|M.Sc Student||Kozachkevichh Stanislav|
|Subject||The Hierarchical Structure of Biogenic Calcite:|
The Case of the Brittle Star Ophiocoma Wendtii
|Department||Department of Materials Science and Engineering||Supervisor||Professor Boaz Pokroy|
|Full Thesis text|
Biomineralization is the process of minerals formation by living organisms. Throughout time, living organisms have evolved and adapted to environmental changes by developing functional skeletal parts that enable them to survive. In many cases, organisms employ sophisticated crystallization strategies and structural designs to reach their desired properties, from which we can learn how to produce novel functional materials.
In this study, we research some of the skeletal parts of the marine brittle star Ophiocoma wendtii. The first work about this species has been done by Gordon Hendler, who discovered that O.wendtii performs phototaxis, i.e. movement in response to light. He suggested that this species responds to light by having a vision system comprised of hundreds of lenses, nerves, and chromatophores that altogether serve as a compound eye. Joanna Aizenberg (2001) showed in her proceeding research that the lenses are single crystals, made of calcite oriented along the c-axis parallel to the lens optical axis in order to avoid birefringence. She proved that each lens is designed to correct spherical aberrations and focus the incoming light in a distance that corresponds to the nerves position, thus undoubtedly proving the optical function of the lenses. Our group had studied the structure of the lenses on the micro- and atomic scales and demonstrated that the single crystal lenses are in fact comprised of two different phases that are coherent with one another: a low-Mg calcite matrix and high-Mg calcite nano-sized inclusions. Since the lattice parameter of calcite decreases in a linear fashion with increasing amounts of magnesium, this resulted in distinct lattice parameters between the two phases where the nano-sized inclusions have smaller lattice parameters. The coherent state, combined with the lattice parameters’ mismatch of the two, induced internal compressive stress into the matrix and formed a pre-stressed ceramic that lends improved mechanical properties to the bulky lenses with enhanced hardness and fracture toughness.
We then studied additional skeletal parts of the brittle star and discovered that the structure described above is common in all of O.wendtii skeletal parts. Moreover, using synchrotron X-ray nano-computed tomography, we have found that the lenses and spicules possess additional structural hierarchy on the micro-scale of periodic Mg-calcite layers with fluctuations in their mass density aligned perpendicular to the surface of the skeletal part.
This hierarchy provides additional improvement for the mechanical properties of O.wendii skeletal parts, and overall, sets forth two hierarchical structures; one on the micro-scale with a layered structure, and the other, on the nano-scale with nano-sized coherent inclusions, that ensure superior mechanical properties for the otherwise brittle ceramic material.