杏吧原创

We have the world at our fingertips. A sense of touch can sometimes be as important as sight, helping us to avoid crushing delicate objects or ensuring that we hold on firmly when carrying hot cups of coffee. Now, for the first time, a person who lost his left hand has had a near-natural sense of touch restored thanks to a prosthesis.

鈥淚 didn鈥檛 realise it was possible,鈥� says Dennis Aabo S酶rensen, who is so far the only person to have been fitted with the new prosthesis. 鈥淭he feeling is very close to the sensation you get when you touch things with your normal hand.鈥�

To restore S酶rensen鈥檚 sense of touch, at the Swiss Federal Institute of Technology in Lausanne and his colleagues implanted tiny electrodes inside the ulnar and median nerve bundles in S酶rensen鈥檚 upper arm. Between them, the ulnar nerve 鈥� which runs down to the little finger and ring finger 鈥� and the median nerve 鈥� which runs down to the index and middle fingers 鈥� carry sensations from most of the hand, including the palm.

The team then connected the electrodes to pressure sensors on the fingertips and palm of a robotic prosthetic hand via cables running down the outside of S酶rensen鈥檚 arm. When he used the hand to grasp an object, electrical signals from the pressure pads were fired directly into the nerves, providing him with a sense of touch.

Getting to grips

The electrical signals were calibrated so that S酶rensen could feel a range of sensation, from the slightest touch to firm pressure just below his pain threshold, depending on the strength of his grip.

According to S酶rensen, the feelings were very natural and close to those in his real hand. So much so, in fact, that he can differentiate objects held in his prosthetic hand by touch alone. For example, he could tell the difference between cotton wool, a plastic cup and a block of wood about 90 per cent of the time while wearing a blindfold.

Knowing how firmly he was grasping an object was a game-changer. 鈥淲ith my existing prosthesis I always have to look at what I鈥檓 doing with it,鈥� says S酶rensen. Without a sense of touch to guide him, picking up a piece of fruit can end messily, he says. 鈥淚t鈥檚 an amazing product, I鈥檇 take one tomorrow if I could.鈥�

However, regulations governing the use of surgically implanted electrodes mean that S酶rensen had to give up the prosthesis after 30 days. Before the treatment becomes clinically available, Micera鈥檚 team needs to test its long-term viability.

One challenge is to ensure that the implanted electrodes continue working long after they have been inserted. For a first attempt, results are promising, says Micera. After 30 days, 90 per cent of the electrodes still worked.

In the meantime, at Case Western Reserve University in Cleveland, Ohio, has already shown that a slightly . Tyler鈥檚 team implanted electrode 鈥渃uffs鈥� that clamped around nerve bundles in the arms of two people who had lost a hand, again restoring a limited sense of touch. The cuffs work in a similar way to S酶rensen鈥檚 implants but, because they do not enter the nerves, greater changes in electrical input are needed for stimulation. This makes the sensations coarser, says Micera.

Personal touch

Tyler agrees that stimulating individual fibres in the nerve bundles leads to finer degrees of sensation. In theory, by precisely stimulating the relevant nerve fibres it could be possible to make a prosthetic limb feel exactly like a real one 鈥� restoring not only fine variations of touch, but also sensations of hot and cold, and even pain. This will be more challenging, however, because the nerve fibres that carry these signals are very small, says Tyler.

Recent studies have shown that macaques can also regain a sense of touch by having electrical signals from a prosthesis routed directly to their brains. But going via the nerves in the arm has the advantage of using the brain鈥檚 usual channels of input, allowing the signal to be interpreted more naturally. 鈥淵ou have the brain helping you do the job,鈥� says Micera.

Micera鈥檚 approach is conceptually similar to how cochlear implants have provided hearing for tens of thousands of people, says at the University of Utah in Salt Lake City. Cochlear implants stimulate nerve fibres in the same way they would normally be activated by sound itself. 鈥淲e don鈥檛 need to know the code that the brain uses to interpret speech for this approach to work,鈥� he says. 鈥淲e just need to provide the basic inputs. Then the brain interprets these inputs in much the way it normally would.鈥�

By doing the same with sensory input from skin and muscles, we are on the way to restoring much more than hearing loss. This will not only have practical benefits, says Clark, but could literally help people with amputations feel whole again.

Journal reference: Science Translational Medicine,