| Natural biomaterials such as tooth enamel and bone usually have excellent integrated mechanical properties due to their hierarchical structure and amorphous intergranular phases(AIP).In particular,the highly mineralized enamel,the hardest tissue in the body,is tough enough to resist impact and vibration during continuous occlusion.This excellent integrated mechanical properties and hierarchical structure of tooth enamel have attracted interest in the field of restorative materials and clinical medical materials.And as the mechanical properties of teeth gradually decline,defect or even fall out with the extension of life span,there is an increasing demand for dental restorative materials and whole tooth regeneration.However,most currently reported bionic enamel materials are prepared by the simple chemical assembly,which is difficult to mimic the multi-level ordered structure of natural tooth enamel;most of them differ greatly from natural tooth enamel in terms of microscopic morphology and macroscopic scale.In this paper,we used artificial hydroxyapatite(HA),silk protein(sf)and biocompatible polyvinyl alcohol(PVA)as raw materials to simulate the composition of tooth enamel to prepare biomimetic enamel materials with similar components and multiscale order as natural tooth enamel by directional freezing.In the first part,hydroxyapatite powders with different particle sizes and surface morphologies were synthesized by hydrothermal and chemical precipitation methods.The feasibility of preparing long rod-shaped hydroxyapatite,magnetic spherical hydroxyapatite and interlaced woven hydroxyapatite with calcium oleate as precursors for drilling mimetic dental enamel was investigated of microscopic morphology,biocompatibility and micromechanical properties,respectively.Among the three,the interlaced woven HA with calcium oleate as precursor had an ordered microstructure similar to tooth enamel and the best micromechanical properties.Young’s modulus and hardness of 3843.33 ± 22.34 MPa and 434.36±20.23 MPa,respectively.Biocompatibility was tested by in vitro cellular assays,demonstrating that all three synthetic HAs were biocompatible.The interlaced woven HA with calcium oleate as the precursor in the previous chapter was selected.The composite scaffolds with enamel fractions similar to tooth enamel were prepared by homogeneous mixing with PVA and silk protein with different cross-linking agents(glutaraldehyde and genipin).The effects of HA and cross-linking agent content in the 3D composite scaffolds on the mechanical strength and biocompatibility of the composite scaffolds were systematically investigated.The results showed that the type and concentration of crosslinker had a great influence on the microscopic mechanical properties of HA/sf/PVA composite scaffolds,and when genipin was used as the crosslinker concentration of 0.05 wt.%with the highest HA content,the mechanical properties of HA/sf/PVA composite scaffolds were significantly improved with a compressive strength of 117±6.12 MPa.The Young’s modulus and hardness measured by nanoindentation were When glutaraldehyde was used as the crosslinking agent at a concentration of 0.1 wt.%and the highest HA content,the compressive strength was 125±8.32 MPa,and Young’s modulus and hardness measured by nanoindentation were 61.61±6.84 GPa and 2.14±0.14 GPa.Compared with the HA/sf/PVA composite scaffold without the cross-linking agent,Young’s modulus and hardness of the nanoindentation were 61.61±6.84 GPa and 2.14±0.14 GPa.PVA composite scaffold without cross-linker,Young’s modulus increased by about 2 times,and the hardness increased by 2.6 and 2.9 times,respectively.The organic contents of the composite scaffolds prepared by the thermogravimetric analysis were 15.22 wt.%,11.62 wt.%,and 10.64 wt.%for the crosslinked scaffolds prepared with genipin and 17.72 wt.%,11.13 wt.%,and 10.03 wt.%for the scaffolds prepared with glutaraldehyde as the cross-linking agent.Combined with the analysis of the mechanical results,it can be seen that the higher the content of interlaced woven HA with calcium oleate as a precursor in the composite scaffold,the better the mechanical properties of the HA/sf/PVA composite scaffold when the cross-linking agent conditions are the same.The results showed that at the concentration of 0.05 wt.%of genipin or 0.1 wt.%of glutaraldehyde and a solid content of about 10 wt.%of interleaved HA,the enamel-like material with micromechanical properties similar to those of human enamel could be prepared.Still,the HA/sf/PVA composite scaffold lacked the characteristics of a multi-level assembly of natural enamel.The overall ordered and oriented structure and compressive strength differed greatly from those of.However,the HA/sf/PVA composite scaffolds lacked the characteristics of a multi-level assembly of natural tooth enamel.The overall ordered and oriented structure and compressive strength were still far from human tooth enamel.Inspired by the formation process of natural tooth enamel,the third part of the thesis further mimics tooth enamel by doping interlaced woven HA with calcium oleate as a precursor with Mg2+ and preparing Mg-doped hydroxyapatite(Mg-HA),PVA and sf into an overall highly ordered composite by the multi-stage assembly and gradient oriented freezing.The structure and component characterization of different assembly stages were analyzed,and finally,the highly rated structure of Mg-HA from microscopic to macroscopic composites was achieved.The results showed that the fabricated bionic tooth enamel material had highly ordered and dense structural characteristics similar to natural tooth enamel.Its microscopic mechanical and compressive strengths were identical to those of human tooth enamel,with Young’s modulus,hardness and compressive strengths of about 66.25 GPa,3.92 GPa and 209 MPa,and the bionic tooth enamel material still had good mechanical stability after 60 days of immersion in acid and alkaline solutions.In vitro cellular experiments showed that the bionic tooth enamel had no significant cytotoxicity. |
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