October 2021

Implantable Magnets May Improve Prosthetic Control

Researchers at MIT’s Media Lab have developed a new strategy that could offer more precise control of prosthetic limbs. Image courtesy of the researchers.

Researchers at the Massachusetts Institute of Technology’s (MIT’s) Media Lab have developed a new strategy, called magnetomicrometry (MM), that they believe could improve control of prosthetic limbs. The strategy is based on the idea that if sensors could measure what muscles in the residual limb are doing, those measurements would offer more precise control of a prosthesis. To achieve that, the researchers inserted pairs of magnets into muscles. By measuring how the magnets move relative to one another, the researchers can calculate how much the muscles are contracting and the speed of contraction.

Two years ago, Cameron Taylor, an MIT postdoc, and Hugh Herr, PhD, a professor of media arts and sciences and head of the Biomechatronics group in the Media Lab, developed an algorithm that greatly reduced the time needed for sensors to determine the positions of small magnets embedded in the body, thus overcoming a major hurdle. They tested their algorithm’s ability to track magnets inserted in the calf muscles of turkeys. The magnetic beads they used were 3mm in diameter and were inserted at least 3cm 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, as they moved the turkeys’ ankle joints. These measurements could be obtained within 3 milliseconds.

For prosthetic control, the magnets would be inserted into the muscle of the residual limbs. The measurements of the magnets 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 prosthesis to move the way the patient wants it to, matching the mental picture that they have of their limb position.

Within the next few years, the researchers hope to do a small study in human patients with transtibial amputations. They envision that 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.

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