Dentin on the nanoscale: Hierarchical organization, mechanical behavior and bioinspired engineering

Abstract

Objective: Knowledge of the structural organization and mechanical properties of dentin has expanded considerably during the past two decades, especially on a nanometer scale. In this paper, we review the recent literature on the nanostructural and nanomechanical properties of dentin, with special emphasis in its hierarchical organization.

Methods: We give particular attention to the recent literature concerning the structural and mechanical influence of collagen intrafibrillar and extrafibrillar mineral in healthy and remineralized tissues. The multilevel hierarchical structure of collagen, and the participation of non-collagenous proteins and proteoglycans in healthy and diseased dentin are also discussed. Furthermore, we provide a forward-looking perspective of emerging topics in biomaterials sciences, such as bioinspired materials design and fabrication, 3D bioprinting and microfabrication, and briefly discuss recent developments on the emerging field of organs-on-a-chip.

Results: The existing literature suggests that both the inorganic and organic nanostructural components of the dentin matrix play a critical role in various mechanisms that influence tissue properties.

Significance: An in-depth understanding of such nanostructural and nanomechanical mechanisms can have a direct impact in our ability to evaluate and predict the efficacy of dental materials. This knowledge will pave the way for the development of improved dental materials and treatment strategies.

Conclusions: Development of future dental materials should take into consideration the intricate hierarchical organization of dentin, and pay particular attention to their complex interaction with the dentin matrix on a nanometer scale.

Keywords: Collagen; Dentin; Intrafibrillar mineral; Organs-on-a-chip; Proteoglycans; Remineralization.

Figure 1

The mechanical contribution of interfibrillar mineral to dentin collagen. A) Healthy dentin had a mean mineral volume of 44.4%, whereas the mineral volume in DI-II dentin varied from 30.9 – 40.5%. B) The mean elastic modulus of healthy dentin measured in dry was of 23.9 GPa, and dropped to 20±1 GPa when wet. DI-II dentin, which lacks intrafibrillar mineral, was 20.4 GPa wet, and dropped drastically to only 5.7 when wet, despite only a moderate decrease in total mineral volume. C) A similar trend is found in some cases of poorly remineralized dentin, where the average elastic modulus in dry can be has high as 18 GPa, and as low as 1.6 in wet. D) A continuous deminerlization of single dentin collagen fibrils reveals the topographical changes in collagen with the loss of intrafibrillar mineral (especially in the gap zones), with a drastic decrease in AFM-indentation elastic modulus.

Figure 2

Schematic representation of potential mechanisms of monomer hydrolysis within the hybrid layer. A) Collagen fibrils (in cross section) require hermetic enveloping with adhesive monomer, however nanoscale voids may allow for diffusion of water from the pulp, which leads to the formation of the so-called water trees. B) Triple-helical collagen molecules, which make up the collagen fibril, contain inherent hydrogen bridges (in C highlighted in red for a single cross-sectional region in a molecule) which allow for diffusion and ‘storage’ of water molecules within the protein. D) The interaction of the water molecules, which is an inherent property of collagen in physiologic conditions, leads to degradation of hydrolysable and degradable ester bonds in common dental monomers (TEGDMA and bisGMA shown – ester bonds highlighted in red).

Figure 3

Proteoglycans (PGs) and glycosaminoglycans (GAGs) in peritubular and intertubular dentin. A) Mild acid etching reveals organic network within peritubular space. B) Enzymatic removal of GAGs reveals the dentin lamina limitans, an organic membrane separating the inter- and peritubular dentin. C) Removal of non-collagenous proteins and PGs with trypsin completely removes the lamina limitans and organic network, suggesting that the peritubular dentin is primarily composed of GAGs (network) and PG protein core (lamina limitans). In second generation harmonic microscopy healthy intertubular dentin appears homogenous, whereas removal of GAGs (E) and PGs (F) reveals voids (red circles) within the intertubular dentin matrix.

Figure 4

Microfibril bundles in dentin collagen. Demineralized fibrils: A) The D-banding pattern of collagen fibrils in demineralized dentin is preserved. Trypsin treated: B) The D-periodic pattern of the fibrils is still visible in some fibrils (arrowheads), while the untwisting phenomenon is seen (white arrow) resulting in thinner (~20 nm) microfibril bundles, which are more clearly seen in (C).

Figure 5

Mechanical contribution of PGs and GAGs. A) Creep response of mineralized dentin before and after digestion of PGs and GAGs. B) Relative creep recovery of mineralized dentin before and after digestion of PGs and GAGs.

Figure 6

Emerging examples of bioinspired engineering and biomimetic materials design: (A–D) Bioinspired materials design. (E–F) Organs-on-a-chip. A) A jigsaw-like interface is engraved in front of the main crack in a glass sample. B) When the crack propagates along the engraved interface the jigsaw tab is pulled out, and normal pressures and frictional tractions develop. C) Further impregnation of polyurethane within the cracks replicates the function of folding-unfolding sacrificial proteins in biological materials, such as enamel, nacre and bone. Combined these bioinspired toughening mechanisms can make standard glass 200 times tougher. E) Prototype to mimic periodontal inflammation on-a-chip. F) The gingival epithelium chambers house engineered cell-laden oral epithelium tissue constructs on a transwell plate insert. Addition of chemicals or inflammatory mediators stimulates exacerbation of inflammation of the epithelium which triggers cells to secrete cytokines that are microfluidically carried to the alveolar bone tissue construct downstream, where inflammation mediated osteoclastic activity is activated and measured in real time via soluble factors.