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Motors & motion control


hour per day, because otherwise your stump is sore or you’re losing skin because of the friction.” For Ortiz Catalan, the best possible medical device is one that does what it’s designed to do without the user noticing. Aesthetics are important, but an emphasis on engineering artificial limbs to match the electric dreams of popular culture misses the point. Prosthetics offer a more profound version of the same design conundrum that all wearable and patient-operated devices must address – how to engineer a productive relationship between a device and its user. The difficulty with prosthetics is that, unlike a smartwatch, for instance, they are not designed to add completely new functionalities to the human body. Evolution didn’t directly produce anything digital, so smartwatches can’t suffer by comparison with its results, but a prosthetic arm succeeds or fails based on its ability to approximate the experience of having one made of bones, nerves, skin and muscles. “Our field is unique, in some ways, because we have a gold standard that everyone is aware of,” says Jacob Segil, director of the Center for Translational Research at the University of Colorado, Boulder. “The size, the weight, the strength, the durability [of the hand] – evolution has made those things optimal and we’re struggling to keep up. That’s exacerbated with our powered limbs, because we’re putting in motors, electronics and mechanical systems that need to be miniaturised to fit within the volume of a limb, but have the strength and speed of our intact bodies. It’s a pretty horrendous engineering problem.” And yet, for all of that, perhaps evolution’s most impressive achievement is the ease with which people identify with their intact limbs. For Segil and Ortiz Catalan, the decades of technological advances in myoelectric prosthetics will truly make a difference when they’re used to tap more directly into that human sense of embodiment. Ortiz Catalan speaks of “hijacking” and Segil of “hot-wiring” the nervous system with more comfortable and controllable devices that, crucially, extend users’ sense of touch.


A mind of its own


In 2020, Ortiz Catalan and his co-authors in the New England Journal of Medicine became the first group to prove the viability and reliability of an arm prosthesis that can be intuitively mind-controlled using implanted (rather than skin contact) electrodes, while also conveying sensations to the user in everyday life. These ‘neuromusculoskeletal’ prosthetics connect directly to the user’s nerves, muscles and skeleton – their body’s control system and mechanical frame, respectively – combining numerous medical device technologies to do so. The skeletal attachment uses titanium, silicone, polyurethane and other biocompatible materials for osseointegration, which,


Medical Device Developments / www.nsmedicaldevices.com


as Ortiz Catalan explains, makes a huge difference for the patient when it comes to comfort, but doesn’t help with control. That, along with the sensation of touch, comes courtesy of the titanium, platinum and iridium electrodes implanted in the muscles and nerves of the amputation stump, and an AI system responsible for translating those signals back and forth between the body and the device. Then there’s Segil’s specialism, the bionic hand itself.


Working with Dustin Tyler from Case Western Reserve University in Cleveland, Ohio – who talked Medical Device Developments through his use of nerve cuff electrodes to introduce the sensation of touch to prosthetics in 2016 – Segil develops multimodal tactile fingertip sensors that record contact, pressure and proximity. In 2020, he was also the lead author on a study in Scientific Reports that showed an amputee could correctly identify hand gestures from electrical signals transmitted by one of Tyler’s neural interfaces. Like Ortiz Catalan, Segil found that the accuracy of these perceptions improved over time. “Our nerves are like wires,” he explains, “and just like we have our digital systems in our computers and phones, we can interface to that electrical system.” Whereas digital systems communicate in streams of zeroes and ones, or bits, the nervous system uses waveforms or ‘spike trains’ made up of action potentials. By zapping nerves that used to or would otherwise connect to the hand, both Segil and Ortiz Catalan’s prosthetics prompt action potentials and spike trains that the brain recognises as originating in that hand.


“This is the fun part,” says Segil. “The brain is this amazing pattern recognition machine, and if it receives a pattern that’s close enough to what it’s expecting, it can do the rest. We don’t think we’re creating the exact same train of action potentials, but we’re getting close enough that the brain actually deciphers it as touch. And that’s going to be our crutch, almost. That’s why we think we’re getting further – because the brain is just really smart.”


Segil adds that last part with a laugh. Truly, human intelligence helps with scientific advances, but in this case, the brain is a particularly willing collaborator. To put it crudely, it often seems the nervous system wants to feel the presence of absent limbs. Not only does it expect patterns of action potentials to emerge from an amputated hand, but it sometimes perceives them all by itself. Tellingly, people who experience these ‘phantom’ sensations find it easier to control their artificial limbs – and the touch-restoring prosthetics both Segil and Ortiz Catalan are working on have been shown to greatly diminish phantom pain. Immense technical complexities notwithstanding, the job of prosthetics engineers might be easier than it first appears: they don’t have to trick the brain with


44%


Proportion of upper-limb amputees


found to have abandoned their powered prosthetics in a recent study.


Disability and Rehabilitation 71


Opposite page: Anchored to the skeleton and linked to implanted electrodes, neuromusculoskeletal prosthetics enable users to feel objects and tell how hard they are touching them, which is essential for imitating a biological hand.


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