Many prosthetic limb users would agree that one of the challenges they face is controlling their prosthetic device, so it moves the same way as a natural limb. Many upper limb prostheses are controlled using electromyography (EMG), which records electrical activity from the muscles. However, this approach allows only limited prosthetic limb control.
Researchers at the Massachusetts Institute of Technology's (MIT) Media Lab recently developed an alternative approach using tiny magnetic beads that offer precise prosthetic limb control. The beads are inserted into the muscle tissue within the residual limb. The beads precisely measure the length of a muscle as it contracts. This feedback is then relayed to a bionic prosthesis within milliseconds.
This alternative approach, called magnetomicrometry (MM), is documented in Science Robotics. The study showed that MM could provide fast and accurate muscle measurements when tested on animals. The researchers hope to try MM in human patients within the next few years.
Magnetomicrometry (MM) provides high signal quality, is minimally invasive, and has low regulatory hurdles and cost.
According to Hugh Herr, head of the Biomechatronics Group in the Media Lab and senior author of the study, the team hopes to eventually replace EMG as the primary way of linking bionic limbs to the peripheral nervous system. MM has more benefits than EMG, including high signal quality, is minimally invasive, and has low regulatory hurdle and cost.
Meanwhile, with existing prostheses, measurements of a person's muscles are collected using electrodes, either surgically implanted in the muscle or attached to the skin's surface. EMG is costly and highly invasive. However, it provides more accurate measurements of the muscles' electrical activity—not their length or speed.
According to Cameron Taylor, the lead author of the study, using control-based EMG allows researchers to see what the brain tells the muscle to do but not what the muscle is doing.
MM is inspired by the idea that prosthetic users could achieve precise control of a prosthetic limb if sensors could measure muscle activity.
MM is inspired by the idea that prosthetic users could precisely control their prosthetic limbs if sensors could measure muscle activity. Pairs of magnets are inserted into the muscles. Researchers then measure what the muscles are doing based on how a magnet moves relative to the other. This way, researchers can calculate how much the muscles are contracting as well as the speed of contraction.
Based on the contractions of the residual limb, these measures could be fed into a computer model to predict where a person's phantom limb is in space. With this strategy, prosthetic devices could move the way the user wants them to, matching their mental picture of their limb's position.
The team hopes to do a small study on humans with below-the-knee limb loss within the next few years. Their broader vision for MM is to use it outside the residual limb, like attaching it to the skin surface, affixing it to the outside of a prosthetic device, or placing it on clothing.
The researchers also said that MM could improve muscle control through functional electrical stimulation—a technique that helps restore mobility in people with spinal cord injuries.
Another possible use for MM is guiding robotic exoskeletons, which can be attached to a joint—like the ankle—to help people who have suffered a stroke or developed muscle weakness.The possibilities are endless for this new strategy. We genuinely hope that this will usher in a world where the quality of life is better for all.