The future of prosthetic limb control may be magnetic
Currently, most powered prosthetic limbs are controlled by electrodes in the user's residual stump. An experimental new MIT system, however, is claimed to work better by replacing those electrodes with implanted magnetic beads.
In a conventional setup, electrodes are either implanted into the muscles of the arm/leg stump, or they're adhered to its skin. Whichever the case, the electrodes proceed to detect electrical signals that are produced by the muscles as the user attempts to perform certain actions. Based on those detected signals, the prosthesis is triggered to move accordingly, allowing the user to perform the desired task.
The technology is known as electromyography (EMG) and according to MIT's Prof. Hugh Herr, it's not an ideal solution. "When you use control based on EMG, you're looking at an intermediate signal," he says. "You’re seeing what the brain is telling the muscle to do, but not what the muscle is actually doing."
Working with postdoctoral student Cameron Taylor and other colleagues, he has developed an alternative system called magnetomicrometry.
It involves implanting two small magnetic beads in each of the stump muscles, then using sensors located outside of the stump to monitor how the distance between the two magnets in each muscle changes. In this way, it's possible to instantaneously detect how much each muscle is stretching or contracting – and at what speed – so that a prosthesis can very quickly be activated to respond.
In lab tests, pairs of 3-mm-wide beads were implanted in the calf muscles of turkeys. When the birds' ankle joints were manually moved by the scientists, external magnetic sensors were able to detect and precisely measure the associated calf muscle movements within just three milliseconds.
"With magnetomicrometry, we’re directly measuring the length and speed of the muscle," says Herr. "Through mathematical modelling of the entire limb, we can compute target positions and speeds of the prosthetic joints to be controlled, and then a simple robotic controller can control those joints."
It is hoped that clinical studies on human amputees could take place within a few years. Ultimately, the technology might also find use in restoring mobility to people with spinal cord injuries, and in the control of robotic exoskeletons.
The system is illustrated in the following video, and is described in a paper that was recently published in the journal Science Robotics.