Mechanical properties of the elemental nanocomponents of nacre structure

Sheet nacre is a nanocomposite with a multiscale structure displaying a lamellar “bricks and mortar” microarchitecture. In this latter, the brick refer to aragonite platelets and the mortar to a soft organic biopolymer. However, it appears that each brick is also a nanocomposite constituted as CaCO3 nanoparticles reinforced organic composite material. What is the role of this “intracrystalline” organic phase in the deformation of platelet? How does this nanostructure control the mechanical behaviour of sheet nacre at the macroscale? To answer these questions, the mechanical properties of each nanocomponents are successively investigated and computed using spherical and sharp nanoindentation tests combined with a structural model of the organomineral platelets built from AFM investigations

La Nacre, les biominéralisations et la pharmacopée

International audienceLa Nacre n'a rien de commun avec les pierres précieuses comme le diamant qui est forgé à des centaines de kilomètres au cœur de la Terre. Rien de commun non plus avec le rubis né des forces tectoniques qui font surgir les montagnes. Non ! La Nacre est une structure fascinante produite par le monde vivant. Comment l'Évolution biologique a-t-elle réussi à produire ce joyau ? Cet article nous montre qu'il s'agit de l'aboutissement d'un long processus qui a commencé il y a des milliards d'années sur Terre avec la vie elle-même ! Les chercheurs l'appellent biominéralisation c'est-à-dire le processus que la vie a élaboré et maîtrisé pour développer les tissus minéralisés comme les coquilles, les dents ou les os

Notes pour une définition des pratiques alternatives et des thérapies douces au Québec

Le phénomène des thérapies douces est d'abord analysé dans le contexte de l'éclatement du champ thérapeutique québécois et occidental et du développement des alternatives en santé. Le bouillonnement et la diversité des thérapies douces rend légitime, pour une meilleure compréhension de leur nature, l'élaboration d'une définition opératoire, définition que nous proposons dans le cadre du présent article.The phenomenon of non-traditional therapies is firstly analyzed within the context of an increasing number of approaches in the therapeutic field in Quebec and the Western hemisphere and of the development of health care alternatives. The growth and diversity of alternative therapies legitimizes the need to establish an operational definition of this phenomenon, which is the framework of this article

Nano-Composite Structure of Nacre Biocrystal.

Pucon - ChiliIntermittent-Contact Atomic Force Microscopy with phase detection imaging reveals a nanostructure within the tablet (Pinctada maxima). A continuous organic framework divides each tablet into nanograins. Their mean extension is 45nm. Transmission electron microscopy performed in the darkfield mode evidences that intracrystalline matrix is highly crystallized and responds like a ‘single crystal'. The organic matrix is continuous inside the tablet, mineral phase is thus finely divided but behaves in the same time as a single crystal. It is proposed that each tablet results from the coherent aggregation of nanograins keeping strictly the same crystallographic orientation thanks to an hetero-epitaxy mechanism

Voronoi Growth Model of Sheet Nacre.

The aim of this work was to study the tiling mode of nacre tablets in the 'brick and mortar' array of sheet nacre. For that purpose, incipient shell nacre (Pinctada margaritifera) was analysed by electron microscopy (SEM) and Raman spectroscopy. Experimental observations pointed out the key role of the stairs-like growing front in sheet-like nacre not only for its long range ordering but also as controlling the hierarchy of local mechanisms. A morphogenesis sequence is proposed taking into account the dynamics of the environment. First, the mantel cells are organised to synthesise and discharge alternatively the extrapallial fluid as batches. Because of the stairs-like feature of the growth front, the extrapallial fluid organizes as successive 'biological films', each of them delayed from the underlying one by 10 to 15µm. Each film is a compartment to prefigure a nacre layer. Then, after individualisation, this film undergoes nucleation and crystallisation of tablets. Finally, the biological film transforms progressively as mature nacre following self assembly mechanisms. The resulting tablets have a shape which responds to a Voronoi growth model, this is shown for the first time: aggregation at the same speed in all directions around single growth centers. This is an efficient model to understand the growth mechanism and rationalise all the experimental observations we have obtained

Nacre biocrystal thermal behavior

International audienceThe thermal behaviour of Pinctada margaritifera nacre was studied at different temperatures by means of thermal gravimetric, thermo-mechanical and Rock-Eval analyses. From the mechanical point of view nacre exhibited a complete reversible behaviour up to 230 °C. The bio-aragonite allotrope was seen to be as stable as the abiotic aragonite up to 470–500 °C. It was also evidenced that the organic phase was keeping cracking oxygen functions at temperatures as high as 650 °C. Nacre thermal behaviour could be described following four distinctive stages and discussed in comparison with previous data obtained in oxidative conditions

Sheet nacre growth mechanism: a Voronoi model.

Xavier Bourrat is in ISTO's lab since January 2005International audienceShell nacre (mother of pearl) of Pinctada margaritifera was analyzed by scanning electron microscopy. The originality of this work concerns the sampling performed to observe incipient nacre on the mantle side. The whole animal is embedded in methyl methacrylate followed by separation of the shell from the hardened mantle. It is revealed this way how each future nacre layer pre-exists as a film or compartment. Experimental observations also show for the first time, the progressive lateral crystallization inside this film, finishing under the form of a non-periodic pattern of polygonal tablets of bio-aragonite. It is evidenced that nuclei appear in the film in the vicinity of the zone where aragonite tablets of the underlying layer get in contact to each other. A possible explanation is given to show how nucleation is probably launched in time and space by a signal coming from the underlying layer. Finally, it is evidenced that tablets form a Voronoi tiling of the space: this suggests that their growth is controlled by an "aggregation-like" process of "crystallites" and not directly by the aragonite lattice growth

Nanoindentation and tribological tests – Suitable tools for modelling the nanostructure of sheet nacre

1. Introduction Nacre (the pearly internal layer of seashells) is a natural nanocomposite currently studied for the design of new organic/inorganic hybrid materials by mimicking biomineralization processes. It is a bioceramic formed at ambient temperature and pressure [1] which displays an exceptional high strength, stiffness and toughness [2] to weight ratio, as well as a natural biocompatibility with human bones [3]

Forming nacreous layer of the shells of the bivalves Atrina rigida and Pinctada margaritifera: An environmental- and cryo-scanning electron microscopy study

International audienceA key to understanding control over mineral formation in mollusk shells is the microenvironment inside the pre-formed 3-dimensional organic matrix framework where mineral forms. Much of what is known about nacre formation is from observations of the mature tissue. Although these studies have elucidated several important aspects of this process, the structure of the organic matrix and the microenvironment where the crystal nucleates and grows are very difficult to infer from observations of the mature nacre. Here, we use environmental- and cryo-scanning electron microscopy to investigate the organic matrix structure at the onset of mineralization in the nacre of two mollusk species: the bivalves Atrina rigida and Pinctada margaritifera. These two techniques allow the visualization of hydrated biological materials coupled with the preservation of the organic matrix close to physiological conditions. We identified a hydrated gel-like protein phase filling the space between two interlamellar sheets prior to mineral formation. The results are consistent with this phase being the silk-like proteins, and show that mineral formation does not occur in an aqueous solution, but in a hydrated gel-like medium. As the tablets grow, the silk-fibroin is pushed aside and becomes sandwiched between the mineral and the chitin layer