Precise sense of touch is still out of reach for bionic hands, say scientists

Bionic prosthetic limbs have seen huge technological advances but Luke Skywalker style robotic devices are still some way off, scientists have warned.

A study of people with amputations using a bionic hand for more than a year revealed reception and relay of sensory information remains a stumbling block.

“Every day, for a year, these subjects saw their prosthetic thumb touching things and felt it in a different location – sometimes close to the thumb, but not on it – and the sensation never budged”, said Sliman Bensmaia, Professor of Organismal Biology and Anatomy at the University of Chicago.

Neuroscientists at the University of Chicago and Chalmers University of Technology in Sweden found even after a year of using the devices participant’s sensation never matched the location of the touch sensors on their devices.

Sensation never matched location

Three participants with above-elbow amputations were equipped with neuroprosthetic devices attached directly to their humerus bone.

Users could control the device via signals received from electrodes implanted in arm muscles and received sensory feedback through another set of implanted electrodes.

A sensor located on the prosthetic thumb triggered stimulation of the nerve, which in turn generated a touch sensation.

In the study, published in the journal Cell Reports, users did not report feeling the sensation on the thumb.

Instead, they felt them in other hand locations such as their middle finger.

Electrode implants

“One problem with current neural electrodes is that you can’t tell during the implantation surgery which part of the nerve corresponds to what sensation, so the electrodes don’t always land in exactly the location in the nerve that would match the location of the sensors in the prosthetic hand,” said lead author Max Ortiz Catalan, PhD, associate professor of bionics at Chalmers University of Technology.

Scientists had hoped over time the brain would resolve the mismatch shifting the perceived sensation to the thumb.

Hard to hit right nerve

But none of the users ever reported feeling the sensation on their thumb.

“There’s been this idea that the nervous system is really plastic, so if you see a mismatch between what you see and what you feel, it’s a great opportunity for neural remapping,” said Professor Bensmaia. “But I think that this idea has been vastly overstated.”

The study highlighted the importance of knowing exactly where to place electrodes, when implanting sensory arrays for neuroprosthetic devices, because it appears unlikely the brain can make adjustments.

“This means that you really have to get it right,” said Professor Bensmaia.

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