Molluscs use thermodynamics to create complex morphologies with exceptional properties

08-10-2018

An international team has found how some molluscs create their complex structures. Their work provides new tools for novel bioinspired and biomimetic bottom-up material design.

  • Share

Nature serves as a source of inspiration for scientists and engineers thanks to the complex material architectures that make up some living organisms. These materials carry out essential functions, ranging from structural support and mechanical strength, to optical, magnetic or sensing capabilities. One example of this are molluscan shells, made of mineralized tissues organised in mineral-organic hierarchical functional architectures.

Molluscs appeared more than 500 million years ago, and they have developed hard and stiff mineralised outer shells for structural support and protection against predation. Their shells consist of mineral-organic composite structures made of calcium carbonates, mostly calcite and aragonite. The different shells exhibit a large variety of intricate three-dimensional  assemblies with superior mechanical properties.

tomography.png

Morphological analysis of the dendritic–prismatic ultrastructure in the shell of U. pictorum. A,B) 2D slices reconstructed from synchrotron‐based microtomography data, (parallel to the surface of the shell), showing the dendritic and the prismatic morphology, closer to the periostracum and nacre, respectively. White areas correspond to the mineral component and black areas to the organic phase. Scale bars are 100 µm. C–E) 3D visualization of tomographic segments showing different stages of structural evolution of the dendritic–prismatic ultrastructure: initial stage showing the dendritic morphology, intermediate stage showing transition to the prismatic morphology and the complete ultrastructure, respectively. Credits: Schoeppler V., et al, Biomineralization as a Paradigm of Directional Solidification: A Physical Model for Molluscan Shell Ultrastructural Morphogenesis, First published: 21 September 2018, DOI: (10.1002/adma.201803855). Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

 “The diversity of mineral morphologies and 3D mineral–organic architectures in molluscs, in combination with their exceptional mechanical efficiency, offers a unique platform to study the formation–structure–function relationship in a biomineralized system”, explains Igor Zlotnikov, scientist from the Technische Universität Dresden (Germany) and leader of the group. Researchers have studied these materials for many years, but they still have not found the physical constraints that define the growth of the architectures.

Now scientists from the Technische Universität Dresden, Institute for Solid State Physics and Optics in Hungary and the ESRF have shown that the formation of the different shell structures follows a self-assembly process guided by the physical and chemical environment that is set by the organism. They demonstrate that the construction of the entire shell of some molluscs is a result of a continuous growth process with gradually changing boundary conditions, similar to the process of directional solidification, well known from classical materials science.

Movie-3-Orientation.gif

Shell morphology as predicted by 2D phase‐field simulation under decreasing supersaturation of the mineral phase. Orientation map: black—fluid, different colors stand for different crystallographic orientations. Credits: Schoeppler V., et al, Biomineralization as a Paradigm of Directional Solidification: A Physical Model for Molluscan Shell Ultrastructural Morphogenesis, First published: 21 September 2018, DOI: (10.1002/adma.201803855). Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

The team carried out experiments on ID19 and ID06, where they studied the structure of the mollusc Unio pictorum. “ID19 is an excellent X-ray imaging beamline where we scanned the shell to visualize the 3D spatial arrangement of the dendritic–prismatic ultrastructure using synchrotron‐based phase-enhanced microtomography”, explains Zlotnikov. The imaging data from ID19 were complemented with diffraction data from ID06 to provide additional information on the crystal structure in different parts of the shell.

ID06 is the test bed and prototype for one of the new beamlines of the Extremely Brilliant Source project, due to come to life in 2020. Zlotnikov’s team is participating in this development: “On ID06 we are currently collaborating with the team to develop the beamline, dedicated to dark-field X-ray imaging and to enable the study of biological structures using this new technique”.

The next step for the team is to try to extend the model to generalise it and to include more complex structures. “We are still very far from reproducing what nature does in bioinspired materials”, explains Zlotnikov, “so far attempts by the research community had a limited success”, he adds. “However, our research provides a model that can greatly contribute to the general knowledge in this field and provides new clues about how to reproduce nature in the future”, he concludes.

Reference:

Schoeppler, V. et al, Adv. Mater. 2018, 1803855. DOI: 10.1002/adma.201803855

Text by Montserrat Capellas Espuny

Top image: Shell morphology as predicted by 2D phase‐field simulation under decreasing supersaturation of the mineral phase. Orientation map: black—fluid, different colors stand for different crystallographic orientations. Credits: Schoeppler V., et al, Biomineralization as a Paradigm of Directional Solidification: A Physical Model for Molluscan Shell Ultrastructural Morphogenesis, First published: 21 September 2018, DOI: (10.1002/adma.201803855). Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.