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.

They describe their findings and the carapace of the boxfish (Lactoria cornuta) in the journal Acta Materialia. Engineers also describe how the structure of the boxfish could serve as an inspiration for body armor, robots and even flexible electronics.

"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," says Wen Yang, with the Swiss Federal Institute of Technology in Zurich. "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.

Proposed mechanical advantage of sutured interface: (a) Straight interface with penetration by sharp tooth separating neighboring scutes; (b) Sutured interface with force F having tangential and normal components; (c) Tangential component of force, Fy, pushing opposite sides of teeth against each other in a locking-in mechanism. Credit: Jacobs School of Engineering/UC San DiegoMost fish have overlapping scales, says 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." The research team is 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 to be very successful in nature," Naleway says.

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 unique protection in the natural world.

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," says Yang. Meanwhile, 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 says.

The most common form of suture structures in nature are those that have a roughly triangular shape and consist of two important components: rigid suture teeth and a compliant interface, he says. "To the best of our knowledge, there is no compliant phase in the interface of the boxfish's sutures.” In addition, the teeth themselves have a much lower aspect ratio—meaning that they are shorter and wider—than most other examples.

Researchers used scanning electron microscopy to characterize the surface structure of the scutes. They also took cross sections and used micro-computer tomography to characterize the dense regions. The results of mechanical testing left the researchers wanting to know why the boxfish would choose a design that excluded overlapping scales.

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