In situ X-ray scattering evaluation of heat-induced ultrastructural changes in dental tissues and synthetic hydroxyapatite

Synchrotron X-ray Studies of the Structural and Functional Hierarchies in Mineralised Human Dental Enamel: A State-of-the-Art Review

Hard dental tissues possess a complex hierarchical structure that is particularly evident in enamel, the most mineralised substance in the human body. Its complex and interlinked organisation at the Ångstrom (crystal lattice), nano-, micro-, and macro-scales is the result of evolutionary optimisation for mechanical and functional performance: hardness and stiffness, fracture toughness, thermal, and chemical resistance. Understanding the physical-chemical-structural relationships at each scale requires the application of appropriately sensitive and resolving probes. Synchrotron X-ray techniques offer the possibility to progress significantly beyond the capabilities of conventional laboratory instruments, i.e., X-ray diffractometers, and electron and atomic force microscopes. The last few decades have witnessed the accumulation of results obtained from X-ray scattering (diffraction), spectroscopy (including polarisation analysis), and imaging (including ptychography and tomography). The current article presents a multi-disciplinary review of nearly 40 years of discoveries and advancements, primarily pertaining to the study of enamel and its demineralisation (caries), but also linked to the investigations of other mineralised tissues such as dentine, bone, etc. The modelling approaches informed by these observations are also overviewed. The strategic aim of the present review was to identify and evaluate prospective avenues for analysing dental tissues and developing treatments and prophylaxis for improved dental health.

I22: SAXS/WAXS beamline at Diamond Light Source - an overview of 10 years operation

Beamline I22 at Diamond Light Source is dedicated to the study of soft-matter systems from both biological and materials science. The beamline can operate in the range 3.7 keV to 22 keV for transmission SAXS and 14 keV to 20 keV for microfocus SAXS with beam sizes of 240 µm × 60 µm [full width half-maximum (FWHM) horizontal (H) × vertical (V)] at the sample for the main beamline, and approximately 10 µm × 10 µm for the dedicated microfocusing platform. There is a versatile sample platform for accommodating a range of facilities and user-developed sample environments. The high brilliance of the insertion device source on I22 allows structural investigation of materials under extreme environments (for example, fluid flow at high pressures and temperatures). I22 provides reliable access to millisecond data acquisition timescales, essential to understanding kinetic processes such as protein folding or structural evolution in polymers and colloids.

X-ray diffraction and in situ pressurization of dentine apatite reveals nanocrystal modulus stiffening upon carbonate removal

Bone-like materials comprise carbonated-hydroxyapatite nanocrystals (c-Ap) embedding a fibrillar collagen matrix. The mineral particles stiffen the nanocomposite by tight attachment to the protein fibrils creating a high strength and toughness material. The nanometer dimensions of c-Ap crystals make it very challenging to measure their mechanical properties. Mineral in bony tissues such as dentine contains 2~6 wt.% carbonate with possibly different elastic properties as compared with crystalline hydroxyapatite. Here we determine strain in biogenic apatite nanocrystals by directly measuring atomic deformation in pig dentine before and after removing carbonate. Transmission electron microscopy revealed the platy 3D morphology while atom probe tomography revealed carbon inside the calcium rich domains. High-energy X-ray diffraction in combination with in situ hydrostatic pressurization quantified reversible c-Ap deformations. Crystal strains differed between annealed and ashed (decarbonated) samples, following 1 or 10 h heating at 250 °C or 550 °C respectively. Measured bulk moduli (K) and a-/c-lattice deformation ratios (η) were used to generate synthetic K_syn and η_syn identifying the most likely elastic constants C₃₃ and C₁₃ for c-Ap. These were then used to calculate the nanoparticle elastic moduli. For ashed samples, we find an average E₁₁=107 GPa and E₃₃ =128 GPa corresponding to ~5% and ~17% stiffening of the a-/c-axes of the nanocrystals as compared with the biogenic nanocrystals in annealed samples. Ashed samples exhibit ~10% lower Poisson's ratios as compared with the 0.25~0.36 range of carbonated apatite. Carbonate in c-Ap may therefore serve for tuning local deformability within bony tissues. STATEMENT OF SIGNIFICANCE: Carbonated apatite nanoparticles, typical for bony tissues, stiffen the network of collagen fibrils. However, it is not known if the biogenic apatite mechanical (elastic) properties differ from those of geologic mineral counterparts. Indeed the tiny dimensions and variable carbonate composition may have strong effects on deformation resistance. The present study provides experimental measurements of the elastic constants which we use to estimate Young's moduli and Poisson's ratio values. Comparison between ashed and annealed dentine samples quantifies the properties of both carbonated and decarbonated apatite nanocrystals. The results reveal fundamental attributes of bony mineral and showcase the additive advantages of combining X-ray diffraction with in situ hydrostatic compression, backed by atom probe and transmission electron microscopy tomography.

Examination of intestinal ultrastructure, bowel wall apoptosis and tight junctions in the early phase of sepsis

Gut hyperpermeability can be caused by either apoptosis of the intestinal epithelium or altered status, permeability or porosity of tight junctions. This project aims to elucidate these mechanisms in the early phase of sepsis. Eighteen male wild type mice were randomized to two groups. All mice received one single gavage of fluorescein isothiocyanate (FITC) dextran 30 min before intervention. One group (n = 10) underwent cecal ligation and puncture to induce sepsis. The other group (n = 8) was sham operated. Septic animals exhibited significantly increased permeability for FITC 8 h post-operatively. Significantly increased serum interleukin-6, tumor-necrosis-factor-alpha and interleukin-1-beta confirmed sepsis. Septic animals showed significant bowel wall inflammation of ileum and colon samples. PCR revealed significantly increased expression of claudin-2 and decreased expressions of claudin-4, tight-junction-protein-1 and occludin-1 resembling increased permeability of tight junctions. However, these alterations could not be confirmed at the protein level. Light microscopy revealed significant dilatation of intercellular spaces at the basal sections of intestinal epithelial cells (IEC) in septic animals confirmed by increased intercellular spaces at the level of tight junctions and adherens junctions in electron microscopy (TEM). In small angle X-ray scattering no increase in number or size of nanopores could be shown in the bowel wall. HOECHST staining and PCR of ileum samples for apoptosis markers proofed no relevant differences in intestinal epithelial cell apoptosis between the groups. Intestinal hyperpermeability in septic animals was most likely caused by alterations of the intercellular contacts and not by apoptosis or increased size/number of nanopores of intestinal epithelial cells in this murine model of early sepsis.

Human bone probed by neutron diffraction: the burning process

The first neutron diffraction study of human burned bone – for understanding heat-induced changes, relevant for archaeology, biomaterials and forensic science.

Heat-induced Bone Diagenesis Probed by Vibrational Spectroscopy

Complementary vibrational spectroscopic techniques - infrared, Raman and inelastic neutron scattering (INS) - were applied to the study of human bone burned under controlled conditions (400 to 1000 °C). This is an innovative way of tackling bone diagenesis upon burning, aiming at a quantitative evaluation of heat-induced dimensional changes allowing a reliable estimation of pre-burning skeletal dimensions. INS results allowed the concomitant observation of the hydroxyl libration (OHlibration), hydroxyl stretching (ν(OH)) and (OHlibration + ν(OH)) combination modes, leading to an unambiguous assignment of these INS features to bioapatite and confirming hydroxylation of bone's inorganic matrix. The OHlib, ν(OH) and ν4(PO43-) bands were identified as spectral biomarkers, which displayed clear quantitative relationships with temperature revealing heat-induced changes in bone's H-bonding pattern during the burning process. These results will enable the routine use of FTIR-ATR (Fourier Transform Infrared-Attenuated Total Reflectance) for the analysis of burned skeletal remains, which will be of the utmost significance in forensic, bioanthropological and archaeological contexts.

In situ monitoring and analysis of enamel demineralisation using synchrotron X-ray scattering

Dental caries is one of the most common chronic diseases that affect human teeth. It often initiates in enamel, undermining its mechanical function and structural integrity. Little is known about the enamel demineralisation process caused by dental caries in terms of the microstructural changes and crystallography of the inorganic mineral phase. To improve the understanding of the carious lesion formation process and to help identify efficient treatments, the evolution of the microstructure at the nano-scale in an artificially induced enamel erosion region was probed using advanced synchrotron small-angle and wide-angle X-ray scattering (SAXS and WAXS). This is the first in vitro and time-resolved investigation of enamel demineralisation using synchrotron X-ray techniques which allows in situ quantification of the microstructure evolution over time in a simulated carious lesion. The analysis revealed that alongside the reduction of mineral volume, a heterogeneous evolution of hydroxyapatite (HAp) crystallites (in terms of size, preferred orientation and degree of alignment) could be observed. It was also found that the rate and direction of dissolution depends on the crystallographic orientation. Based on these findings, a novel conceptual view of the process is put forward that describes the key structural parameters in establishing high fidelity ultrastructure-based numerical models for the simulation of the enamel demineralisation process.

STATEMENT OF SIGNIFICANCE

Hydroxyapatite (HAp) crystallites in the enamel dissolve during dental caries although little is known about the structural-chemical relationships that control the dynamic demineralisation process. For the first time this work investigated the in situ evolution of nano-scale morphology and the spatial distribution of ultrastructural HAp crystallites of human enamel during demineralisation in simulated caries. Advanced synchrotron SAXS and WAXS techniques showed that the heterogeneous evolution of crystallites (size, preferred orientation and degree of alignment) could be attributed to crystallographic-orientation-dependent anisotropic dissolution. Hence we propose a novel conceptual schematic diagram to describe the demineralisation process. These findings have important implications for understanding the detailed mechanisms of enamel demineralisation and provide insight into potential enamel remineralisation that could restore structural integrity and function.

Nanoscale modifications in the early heating stages of bone are heterogeneous at the microstructural scale

Nanoscale studies of bone provide key indicators to evidence subtle structural changes that may occur in the biomedical, forensic and archaeological contexts. One specific problem encountered in all those disciplines, for which the identification of nanostructural cues could prove useful, is to properly monitor the effect of heating on bone tissue. In particular, the mechanisms at work at the onset of heating are still relatively unclear. Using a multiscale approach combining Raman microspectroscopy, transmission electron microscopy (TEM), synchrotron quantitative scanning small-angle X-ray scattering imaging (qsSAXSI) and polarized light (PL) microscopy, we investigate the ultrastructure of cortical bovine bone heated at temperatures < 300°C, from the molecular to the macroscopic scale. We show that, despite limited changes in crystal structure, the mineral nanoparticles increase in thickness and become strongly disorganized upon heating. Furthermore, while the nanostructure in distinct anatomical quadrants appears to be statistically different, our results demonstrate this stems from the tissue histology, i.e. from the high degree of heterogeneity of the microstructure induced by the complex cellular processes involved in bone tissue formation. From this study, we conclude that the analysis of bone samples based on the structure and organization of the mineral nanocrystals requires performing measurements at the histological level, which is an advantageous feature of qsSAXSI. This is a critical aspect that extends to a much broader range of questions relating to nanoscale investigations of bone, which could also be extended to other classes of nanostructured heterogeneous materials.

Understanding the Crystallinity Indices Behavior of Burned Bones and Teeth by ATR-IR and XRD in the Presence of Bioapatite Mixed with Other Phosphate and Carbonate Phases

We have critically investigated the ATR-IR spectroscopy data behavior of burned human teeth as opposed to the generally observed behavior in human bones that were subjected to heat treatment, whether deliberate or accidental. It is shown that the deterioration of the crystallinity index (CI) behavior sometimes observed in bones subjected to high temperature appears to be of higher frequency in the case of bioapatite from teeth. This occurs because the formation of the β-tricalcium phosphate (β-TCP) phase, otherwise known as whitlockite, clearly ascertained by the X-ray diffraction (XRD) patterns collected on the same powdered specimens investigated by ATR-IR. These results point to the need of combining more than one physicochemical technique even if apparently well suitable, in order to verify whether the assumed conditions assessed by spectroscopy are fully maintained in the specimens after temperature and/or mechanical processing.

Osteometrics in burned human skeletal remains by neutron and optical vibrational spectroscopy

Vibrational spectroscopic techniques are applied to the osteometric study of burned human skeletal remains.

Mid-Infrared Spectroscopy Analysis of the Effects of Erbium, Chromium:Yattrium-Scandium-Gallium-Garnet (Er,Cr:YSGG) Laser Irradiation on Bone Mineral and Organic Components

The effects of varying the energy density of a high-intensity erbium, chromium: yttrium-scandium-gallium-garnet (Er,Cr:YSGG) laser on the mineral and organic components of bone tissue were evaluated using Fourier transform infrared spectroscopy. Bone samples obtained from the tibias of rabbits were irradiated with five energy densities (3, 6, 8, 12, and 15 J/cm(2)), and the effects on the carbonate to phosphate ratio and in the organic components were compared with those of nonirradiated samples. The increased temperature during the laser irradiation was also measured using infrared thermography to relate the observed spectral changes to the laser thermal effects. The analyses of the infrared spectra suggests that the irradiation with Er,Cr:YSGG promoted changes in bone tissue in both the mineral and organic components that depend on the laser energy density, pointing to the importance of using the proper energy density in clinical procedures.

X-ray Study of Human Dental Tissues Affected by Erythroblastosis Fetalis

Numerous diseases are known to cause microstructural alteration of dental tissues structure. One type in particular is associated with neonatal jaundice and circulation of bilirubin in blood at high concentration due to increased hemolysis in conditions such as erythroblastosis fetalis, septicemia, biliary atresia, and other causes of hyperbilirubinemia. In those conditions, the products of the catabolism of hemoglobin end up deposited in various tissues, including teeth, where they can present clinically as visibly stained brown/green teeth. There is almost no information on the nature or extent of the structural changes taking place in these conditions. Here, advanced nondestructive wide-angle synchrotron X-ray scattering techniques combined with scanning microscopy methods were used to investigate for the first time the ultrastructure of the dental hard tissues in an archival case of intrinsically pigmented green teeth. Despite no obvious elemental variation across the pigmented tissue region, the high-resolution crystallographic properties probed by wide-angle synchrotron X-ray scattering revealed an ultrastructural variation (orientation, particle size, and lattice parameter of hydroxyapatite crystallites) associated with a pigmentation line in dentine and with a distinct neonatal line in enamel.