Black Tahitian pearls

Secrets of pearl perfection could unlock high-precision nanomaterials

Image credit: Dreamstime

Researchers from the University of Michigan have uncovered for the first time the process by which molluscs build ultradurable structures with a level of symmetry that outstrips everything in the natural world above the size of an individual atom.

For centuries, scientists have tried to understand how molluscs produce such perfect objects. “We humans, with all our access to technology, can’t make something with a nanoscale architecture as intricate as a pearl,” said Professor Robert Hovden, a materials science and engineering expert. “So, we can learn a lot by studying how pearls go from disordered nothingness to this remarkably symmetrical structure.”

As a pearl is built, its symmetry edges towards perfection, coaxing order from unpredictable layers of nacre, the durable organic-inorganic composite that gives it its pearlescent sheen. The process begins when nacre covers a shard of aragonite that surrounds an organic centre. The layers of nacre, which comprise more than 90 per cent of a pearl’s volume, become progressively thinner and more closely matched as they grow outward from the comparatively clumsily built centre.

By adjusting the thickness of each layer of nacre, molluscs maintain symmetry of their pearls. For instance, if one layer is thicker, the next tends to be thinner.

Hovden and his colleagues studied Akoya 'keshi' pearls, which are produced by East Australian oysters. They selected these pearls as they form naturally, as opposed to bead-cultured pearls with their artificial centres. Each keshi pearl was sliced with a diamond wire saw into sections 3-5 mm in diameter, then polished and examined using an electron microscope.

They found that pearls lack true long-range order (such as the planned symmetry that keeps layers of bricks in large buildings consistent). Instead, they show medium-range order, maintaining symmetry for around 20 layers at a time; this is sufficient for maintaining consistency and durability over the thousands of layers of nacre within a pearl.

The pearl pictured contains 2,615 finely matched layers of nacre deposited over 548 days.

Nacre layers inside pearl

University of Michigan

Image credit: University of Michigan

“These thin, smooth layers of nacre look a little like bedsheets, with organic matter in between,” said Hovden. “There’s interaction between each layer, and we hypothesise that that interaction is what enables the system to correct as it goes along.”

The team also uncovered details about how the interaction between layers works. A mathematical analysis of the pearl's layers show how they follow a phenomenon known as 1/f noise, in which a seemingly random series of events are connected with each new event influenced by the one before it. This phenomenon has been shown to govern a wide variety of natural and human-made processes including seismic activity, financial markets, electricity and classical music.

Hovden explained: “When you roll dice, for example, every roll is completely independent and disconnected from every other roll. But 1/f noise is different in that each event is linked. We can’t predict it, but we can see a structure in the chaos. And within that structure are complex mechanisms that enable a pearl’s thousands of layers of nacre to coalesce toward order and precision.”

The researchers hope their findings could help nanoengineers catch up with oysters, possibly by suggesting how next-generation materials with precisely layered nanoscale architectures can be manufactured.

“When we build something like a brick building, we can build in periodicity through careful planning and measuring and templating,” said Hovden. “Molluscs can achieve similar results on the nanoscale by using a different strategy. So, we have a lot to learn from them, and that knowledge could help us make stronger, lighter materials in the future.”

Earlier this year, a team from Technische Universitaet Dresden published a study with some similarities; this study looked specifically at how the structural defects in mother-of-pearl attract and cancel each other out, permitting the assembly of perfect architecture. They found defects of opposite directions were attracted to each other from great distances; the right-handed and left-handed defects shifted through the structure until they met, resulting in mutual annihilation. This leads to a tissue-wide synchronisation which gave rise to mother-of-pearl’s regular and flawless architecture.

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