Nature has engineered ceramic shells of molluscs like clams, oysters, and snails to withstand high pressures at the ocean bottoms, shows a recent study1. The findings could help engineers build stronger materials for use in transport or defence.
Researchers (clockwise from left) K. Chattopadhyay, C. S. Tiwary, P. M. Ajayan, S. Kishore and S. Sarkar. Credit: Tiwary, C. S. et al
Cloaking themselves with shells made of calcium carbonate, nacre — also known as mother-of-pearl — and other organic and inorganic substances, molluscs protect their soft innards from predators or high pressures at ocean bottoms. Scientists previously thought that specific microstructuring — the means by which the chemicals are layered — must be keeping them sturdy against all odds. So, “Shapes of the seashells were left unexplored”, says Chandra Sekhar Tiwary, lead author of the paper.
For this study, the team from from the Indian Institute of Science, Bangalore, in collaboration with the Ajayan Pulickel lab at Rice University, Texas, USA, modelled and printed two types of shells using a 3D-printer. They looked at a fan-shaped shell, like that of clams; and a cork-screw-like shell of auger snails. By subjecting both the natural and printed shells to high pressure tests, and observing their cracks, the team inferred that it is Nature’s selection of shell shapes, rather than just their chemical composition that holds the clue for the sturdiness.
Some of the varieties of shells studied. Credit: Tiwary, C. S. et al
The researchers found that fan-shaped shells hold stress at the hinge region that closes a shell, which is relieved as cracks near the edge. The ribbed shells outperformed the rib-less shells, whose crack patterns were slightly different. For cork-screw shaped shells, they found the strengths and wear patterns to be totally different.
While the animal hides within the narrow region of a screw-like shell, it relieves the load as cracks at the wider top area of the shell, thus protecting the animal inside. The interesting finding is that they found the same crack patterns in both the natural and printed shells under pressure test, explains Tiwary. “[Our study] shows how such shapes play an important role in load transfer in the structure."
The team is now studying other shapes of mollusc shells. Tiwary is optimistic about the use of such shells in building protective structures and similar applications.
Marc Meyers at the University of California, San Diego, says the study has looked only into macrostructures to explain the hypothesis. Future studies should go into the finer micro- or nano-structure levels. Vincent Sherman, Meyer's graduate student, thinks the study’s strength lies in the choice of the methodology to show the stress-handling regions of the shells.
