Scientists at MIT have found a way to track muscle movement using tiny magnetic beads implanted inside the body. The goal is to help people with amputations control their artificial limbs much more naturally.
For years, researchers have tried to make robotic limbs respond to muscle signals. These traditional methods, however, relied on surface electrodes attached to the skin. The sensors pick up electrical signals, but they are not always reliable. For example, sweat can disrupt the readings, and the sensors sometimes pick up signals from neighboring muscles by mistake. As a result, scientists struggle to build a prosthetic leg that is completely safe for everyday walking.
Instead of reading electrical signals from the outside, the MIT team put tiny magnets directly into the muscle. When the muscle moves, external sensors on the skin track the distance between the magnets. This provides instant, precise data on the muscle’s length and speed.
“For four decades, the standard methodology for neurally controlling prostheses has been to use skin electrodes to measure muscle activation,” said Hugh Herr, a professor at MIT and senior author of the study. “However, to control a prosthetic limb, joint position, torque, or impedance has to be controlled, and one cannot directly determine such mechanical signals using electrical activation alone.”
Testing the Technology on Prosthetics
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The researchers tested the system on three participants with below-knee amputations. Each person had pairs of magnets, about the size of a sesame seed, placed into their ankle muscles.
“This interface is minimally invasive so that it could be accessible to more people. You place two beads in the muscle, and you have access to all of this rich information about the muscle dynamics,” said Christopher Shallal, a lead author on the study. “Magnets can be tracked wirelessly through the body, so you can get very specific muscle signals and measure the mechanical output of the muscle without needing to be extremely invasive.”
To test the accuracy, participants moved a cursor across a computer screen using their muscle movements. The magnetic beads gave cleaner, more stable control than skin or implanted electrodes.
“The cursor on the screen represented the degrees of freedom that they would control on a robotic ankle-foot prosthesis,” Shallal explained. “This virtual neuroprosthetic task allowed us to compare the performance across different neural interfaces, and our results suggest that the magnet-based approach can offer more accurate and stable control.”
Now, the team is working on a larger study in the U.S. to see how well these magnets help people control actual prosthetic limbs in real time.
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