Magnets could offer better control of prosthetic limbs
System uses tiny magnetic beads to rapidly measure the position of
muscles and relay that information to a bionic prosthesis

Date:
August 18, 2021
Source:
Massachusetts Institute of Technology
Summary:
Researchers have developed a new strategy that could offer much
more precise control of prosthetic limbs. After inserting small
magnetic beads into muscle tissue, they can accurately measure
the length of a muscle as it contracts, and this measurement can
be relayed to a robotic prosthesis within milliseconds.



FULL STORY ==========================================================================
For people with amputation who have prosthetic limbs, one of the
greatest challenges is controlling the prosthesis so that it moves the
same way a natural limb would. 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 MIT's Media Lab have now developed an alternative approach
that they believe could offer much more precise control of prosthetic
limbs. After inserting small magnetic beads into muscle tissue within
the amputated residuum, they can precisely measure the length of a muscle
as it contracts, and this feedback can be relayed to a bionic prosthesis
within milliseconds.

In a new study appearing today in Science Robotics, the researchers tested their new strategy, called magnetomicrometry (MM), and showed that it
can provide fast and accurate muscle measurements in animals. They hope
to test the approach in people with amputation within the next few years.

"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," 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.

Cameron Taylor, an MIT postdoc, is the lead author of the study. Other
authors include MIT postdoc Shriya Srinivasan, MIT graduate student
Seong Ho Yeon, Brown University professor of ecology and evolutionary
biology Thomas Roberts, and Brown postdoc Mary Kate O'Donnell.

Precise measurements With existing prosthetic devices, electrical
measurements of a person's muscles are obtained using electrodes that can
be either attached to the surface of the skin or surgically implanted
in the muscle. The latter procedure is highly invasive and costly, but
provides somewhat 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," Taylor says.

The new MIT strategy is based on the idea that if sensors could 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.

"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.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Anne
Trafton. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Zachary F. Caffall, Bradley J. Wilkes, Ricardo Herna'ndez-Martinez,
Joseph E. Rittiner, Jennifer T. Fox, Kanny K. Wan, Miranda
K. Shipman, Steven A. Titus, Ya-Qin Zhang, Samarjit Patnaik,
Matthew D. Hall, Matthew B. Boxer, Min Shen, Zhuyin Li, David
E. Vaillancourt, Nicole Calakos. The HIV protease inhibitor,
ritonavir, corrects diverse brain phenotypes across development in
mouse model of DYT-TOR1A dystonia. Science Translational Medicine,
2021; 13 (607): eabd3904 DOI: 10.1126/ scitranslmed.abd3904 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210818153638.htm

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