Sesame-Seed Magnets in Muscle Beat Skin Electrodes for Prosthetic Control in MIT Study of Three Amputees

MIT researchers have shown in three people with below-knee amputations that tiny magnetic beads implanted in muscle can track how the muscle moves more accurately than electrodes, a step toward prosthetic limbs that respond more like natural ones, according to a study published Thursday in Nature Sensors and described by MIT News.

The technique, called magnetomicrometry, places pairs of magnets in a muscle. When the muscle contracts or stretches, sensors worn on the skin measure the changing distance between them, from which the muscle's length and velocity can be calculated.

"This interface is minimally invasive, so 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 graduate student in the Harvard-MIT Program in Health Sciences and Technology and a lead author. The other lead author is Cameron Taylor, PhD '20, now an assistant professor of biomedical engineering at the University of North Carolina at Chapel Hill.

The senior author, Hugh Herr, a professor of media arts and sciences and co-director of the K. Lisa Yang Center for Bionics at MIT, framed the problem. "For four decades, the standard methodology for neurally controlling prostheses has been to use skin electrodes to measure muscle activation," he said. "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."

Skin electrodes, a method called electromyography, have known weaknesses, MIT News explains. The electrical signal is an imperfect stand-in for muscle movement; when a muscle is held at a fixed length for several seconds, the signal fades. Electrodes can also pick up neighboring muscles, and sweat can interfere. Those measurements have not been reliable enough to ensure safety while walking on a neuroprosthetic leg, according to MIT.

The details of the implants: beads 3 millimeters across, about the size of a sesame seed, made of a neodymium-iron-boron alloy and coated in gold and a polymer called parylene C to make them biocompatible. They can be read from as deep as 6 centimeters below the skin with a custom external sensor. Participants had magnets placed in up to four ankle-joint muscles in surgeries at Brigham and Women's Hospital with plastic surgeon Matthew Carty, a co-author.

All three participants had undergone a procedure called an agonist-antagonist myoneural interface, in which muscle pairs are reconnected during amputation so they still communicate in the residual limb, preserving the sense of limb position known as proprioception. Each also had electrodes implanted in the same muscles, which let the team compare three methods head to head: magnets, implanted electrodes and skin electrodes.

The participants were not wearing prostheses during the tests. Instead, they used muscle movements to steer a cursor to targets on a screen, standing in for the controls of a robotic ankle-foot prosthesis. "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," Shallal said.

The work builds on a 2021 animal study led by Taylor, and on Herr's "bionic knee" reported last year, which used electrodes implanted in muscle and succeeded in a small clinical study. Magnets offer a less invasive path: "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," Shallal said.

Why mechanical signals matter: a prosthetic ankle has to decide, step by step, where the foot should point and how stiff it should be. Herr's argument is that electrical activity tells a controller roughly how hard a muscle is trying, but not how long it is or how fast it is changing, which are the quantities the joint actually needs. Magnetomicrometry measures those directly. And because the beads are passive magnets read by a sensor worn outside the body, the system is wireless, which is why the team describes it as minimally invasive compared with implanted electrodes.

The caveats are in the design: three participants, all with the same specialized amputation surgery, and a screen task rather than walking. The team is now running a larger study of how well people can control actual prosthetic limbs with the magnetic signals. The research was funded in part by the Yang Tan Collective at MIT and a National Science Foundation Graduate Research Fellowship.

Next: results from the larger prosthesis study.