MIT researchers have shown that tiny magnetic beads implanted in muscle can accurately track how that muscle moves, a step they hope will help people with certain amputations control prosthetic limbs more naturally.
The study, led by the lab of MIT professor Hugh Herr, appeared Oct. 29 in the journal Nature Sensors, according to MIT News.
Why skin electrodes fall short
For about four decades, neuroprosthetic limbs have relied on electrodes placed on the skin to read the electrical activation of the muscles left after an amputation. MIT says those signals have not proven reliable enough to ensure safety while walking with a neuroprosthetic leg. The signal fades when a muscle is held at a fixed length, electrodes can pick up signals from nearby muscles, and sweat can interfere.
“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 Herr, a professor of media arts and sciences and co-director of the K. Lisa Yang Center for Bionics at MIT.
How the magnets work
With magnetomicrometry, pairs of magnets are implanted in a muscle. When the muscle contracts or stretches, sensors on the skin measure the changing distance between the magnets, which lets researchers calculate the muscle’s length and speed.
The beads are 3 millimeters in diameter, about the size of a sesame seed. They are made from an alloy of neodymium, iron and boron and coated with gold and a polymer called parylene C so the body tolerates them. A custom sensor worn over the skin can read them from as deep as 6 centimeters.
“This interface is minimally invasive, so it could be accessible to more people,” said Christopher Shallal, an MIT graduate student in the Harvard-MIT Program in Health Sciences and Technology and a lead author of the paper. “You place two beads in the muscle and you have access to all of this rich information about the muscle dynamics.”
The human test
Three participants with below-knee amputations had magnets implanted in up to four muscles of the ankle joint. The surgeries were performed at Brigham and Women’s Hospital with plastic surgeon Matthew Carty, a co-author of the study. Each participant also had electrodes implanted in the same muscles, so the team could compare the magnets with implanted electrodes and with electrodes on the skin.
Participants used their muscles to steer a cursor on a screen, standing in for the controls of a robotic ankle-foot prosthesis. “Our results suggest that the magnet-based approach can offer more accurate and stable control,” Shallal said.
The team is now running a larger study to see how well people can control actual prosthetic limbs with the magnetic measurements. The paper’s other lead author is Cameron Taylor, an MIT PhD graduate who is now an assistant professor of biomedical engineering at the University of North Carolina at Chapel Hill.