MIT researchers report that pairs of tiny magnets implanted in muscles can offer more accurate and stable control signals than standard electrode-based measurements. In a study published today in Nature Sensors, three people with below-knee amputations used the technique, known as magnetomicrometry, to move a cursor on a screen.
The approach works by placing pairs of magnets into muscles. When a muscle contracts or stretches, external sensors worn on the skin measure the change in distance between the magnets, allowing muscle length and velocity to be calculated.
For four decades, the standard method for neurally controlling prostheses has been electromyography, which uses skin electrodes to measure muscle activation. Hugh Herr, senior author of the study and a professor of media arts and sciences at MIT, said such electrical measurements cannot directly determine the mechanical signals, such as joint position, torque or impedance, needed to control a prosthetic limb.
Christopher Shallal, an MIT graduate student and one of the lead authors, called the new interface minimally invasive. "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," he said.
In the study, the three participants had magnets implanted in up to four muscles of the ankle joint. The beads are 3 millimeters in diameter, made from an alloy of neodymium, iron and boron, and coated with gold and the polymer parylene C to make them biocompatible. Their signals can be read from a depth of up to 6 centimeters. The surgeries were performed at Brigham and Women's Hospital in collaboration with plastic surgeon Matthew Carty, a co-author of the study.
Each participant also had electrodes implanted in the same muscles, allowing the researchers to compare magnetomicrometry with electromyography using both implanted and surface electrodes. During the experiments, muscle movements were translated into control signals that moved a cursor on a screen, representing the degrees of freedom the participants would control on a robotic ankle-foot prosthesis. Shallal said the results suggest the magnet-based approach can offer more accurate and stable control.
The paper's other lead author is Cameron Taylor, an assistant professor of biomedical engineering at the University of North Carolina at Chapel Hill, who led a 2021 animal study demonstrating the technique. Last year, Herr's lab reported a bionic knee prosthesis using implanted electrodes. The researchers are now performing a larger study evaluating how well people can control prosthetic limbs using measurements from the magnetic beads.
Analysis
Why This Matters
- The interface is minimally invasive, which could make richer prosthetic control accessible to more people.
- Magnetomicrometry measures muscle length and velocity directly, providing mechanical signals that electrical activation alone cannot.
- A larger study is now testing how well people control prosthetic limbs with the magnetic beads.
Background
Herr's lab at MIT has spent years developing human-machine interfaces to make artificial limbs behave more like natural ones. The standard approach, electromyography, estimates muscle contraction from electrical activation, but the signal can fade when a muscle is held at a fixed length, may pick up signals from other muscles, and can be disrupted by sweat. The magnetic technique was first demonstrated in animal models in a 2021 study, and last year the lab reported a bionic knee using implanted electrodes, a more invasive alternative.
What to Watch
- Findings from the larger study now under way on controlling prosthetic limbs with magnetic bead measurements.
- Whether the minimally invasive magnetic interface moves toward wider clinical access.
- How long-term magnetic implants perform in humans beyond the initial participants.
Checked
12 claims in this story checked against 1 source: 12 supported.
Show each claim
- SupportedMagnetic muscle tracking could improve prosthetic-limb control, according to the MIT study.MIT News: ““our results suggest that the magnet-based approach can offer more accurate and stable control,” Shallal says.”
- SupportedResearchers say implanted muscle magnets can offer more accurate and stable control than electrode-based measurements.MIT News: ““our results suggest that the magnet-based approach can offer more accurate and stable control,” Shallal says.”
- SupportedThe study appeared today in Nature Sensors.MIT News: “In the new study, which appears today in Nature Sensors”
- SupportedThree people with below-knee amputations used magnetomicrometry to move a cursor on a screen.MIT News: “During the experiments, the participants were not wearing a prosthetic limb but were asked to perform tasks such as moving a cursor on a screen to a particular target using their muscle movements.”
- SupportedFor four decades, electromyography using skin electrodes has been the standard method for neurally controlling prostheses.MIT News: ““For four decades, the standard methodology for neurally controlling prostheses has been to use skin electrodes to measure muscle activation.””
- SupportedHugh Herr said electrical activation alone cannot directly determine mechanical signals such as joint position, torque or impedance needed to control a prosthetic limb.MIT News: ““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,” says Hugh Herr”
- SupportedChristopher Shallal called the interface minimally invasive and said it could be accessible to more people.MIT News: ““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,” says Christopher Shallal”
- SupportedThe three participants had magnets implanted in up to four muscles of the ankle joint.MIT News: “Participants had magnets implanted in up to four muscles of the ankle joint.”
- SupportedThe beads are 3 millimeters in diameter and made of an alloy of neodymium, iron and boron, coated with gold and parylene C.MIT News: “The beads are 3 millimeters in diameter, about the size of a sesame seed. The magnets are made from an alloy of neodymium, iron, and boron, and are coated with gold and a polymer called parylene C”
- SupportedThe magnetic signals can be read from a depth of up to 6 centimeters.MIT News: “The magnetic signals can be read from a depth of up to 6 centimeters from the skin’s surface”
- SupportedParticipants also had electrodes implanted in the same muscles, allowing comparison with magnetomicrometry and surface-electrode electromyography.MIT News: “Each of the participants in this study also had electrodes implanted into the same muscles that received magnetic beads. This allowed the researchers to compare the accuracy of three different types of measurements: magnetomicrometry, electromyography with implanted electrodes, and electromyography with electrodes on the surface of the skin.”
- SupportedThe researchers are conducting a larger study of how well people can control prosthetic limbs using magnetic-bead measurements.MIT News: “The researchers are now performing a larger study in which they are evaluating how well people can control prosthetic limbs using measurements from the magnetic beads.”
