Academician Jiang Lei of Beihang University, Professor Lin Guo/Professor Xuliang Deng of Peking University Stomatological Hospital, Science: The strongest artificial enamel in history

Tooth enamel is the hardest natural biological material in the human body, with a hardness only slightly lower than diamond, comparable to crystal, and excellent resistance to deformation and vibration damage.The translucent material covering the surface of our teeth is about 2mm thick (Figure 1A) and contains up to 96% inorganic minerals, consisting of tightly arranged hydroxyapatite (HA) nanowires.With a diameter of 30 to 50 nm self-assembly HA nanowires are aligned with each other, through close together first glaze column about 5 microns in diameter (figure 1 b), and then the further cross arrangement in the glaze formation of highly ordered hierarchical structure (FIG. 1 c and D), make the enamel rock solid, so that we can freely cutting, grinding, food.In natural tooth enamel, most crystalline fragments of HA nanowire are connected with each other through amorphous intercrystalline phase (AIP, Mg substituted amorphous calcium phosphate), and the interface has strong chemical bond cooperation (Figure 1E) to meet the required mechanical strength and toughness.Figure 1. Complex hierarchical structure of natural tooth enamel However, enamel, as a highly mineralized biological tissue, can be considered almost purely inorganic and cannot regenerate due to the lack of a bioorganic substrate, including cells.From the first days of permanent tooth growth, tooth enamel is slowly worn down, accelerated by the release of acids from sugars in bacteria’s fermented food and acidic drinks.Once the enamel’s defenses are breached, the whole tooth is left unprotected.Tooth decay, which is a nightmare for many people, starts with the loss of enamel.Repairing tooth enamel is one of the most hardcore challenges in bionics.Biomineralization, inorganic templates can be used synthetic methods of bionic structure of natural tooth enamel, but because of the complexity of the structure of the enamel, is also unable to effectively obtain the same as the natural enamel multilevel structure of large areas of repair layer, also did not various properties of natural teeth and name, from the clinical application and the far distance.In order to solve these problems, Prof. Lei Jiang, Prof. Lin Guo, Peking University Stomatological Hospital, Prof. Xuliang Deng, And Prof. Nicholas A. Kotov, Institute of Biological Interfaces, University of Michigan, United States of America,Through two-way frozen alignment assembly formed the hydroxyapatite nanowires and polyvinyl alcohol intergranular phase (AIP) of amorphous coating, design a with multi-scale highly ordered HA hierarchy of artificial enamel (ATE), and to realize the natural tooth orderly (close) the composition, structure and performance (mineralization) mechanics and the perfect moment.The obtained ATE exhibits excellent mechanical properties, achieving a perfect combination of high stiffness (105.6 ± 12.1 GPa), hardness (5.9 ± 0.6 GPa), strength, viscoelasticity and toughness, which not only exceeds the reported HA matrix composites and ceramic-polymer matrix composites, but even exceeds natural enamel.The results were published in Science as “Multiscale Engineered Artificial Tooth Enamel,”Hewei Zhao, Shaojia Liu and Yonghai Yue from Beihang University and Yan Wei from School of Stomatology, Peking University were co-first authors of the paper.Accurately replicating such complex layers and the functions of natural teeth is a huge challenge.To this end, we first synthesized HA nanowires growing along the direction [001] without obvious defects by solvothermal method.The ZrO2 (A-Zro2) amorphous layer with A thickness of about 3 nm was grown in situ by in-situ hydrolysis of Zr precursor.The fracture strength and strain of HA@A-ZrO2 nanowires obtained are ~ 1.6gpa and ~6.2% (Figure 2G), which are 2.5 times and 1.6 times (~ 0.65gpa and ~4%) of pure HA nanowires, respectively.Moreover, the nanowires can withstand up to ~5.2% tensile deformation before fracture.The results show that the amorphous zirconia coating can effectively improve the interface properties and mechanical properties of the materials.Finally, the researchers freeze HA@A-ZrO2 nanowire dispersions in the presence of polyvinyl alcohol (PVA) bidirectionally and self-assemble to form macrocomposites with parallel nanowires.After freeze-drying and mechanical compression, dense artificial enamel (ATE) was obtained (Figure 2H).Polydimethylsiloxane (PDMS) wedges generate bidirectional temperature gradients that drive ice crystals to grow vertically and horizontally (Figure 2H).Vertical growth of ice crystals forces HA@A-ZrO2 nanowires and PVA to occupy the gap between the ice sheets, and parallel growth forces them to acquire parallel orientations.It is important to note that ATE is machined and can form toothlike macroshapes (FIG. 2I), with densely arranged parallel columns and microscale arrangements (FIG. 2J).The AIP layers between HA nanowires were almost identical to those in enamel (Figure 2K), which perfectly reproduced the multi-scale hierarchical structure of natural enamel.Figure 2. Synthesis and characterization of artificial tooth enamel (ATE) To explore the role of each structural element, the researchers prepared two ha-based reference composites,That is, ATE (ATE-NAIP) of nanowires without AIP and ATE (ATE-NOM) of composites loaded with HA@A-ZrO2 nanowires without ordered microstructure.The average Young’s modulus (E) and hardness (H) of ATE were 105.6 ± 12.1 GPa and 5.9 ± 0.6 GPa, respectively, according to the results of nano-indentation (stiffness, hardness and viscoelasticity, loading direction parallel to the nanowires) and three-point bending (strength and toughness, loading direction perpendicular to the nanowires) tests.It is not only higher than HA matrix composites and ceramic-polymer matrix composites reported, but also higher than natural biological materials such as teeth, nales and animal bones (Figure 3).The stiffness and hardness increased by two times compared with ATE-NAIP and ATE-NOM, indicating that the parallel arrangement of nanocrystal-amorphous interfaces and micron columns helps improve the strength and hardness of the composites.At the same time, ATE also exhibits high viscoelasticity, with an average energy storage modulus (E’) of 78.6 ± 9.8gpa (FIG. 3C), and an average damping coefficient (Tan δ) of ~0.07 (FIG. 3A), which exceeds the limit of conventional engineering materials with similar energy storage modulus.In addition, the bending strength of ATE is about 142.9mpa, and the fracture strain is 0.018, which are about 2 times and 10 times of HA based ceramics respectively (FIG. 3A).Compared with ATE, the bending strength of ATE-NAIP and ATE-NOM is reduced by nearly two times, which further demonstrates the necessity of multi-scale engineering design of organic-inorganic composites.FIG. 3 Mechanical properties of artificial enamel (ATE) ATE shows a perfect combination of high strength and toughness. Such excellent mechanical properties can be attributed to the following reasons: 1.The in-situ growth of amorphous ZrO2 layer can penetrate into the damaged enamel column to form physical anchoring connection and enhance the interfacial force.At the same time, chemical bond cooperation (such as hydrogen bond, covalent bond, etc.) is formed between ZrO2 layer and HA nanowire interface to further improve the adhesion of the interface and enhance the stability.2. Both HA@A-ZrO2 nanowires and PVA matrix are tightly bonded to AIP through chemical bonds. AIP provides a buffer layer that not only promotes stress transfer, but also enhances the inorganic-organic interface, thus giving ATE excellent mechanical properties.3. PVA organic phase fills the gap of amorphous HA nanowires, which not only maximizes the slip of HA@A-ZrO2 nanowires, but also has chemical bonds at the A-Zro2 /PVA interface, thus strengthening the interface connection (FIG. 4).4. Multi-scale hierarchical structure design can dissipate energy by transferring energy from one nanowire to the organic layer and adjacent nanowire, thus avoiding structural collapse and simultaneously improving the stiffness, hardness, and viscoelasticity of ATE (Figure 5).Figure 4. Organic phase confinement and interface interaction in ATE.Figure 5. Deformation and failure modes in ATE.In conclusion, this work provides a new engineering design method and approach for the macroscopic assembly of artificial tooth enamel to perfectly reproduce the multiscale hierarchical structure of natural biological materials.The designed biomimetic composite retains the structural complexity of the biological prototype, and achieves a perfect combination of high stiffness, hardness, strength, viscoelasticity and toughness.Zhaoet al., Multiscale engineered artificial tooth enamel. Science375, 551 — 556 (2022). DOI:Source: Frontiers of Polymer Science

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