MIT researchers have demonstrated that implanting tiny magnetic beads inside muscle can track muscle motion with high precision. This magnetic measurement approach could enable people with certain amputations to control prosthetic limbs more naturally.
Traditional neuro‑prosthetic systems rely on surface electromyography (EMG) electrodes to sense muscle electrical activity, but the signals are vulnerable to sweat, cross‑talk from neighboring muscles, and decay when a muscle holds a fixed length, making it hard to obtain reliable length, velocity, or torque information.
Magnetomicrometry places a pair of ~3 mm neodymium‑iron‑boron beads inside the muscle. External skin‑mounted sensors measure the distance change between the beads, allowing real‑time calculation of muscle length and contraction speed. The beads are coated with gold and parylene C for biocompatibility, and the magnetic field can be detected up to 6 cm beneath the skin.
In the latest Nature Sensors paper, three below‑knee amputees received magnetic beads (up to four ankle‑joint muscles per participant) and, for comparison, implanted and surface EMG electrodes. Participants performed a cursor‑control task on a screen without wearing a prosthesis; the cursor movement represented the degrees of freedom of a future robotic ankle‑foot prosthesis.
Results showed that magnetic signals provided more accurate and stable control than both implanted and surface EMG, suggesting a more reliable translation of muscle dynamics into command signals. The team is now conducting a larger clinical study to evaluate magnetic bead measurements in real prosthetic control.
The minimally invasive nature of the approach—requiring only two beads—makes it a promising candidate for a broader amputee population. Funding came from the MIT Yang‑Lisa Center, the National Science Foundation Graduate Research Fellowship, and other sources.
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