Researchers are developing new technology that could enable better control systems for prosthetics

PROVIDENCE, RI [Brown University] — Researchers at Massachusetts Institute of Technology and Brown University have developed a sophisticated way to monitor muscle movement using a simple set of magnets that they hope will make it easier for amputees to control their prosthetic limbs.

In two new papers published in the journal Frontiers in Bioengineering and Biotechnology, the researchers demonstrated the accuracy and safety of their magnet-based system, which can track muscle length during movement. The studies, conducted using animal models, raise hopes that the strategy could be used to help people with prosthetic limbs control them to better mimic natural limb movement.

The findings improve understanding of how muscles change length, generate force and generate force during physical exercise, said Thomas Roberts, a professor of biology at Brown’s Department of Ecology, Evolution and Organismal Biology, affiliated with Warren Alpert Medical School Co-author of both papers.

“This technique gives us the ability to measure mechanical muscle function during ordinary movements, which is essential for understanding how muscles work to move us,” said Roberts, whose research aims to enhance understanding of muscle physiology with modern approaches to integrate in functional morphology and biomechanics.

The new technique also has practical applications outside of the laboratory.

“These recent results demonstrate that this tool can be used outside of the laboratory to track muscle movement during natural activity, and they also suggest that the magnetic implants are stable, biocompatible and cause no discomfort,” said Cameron Taylor, a Research Scientist at MIT and co-lead author of both articles.

In one of the studies, the researchers showed that they could accurately measure the length of the turkeys’ calf muscles while the birds ran, jumped and performed other natural movements. In the other study, they showed that small magnetic beads used for the measurements do not cause inflammation or other adverse effects when implanted in the muscle.

“I am excited about the clinical potential of this new technology to improve bionic limb control and effectiveness for people with limb loss,” said Hugh Herr, professor of media arts and sciences, co-director of MIT’s K. Lisa Yang Center for Bionics and Associate Fellow of MIT’s McGovern Institute for Brain Research.

track movement

Researchers have long focused on checking prostheses using an approach known as surface electromyography (EMG). Electrodes, which are attached to the skin’s surface or surgically implanted into the residual muscle of the amputated limb, measure electrical signals from a person’s muscles, which are fed into the prosthesis to help it move like the person wearing the prosthesis limb wears, intended.

However, this approach does not take into account information about muscle length or speed that could help make the prosthetic movements more accurate.

A few years ago, the MIT team, along with Brown collaborators, began working on a novel way to make these types of muscle measurements using an approach they call magnetomicrometry. This strategy uses the permanent magnetic fields surrounding small beads that are implanted in a muscle. Using a credit card-sized, compass-like sensor attached to the outside of the body, their system can track the distances between the two magnets. When a muscle contracts, the magnets move closer together, and when it flexes, they move farther apart.

In a study published last year, the team showed that the system can be used to accurately measure small ankle movements when the beads are implanted in the calf muscles of turkeys. In one of the new studies, the researchers looked at whether the system could make accurate measurements during more natural movements in an environment outside of the laboratory.

To do this, they built an obstacle course consisting of ramps for the turkeys to climb and boxes for them to jump on and off. The researchers used their magnetic sensor to track muscle movements during these activities and found that the system could calculate muscle lengths in less than a millisecond.

They also compared their data to measurements made using a more traditional approach known as fluoromicrometry, a type of X-ray technology that requires much larger equipment than magnetomicrometry. The magnetomicrometry measurements differed from those produced by fluoromicrometry by less than a millimeter on average.

“We’re able to provide the muscle length tracking functionality of room-sized X-ray machines in a much smaller, portable package, and we can collect the data continuously rather than being limited to the 10-second bursts that fluoromicrometry is limited to.” said Taylor.

Biocompatibility assessment

In the second work, the researchers focused on the biocompatibility of the implants. They found that the magnets did not create tissue scarring, inflammation, or other harmful effects. They also showed that the implanted magnets did not alter the turkeys’ gait, suggesting they did not cause any discomfort.

The researchers also showed that the implants remained stable throughout the eight-month study and did not migrate toward each other as long as they were implanted at least 3 centimeters apart. The researchers envision the beads, which consist of a magnetic core coated with gold and a polymer called parylene, could remain in tissue indefinitely after implantation.

“Magnets do not require an external power source, and once implanted in the muscle, they can maintain the full strength of their magnetic field throughout the patient’s lifetime,” Taylor said.

Researchers now plan to apply for US Food and Drug Administration approval to test the system on people with prosthetic limbs. They hope to use the sensor to control prosthetics, much like it’s now used with surface EMG: measurements of muscle length are fed into a prosthetic’s control system to guide it into the position the wearer intends.

“Measurements of the distance between spheres could be used as an indicator of user intent, allowing for improved control over a prosthesis,” Roberts said. This would help the prosthesis adapt to different conditions, he said, such as: B. Increases or decreases in speed, suddenly changing terrain or unexpected obstacles in the user’s path.

“The place where this technology fills a need is to transmit these muscle lengths and speeds to a wearable robot so that the robot can operate in a way that works with humans,” Taylor said. “We hope that magnetomicrometry will allow a person to control a wearable robot with the same level of comfort and ease as someone would control their own limbs.”

In addition to prosthetic limbs, these wearable robots could include robotic exoskeletons worn outside the body to help people move their legs or arms more easily.

The research was funded by the Salah Foundation, the K. Lisa Yang Center for Bionics at MIT, the MIT Media Lab Consortia, the National Institutes of Health, and the National Science Foundation.

William Clark, a former postdoc in Roberts’ lab at Brown, is the co-lead author of the biocompatibility study. Seong Ho Yeon, an MIT graduate student, is also one of the lead authors of the measurement study. Additional authors are Ellen Clarrissimeaux, MIT Research Support Associate, and Mary Kate O’Donnell, former postdoctoral fellow at Brown University.

This story was adapted from an article written by Anne Trafton for the MIT News Bureau.


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