At your fingertips

Advanced prosthetics have a people problem. Cyborgian engineering wonders, they’re great conversation starters and inspiring fodder for online videos, but, for a significant portion of their intended market, they don’t have much else to recommend them. There’s been a lot of technological progress in the 75 years since someone was first strapped into one (it was non-portable), but prostheses controlled by the electrical activity of users’ muscles still aren’t widespread. It’s not just that they’re expensive: a recent Disability and Rehabilitation study found that, once the party’s over and the cameras are gone, 44% of advanced upper-limb prosthetics are rejected.

The most commonly cited issues with myoelectric prosthetics are fragility, weight, fit and control. Users of the current generation of ‘dual-site’ myoelectric upper-arm prosthetics have to flex their muscles to cycle through preprogrammed grips until they find the one they want, as if daily tasks were carnival games. The mechanical attachment is another sticking point – shoving one’s arm into a socket for hours on end can be extremely uncomfortable. Often, it simply isn’t worth the hassle: in the American Journal of Physical Medicine & Rehabilitation in 2007, 60% of individuals who frequently used an upper-limb prosthetic reported that they are just as or more functional without it, as did 98% of those who had rejected one. In these cases, an uncomfortable or unreliable prosthetic turns a neutral part of the user’s body into a problem or vulnerability that would not otherwise exist. As Britt H Young, an owner, if not a regular user, of an Ottobock Bebionic, put it in the online publication Input in March 2021, “Prosthetic arm technology is still so limited that I become more disabled when I wear one.”

“Prosthetics are meant to restore function,” stresses Max Ortiz Catalan, head of the bionics research unit at Chalmers University of Technology in Sweden, and the founding director of the Center for Bionics and Pain Research. In that regard, they’re underperforming. “It’s quite useless to have a very sophisticated hand or a very sophisticated leg if you can only wear it for one 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, 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 anything perfect, but give it something functional with which to work.

“What I like to argue,” says Segil, “is that the other half of the equation in terms of why rejection rates are so high is the fact that the limb is external to the person, it’s not a part of them. If we can elicit the sense of embodiment – if we can allow the person to understand the limb as oneself, then they’ll be more forgiving and more accepting of the limb.”

Take control

With the recent development of closed-loop prosthetic control and sensory feedback, talk of ‘embodiment’ has become inescapable. The same can’t be said for specific definitions of the term. One of Ortiz Catalan’s most recent projects has been the development of a practical understanding of “prosthetic embodiment” for use across the field. Although it has previously been used interchangeably with ‘ownership’, which is a person’s sense that the parts that make up their body belong to them, Ortiz Catalan and his colleagues stress that any model of embodiment also has to include ‘agency’ – a person’s sense that they initiate and control their body’s movements.

“If I want to reach down to grab something, my hand will do it right away, so I know I’m in control of it,” he explains. “That’s agency. And when I touch something, I have sensations that I don’t even need to think about. That’s called ownership. If your prosthesis is responsive and it provides a sensation, then you feel it’s a part of your body.”

The dual senses of ownership and agency need to be continually reinforced to maintain any sense of embodiment. Clearly, the absence of sensing technology in the current generation of widely available prosthetics limits the potential for them to instil a sense of ownership, as do their uncomfortable sockets, which draw attention to the difference between body and prosthetic. On top of that, their propensities for breaking down and moving of their own accord also regularly short the circuit of agency. It’s not hard to see why users who feel they have other options choose to forego the expense and discomfort. By contrast, Segil’s favourite thing about working on sense-restoring prosthetics is how closely their users seem to bond with them. “They refer to this device – which is plastic and metal – as themselves,” he says. “They talk about ‘my hand’ or they say ‘that felt like you touching me’. And those comments are, for me, somehow more impactful than the everyday functionality of the limb. It shows that there’s broader implications if we can restore sensation, and hopefully, eventually, embodiment of the device, for the person as a whole.”

More generally, he’s learning how intimately psychology and engineering can be enmeshed. “What these feelings of ownership and agency are built upon is actually not far away from a lot of engineering principles,” he notes. “Ownership has to do with multisensory integration, which is like sensor fusion. You’re just combining multiple sensations together, and when they happen in synchrony, we make a decision based upon it. What we’re noticing is that we can apply some engineering principles to study these psychological ideas, because we have this portal into the body – because we have this neural interface and other experimental methods – to probe them.”

As tantalising as that might be for psychologists and medical device designers, Ortiz Catalan warns that it’s a secondary concern. The problem with prosthetics has as little to do with the fact engineers haven’t read enough phenomenology as the fact not all artificial limbs have Iron Man paint jobs or 5G capabilities. The problem is they don’t work for patients.

“If you focus on function, and that function is intuitive, then it’s rather logical that the patient will adopt the technology and feel it as part of their arm,” Ortiz Catalan says. “My concern is that there are a lot of engineering projects that are not considering the patient from the viewpoint of what their needs are. If you focus on the needs, and you satisfy those, everything else will fall into place.” There’s a mantra for every medical device manufacturer, no matter their product.