With the recent announcement of a prosthetic limb that allows patients to regain their sense of touch, Aoife Hardesty discusses how these prosthesis communicate with the human body.

People have been trying to find suitable ways of replacing lost limbs for years. Conjure up an image of a pirate in your mind and I’m guessing that that pirate will have a hook for a hand and a wooden leg. Well hooks and peg legs have actually been the standard model of prosthetic for years and are still in use today. In fact, the oldest evidence of prosthetics is a prosthetic toe made of wood and leather which was found on a 3000 year old mummy in Cairo.

Life is made very difficult for those with missing limbs, which is why researchers attempt to design better and more efficient prosthetics. Since people started using replacements for missing limbs, many improvements and updates to the replacements have been made as the technology advanced. Nowadays prosthetics are made out of advanced plastics and are moveable thanks to electrical technology. Now, as revealed in an article published in Science Translational Medicine on Oct 8th 2014, a research team has been successful in allowing amputees to feel sensations through their prosthetic limbs.

First of all, let’s have a look at how we perceive touch. When you brush your hand off a surface, receptors in your hand send electrical signals via nerves to the brain. These signals are sent to the part of the brain that deals with sensory processing where they are perceived to mean the surface you are touching is soft, hard, bumpy etc. If you were to lose your hand, the receptors would no longer be connected to the nerves that travel up your arm to your spinal cord and to your brain, and so your receptors would be unable to send signals along these nerves to the brain. However, the nerves would still remain. So, if you were to stimulate those nerves in certain ways, theoretically you should still be able to transmit signals along the nerves to the sensory component of the brain. Even after a hand or other body part is lost, signals continue to be sent along the nerves and this is what causes phantom limb pain.

The experiment was conducted with two individuals who had prosthetic hands. They were fitted with a cuff of electrodes that surrounded the three main nerves that transmit sensory information from the hand. In one individual, two cuffs were implanted in his forearm, allowing him to feel 19 points. The other individual had three cuffs implanted in his upper arm providing him with feeling of 16 points. Signals can be sent via the electrodes to stimulate the nerves into sending electrical signals to the brain. At the beginning of this study, the initial stimulations were perceived by the subjects as tingling or prickling sensations, but the success was in the fact that they didn’t perceive these sensations to come from their arms where the electrodes were stimulating the neurons, but rather from their missing hand.

Simply restoring some sort of feeling wasn’t all the researchers had in mind; they wanted to make the subjects feel they were sensing real objects, so they developed algorithms which were able to convert information from sensors attached to the patients’ hands into different complex patterns of electrical signals which were transmitted via the electrode into the nerves and then sent on to the brain. The brain was then able to perceive these signals. So when the prosthetic hands were brushed with cotton balls, for example, the sensors picked up this input and sent signals back to a computer where a computer algorithm produced an electrical pattern to simulate the correct sensation. This electrical pattern was then sent via the electrode into the nerve and along to the brain which was able to perceive it as the soft touch of a cotton ball. In order to ensure the subjects weren’t just looking at what they were being touched with, they were blindfolded to ensure they were relying purely on the signals their brains received from the electrodes. The subjects were able to tell the difference between various surfaces, such as sandpaper and smooth and hard surfaces.

The feedback of sensory information allows for the development of finer motor control of the prosthetic hands. In a test, one of the subjects was unable to pluck cherries from their stems because they could not gauge the right amount of pressure to hold the cherries with. However, when they had the sensory information being transmitted, they were able to adjust their grip on the cherries to hold them tight enough so that they could pluck the cherries off the stem without squashing them. An unexpected result of the test was that the subjects’ phantom limb pain reduced until it was non-existent. This is most likely due to the fact that nerves that were sensing the missing limb were once again receiving actual signals, and so were happy once more. Essentially what the study has shown is that these people can experience sensations through their prosthetic hands with the help of an electrical interface connecting the hands to sensory nerves in their arms. At the moment this test has only been carried out in the lab, however the heads of the study are hopeful that it will be developed for home use within 5 years. For the people in the test, the results are amazing; they can feel things with their hands once more, allowing vastly improved motor skills and ultimately, a higher quality of life.