The egg case of the swell shark seems like the stuff of fairytales. Often referred to as “mermaid’s purses,” they’re paradoxically tough and leathery to protect the shark embryo, yet permeable enough to allow for the gaseous and metabolic waste exchange necessary for survival. To understand how the biomaterial’s nanoarchitecture contributes to its toughness, researchers from UC Santa Barbara, Duke University, and the Advanced Light Source (ALS) used a combination of in situ x-ray scattering and electron microscopy.
Electron microscopy showed that the hierarchical structure of the egg case comprises nanoribbons with a nanolatticed architecture, which are unidirectionally aligned to form layers. The layers comprising the nanoribbons are then twisted into a spiral known as a Bouligand-like arrangement.
Using in situ small-angle x-ray scattering (SAXS) at ALS Beamline 7.3.3, the researchers were able to observe changes in the nanoarchitecture of the egg cases as they were deformed in real time and in the egg cases’ native hydrated condition. Corroborating their SAXS measurements with transmission electron microscopy allowed the team to determine the deformation mechanisms that contribute to the material’s toughness.
The SAXS diffraction patterns revealed an extreme degree of long-range order in the nanoarchitecture within the egg case wall. “The long-range ordering—the likes of which have been a challenge for fabrication of macroscopic nanoarchitectured materials—represents a tremendous achievement by the sharks,” said Rubayn Goh, the study’s first author who is now a scientist at the Institute of Materials Research and Engineering in Singapore.
The researchers also observed the formation of shear bands. “It’s reminiscent of toughening strategies seen in metallic glass composites,” Goh said. “At the nanoscale, this efficiently distributes strain and prevents the formation of critical strain localization, which leads to greater deformation before failure.”
Shark egg cases evolved to withstand relentless ocean conditions, and insights into their nanoarchitecture could advance the development of synthetic materials. “In looking to nature for design inspirations,” said Herb Waite, professor of biochemistry at UC Santa Barbara and the study’s co-corresponding author, “we sought to reduce our learning curve in developing high-performance multifunctional materials.” The researchers envision a future with improved materials for purifying air and water, medical dialysis, drug delivery, and many other applications—all without needing to pilfer a mermaid’s purse.
R. Goh, S.P.O. Danielsen, E. Schaible, R.M. McMeeking, and J.H. Waite, “Nanolatticed Architecture Mitigates Damage in Shark Egg Cases,” Nano Lett. 21, 19 (2021), doi: 10.1021/acs.nanolett.1c02439