Magnetic bead implants could offer more accurate way to control prosthetic limbs

MIT researchers have developed a novel magnetic tracking system to accurately monitor muscle movement, which could enhance the control of prosthetic limbs for individuals with certain types of amputations. For many years, neuroprosthetic limbs have been developed using neural information from remaining muscles to guide the prosthetic.

Traditionally, surface electrodes have been used to measure muscle activity, but this method has not been reliable enough for safe walking with a neuroprosthetic leg. The MIT team's new system, which employs tiny magnetic beads implanted in the muscle, offers more precise measurements and could pave the way for the commercial development of such limbs.

The study, published in Nature Sensors, involved three participants with below-knee amputations who had a specific type of amputation called an agonist-antagonist myoneural interface (AMI). This surgical procedure reconnects muscle pairs, allowing dynamic communication within the residual limb, which aids the user in positioning and moving the prosthetic limb.

Magnetic beads, each approximately the size of a sesame seed, were implanted in up to four muscles of the ankle joint. These beads, coated with gold and parylene C for biocompatibility, are surrounded by 3 millimeter diameter magnets. A customized external sensor, worn over the skin, measures the change in distance between the magnets when a muscle contracts or stretches, allowing for the calculation of muscle length and velocity.

During the study, participants also had electrodes implanted into the same muscles for comparison purposes. The researchers measured three types of muscle movement: magnetomicrometry, electromyography with implanted electrodes, and electromyography with surface electrodes. The findings suggest that the magnetic tracking system provides more accurate measurements, paving the way for safer and more effective prosthetic limbs.