How a Single Finger Snap Became a Scientific Game-Changer

February 24, 2022 – Raghav Acharya, a student at the Georgia Institute of Technology in Atlanta, watched Avengers: Infinity War a few years ago when he had a thought: how was the supervillain Thanos able to snap his fingers while putting on a metal glove? Acharya, a chemical engineering student, posed this question to Associate Professor Saad Bhamla, Ph.D. As he and Bhamla sank deeper into their edgy supervillain, more questions arose: could a metal glove dampen the vibrations? Could Thanos have created more power with metal fingers? And in general, what is important for a snap of the fingers to occur?

Elio Challita, a doctoral student in bioengineering, then joined them and tested their questions. Using high-speed cameras, the researchers recorded three people snapping fingers in five scenarios. What they found surprised them.

“The snap of a finger is one of the fastest angular movements we have seen so far in the human body,” says Acharya. In fact, it’s about 20 times faster than the blink of an eye.

I wonder what we care about the speed of the click of a finger? Our survival certainly does not depend on it. But these researchers have found that friction and compressibility, the ability of something to flatten or shrink under pressure—essential components of a successful finger snap—could potentially play an important role in the development of prostheses and microrobots in medicine and other fields.

“Using fingers is like Mount Everest in prosthetics,” says John Long, Ph.D., program director at the National Science Foundation, which funds the Bhamla lab.

According to Long, the researchers were the first to simulate a snap of their fingers. Compression and friction between the thumb and middle fingers allow energy to be stored through the movement of tendons and muscles. The fingers then act as latches and instantly release (or unlock) the stored energy.

To get the loudest click flick of a finger, the fingertips need an even amount of rubbing and squeezing the skin. The researchers proved this by changing their variables: coating the fingers with grease (eliminating friction), replacing the fingers with a metal thimble (eliminating compressibility), and coating the fingers with rubber (increasing friction). In each case, there was not enough energy to mimic what only human skin could achieve.

Today’s prosthetics focus on functionality and aesthetics using rigid materials such as metal or plastic. Friction as a variable is usually excluded from biomechanical designs because it can lead to material wear. But at the snap of our fingers, we know how friction and compression contribute to motion. If a prosthetic arm (or microrobot) can click, it is indicative of an advanced level of dexterity, suggesting that it can perform other complex tasks with similar accuracy. Prostheses designed with a more flexible material such as silicone and robust modeling of similar finger-click dynamics could mean that the overall performance of prostheses will be more similar to human skin.

By understanding the key functions of fast finger snaps, those same principles can better inform us about how we build other systems. Long believes the results of this research could, for example, help develop micromanipulators that would allow surgeons to spring-load a motor, quickly releasing more power in a tight space. Bhamla suggests that finger snapping can be used as a diagnostic tool to detect the early onset of certain diseases that lead to muscle weakness, such as arthritis.

After the finger snap experiment, the trio of researchers teamed up with Mark Ilton, Ph.D., assistant professor of physics at Harvey Mudd College in Claremont, California, who helped them develop a mathematical model that provides other scientific fields with fundamental physics their experiment. By simplifying their work, people like roboticists and engineers can understand the key ways to achieve ultra-fast acceleration and use this equation in their work.

The researchers not only demonstrated the fastest human movement but also touched many areas of science, from biology to physics and engineering, by providing a lens in complicated operation for what is often considered a simple, everyday movement.

“Now we have put a finger click on the map,” says Bhamla.

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