We humans love symmetry. It’s at the root of ‘beauty’. If a structure is not symmetrical then something may be wrong. In biology symmetry appears strongly at the cellular level: witness the delight of early microscopists, fascinated by the polycrystalline shapes of diatoms. But when single cells and their structures start to join forces, symmetry and pattern degrade. Is this nature’s carelessness, lack of skill? Or is it design by natural selection? Instead of high performance materials, nature has evolved high-performance structures where symmetry is more easily adjusted and controlled. In the long run, this is cheaper.
Mother-of-pearl (nacre) is one of the simplest natural materials. It’s made of crystalline platelets of aragonite (calcium carbonate) in a protein matrix. That’s 95% chalk, 5% glue – how cheap can you get? It’s stiff (Young’s modulus up to 100 GPa) and very resistant to cracking. Dry nacre conforms with composite theory for platelets in an epoxy matrix. When wet it’s still very stiff (70-80 GPa), but also ductile, especially in tension along the platelet layers. Stretched in transmitted light its appearance changes from translucent to milky as voids open up internally. The platelets separate laterally, lubricated by the wet matrix, but the nacre sheet doesn’t break until it reaches strains of 0.015 or more, very high for a ceramic. The platelets have randomly uneven upper and lower surfaces that jam as they slide against each other and lock, forcing other parts of the sheet to take up the deformation.
Eventually the whole sheet has deformed, absorbing lots of strain energy. So all diagrams of nacre with the platelets shown with flat surfaces are wrong. They miss a randomness that multiplies the work of fracture by up to 3,000-fold.
Nacre goes one better still. Most physical models of nacre have the platelets of uniform size, hexagonal so that they form an ‘ideal’ tiled surface with no gaps. This ideal performs significantly worse than the real structure in which the platelets, more randomly sized and shaped, overlap unevenly from layer to layer. Failure is then randomly and widely distributed; the nacre deforms further before cracks can join up and the entire sheet fails.
Researchers at KTH Stockholm have incorporated the platey clay mineral, montmorillonite, into the structure of paper at a volume fraction of up to 0.9. The resulting material has similar structure and mechanical properties to nacre and is an excellent oxygen barrier and fire shield. For comparison, the best oxygen barrier in biology is the envelope of a fish swim bladder, a hydrostatic organ filled with oxygen secreted from the blood. It’s made of sheets of tightly bonded collagen fibres (the stuff of tendons) alternating with layers of plate-like crystals of guanine, an excretory product.
Less emphasis on the materials that make up these structures makes recycling easier. Our current technology uses 300 or so polymers; nature mainly uses two – polysaccharides and proteins. This greatly reduces the complexity of chemistry for degradation. Ceramics are largely based on calcium and so can be soluble; silica is used in some critical applications
Barthelat, Francois. 2010. “Nacre from mollusk shells: a model for high-performance structural materials.” Bioinspiration & Biomimetics 5 (3):035001.
Vincent, J F V. 2002. “Survival of the cheapest.” Materials Today (December):28-41.
Walther, A, I Bjurhager, J-M Malho, J Ruokolainen, L A Berglund, and O Ikkala. 2010. “Supramolecular control of stiffness and strength in lightweight high-performance nacre-mimetic paper with fire-shielding properties.” Angew. Chem. Int. Ed. 49:1-7. (https://tinyurl.com/2h5b79m8)
Vincent, J F V. 2012. Structural Biomaterials. Third ed. Princeton: University Press.
This article first appeared in Professional Engineering No 2, 2022