The vertebral skeleton is probably the most remarkable example of the efficiency of living organisms in forming robust structures which closely combine organic and mineral materials, in this case calcium phosphate. However, in the submarine environment, numerous and frequently single-cell organisms can achieve similar exploits by using silica to produce carapaces and spines to protect themselves, or spicules that are fibers which direct light to their neurons as effectively as the best optical fibers. With a complex architecture and shape, these natural structures are even more astonishing in that they develop spontaneously in water under moderate conditions of temperature and pressure, according to mechanisms which are still largely unknown. This feat is a dream for chemists who are often obliged to heat, extrude or compress materials under aggressive conditions in order to endow them with a shape.
In the context of their studies on the physical chemistry of a therapeutic peptide, lanreotide, researchers from CNRS and the University of Rennes have discovered that this peptide could serve as a scaffold for the spontaneous formation of silica nanotubes by simple mixing with a silica precursor in water. These hybrid tubes consist in a perfect helical assembly of molecules of the drug in a 24 nm diameter tube, the internal and external surfaces of which are covered with two thin and uniform layers of 2 nm silica. The tubes are several micrometers long and aligned in fibers of a few millimeters. Their organization is thus controlled hierarchically over more than 6 orders of magnitude, or the same ratio in length as the diameter of a hair and the height of the Eiffel Tower.
To achieve this detailed study, the team of scientists (including physicists, biologists and chemists) developed a slow technique which enabled the coating with silica of nanotubes of biological molecules forming in water. They were surprised to observe that the silica deposit favored the gradual lengthening of the organic nanotube, the new tip of which could then serve again as a scaffold for further silica deposits. This recurrent process ensured both control of the organization at a molecular scale and the growth of an organic scaffold as the mineral was deposited. This process is astonishingly similar to the construction of a sky-scraper, during which assembly of the metallic framework and the application of concrete are alternated with precision, except that there are no laborers and that the silica nanotubes are infinitely smaller.
This work opens two new perspectives. Firstly, based on a simplified system, it provides a clearer understanding of some of the astute, but still mysterious, mechanisms developed by nature to produce skeletons and spicules. Secondly, it opens the way to novel materials with nanometric dimensions, whose organization in space can be controlled up to macroscopic scales, thus endowing them with unique properties. In the case of silica nanotubes, the scientists hope that they will be able to demonstrate their ability to conduct light as effectively as the natural spicules of sea sponges...
Pursuit of this work now aims to gain a clearer understanding of the mechanisms which underlie the formation of natural mineral architectures in living organisms, and the synthesis of increasingly sophisticated models. This should enable further discoveries concerning the design and development of still more refined and "intelligent" materials for biological or technological applications.
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