Bioinspired Materials Can Take a Punch

Some of the most innovative and useful inventions have been inspired by nature. Take the Shinkansen bullet train in Japan, whose aerodynamic design is modeled after the kingfisher bird. Or Velcro, which a Swiss engineer invented after observing that the burrs that stick to a dog’s fur have tiny hooks in them.

Now, scientists have turned to a small underwater predator for inspiration. The mantis shrimp is a colorful invertebrate that packs a powerful punch. It can crack clamshells with the force of a .22 caliber bullet, thanks to unique structures that make its exoskeleton surprisingly strong.

Researchers at the National Institute of Standards and Technology (NIST) have made synthetic versions of these structures and tested their impact performance by blasting microprojectiles at them. They discovered that adjusting specific parameters of the structures changed how they absorbed and dissipated the impact energy.

“The results and insights of this research mark an important advance in bioinspired materials design with applications for aerospace, such as helping spacecraft survive the impact of micrometeoroids and protecting orbiting satellites that collide with debris,” said NIST materials research engineer Edwin Chan.

Other potential applications include better bullet-resistant glass, blast-resistant building materials, and more protective helmets.

Chan and his colleagues published their findings in the Proceedings of the National Academy of Sciences.

This research idea came from Sujin Lee, who came to NIST as a National Research Council (NRC) postdoctoral fellow. Lee wanted to understand why the mantis shrimp’s appendage didn’t break as it smashed the shells of other creatures. Chan was also intrigued by this concept, and the two developed a research project to find out.

“When a person punches someone, their hand hurts, but with a mantis shrimp, it doesn’t,” said Chan. Or it doesn’t seem to, anyway. Lee and Chan already knew that this was related to microscopic “Bouligand structures” in the shrimp’s exoskeleton.

“Bouligand structures are a universal material platform for impact resistance in nature, and we wanted to learn more about them, so we produced and tested them in the lab,” said Chan.

Lee and Chan synthesized the structures from cellulose nanocrystals, which are found in plant fibers. The nanocrystals self-assembled into plates, which layered on top of each other like rotating stacks of plywood.

Those stacks formed their synthetic Bouligand structures. Researchers then modified the crystals using high-frequency sound waves before assembling them into thin films that served as their test material.

Next, they tested the impact resistance of the thin films by firing microprojectiles at them at speeds of up to 600 meters per second. The microprojectiles, made of silica, were propelled toward their target by a high-intensity laser. The researchers recorded images of the microprojectiles impacting the thin films with an ultrafast camera.

Microprojectile Hitting CNC Film

Based on those images, the researchers observed that a microprojectile can leave a permanent indentation while also bouncing back like a tennis ball hitting the ground. The degree of indentation and the amount of bounce-back depended on how the energy dissipated or spread out in shockwaves after the microprojectile’s impact.

The researchers discovered that they could adjust how the energy dissipated by fine-tuning various factors that affected the sample's mechanical properties, such as making the nanocrystals thicker or changing their density. They found that the microprojectiles left permanent indentations in the thinner films, but the thicker films excelled at redirecting the shockwaves from the impact.

NIST worked on this project as part of its mission to develop advanced measurement methods that can be useful to U.S. industry. Researchers can use the measurement methods developed for this project to further develop impact-resistant materials based on Bouligand structures as well as other types of advanced materials with special properties.