Using new technology, researchers hope to create better control systems for prosthetic limbs

Using a simple set of magnets, MIT researchers have developed a sophisticated way to monitor muscle movement, which they hope will make it easier for amputees to control their prosthetic limbs.

In a new pair of papers, the researchers demonstrated the accuracy and safety of their magnet-based system, which can track the length of muscles during movement. The studies, done on animals, offer hope that this strategy could be used to help people who wear prostheses control them in a way that more closely mimics natural limb movement.

“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 magnetic implants are stable and biocompatible and do not cause discomfort,” says Cameron Taylor, an MIT researcher and co-lead author of both papers.

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

“I am very excited about the clinical potential of this new technology to improve the control and efficiency of bionic limbs for amputees,” says Hugh Herr, Professor of Media Arts and Sciences, Co-Director of the K. Lisa Yang Center for Bionics at MIT and an associate member of the McGovern Institute for Brain Research at MIT.

Herr is one of the principal authors of the two articles, which appear today in the journal Frontiers in bioengineering and biotechnology. Thomas Roberts, a professor of ecology, evolution and organismal biology at Brown University, is a lead author of the measurement study.

Follow movement

Currently, motorized prosthetic limbs are typically monitored using an approach known as surface electromyography (EMG). Electrodes attached to the surface of the skin 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 in the way the person wants carrying the member.

However, this approach does not take into account any information about muscle length or speed, which could help make prosthetic movements more precise.

Several years ago, the MIT team began working on a new way to perform these kinds of muscle measurements, using an approach they call magnetomicrometry. This strategy takes advantage of the permanent magnetic fields surrounding the small beads implanted in a muscle. Using a compass-like sensor the size of a credit card 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 and when it flexes, they move apart.

In a study published last year, the researchers showed that this system could be used to accurately measure small ankle movements when the beads were implanted in the calf muscles of turkeys. In one of the new studies, the researchers investigated whether the system could make accurate measurements during more natural movements in a non-laboratory environment.

To do this, they created an obstacle course consisting of ramps that the turkeys can climb up and boxes that they can jump up and down. 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 taken using a more traditional approach known as fluoromicrometry, a type of X-ray technology that requires much larger equipment than magnetomicrometry. The magnetomicrometric measurements differed from those generated by fluoromicrometry by less than one millimeter on average.

“We are able to deliver the muscle length tracking functionality of coin-sized x-ray equipment using a much smaller portable package, and we are able to collect data continuously instead of being limited to 10 second bursts. that fluoromicrometry is limited,” says Taylor.

Seong Ho Yeon, an MIT graduate student, is also co-lead author of the measurement study. Other authors include MIT research support associate Ellen Clarrissimeaux and former Brown University postdoctoral fellow Mary Kate O’Donnell.


In the second article, the researchers focused on the biocompatibility of the implants. They found that the magnets did not produce tissue scarring, inflammation, or other harmful effects. They also showed that the implanted magnets did not alter the gaits of the turkeys, suggesting that they did not produce discomfort. William Clark, a postdoctoral fellow at Brown, is the co-lead author of the biocompatibility study.

The researchers also showed that the implants remained stable for eight months, the duration of the study, and did not migrate towards each other, as long as they were implanted at least 3 centimeters apart. other. The researchers envision that the beads, made of a gold-coated magnetic core and a polymer called parylene, could remain in tissue indefinitely once implanted.

“The magnets do not require an external power source, and after implanting them in the muscle, they can maintain the full strength of their magnetic field throughout the life of the patient,” says Taylor.

The researchers now plan to seek FDA approval to test the system in people with prosthetics. They hope to use the sensor to control prostheses in the same way surface EMG is used today: measurements of muscle length will be fed into a prosthesis’ control system to help guide it to position desired by the wearer.

“Where this technology fills a need is to communicate those muscle lengths and speeds to a wearable robot, so the robot can operate in a way that works in tandem with the human,” Taylor says. “We hope magnetomicrometry will allow a person to control a wearable robot with the same level of comfort and ease that someone would control their own limb. »

In addition to prosthetic limbs, these wearable robots could include robotic exoskeletons, which are 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.

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