Feb. 24, 2022 -- Raghav Acharya, a student at the Georgia Institute of Technology in Atlanta, was watching Avengers: Infinity War a few years back when he had a thought: How was supervillain Thanos able to snap his fingers while wearing a metal gauntlet?
Acharya, an undergraduate studying chemical engineering, took that question to assistant professor Saad Bhamla, PhD. As he and Bhamla dived deeper into their snapping supervillain, more questions surfaced: Would the metal gauntlet have dampened the vibrations? Could Thanos have built more force with metallic fingers? And what’s important for a finger snap to occur, anyway?
Then, joined by Elio Challita, a doctoral student in bioengineering, they put their questions to the test. Using high-speed cameras, the researchers recorded three people snapping their fingers in five scenarios. What they found surprised them.
“The finger snap is one of the fastest angular motions we’ve observed in the human body so far,” says Acharya. In fact, it’s about 20 times faster than the blink of an eye.
Interesting, but why should we care about the speed of a finger snap? Our survival doesn’t exactly depend on it. But these researchers found that friction and compressibility, the capacity of something to be flattened or reduced by pressure -- essential parts of a successful finger snap -- could potentially play an important role in the design of prosthetics and microrobots in medicine and other fields.
“Using fingers is like the Mount Everest of prosthetics,” says John Long, PhD, a program director at the National Science Foundation, which funds Bhamla’s lab.
The researchers were the first to model the finger snap, Long says. The compression of, and friction between, the thumb and middle finger allows energy to build up from the movement of tendons and muscles. The fingers then act as a latch and instantly release (or unlatch) the stored energy.
To achieve the loudest click of a finger snap, the fingertips need just the right amount of skin friction and compression. The researchers proved this by changing their variables: coating the fingers with a lubricant (removing friction), replacing the fingers with a metallic thimble (removing compressibility), and covering the fingers with rubber (adding too much friction). In every case, not enough energy was stored to mimic what the human skin alone could achieve.
Today’s prosthetics focus on function and aesthetics, using rigid material like metal or plastic. As a variable, friction is typically excluded from biomechanical designs, as it can lead to the wear and tear of materials. But based on a finger snap, we know how much friction and compression contribute to movement. If a prosthetic hand (or microrobot) can snap, it shows an advanced level of dexterity, suggesting that it might achieve other complex tasks with equal precision. Prosthetics designed with more agile material, such as silicone, and robust modeling of similar dynamics of a finger snap, could mean that the overall performance of prosthetics could more closely resemble that of the human skin.
By understanding the key functions of fast finger snaps, these same principles can better inform the way we build other systems. Long believes that learnings from this research could, for example, help develop micro manipulators that allow surgeons to spring-load a motor, quickly releasing a lot of power within a confined space. Bhamla speculates that finger snapping could be used as a diagnostic tool to identify the early start of certain muscle weakening diseases, like arthritis.
After the finger-snap experiment, the trio of researchers teamed up with Mark Ilton, PhD, an assistant professor of physics at Harvey Mudd College in Claremont, CA, who helped them devise a mathematical model that provides other scientific fields with the essential physics of their experiment. By simplifying their work, people like roboticists and engineers can understand the key ways of achieving ultrafast acceleration and use the equation to build upon their own work.
The researchers have not only showcased the fastest human-powered motion, they’ve also touched many realms of science, from biology to physics and engineering, by providing a lens into the complex operation behind what’s often considered a simple, everyday motion.
“We have now put the finger snap on the map,” says Bhamla.
Have some of that, Thanos.