The hierarchical structure of biogenic calcite for improved mechanical properties: the case of the brittlestar Ophiocoma wendtii

Seminars
21.01.2018
14:30
David Wang Auditorium, 3rd floor Dalia Maydan Bldg.
Mr. Stas Kozachkevich, M.Sc. candidate

Mr. Stas Kozachkevich, M.Sc. candidate

Department of Materials science and engineering, Technion – Israel institute of technology , Haifa, Israel. staskoza@gmail.com

Living organisms demonstrate functional skeletal parts produced via biomineralization, which is the formation of minerals by living organisms. In many cases, these organisms employ sophisticated crystallization techniques and mechanical designs to obtain desired properties. Scientists and engineers can learn from these biominerals how to produce novel materials with tailored properties.

In this study, we investigated some of the skeletal parts of the marine brittlestar Ophiocoma wendtii. Hendler1 and Aizenberg2 showed that the lenses array found along the brittlestars’ arms are single crystals of calcite, which are oriented along the c-axis parallel to the lens’ optical axis. This avoids birefringence and so undoubtfully proves the optical function of the lenses and the ability of the brittlestar to respond to light. We studied the lens structure on the atomic to the macro level and showed that the lenses have a low Mg-calcite (calcite doped with Mg) matrix which contains nanometric and coherent high Mg-calcite inclusions3. This structure is formed during the crystallization process from an amorphous precursor. The unique 3D organization of the matrix and the nano-inclusions induce internal compressive stress in the matrix and create a prestressed ceramic material with improved mechanical properties such as increased hardness and fracture toughness. We further investigated the lenses and additional skeletal parts of the brittlestar and discovered that the structure of the lenses is a widespread phenomenon in all of O.wendtii skeletal parts. Moreover, by using synchrotron nano-HoloTomography with ex-situ heating experiments, revealed that the skeletal parts possess additional structural hierarchy of undulating density Mg-calcite layers aligned perpendicular to the surface of a skeletal part. This hierarchy induces further improvement of the mechanical properties to the otherwise brittle O.wendii ceramic skeletal parts.

References:

1. Gordon Hendler, Maria Byrne, Zoomorphology (1987) 107: 261-272. 2. Joanna Aizenberg, Alexei Tkachenko, Steve Weiner, Lia Addadi & Gordon Hendler, Nature ,Vol 412 , August 2001.
3. Iryna Polishchuk, Avigail Aronhime Bracha, Leonid Bloch, Davide Levy, Stas Kozachkevich, Yael Etinger-Geller, Yaron Kauffmann, Manfred Burghammer, Carlotta Giacobbe, Julie Villanova, Gordon Hendler, Chang-Yu Sun, Anthony J. Giuffre, Matthew A. Marcus, Lakshminath Kundanati, Paul Zaslansky, Nicola M. Pugno, Pupa U. P. A. Gilbert, Alex Katsman, Boaz Pokroy, Science 358, 1294–1298 (2017).

Advisor: Associate Prof. Boaz Pokroi