A touching mystery: why do scientists struggle to make bionic limbs feel?

Scientists have developed the first bionic arm that can feel, but they are yet to master touch. 

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The biggest plot point in the latest Star Wars film revolves around two people touching hands. Touch, the film pontificates, is the ultimate expression of humanity. 

Last week it was reported that for six months an Italian woman and amputee named Almerina Mascarello had been testing a bionic arm. And this was not just any bionic arm. This one could feel.

The bionic arm was created by scientists in Rome and Switzerland, and is a continuation of an earlier model. Previously, the computer accompanying the arm was so large that the participant had to remain in the lab. Mascarello’s computer, however, fits into her backpack.

Prosthetic limbs have been a feature of medical science since the era of the Ancient Egyptians, although in recent decades the options have improved dramatically from the stumps of wood that used to make do for a leg. In one respect, though, these limbs have not changed. None of them possess the ability to feel. In other words, they lack the ability to send back information to the brain that says: yes this feels rough, this feels cold or this feels gooey.

As Adrien Rapeaux, a PhD student and a researcher in neuroprosthetics at Imperial College London points out, someone wearing a prosthetic limb today cannot “tie their laces without looking, or adjust their posture to the slope and layout of the ground without breaking stride”. These are aspects of touch that most of us take for granted. The science of neuroprosthetics however is complicated. Motor control is one thing, but sensory feedback is a very different and a whole lot more complicated.

This is highlighted by the fact that, despite the breakthrough it represents, Mascarello's bionic arm can only differentiate between hard and soft. 

These bionic limbs are also not yet talking directly with the brain. Instead, artificial sensors in the bionic arm send signals to a computer, which converts the signals into a electrical stimulation that nerve bundles which Mascarello still has in her stump can understand. This is called closing the loop. 

The easy-to-do and fun party trick rubber hand illusion plays on a similar idea.

In the original study that discovered the illusion, scientists placed a lifelike rubber hand in front of their able-bodied subjects. The subjects' arms were covered by a dark cloth. They could see the rubber hand being stroked, and simultaneously the scientists would stroke the right hand of the subjects under the cloth. Eventually, the subject would believe the fake hand was part of their body, and that the sensation of being stroked was coming from that hand.

This illusion would not work if your real hand wasn’t being stroked, and your brain wasn't receiving that sensory information. For this reason, amputees often do not “believe” their artificial limb is a part of their body, as there is no sensation of touch.

Bidirectional limbs may one day (a long time into the future) solve this problem.

Dr Ed Chadwick, one of the researchers of Senseback, a multi-university project in Britain which is using fingertip sensors to expand on the research behind Mascarello's arm, notes the enormity of the task set. As he puts it: “The nervous system is largely a 'black box' - we have no way of knowing which neurons are carrying which information.”

While the science should be lauded, the research is still in its infancy. Professor Nick Donaldson, a bioengineer at UCL, says the problem is that “we have no method yet for chronically activating single axons, let alone the huge number that there is normally involved”. In other words, touch is already extremely complicated, and electrical stimulation is not the most precise or specialised tool for communicating with the brain.  

So why has an arm that can only differentiate between hard and soft caused such an international storm? Stories about machines that can do this most human of things, seem to touch a nerve (no pun intended). Chadwick goes onto consider whether it is due to how touch is "inherent in the way many of us communicate and express emotion".

So that's what Star Wars was talking about.