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Neural engineering

Neural engineering

Smart inflatable hand offers lighter, more affordable prosthetic

01 Sep 2021 Ben Lewis 
Inflatable robotic hand
Smart hand: The soft, elastic neuroprosthetic hand enables precise movement and delivers feedback, at a fraction of the weight and cost of comparable neuroprosthetics. (Courtesy: Xuanhe Zhao, Shaoting Lin et al)

So many of the actions that we perform every day rely on the precise movements of our hands. For people who have had an upper-limb amputation, prosthetics could give back some amount of this function that most take for granted. However, these prosthetics are often heavy, rigid and expensive. Researchers from Massachusetts Institute of Technology (MIT) and Shanghai Jiao Tong University aim to restore function with their high-tech, inflatable neuroprosthetic hand, which they describe in Nature Biomedical Engineering.

More than a feeling

Neuroprosthetics are smart bionic limbs that not only look like the missing body part, but also use the person’s own remaining nerve signals to control movement in the prosthetic. This gives the user back some of the functional movements of their hand. But we don’t only use our hands as grabbing devices – hands also provide tactile feedback on whatever we are touching. To best replicate the human hand, an ideal neuroprosthetic would combine both of these features in a light and flexible package.

The research team explored a new direction when designing their prosthetic. They replaced rigid metal elements with soft and stretchy elastomer controlled by precise inflation of the balloon-like fingers, using a simple pneumatic system instead of electrical motors. Using computer modelling of the required pressures, the pneumatic pumps can achieve five different common grips. Sensors detecting electrical signals from the user’s limb control these pumps to deliver whichever movement the user wishes to perform.

And the user can do more than just control the position of the prosthetic fingers. Thanks to pressure sensors in the fingertips, an electrical signal sent back up the limb provides feedback about what each finger is touching. These electrical impulses enable the user to “feel” the pressure on the artificial fingers and to know, for instance, which finger on the hand is being touched.

Getting hands-on

When the researchers tested this new smart prosthetic hand with volunteers, it worked at least as well as traditional neuroprosthetics in typical tests of hand function. The users were able to handle food, objects and tools, and use them naturally, whilst also being able to interact with people, animals and their environment. Even delicate tasks, such as precisely inserting complex shapes into corresponding slots, were possible.

Going further than typical neuroprosthetics, the researchers also demonstrated that the tactile feedback worked in a blindfolded test, where the user could feel whether an object was in their grasp and lift it if they felt they were holding onto it. This step-change in prosthetic capabilities is made possible by the addition of the pressure sensors.

Importantly, the lightweight design doesn’t sacrifice durability. Despite weighing less than one third of a kilogram and being made of an elastomer, the prosthetic resisted being hit with a hammer or run over with a car,  recovering to remain functional.

Whilst further work is needed to make this new prosthetic a viable option for patients who have undergone amputation, the researchers are optimistic about its potential. “This is not a product yet, but the performance is already similar or superior to existing neuroprosthetics, which we’re excited about,” says co-senior author Xuanhe Zhao. “There’s huge potential to make this soft prosthetic very low cost, for low-income families who have suffered from amputation.”

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