Scientists discover how mother-of-pearl self-assembles into a perfect structure

In a new study published in Physics of nature, researchers from the B CUBE – Center for Molecular Bioengineering at TU Dresden and European Synchrotron Radiation Facility (ESRF) in Grenoble describe, for the first time, that the structural defects of self-assembling nacre attract and cancel each other out, ultimately leading to a perfect periodic structure.

Molluscs build shells to protect their soft tissues from predators. Mother of pearl, also known as mother of pearl, has a complex and very regular structure that makes it an incredibly strong material. Depending on the species, nacres can reach tens of centimeters in length. Regardless of the size, each mother of pearl is constructed from material deposited by a multitude of individual cells in several different places at the same time. How exactly this highly periodic and uniform structure emerges from the initial disorder was until now unknown.

Mother-of-pearl formation begins uncoordinated, with cells depositing material simultaneously in different places. Not surprisingly, the structure of the early mother-of-pearl is not very regular. At this point it is full of flaws. “At the very beginning, the layered mineral-organic tissue is full of structural flaws that propagate through a number of layers like a helix. In fact, they look like a spiral staircase, having a right or left orientation. “, says Dr Igor Zlotnikov, research group leader at B CUBE – Center for Molecular Bioengineering at TU Dresden. “The role of these defects in the formation of such periodic tissue has never been established. On the other hand, mature mother-of-pearl is flawless, with a regular and uniform structure. How could perfection emerge from it. ‘such a mess? “

Zlotnikov group researchers collaborated with the European Synchrotron Radiation Facility (ESRF) in Grenoble to examine in great detail the internal structure of early and mature mother-of-pearl. Using synchrotron-based x-ray holographic nanotomography, researchers were able to capture the growth of nacre over time. “Mother of pearl is an extremely fine structure, with organic characteristics below 50 nm. ESRF’s ID16A beamline provided us with an unprecedented ability to visualize mother-of-pearl in three dimensions, ”says Dr Zlotnikov. “The combination of electron-dense, highly periodic inorganic platelets with delicate, thin organic interfaces makes mother-of-pearl a difficult structure to image. Cryogenic imaging helped us get the resolving power we needed, ”says Dr Pacureanu of the X-ray Nanoprobe group at ESRF.

Analyzing the data was quite a challenge. The researchers developed a segmentation algorithm using neural networks and trained it to separate different layers of nacre. That way, they were able to track what happens to structural faults as the nacre grows.

The behavior of structural defects in a growing nacre was surprising. Opposite screw direction defects were attracted to each other from great distances. The faults of both right and left handed people moved through the structure until they met and canceled out. These events led to tissue-wide synchronization. Over time, this allowed the structure to develop into a perfectly regular and flawless structure.

Periodic structures similar to mother of pearl are produced by many different animal species. The researchers believe that the newly discovered mechanism could lead not only to the formation of nacre but also to other biogenic structures.