Towards understanding the atomistic disorder of synthetic bone mineral
Date
2018
Authors
Marisa, Mary E., author
Neilson, James R., advisor
Bernstein, Elliot, committee member
Popat, Ketul, committee member
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Abstract
Biominerals are an interesting class of materials due to their complex structures and superior properties as compared to similar materials produced under laboratory settings. These complex structures often demonstrate a high level of control from the nano- to macroscopic scale. As a result, it is very difficult to create mimetic materials with hierarchical structures under laboratory conditions. Bone mineral, nominally calcium hydroxyapatite [Ca10(PO4)6(OH)2], shows a distinct, well known hierarchical structure from the individual nanoparticles of hydroxyapatite in the collagen matrix to the macroscopic bone. However, the atomistic structure of the apatite is not as well understood. This is due to the high level of chemical substitution and atomistic disorder. One of the most common chemical substitutions in bone mineral is the replacement of the tetrahedral phosphate ion with a planar carbonate ion. While several studies have attempted to understand this chemical substitution, there is not a consensus on the orientation of the carbonate ion in the phosphate site. Using X-ray or neutron diffraction as a structural determination tool is very useful for highly crystalline materials. However, the usefulness of these diffraction techniques decreases with increased disorder due to broadening of reflections which can obscure structural information. Instead, a total scattering technique, such as pair distribution function analysis, can be used to obtain an understanding of the local coordination environment. This, in conjunction with calculations of energy, can be used to identify the most likely substitution orientation. Using this method of structural determination, it is possible to conclude that the lowest energy substitution is the substitution of the planar ion into the mirror plane of the tetrahedral phosphate. Many biominerals formed in aqueous media, such as those found in bone, are synthesized via metastable or amorphous precursors. Crystallization pathways can be dependent on the species initially present in solution and other chemical factors such as pH. Bone mineral is of importance because of the medical implications in connection with various bone tissue diseases. Understanding the pathway through which biomimetic bone mineral is formed may inform targets for bone disease or improve processing for synthetic grafting materials. Here, the crystallization of biomimetic bone mineral is monitored via ex situ X-ray diffraction to determine the precursor phases. Samples prepared with and without exogenous carbonate are studied to determine possible factors which influence the rate of crystallization. Carbonate is chosen because of the known substitution for phosphate in bone mineral. This synthesis pathway from low pH to high pH shows that brushite, a hydrated calcium phosphate phase, is initially formed prior to precipitation of the desired apatite phase. However, the apatite phase appears more slowly in the carbonated samples. Analysis of the phosphate concentration via an ammonium molybdate assay shows that the non-carbonated synthesis has a steady decline of the phosphate throughout the reaction while the carbonated synthesis shows an induction period during which the phosphate concentration remains steady before having a sharp decrease.
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Subject
bone mineral
atomistic structure