Magnets offer better control of prosthetic limbs

Currently, most prosthetic limbs are controlled using electromyography (EMG), a way of recording electrical activity from the muscles, but it provides only limited control of the prosthesis. Researchers at MIT’s Media Lab have now developed an alternative strategy – magnetomicrometry (MM) – that could offer more precise control of prosthetic limbs.

As reported in Science Robotics, after inserting small magnetic beads into muscle tissue within the amputated residuum, muscle contraction can be precisely monitored with feedback relayed within milliseconds to a bionic prosthesis.

“Our hope is that MM will replace EMG as the dominant way to link the peripheral nervous system to bionic limbs. And we have that hope because of the high signal quality we get from MM, and the fact that it’s minimally invasive and has a low regulatory hurdle and cost,” says Hugh Herr, a professor of media arts and sciences, head of the Biomechatronics group in the Media Lab, and the senior author of the paper.

Precise measurements

Existing prosthetic devices take electrical measurements of muscles using electrodes attached to the skin surface or surgically implanted in the muscle, the latter being highly invasive and costly, but providing somewhat more accurate measurements. However, in either case, EMG offers information only about muscles’ electrical activity, not length or speed.

The new strategy is based on the idea that if sensors could measure muscle activity, those measurements could more precisely control a prosthesis. Researchers inserted pairs of magnets into muscles, and by measuring the magnets’ relative movement, calculated how much and how fast the muscles contract.

Researchers tested their algorithm’s ability to track magnets inserted in turkey calf muscles. The magnetic beads were 3mm in diameter and inserted at least 3cm apart – any closer and the magnets tend to migrate toward each other.

Using an array of magnetic sensors placed on the legs’ exterior, researchers determined the position of the magnets within 3ms and with a precision of 37µm as they moved the turkeys’ ankle joints.

To control a prosthetic limb, these measurements could be fed into a computer model that predicts where the patient’s phantom limb would be in space, based on the remaining muscle’s contractions. This would direct the prosthetic device to match the patient’s mental picture of their limb position.

“With magnetomicrometry, we’re directly measuring the length and speed of the muscle,” Herr says. “Through mathematical modeling of the entire limb, we can compute target positions and speeds of the prosthetic joints to be controlled, and then a simple robotic controller can control those joints.”

Muscle control

Within a few years, the researchers hope to study human patients with amputations below the knee. Sensors to control the prosthetic limbs could be placed on clothing, attached to the skin, or affixed to the outside of a prosthesis.

MM could also improve muscle control with functional electrical stimulation, which is now used to restore mobility in people with spinal cord injuries. Another possible use is guiding robotic exoskeletons to help people who have suffered a stroke or developed other muscle weakness.

A final advantage of MM is it’s minimally invasive. Once inserted in the muscle, the beads could remain in place for a lifetime without needing to be replaced, Herr concludes.

Massachusetts Institute of Technology (MIT)
https://www.mit.edu