One of the greatest challenges for people with amputations is to control the prosthesis so that it moves the same as a natural limb.
Most prosthetic limbs are controlled using electromyography, a way of recording electrical activity from the muscles, but this approach provides only limited control of the prosthesis. Researchers at Massachusetts Institute of Technology’s (MIT) Media Lab have developed an alternative approach they believe could offer more precise control of prosthetic limbs. After inserting small magnetic beads into muscle tissue within the residual limb, they can precisely measure the length of a muscle as it contracts—feedback that can be relayed to a bionic prosthesis within milliseconds.
Researchers at MIT’s Media Lab have developed a new strategy that could offer more precise control of prosthetic limbs.
Photograph courtesy of the researchers.
In a new study published in Science Robotics, the researchers tested their new strategy, magnetomicrometry (MM), and showed that it can provide fast and accurate muscle measurements in animals.
“Our hope is that MM will replace electromyography as the dominant way to link the peripheral nervous system to bionic limbs. And we have that hope because of the high signal quality that we get from MM, and the fact that it’s minimally invasive and has a low regulatory hurdle and cost,” said Hugh Herr, PhD, a professor of media arts and sciences, head of the Biomechatronics group in the Media Lab, and senior author of the paper.
With existing prosthetic devices, electrical measurements of a person’s muscles are obtained using electrodes that can be attached to the surface of the skin or surgically implanted in the muscle. The latter procedure is invasive and costly but provides more accurate measurements. However, in either case, electromyography (EMG) offers information only about muscles’ electrical activity, not their length or speed.
“When you use control based on EMG, you’re looking at an intermediate signal. You’re seeing what the brain is telling the muscle to do, but not what the muscle is actually doing,” said Cameron Taylor, an MIT postdoc, and lead author of the study.
In the Science Robotics paper, the researchers tested their algorithm’s ability to track magnets inserted in the calf muscles of turkeys. The magnetic beads they used were 3 millimeters in diameter and were inserted at least 3 centimeters apart. Using an array of magnetic sensors placed on the outside of the legs, the researchers found that they were able to determine the position of the magnets with a precision of 37 microns (about the width of a human hair), as they moved the turkeys’ ankle joints.
For control of 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 contractions of the remaining muscle. This strategy would direct the prosthetic device to move the way the patient wants it to, matching the mental picture they have of their limb position.
“With magnetomicrometry, we’re directly measuring the length and speed of the muscle,” Herr said. “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.”
Within the next few years, the researchers hope to do a small study in human patients with transtibial amputations. They envision the sensors used to control the prosthetic limbs could be placed on clothing, attached to the surface of the skin, or affixed to the outside of a prosthesis.
Editor’s Note: The story was adapted by materials provided by MIT.
