MIT Implants Magnetic Beads in Muscle to Steer Prosthetic Limbs

Researchers at MIT have shown that two tiny magnetic beads implanted in a muscle can track how that muscle moves accurately enough to steer a virtual prosthetic ankle, a step toward artificial limbs that respond more like natural ones. The study, published Oct. 29, 2026, in the journal Nature Sensors, tested the technique in three people with below-knee amputations, according to MIT News.

Key figures: 3 participants · beads 3 mm wide · up to 4 ankle muscles tracked · signals read from up to 6 cm deep

Why prosthetic control needs a better signal

For decades, engineers building neuroprosthetic limbs have tried to read what the muscles left after an amputation are doing, then feed that information to the motors in a robotic leg or arm. If the controller knows a muscle’s length, speed and activation, a person with a leg amputation can walk with a more natural gait, and a person with an arm amputation can handle objects more precisely.

Most systems have relied on electromyography, which uses electrodes on the skin to pick up the electrical activity of the muscle underneath. MIT’s researchers say that signal is not a perfect stand-in for what the muscle is physically doing. When a muscle holds a fixed length for several seconds, the electrical signal fades. Skin electrodes can also pick up neighboring muscles, and sweat can interfere. According to MIT, those measurements have not proven reliable enough to keep a person safe while walking on a neurally controlled prosthetic leg.

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

How magnetomicrometry works

The technique is called magnetomicrometry. Surgeons implant pairs of magnets in a muscle. When the muscle contracts or stretches, a sensor worn over the skin measures the changing distance between the two magnets, and software converts that into the muscle’s length and velocity. Herr’s lab first demonstrated the approach in animals in a 2021 study led by Cameron Taylor, who earned his MIT doctorate in 2020 and is now an assistant professor of biomedical engineering at the University of North Carolina at Chapel Hill. The new paper is the first to extend it to people with long-term implants.

The beads are about the size of a sesame seed. They are made of a neodymium-iron-boron alloy and coated with gold and a polymer called parylene C so the body tolerates them. The custom external sensor can read their signals from as deep as 6 centimeters below the skin.

“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, an MIT graduate student in the Harvard-MIT Program in Health Sciences and Technology and co-lead author with Taylor.

A Boston hospital, three volunteers and a cursor test

The implant surgeries took place at Brigham and Women’s Hospital in Boston, working with plastic surgeon Matthew Carty, a co-author. All three volunteers had an amputation that creates what is called an agonist-antagonist myoneural interface, a procedure in which paired muscles are reconnected so they keep working against each other inside the residual limb. That arrangement preserves some sense of limb position, known as proprioception. Each participant received beads in up to four muscles that control the ankle.

Each volunteer also had electrodes implanted in the same muscles. That let the team compare three methods side by side: the magnets, implanted electrodes and skin electrodes. Participants were not wearing a prosthesis during the tests. Instead, they moved a cursor toward targets on a screen using only their muscles, a stand-in for steering 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.

What the study does and does not show

The result is an early one. Three participants is a very small group, and the test used a screen rather than a working leg. MIT says the team is now running a larger study to measure how well people can control actual prosthetic limbs with the bead signals. Last year, Herr’s group reported a “bionic knee” that used implanted electrodes and succeeded in a small clinical study, according to MIT. The magnet approach is meant as a less invasive alternative, and the team hopes the extra accuracy will help companies bring such limbs to market.

The research was funded in part by the Yang Tan Collective at MIT and a National Science Foundation Graduate Research Fellowship. It adds to a busy month for Boston-area medical technology, from Burlington’s Butterfly Network and its handheld ultrasound business to Cambridge startup DropGenie’s gene-editing chip.

Sources

• MIT News: Magnetic sensors track muscle movement, for better prosthetic limb control (Oct. 29, 2026)
• Nature Sensors: A wireless magnetic implant system for continuous neuromuscular sensing (Oct. 29, 2026)
• MIT Media Lab: study announcement

Additional sources

• medRxiv preprint of the study (2025)
• K. Lisa Yang Center for Bionics at MIT
• Brigham and Women’s Hospital