Strength and shimmering beauty: Mother-of-pearl owes its fascinating properties to an astonishingly regular fine structure. Researchers have now discovered an interesting mechanism of self-organization through which a perfectly regular structure develops from initial disorder in mother-of-pearl. Accordingly, at the beginning of mother-of-pearl formation, defect structures that are twisted in opposite directions arise. When they meet, they cancel each other out and lead to a change in the tension system, so that the mother-of-pearl structure is synchronized.
A biomaterial that has long fascinated researchers: mother-of-pearl is a refined composite material made of calcium carbonate and organic substances, which is characterized by its amazing strength and optical characteristics. A microscopic examination reveals a kind of brick wall structure that is very regular. Researchers have long tried to imitate this principle in order to make the properties of mother-of-pearl usable for applications. Technical manufacturing processes have already led to considerable success. So far, however, it has remained unclear how the natural mother-of-pearl producers – shell and co – produce the complex unitary structure.
Amazing regularity in sight
One thing is clear: the shell and the mother-of-pearl that lines it are produced by cells in the mollusc’s mollusc tissue. A large number of individual cells simultaneously form the material at different points and it is deposited. This gradually creates the layers that ultimately create the protective shield for the sensitive mollusc. As part of their study, the researchers led by Igor Zlotnikov from the Technical University of Dresden investigated the question of how this educational process can lead to the highly periodic and uniform structure of mother-of-pearl.
As part of their study, they examined the shells of mussels of the species Unio pictorum using synchrotron-based holographic X-ray nanotomography in order to record the growth of mother-of-pearl over time. “Mother-of-pearl is an extremely fine structure with organic features that are less than 50 nanometers in size. Our modern examination technology now opened up ideal possibilities for making mother-of-pearl visible in three dimensions, ”says Zlotnikov.
Their investigations initially made it clear: The formation of mother-of-pearl begins uncoordinated with the cells that deposit the material in different places at the same time. As a result, the early mother-of-pearl structure is also rather irregular – it is characterized by defects. “Right at the beginning, the layered, mineral-organic fabric is full of structural defects that spread like a helix through several layers. In fact, they look like a spiral staircase that is either right-handed or left-handed, ”explains Zlotnikov. Ripe mother-of-pearl, on the other hand, no longer shows these defects and has a regular, even structure.
opposites attract
To track what happens to the structural defects when mother-of-pearl grows, the researchers used artificial intelligence: They trained a neural network to analyze and trace the fine structure of the various layers of mother-of-pearl. It was found that the defect structures with opposite screw directions meet in the early mother-of-pearl structure. They apparently attract each other from comparatively great distances.
The scientists’ modeling shows that these meetings play an important role in the development of the periodic structure of mother-of-pearl: when the right-handed and left-handed defects meet, their respective torsional stresses cancel each other out. These events then lead to a tissue-wide synchronization of the structures. In the course of time, the mother-of-pearl can develop a completely regular and defect-free structure, the scientists explain.
As they finally emphasize, their results may not only have a meaning for the understanding of the origin of the super material of mussels and Co. Because periodic, mother-of-pearl-like structures are also produced by many other animals. The researchers believe that the newly discovered mechanism could therefore also play a role in the formation of these bio-structures.
Source: Technical University of Dresden, specialist article: Nature Physics, doi: 10.1038 / s41567-020-01069-z