Mother-of-pearl is an incredible material: from its beauty to its solidity, its virtues are numerous. This fascinating material continues to surprise and inspire us. Welcome to this guided tour of the virtues of mother-of-pearl!
Mother-of-pearl and its benefits for molluscs
Definition of mother-of-pearl
Mother-of-pearl is a biomaterial synthesized by many molluscs. Certain univalve shellfish, such as abalone, or bivalves, such as oysters or pinna nobilis, the largest bivalve in the Mediterranean, also known as “noble pen shell” or "fan mussel", are known to secrete pearl. But this material is common to most shelled molluscs: even snails produce it! The mother-of-pearl constitutes the inner layer of the shell, directly in contact with the mollusk. Like the rest of the shell, it is mainly composed of calcium carbonate. This mineral is found in the mother-of-pearl in the form of aragonite, while it can, in the rest of the shell, also be found in the form of calcite, its stable form under ambient conditions of pressure and temperature. The acidification of the oceans, caused by global warming, puts these molluscs at risk, in the same way as corals which also contain aragonite. Indeed, the acidity of the water changes the balance between the crystallized forms, calcite or aragonite, of the limestone, and its presence in solution in the water. Water that is too acidic can then prevent these organisms from creating their shell and pearl, or even destroy this external skeleton necessary for their survival.
Structure and role of mother-of-pearl
Mother-of-pearl has a protective role for the molluscs that secrete it. Aragonite is a very hard material, but also very brittle. In nacre, aragonite crystals are organized in layers, bound together by conchiolin. This organic material brings a relative flexibility to the whole, which translates into a breaking strength 3,000 times greater than that of aragonite alone. And to obtain this performance, 5 to 6% of conchioline in the mother-of-pearl is enough, while allowing it to preserve the hardness of the aragonite! Secreted throughout the life of the mollusk, mother-of-pearl therefore protects the inside of the shell. It is also this protective role that allows the creation of beautiful mother-of-pearl beads. Indeed, when a foreign body penetrates inside the shell, it will trigger the production of nacre around it. Layer by layer, it will be covered in this way so as not to irritate the shell, which will give rise to a pearl. The iridescent appearance of these pearls and of the interior of the shells is directly linked to the layered structure of the mother-of-pearl. Indeed, the alignments of aragonite will lead to particular reflections of light and thus produce the beautiful reflections of nacre. To these structural colors can be added pigmentary colors, brought to the mother-of-pearl by carotenoids (pigments which, as their name indicates, are also present for example in carrots) in the conchioline.
Mother-of-pearl and its virtues for men
Mother-of-pearl: a precious material for social uses
The beauty of mother-of-pearl was the first reason for its use by men. Some shells with a particularly beautiful nacre have thus been used directly as currency, such as cowrie shells in China, India or even Africa. The development of new techniques, as well as the breeding of certain species for their mother-of-pearl specifically, have allowed the development of new uses for nacre. This semi-precious stone is then used in jewelry or marquetry, but also to make buttons or various decorations. Its milky white appearance has even led to its use in realistic sculpture, to represent the whites of the eyes! Finally, musical instruments do not escape the use of mother-of-pearl, whether for their decoration, like certain guitars, or for a more direct role: the keys of accordions are thus made of mother-of-pearl.
Mother-of-pearl and its therapeutic virtues
Mother-of-pearl also gives rise to less common uses. Its color and solidity reminiscent of that of teeth, the Mayans used to use it to replace their damaged teeth. Studies of these implants have shown an amazing connection between these new teeth and the jawbone: these implants were not only not rejected by the patients but on the contrary became an integral part of the dentition. These amazing properties were then studied to consider the use of mother-of-pearl for bone implants. And in fact, mother-of-pearl has a structure similar to that of bones, and above all similar growth mechanisms. So the same kinds of chemical messages induce nacre and bone growth. The presence of mother-of-pearl therefore promotes bone regeneration, which opens up promising avenues for the development of biomaterials effective in bone repair.
Mother-of-pearl and its virtues for biomimicry
In search of solidity
What if the virtues of mother-of-pearl for humans went beyond its use as a material, and extended to the inspirations it can generate, as many other marine organisms? As we have said, the structure of mother-of-pearl gives it resistance and solidity. Taking inspiration from this brick and mortar structure, combining hard bricks such as aragonite and a more flexible mortar such as conchiolin can thus make it possible to develop new, more solid materials. This idea is applicable to many types of materials! For example, a bio-based plastic was developed inspired by mother-of-pearl. Sheets of mica play the role of bricks, and cellulose (constituent of plants) plays that of conchiolin to give this new biodegradable plastic good mechanical properties. Just as conchiolin is only a small proportion of mother-of-pearl, the addition of a very small amount of softer material can be enough to have impressive results. Thus, the addition of 7% polymer in glass makes it possible, by reproducing the structure of mother-of-pearl, to obtain a 700 times more crack resistant glass! Finally, a last mechanism improves the resistance of mother-of-pearl: sacrificial bonds. Their principle? These are fairly weak bonds, which will break preferentially without threatening the integrity of the entire structure. As they are also easily reformable, this offers interesting healing capacities. This mechanism can be reproduced at different scales to provide self-repairing capabilities to different materials.
Towards new applications
If we take a step back, the structure of mother-of-pearl can be interesting in applications further away from its primary usefulness. For example, an effective water filter is inspired by mother-of-pearl. This filter combines a filtering protein, through the pores of which is carried out the filtration, and a mineral structure, which ensures the mechanical solidity of the filter. Thus, it is possible to pass large water flows through this robust filter while having a good selectivity of the filtered molecules. Finally, mother-of-pearl will perhaps accompany the next space explorations! Indeed, a new solid rocket fuel is inspired by it to improve its characteristics. The addition of conductive and resistant bricks within the fuel polymer improves both the thermal conductivity and the resistance of the assembly. Inspired by mother-of-pearl, this provides solid fuel with better resistance to the stresses exerted on it, especially during take-off. But in addition, combustion is made more reliable thanks to the better distribution of heat within the material: hot spots and the risk of accidents are limited.
Conclusion
Decorative and solid, mother-of-pearl has many virtues that make it a very popular material. Given the number of innovations it inspires, mother-of-pearl still has a bright future ahead of it in terms of biomimicry!