Boxfish armor inspires researchers

The boxfish gets its name from its boxy shape.

The boxfish’s unique armor draws its strength from hexagon-shaped scales and the connections between them, engineers at the University of California, San Diego, have found.

“The boxfish is small and yet it survives in the ocean where it is surrounded by bigger, aggressive fish, at a depth of 50 to 100 meters,” said Wen Yang, a UC San Diego alumna now working at Swiss Federal Institute of Technology in Zurich in Switzerland and the paper’s first author. “After I touched it, I realized why it can survive - it is so strong but at the same time so flexible.”

The boxfish’s hard frame and flexible body make it an ideal animal to study for inspiration for armor materials. The hexagon-shaped scales are called scutes. They are connected by sutures, similar to the connections in a baby’s skull, which grow and fuse together as the baby grows.

Most fish have overlapping scales, said Steven Naleway a materials science and engineering Ph.D. student and co-author on the paper. “That means that there are no weak points, should a bite from a predator land exactly in between scales,” he said. “We are currently investigating what mechanical advantage scutes and sutures might provide. We know that the boxfish has survived for 35 million years with this armor, so the design has proved very successful in nature.”

Each hexagonal scale, or scute, has a raised, star-like structure in the center that distributes stress across the entire surface. Under the scutes, the team found an inner layer that forms a complex structure in which collagen fibers interlock. This structure creates a flexible inner layer in the armor, which is difficult to penetrate due to the interlocking collagen fibers. Together, the outer and inner layers of the boxfish armor provide the fish with protection unique in the natural world.

The boxfish's carapace (or shell) is composed of several hexagonal scutes that provide body support and armored protection (center). These scutes are connected by tooth-like joints called sutures, which provide some level of combined strength and flexibility (right). Credit: Michael M. Porter/Clemson University.

The team also tested the scutes’ ability to withstand tension by pulling them apart both horizontally and vertically, as well as their ability to withstand penetration. “We were able to demonstrate that even if a predator manages to generate a crack in the outer layer, the collagen fibers will help to prevent the structure from failing,” said Yang. Her current research focuses on the characterization of bio-inspired materials.  

Meanwhile, the connections between the scutes, called sutures, make the armor even stronger. Upon impact, the sutures’ zigzag patterns essentially lock in and keep the scutes from breaking apart. These sutures are different from many of those found elsewhere in nature, Naleway said.

Read more about the boxfish's sutures and their applications here.