Free R, DeRocher K, Xu R, Joester D, Stock S R
Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, USA.
Argonne National Lab, Advanced Photon Source, Lemont, Illinois 34ID-E, USA.
Powder Diffr. 2020 Jun;35(2):117-123. doi: 10.1017/s0885715620000251. Epub 2020 May 8.
Tooth enamel, the outermost layer of human teeth, is a complex, hierarchically structured biocomposite. The details of this structure are important in multiple human health contexts, from understanding the progression of dental caries (tooth decay) to understanding the process of amelogenesis and related developmental defects. Enamel is composed primarily of long, nanoscale crystallites of hydroxyapatite that are bundled by the thousands to form micron-scale rods. Studies with transmission electron microscopy show the relationships between small groups of crystallites and X-ray diffraction characterize averages over many rods, but the direct measurement of variations in local crystallographic structure across and between enamel rods has been missing. Here, we describe a synchrotron X-ray-based experimental approach and a novel analysis method developed to address this gap in knowledge. A 500-nm-wide beam of monochromatic X-rays in conjunction with a sample section only 1 μm in thickness enables 2D diffraction patterns to be collected from small well-separated volumes within the enamel microstructure but still probes enough crystallites (300 per pattern) to extract population-level statistics on crystallographic features like lattice parameter, crystallite size, and orientation distributions. Furthermore, the development of a quantitative metric to characterize relative order and disorder based on the azimuthal autocorrelation of diffracted intensity enables these crystallographic measurements to be correlated with their location within the enamel microstructure (e.g., between rod and interrod regions). These methods represent a step forward in the characterization of human enamel and will elucidate the variation of the crystallographic structure across and between enamel rods for the first time.
牙釉质是人类牙齿的最外层，是一种复杂的、具有层次结构的生物复合材料。这种结构的细节在多种人类健康背景下都很重要，从理解龋齿（蛀牙）的发展到理解釉质形成过程及相关发育缺陷。牙釉质主要由长的纳米级羟基磷灰石微晶组成，这些微晶数以千计地聚集在一起形成微米级的棒状物。透射电子显微镜研究显示了小群微晶之间的关系，X射线衍射则表征了许多棒状物的平均值，但牙釉质棒之间和内部局部晶体结构变化的直接测量一直缺失。在此，我们描述了一种基于同步加速器X射线的实验方法和一种新开发的分析方法，以填补这一知识空白。一束约500纳米宽的单色X射线与仅1微米厚的样品切片相结合，能够从牙釉质微观结构中分离良好的小体积区域收集二维衍射图案，但仍能探测到足够数量的微晶（每个图案约300个），以提取诸如晶格参数、微晶尺寸和取向分布等晶体学特征的总体统计数据。此外，基于衍射强度方位自相关开发的一种定量指标来表征相对有序和无序，使得这些晶体学测量能够与它们在牙釉质微观结构中的位置相关联（例如，在棒状区域和棒间区域之间）。这些方法代表了人类牙釉质表征方面的一个进步，并将首次阐明牙釉质棒之间和内部晶体结构的变化。
