Using Springs to Bypass Traditional Speed Limits

Carnivorous trap-jaw ants clamp down on prey in a split second, with jaw speeds approaching 145 mph. Like a bullet from a gun, a chameleon’s tongue shoots out with amazing accelerations to capture flies in midair. Animals like these are fascinating studies of physics and biology. How do these little guys pack so much speed compared to the rest of us?

Chameleons Tongue
Animated gif of a chameleon tongue, photographed at the Peyrieras Reptile Reserve, Madagascar, July 2014. Image Credit: By SurreyJohn (CC BY-SA 4.0)

Trap-jaw ants and chameleons are just two of the many organisms—animals, plants, and fungi alike—that accomplish extraordinary feats of speed. To reveal the secrets behind this super-fast motion, a team of scientists led by Sheila Patek at Duke University studied more than 100 examples of these biological systems and modeled how they move.

“Like using a bow to launch an arrow, some animals use springs in their bodies to drive amazingly fast movements,” says Mark Ilton, a soft matter physicist from the University of Massachusetts Amherst who worked on this new research.

A spring is any elastic object that stores energy, not just a bouncy steel coil. In archery, the bow is a spring, storing energy generated by your arm muscles as you draw an arrow back. In biological organisms, elastic materials are springs, storing energy generated by muscles or other avenues within the body. The tongue of a chameleon, tendons of a frog, and exoskeleton of a mantis shrimp are all springs—and understanding how they’re used in nature could help us tune the abilities of tiny robots and other synthetic systems.

By storing energy in a bow and releasing it quickly, you can shoot an arrow much faster and with more force than you can throw it. Similarly, by storing energy in elastic biological tissue and releasing it quickly, animals can flick, jump, and strike at speeds and forces that surpass traditional limits.

Although both feats involve springs, the remarkable snap of a Venus fly trap and jump of a locust utilize different biological structures and materials. To create a general foundation for understanding such different systems, the researchers boiled the systems down to their essential features:

• Motors, which put energy into a system
• Springs, which store that energy, and
• Latches, which can suddenly release stored energy

Using simple equations to represent springs, latches, and motors, the team mathematically explored the different ways in which these components interact in nature and in manmade systems. They analyzed the limits and trade-offs of different configurations, asking questions such as:

• When is a projectile best launched by a motor versus a spring?
• What are the implications of using a spring to amplify the power of a system?
• How do latches impact energy release?
• What are the general principles behind using these components to improve performance?

Several findings emerged as the researchers analyzed this model. For example, spring-based systems can launch lightweight projectiles at faster speeds than motor-only systems, but motor-only systems do better with heavy projectiles. In spring-based systems, the output is dependent on the geometry and stiffness of the elastic material. The shape of a latch influences how quickly the energy it holds back is released. For greatest power amplification, the properties of the spring should be tuned to the motor, and the motor’s properties should be tuned to produce the highest force possible.

From a broader perspective, this model provides a framework for describing how springs, motors, and latches interact within a system—whether it’s inside of a trap jaw ant, a Venus fly trap, or a small robot. In fact, robot design is one of the motivating factors for this research, says Ilton. Scientists and engineers are developing small “microrobots” for a variety of applications, ranging from scoping out your digestive tract to searching through storm wreckage for survivors. These robots are about the same size as many animals that display rapid motion, so scientists are engineers are looking to those animals for design inspiration. Nothing says “inspirational” like an ant’s crushing bite!

—Kendra Redmond

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