Magnetics Business & Technology - September/October 2022 - 19

RESEARCH & DEVELOPMENT
measure what muscles are doing, those measurements would
offer more precise control of a prosthesis. To achieve, that, the
researchers decided to insert 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, Herr and Taylor developed an algorithm that
greatly reduced the amount of time needed for sensors to determine
the positions of small magnets embedded in the body. This
helped them to overcome one of the major hurdles to using MM to
control prostheses, which was the long lag-time for such measurements.
In
the new 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 - if they are closer than that, the magnets
tend to migrate toward each other.
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. These measurements could be obtained
within three milliseconds.
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 that the patient wants it to,
matching the mental picture that they have 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 the next few years, the researchers hope to
do a small study in human patients who have amputations
below the knee. 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.
MM could also be used to improve the muscle
control achieved with a technique called functional
electrical stimulation, which is now used to help
restore mobility in people with spinal cord injuries.
Another possible use for this kind of magnetic control
would be to guide robotic exoskeletons, which
can be attached to an ankle or another joint to help
people who have suffered a stroke or developed
other kinds of muscle weakness.
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" Essentially the magnets and the exoskeleton act as an artificial
muscle that will amplify the output of the biological muscles in
the stroke-impaired limb, " Herr says. " It's like the power steering
that's used in automobiles. "
Another advantage of the MM approach is that it is minimally
invasive. Once inserted in the muscle, the beads could remain in
place for a lifetime without needing to be replaced, Herr says.
The research was funded by the Salah Foundation, the MIT
Media Lab Consortia, the National Institutes of Health, and the
National Science Foundation.
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September/October 2022 * Magnetics Business & Technology 19
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Magnetics Business & Technology - September/October 2022

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