Abstract
Peritubular dentine (PTD) and intertubular dentine (ITD) were investigated by 3D correlative Focused Ion Beam (FIB)-Scanning Electron Microscopy (SEM)-Energy Dispersive Spectroscopy (EDS) tomography, tapping mode Atomic Force Microscopy (AFM) and scattering-type Scanning Near-Field Optical Microscopy (s-SNOM) mapping. The brighter appearance of PTD in 3D SEM-Backscattered-Electron (BSE) imaging mode and the corresponding higher grey value indicate a greater mineral concentration in PTD (~160) compared to ITD (~152). However, the 3D FIB-SEM-EDS reconstruction and high resolution, quantitative 2D map of the Ca/P ratio (~1.8) fail to distinguish between PTD and ITD. This has been further confirmed using nanoscale 2D AFM map, which clearly visualised biopolymers and hydroxyapatite (HAp) crystallites with larger mean crystallite size in ITD (32 ± 8 nm) than that in PTD (22 ± 3 nm). Correlative microscopy reveals that the principal difference between PTD and ITD arises primarily from the nanoscale packing density of the crystallites bonded together by thin biopolymer, with moderate contribution from the chemical composition difference. The structural difference results in the mechanical properties variation that is described by the parabolic stiffness-volume fraction correlation function introduced here. The obtained results benefit a microstructure-based mechano-chemical model to simulate the chemical etching process that can occur in human dental caries and some of its treatments.
Highlights
Human dentine is a tough natural mineralized material which consists of ~50% by volume of mineral hydroxyapatite (HAp), ~30% organic matter, and up to ~20% water
We explore the principal difference between intertubular dentine (ITD) and peritubular dentine (PTD) using simultaneous Focused Ion Beam (FIB)-Scanning Electron Microscopy (SEM)-Energy Dispersive Spectroscopy (EDS) nanotomography and Atomic Force Microscopy (AFM)/scattering-type Scanning Near-Field Optical Microscopy (s-SNOM) as a correlative technique to achieve 3D spatially resolved structural and compositional characterization of the arrangement of mineral and organic phases within ITD and PTD of human dentine from micron to nanometre scales
The serial sectioning experiment was performed on the volume of interest (VOI) (~20 × 20 × 10 μm3 ) by removing 200 nm thick layers of material by FIB milling at 30 keV and 505 pA of current, followed by optimised in-Beam BSE imaging at 10 kV on each newly exposed surface with simultaneous EDS mapping
Summary
Human dentine is a tough natural mineralized material which consists of ~50% by volume of mineral hydroxyapatite (HAp), ~30% organic matter, and up to ~20% water. It has a typical well-oriented microstructure consisting of an arrangement of dentinal tubules filled with odontoblast processes or their remnants with an area density of around (19–45) × 1000/mm and a mean diameter range of 0.8–2.5 μm. The greater part of the dentine volume is occupied by ITD which is a composite consisting of collagen fibrils discontinuously reinforced with. Materials 2018, 11, 1493 nanoplatelets of carbonated HAp. Materials 2018, 11, 1493 nanoplatelets of carbonated HAp This arrangement makes the dentine tough and strong. The PTD is more mineralized than ITD, contains no collagen, and is harder and stiffer than ITD [4,5,6]
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