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The Nanomechanics of Biomineralized Soft-Tissues and Organic Matrices

  • Author(s): Bezares-Chávez, Jiddu
  • et al.
Abstract

The research reported on in this dissertation is concerned with the macro-molecular constitution and geometrical organization of the soft-tissue comprising the matrix of the nacreous portion of the shell of Haliotis rufescens, the Red abalone. Nacre is one of literally legions of intricate biomineralized structures that exist in nature and has long served as a paradigm for elegant and optimized structural design. Biomineralization involves, inter alia, the uptake and synthesis of elements and compounds from the environment and their incorporation into highly optimized functional structures. Nacre has a structure described as a brick wall like with a matrix of biopolymer layers that are preformed and serve as a template into which nanocrystalline tiles of CaCO₃ precipitate. The matrix, or what are known as inter- lamellar layers, are of particular interest as they impart both toughness and strength to the composite ceramic nacre structure. The work first involved a histochemical mapping of the macromolecular structure of the interlamellar layers; this revealed the locations of proteins and functional molecular groups that serve as nucleation sites for the ceramic tiles. Parallel studies on the nacre of Nautilus pompilius, the Chambered Nautilus, revealed the generality of the findings. Of particular interest was determining both the content and layout of chitin within these layers. In fact it was determined that chitin was organized as mostly unidirectional architecture of fibrils, with a certain fraction of fibrils laying at cross directions. Most remarkably, it was found that the fibrils possessed a very long range connectivity that spanned many tiles. This was determined by systematic atomic force (afm) and analytical optical histochemical microscopy. These findings were further verified by a unique form of mechanical testing whereby tensile testing was conducted on groups of interlamellar layers extracted from nacre. Mechanical testing led to a quantitative visco-elastic constitutive model for these layers and, in turn, to a complete mechanical/structural model for the complete nacre composite. Further verification was obtained via micro- and nano-indentation experiments which were modeled via detailed FEM numerical simulations. Nano-indentation also allowed a detailed assessment of the nano-structure and properties of the ceramic tiles which are best described as nano-scale composites composed of protein infiltrated CaCO₃ nano-grains within a biopolymer matrix. The role of water content, i.e. moisture content, was also determined via, in part experiments conducted on dehydrated nacre. These findings lead to a pathway for specifying optimal bio-mimicked or bio-inspired synthetic materials

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