The prostheses of tomorrow won’t just replace a limb. They’ll move and act like one too.
But not without the innovation of Team Alpha, a team of George Fox University students, competing in this year’s Invent Oregon Collegiate Challenge, that has designed a special sensor to detect human movement with liquid metal.
It’s like something out of science fiction, and Team Alpha lacks no imagination.
“How our arm works, in general, is that your body sends electricity through your neurons to control your muscles,” said Andy Wang, a rising senior in computer science. "The sensor we're creating picks up the electricity from your muscles, calculates that signal and sends it to a prosthetic arm to tell it what to do.”
Artificial limbs used today are designed to fit the individual wearer, and they serve as an extension of the body. But new, advanced prostheses will also be able replace the function of a lost limb by using patterns of muscle movement that still persist even when a limb is gone.
Part of the phantom limb effect, prostheses just need to be able to measure how those muscles contract. But the sensors of today are expensive, rigid plastic devices that lack comfort and wearability.
Team Alpha will change all that.
In a medical technique called electromyography, the signals sent between the nervous system and muscles can be detected on the skin with sensors that look like suction cups—even when a part of the body is no longer there. These electromyographical sensors, or surface electrodes, use these signals to reproduce movements in a prosthesis with varying levels of accuracy.
“You put one electrode here and then another other electrode pretty close to it,” said Issac Edminster, placing an index and middle finger on his forearm. Edminster is a computer science and biochemistry major. Team Alpha also includes biomedical engineering student Julio Lopez-Hernandez.
“It takes the voltage difference between the two locations, because when you're flexing a muscle, then you can actually pick up those minute signals between the two nodes."
The most simple prostheses that Team Alpha interacts with are like light switches: they use binary, opposing muscle movements, like a wrist that is either flexed or extended, to reproduce useful movements like gripping.
“Let's you say you have an imaginary arm,” Wang said, extending his wrist to demonstrate how the muscles of the forearm create the movement. "You do this and you tell the prosthetic arm to open your hand. And you do this—” He flexed his wrist in the opposite direction, pointing to the muscles of the forearm on the opposite side. “You tell your prosthetic hand to close the grip."
A normally functioning forearm is far more complex, capable of many more movements. Team Alpha’s technology can accurately reproduce some of these by increasing the number of sensors used to measure nerve signals, kind of like the difference in detail between 2-bit, 8-bit, and 16-bit graphics in computers, and they are right at the horizon of what is possible with a surface electrode.
Greater accuracy requires more invasive technology that detects signals by connecting directly to muscle tissue. To Edminster, surface electrodes are more ideal. “It just sits on top of the muscle, which is obviously more comfortable, more practical for everyday life."
This is possible because Team Alpha is made out of metal that remains in a liquid state at room temperature—a technology that attracted Edminster to begin with. “When we all first met our professor,” Edminster said, referring to the team’s faculty advisor, “I said, ‘Listen, I want to do something with liquid metal.’"
It does have an otherworldly ring to it. But liquid metal isn’t just something out of a science fiction movie. The active material in Team Alpha’s invention is an alloy called galinstan, composed of gallium, indium and tin.
"Gallium has a really cool property,” Edminster said, calling to mind mercury thermometers. "In that same way, gallium has a really low melting point. When mixed with a couple other metals, you can make what's called a eutectic, which creates a metal that is actually liquid at room temperature. It's still highly conductive, but it flows completely freely."
Galinstan typically melts at 52 degrees, well below the body’s normal internal temperature and, for the most part, below skin surface temperature in most environments.
“It makes the sensor very, very flexible,” Wang said. “That means you are extremely comfortable." Sensors in protheses today are typically rigid, and that makes them harder to wear.
But prostheses are also just the beginning of Team Alpha’s applications. Let your imagination run wild for a moment and think of a sensor that essentially digitizes movement.
The technology could be used for so much more, and that isn’t lost on Team Alpha. Like helping amputees remember old movement patterns to gain new functionality. Like a full-body exoskeleton that calls science fiction to mind. Like creating virtual or augmented-reality representations of people to move or manipulate things in a computer-generated world.
“Imagine being able to individually control fingers or hands with just like, you know, your Oculus,” Edminster said. “We're doing a really cool project and dealing with super cool anatomical, physiological, medical—I don't know where to start.”
These technologies are down the road, and Team Alpha is excited to be a part of the innovation. The excitement is palpable, and Edminster and Wang have had access to technology and people they never thought possible.
"When I walked in last summer on an engineering team, I did not expect this was actually going to happen a year later,” Wang said. “Knowing that that's a project you're working on, you can create something like that. It just gets me so excited and hyped up for this project.”
And importantly, they are using their knowledge to improve the lives of others.
“An adaptable sensor not costing $20,000. A direct way of helping people, of having these people being able to have more movement, more flexibility in their prosthetic limbs without having to mortgage their house or sell it,” Edminster said.
“It's the idea of being able to really make an applicable difference in someone's life. That's what most excites me.”