Biological organisms have the extraordinary ability to form mineral structures with unparalleled control. From curved single crystals to hierarchically structured skeletal parts, biomineralization processes produce materials with properties that are highly optimized for their intended applications. These processing techniques occur at ambient conditions using materials abundant in the environment, and therefore represent a much more sustainable synthesis process than many industrial manufacturing techniques. Herein we report the characterization of one such biomineral: human dental enamel. Human enamel exhibits an ordered structure across many length scales, from the millimeter down to the nanometer. Enamel must bear masticatory forces for decades, and its mechanical properties are therefore optimized for toughness, hardness, and wear resistance. Using scanning transmission electron microscopy (STEM) and atom probe tomography (APT), we established that the hydroxyapatite (OHAp) crystals in enamel have a core-shell architecture with a core where the concentration of impurity elements (Mg, Na, F, C) is relatively high, surrounded by a relatively pure shell region. The chemical gradients in the core may contribute to toughening of the crystallites, strengthening the material as a whole. In order to better understand how an insulating material such as hydroxyapatite evaporates in the atom probe, we performed an analysis of multi-hits generated during an experimental run. This analysis suggested that there is some mobility of ions on the sample tip, and that phenomena such as the generation of neutral oxygen atoms may contribute to slight compositional inaccuracies in APT data. At larger length scales, synchrotron x-ray microdiffraction experiments on thin sections of enamel revealed that crystallites are roughly co-aligned along their crystallographic c-axes in enamel rod head regions, with more deviations in the tail and interrod regions. This analysis provided evidence that the crystallite volume changes systematically within a rod, pointing to possible compositional differences in crystallites within the same rod. Additionally, crystallite counting suggests that the crystallites are not a single coherent domain, but are divided by incoherent domain boundaries along the long dimension of the crystallites. Finally, preliminary APT analysis of teeth affected by molar incisor hypomineralization (MIH) and fluorosis allows us to determine what changes result structurally and chemically from these conditions. MIH and fluorosis do not seem to disrupt the core-shell architecture of crystallites, but do affect the bulk concentration of trace elements. Analysis of the surface zone and body of an artificial sub-surface carious lesion suggests that the crystallite structure has been more dramatically altered. There is also evidence that ions and potentially organic molecules from the solutions used to create the lesion infiltrated into the enamel and were found in the body of the lesion. Improved understanding of how these conditions affect enamel on the nanoscale may aid in the development of novel prevention and treatment techniques.

Creator

• DeRocher, Karen

DOI

• https://doi.org/10.21985/n2-1338-nz08

Subject

• Materials Science

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:15281
• etdadmin_upload_764846

Keyword

• Enamel
• Atom Probe Tomography
• Biominerals

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright