For people with 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 developed an alternative approach that 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. The strategy, called magnetomicrometry (MM), can provide fast and accurate muscle 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. In either case, electromyography (EMG) offers information only about muscles’ electrical activity, not their length or speed.
The new 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 inserted pairs of magnets into muscles. By measuring how the magnets move relative to one another, they can calculate how much the muscles are contracting and the speed of contraction.
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. Through mathematical modeling of the entire limb, the researchers can compute target positions and speeds of the prosthetic joints to be controlled and then a simple robotic controller can control those joints.
The researchers 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. 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.