Researchers Seek to Enhance Integration of Prosthetics

by James Cavuoto, editor

August 2021 issue

One of the most promising segments of the neuroprosthetics market is the neural integration of prosthetic limbs. First-generation neuroprosthetics from companies like Ossur and Ottobock relied on EMG signals to control artificial limbs. Recently, two different research teams demonstrated alternative means of integrating neuroprosthetic limbs, one relying on magnetic signaling and the other using peripheral nerve stimulation.

Stanisa Raspopovic and colleagues at ETH Zurich are developing neurorobotic lower limb prostheses that can communicate critical sensory information back to the wearer. By providing more realistic physical feedback and helping to alleviate phantom limb pain, Raspopovic’s system led test users to report a greater sense of “embodiment,” or the feeling that a prosthetic is a true extension of the wearer’s body.

The ETH team designed a “sensing leg,” imbued with sensors that can detect pressure and movement. This information is communicated to the wearer through a PNS interface, and the researchers used computational modeling to determine the optimal number of connections to implant in a targeted nerve. The sensors enabled the test users to avoid a number of obstacles when wearing special glasses to obscure their lower field of vision, and substantially enhanced their abilities to navigate stairs and sandy terrain, all while reducing their metabolic energy use. The PNS interface also includes a “neuro-pacemaker” mode that therapeutically stimulates the wearer’s remnant nerves without connecting to the prosthesis. The precise targeting of neural pathways, informed by computational modeling, enabled the implant to reproduce more naturalistic nerve inputs, leading test wearers to report a notable reduction in phantom limb pain.

Meanwhile, researchers at MIT’s Media Lab have developed an alternative approach to EMG control that could offer 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 in Science Robotics, the researchers tested their new strategy, called magnetomicrometry, and showed that it can provide fast and accurate muscle measurements in animals. They hope to test the approach in amputees 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,” said Hugh Herr, head of the biomechatronics group in the Media Lab, and senior author.

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, EMG offers information only about muscles’ electrical activity, not their length or speed.

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, they calculate how much the muscles are contracting and the speed of contraction.

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 mm in diameter and were inserted at least 3 cm 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, 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.

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 functional electrical stimulation. 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.

In addition to these approaches, a Cleveland Clinic research team demonstrated progress with more traditional means of interfacing with prosthetics. They engineered a bionic arm that allows wearers to think, behave, and function like a person without an amputation.

The system combines three functions—intuitive motor control, touch, and grip kinesthesia. “We modified a standard-of-care prosthetic with this complex bionic system which enables wearers to move their prosthetic arm more intuitively and feel sensations of touch and movement at the same time,” said lead investigator Paul Marasco.