"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 100m," said Wen Yang, a UC San Diego alumna. "After I touched it, I realised 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 armour materials. The hexagon-shaped scales are called scutes that are connected by sutures, similar to the connections in a baby's skull, which grow and fuse together as the baby grows.
Each hexagonal scute has a raised, star-like structure in the centre that distributes stress across the surface. Under the scutes, the team found a flexible yet hard to penetrate inner layer formed by a complex structure of interlocking collagen fibres. Together, the outer and inner layers of the boxfish armour provide the fish with protection unique 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 fibres will help to prevent the structure from failing," said Yang.
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.
"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," Steven Naleway a materials science and engineering Ph.D. student said. "To the best of our knowledge, there is no compliant phase in the interface of the boxfish's sutures."
The engineers used scanning electron microscopy to characterise the surface structure of the scutes. They also took cross sections and used micro-computer tomography to characterise 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.
"We are currently investigating what mechanical advantage scutes and sutures might provide (over overlapping scales)," Naleway commented. "We know that the boxfish has survived for 35million years with this armour, so the design has proved very successful in nature."
Marc Meyers, Distinguished Professor of Materials Science at UC San Diego, added: "These damage-resisting structures are being studied with support of the U.S. Air Force to hopefully guide us to bioinspired designs that will offer more protection against impact than our conventional ones."
