Engineering news
A far cry from traditional wooden legs, some prosthetics can now detect and act on signals from nerves in the spine, automatically grasp objects after “seeing” them with webcams, recreate a sense of touch and even generate electricity with photovoltaic skin.
However, despite the advances, about 20% of new limbs are still rejected by the user. A new surgical technique from a team of MIT researchers aims to reduce this significant number by making smart prosthetics such as bionic hands feel “much more like natural limbs”. The system, which involves grafting pairs of muscles on to the amputation site, will allow amputees to sense where their limbs are in space and how much force is applied on them.
Professor Hugh Herr, a senior author of the study, is an amputee himself after both his legs were amputated below the knee when he was 17. An acclaimed rock climber who has designed his own prosthetics, he plans on having the technique applied to his own limbs.
Conventional amputations sever muscles which normally help people control their limbs and know where they are in space, leading many patients to reject replacements. “The greatest risk of a prosthetic device not feeling like a natural limb is that a patient will feel so uncomfortable that they will abandon it, potentially constraining their mobility and quality of life,” said lead author Shriya Srinivasan to Professional Engineering.
“Despite the rapid progression of prosthetics in the last few decades, the rate of rejection of prosthetic devices by users has not significantly declined. Much of this has been attributed to a lack of a sense of ownership.”
"Patient will not have to think"
The lack of knowledge of a limb’s position in space or forces being applied can “completely hinder one's ability to move, to successfully balance, or to manipulate objects” said Herr. The new technique, which has been successfully tested on rats, aims to resolve this by grafting 4cm by 1.5cm pairs of muscles to nerves where the limb has been amputated. When the user moves, one muscle contracts and the other extends, sending feedback to the brain about how much the limb moved and the forces applied to it.
The team has also developed parts of a control system which will translate the nerve signals into instructions for moving the prosthetic limb. The instructions and resulting action will be followed by the neural feedback from the muscles, letting the brain quickly learn how much control it has to exert to move the limbs in particular ways.
“Using this framework, the patient will not have to think about how to control their artificial limb,” said Herr. “We think that because the brain is so good at remapping and it's so plastic, it will quickly adapt to knowing how much it has to contract each muscle graft for natural prosthetic control.”
The researchers anticipate the technique will work for “nearly any” amputee, including those whose operations were performed many years ago, as long as there is a small piece left of a healthy nerve.
The study was published yesterday in Science Robotics.