Amputees gain dextrous touch and movement with bionic hands

Research into life-like bionic limbs is advancing in leaps and bounds. Recently, we reported on a partially paralysed man who was able to regain sense of touch in his real and an artificial hand. And today, US researchers report two men, each with a robotic limb in place of an amputated hand, were able to adjust their grip depending on if they were picking up an egg or wielding gardening equipment.

Ohioans Keith Vonderhuevel of Sidney and Igor Spetic from Madison could do all this – as well as dextrously squeeze sauce on a hot dog and hold a dog on a leash – with their pressure-sensing prosthesis, thanks to Emily Graczyk from Case Western Reserve University in Cleveland and her colleagues.

Normally, amputees receive no feedback from their prosthetics. As a result, they don’t like to shake hands, touch a face or hold a loved one for fear of hurting them.

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On an intact hand, pressing a patch of skin – such as the fingertips as you’re picking up a biscuit – increases the firing rate of nerve fibres at that spot. But it also activates fibres nearby, and until now, no one knew for sure how each of these neural responses influenced the perceived magnitude of pressure so you don’t crush the object you’re holding or let it slip from your grasp.

Graczyk and her colleagues found a single feature of electrical stimulation – dubbed the activation charge rate – affected the strength of pressure sensation.

Reporting in Science Translational Medicine, the researchers implanted the subjects with neural interfaces, devices embedded with electrodes, to the median, ulnar and radial nerves of the arm – the same nerves that would carry signals to the brain from the hand, were it still attached.

The prosthetic system started with pressure sensors on the prosthetic hand, which sent signals to a portable stimulator. It, in turn, sent electrical pulses to cuffs that encircle nerve bundles in the arm.

Contact points on the cuffs transferred the electrical pulses, stimulating the nerves, which carried signals to the brain. These signals let the brain discern different levels of intensity.

The work helps the researchers understand how people perceive what’s in their hands, which will lead to better prosthetics and robotics in general, co-author Dustin Tyler, also from Case Western Reserve University, says.

“The more natural the feedback they receive, the less someone has to concentrate and look at the prosthetic hand, and the more normal the operation.

“But the real implications go well beyond that. This may enable us to retrieve tactile information remotely, the way we use cameras to see remotely.”

In the future, he adds, the operator of a robotic explorer on the ocean floor or a bomb-squad robot could feel through the controls what the robots touch.