-resonance imaging, which records blood flow in different parts of the brain, to look at one’s head—hands off.

It’s not what you think

Dr. Makin asked them to look at pictures of three different types of objects: biological hands, artificial objects, and hand-held tools such as brushes and spoons. By comparing which bits of the subjects’ brains responded, Dr Makin hoped to establish whether the artificial hand was interpreted as a biological hand or as an object closer to a tool.

To Dr. Makin’s surprise, the answer was no. Instead, he says, the brain “puts prosthetics entirely in their own category, distinctly different from the others.” It was true whether the prosthesis was cosmetic, designed to look like a biological hand or a functional one, a sophisticated robotic version or even a simple hook.

This does not mean that an artificial limb cannot be embodied. Many amputees complain of some degree of sculpting of their prostheses. But what it does show is that whether a prosthesis is embodied has little to do with whether it looks like a biological arm or leg.

This has several implications for the design of prosthetics. One is to open it up to all sorts of innovations in materials, colors and shapes. This is where Mrs. De Oliveira Barata comes in. He runs the Alternative Limb Project (ALP), a British studio that makes custom prosthetics. Miss de Oliveira Barata, who has a background in make-up and special effects, began by creating realistic limbs for the limbs. But a project with a young girl who was determined to cover her replacement leg in cartoons changed her perspective.

this day ALP Creates attractive, high-concept prosthetics for its clients. Its first major commission came from Victoria Modesta, a British singer whose leg was amputated below the knee. Inspired by an idea that came to Ms. Modesta in a dream, Ms. de Oliveira Barata designed a metallic black prosthesis in the shape of a long, conical spike that vaguely resembled a giant stiletto heel. Ms. Modesta performed at the closing ceremony of the 2012 Paralympic Games wearing a silver and glass crystal covered leg, another Ms. de Oliveira Barata creation.

And if a prosthetic limb does not need to be imitated, then perhaps prosthetics can be more than just replacements? Ms. Clode is an expert in the design of robotic prostheses controlled by artificial tendons. He is interested in exploring the possibility of augmenting existing bodies with new capabilities, making prosthetics “a technology that can be used by everyone, not just limbs.” To that end he designed the “Third Thumb,” a small and powerful artificial digit that does exactly what it says on the tin. Controlled, like Mrs. Knox’s vine-arm, by a pair of shoe pressure sensors, the thumb can be used to replace the lost one. But it can be added to an intact hand on the opposite side from its existing, biological thumb.

Third thumb by Dani Claude.
Photo: Dani Claude

Having two thumbs “expands what the human hand can do in amazing ways,” Ms. Claude said. Many hold and manipulate an object at the same time, such as peeling a banana or opening a soft drink bottle with one hand. Using toe control may seem primitive compared to some of the more sophisticated prosthetic hands on the market, which are designed to take control by picking up signals from the wearer’s remaining muscles.

But Ms Clode says the opposite is true. Myoelectric prostheses can be slow and clunky. And because the muscles left at the amputation site are unlikely to be the same as those used to move the missing limb, it can take months to learn how to use them with a prosthesis. Shoe-mounted controls, in contrast, can be mastered relatively quickly.

In the future, it may be possible to provide such artificial sensory feedback. In a paper published in November 2023 Science Robotics, a team of Chinese researchers led by Beihang University roboticist Li Wen, described a flexible robotic arm that mimics an octopus tentacle. Although not designed as a prosthetic, the tentacle is controlled by a finger-glove that fits over the wearer’s index finger and provides touch feedback through the same mechanism that controls it.

Neither Ms de Oliveira Barata nor Ms Clode are neuroscientists. But academic researchers are also starting to take an interest. Ms Clode is working with Dr Makin to investigate how the brain adapts to physical growth. They are particularly interested in how the brain reallocates its cognitive resources to accommodate new appendages and whether such appendages can be embodied. “I don’t think we’re even close to the limit of what’s possible yet,” Ms. Claude said. “But we definitely want to find out.”

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