Construction Of Bioinspired Ordered Structured Materials By Controlled Assembly Of Ultralong Hydroxyapatite Nanowires And Their Properties

Living organisms in nature have undergone continuous evolution over billions of years,gradually evolving optimized structures with functions such as mechanical support,protection and motion,thus many natural materials exhibit excellent mechanical properties.For example,in human hard tissues,one-dimensional hydroxyapatite nanostructures with poor mechanical properties self-assemble into high-performance structural materials with high strength,high modulus and high toughness.In addition,hydroxyapatite materials have the advantages of high biocompatibility,high bioactivity,and high thermal stability,thus has a great potential in various applications.However,the biomimetic ordered assembly of hydroxyapatite nanostructures is still at the stage of low-dimensional or simple ordered structure imitation,and the obtained macroscopic assembly structures and properties are far behind those of natural materials,which greatly limits the practical applications of hydroxyapatite-based materials.The challenges for this are,on the one hand,the difficulty of further assembly of hydroxyapatite nanostructures into macroscopic ordered materials,and multi-scale and multi-level orderly regulation of hydroxyapatite nanostructures are difficult.On the other hand,the structural and strength regulations of the interfaces are not precise enough to coordinate the conflict of strength,stiffness and toughness.In this dissertation,ultralong hydroxyapatite nanowires with high flexibility are used as the structural units for controlled ordered assembly into high-performance materials with multi-scale and multi-level ordered structures,and the properties of the as-prepared materials are investigated.The main research contents are as follows:(1)Rapid synthesis of ultralong hydroxyapatite nanowires by microwave-assisted hydrothermal method.Ultralong hydroxyapatite nanowires with high aspect ratios and high flexibility are synthesized by the microwave-assisted calcium oleate precursor hydrothermal method.The as-prepared ultralong hydroxyapatite nanowires have lengths of hundreds of micrometers,and spontaneously assemble into nanowire bundles along their axial direction.The effects of reaction parameters on the product are investigated,including the microwave heating time,temperature,type of reactants,solvent type,and calcium/phosphorus ratio.Compared with the traditional hydrothermal method,this method has the advantages of high efficiency,rapidness,and energy saving,and the reaction time can be shortened by two orders of magnitude.The as-prepared ultralong hydroxyapatite nanowires are the ideal building material for the preparation of the highly flexible fire-resistant paper,which is promising for applications in various fields.(2)Preparation of multiscale ordered enamel-like structures by program-controlled injection method.Inspired by the multiscale ordered self-assembly structure of tooth enamel,we developed a novel bottom-up multi-stage assembly strategy to construct a enamel-mimetic structural material based on ultralong hydroxyapatite nanowires,achieving a controlled multiscale ordered structure:firstly,the ultralong hydroxyapatite nanowires are synthesized by the calcium oleate precursor solvothermal method,and ultralong hydroxyapatite nanowires are orderly self-assembled into nanowire bundles at the nanoscale.The solvothermal slurry containing ultralong hydroxyapatite nanowires with liquid crystal behaviour is injected using a syringe to produce highly ordered ultralong hydroxyapatite nanowire macrofibres,which are further used to construct the macroscopic bulk sample by a well-controlled procedure,and thus achieving the ordered arrangement of ultralong hydroxyapatite nanowires from the nanoscale to the microscale to the macroscale.The as-prepared structural material exhibits a significant mechanical enhancement after reinforced by organic compounds(its compressive modulus is 28 times that of the unreinforced one),and shows excellent mechanical stability and high biocompatibility.It is found that the weak interface facilitates the strain transmission under external forces,and the fracture of the ordered ultralong hydroxyapatite nanowires can effectively absorb the energy,which can improve the mechanical properties of the material.Different from those conventional enamel-like structural materials with microscale ordered structures,the enamel-like structural materials prepared by our strategy can achieve a high degree of internal structural order at the centimetre scale.In addition,this strategy is applicable to prepare enamel-like structural materials with diverse shapes,complex structures and excellent properties by changing the program and organic species,further extending the applications of hydroxyapatite materials.(3)Multistage assembly of ordered fiberboard-and-mortar structural materials with the combination of high strength and high toughness.Inspired by the brick-and-mortar structure of nacre,the enamel-mimetic multiscale ordered ultralong hydroxyapatite nanowires are pressed into the fiberboard,and we have designed a novel fiberboard-and-mortar multiscale ordered structure based on the multi-level ordered assembly of ultralong hydroxyapatite nanowires.The flexural strength,stiffness and toughness of the as-prepared structural material are 308MPa,35 GPa and 4.17 MPa·m^1/2,respectively,which are much better than those of the reported hydroxyapatite-organic composites and many natural and other synthetic materials.The lower density(lightweight)of the as-prepared material endows it with ultrahigh specific strength and specific toughness,which is more competitive in practical applications.The as-prepared structural material exhibits both enamel and nacre strengthening and toughening mechanisms:the highly ordered ultralong hydroxyapatite nanowires within the fiberboard can effectively transfer the stress and avoid stress concentration;highly ordered ultralong hydroxyapatite nanowires are well infiltrated with the organic phase and form the moderate-strength interface at the nanoscale to increase the bonding and frictional forces which need to be overcome during the pull-out of nanowire bundle;and the layered structure allows the interfacial debonding during loading.In addition,the layered structure can deflect the crack direction and prolong the crack extension path.Furthermore,the as-prepared material exhibits high impact resistance,good damping properties and durability,and has good potential for various applications in biomedical and safety protection fields,expanding the application scope of the hydroxyapatite materials.(4)Biomimetic flexible,high-strength and versatile fiberboard-and-mortar structural hydrogel.Water participates in the transportation of substances in the living body,thus endows the tissues with multifunctions.Inspired by this,we have prepared a flexible,high-strength and versatile fiberboard-and-mortar structural hydrogel based on highly ordered ultralong hydroxyapatite nanowires and polyacrylic acid,thus extending diversified applications of hydroxyapatite-based materials.Due to the superiority of the fiberboard-and-mortar structure,the as-prepared hydrogel exhibits high strength and excellent flexibility.The as-prepared hydrogel has high water content and high strength because of the strong interactions between ultralong hydroxyapatite nanowires and polyacrylic acid.The water content,strength and Young's modulus of the as-prepared hydrogel are comparable to those of human cartilage,which are better than those of many reported hydrogel materials.The typical energy dissipation behaviors of the hydrogel such as the massive nanowire fracture,nanowire pull-out,nanowire-organic adhesion and nanowire deformation,pore rupture and crack bridges during the tensile fracture make the hydrogel effective in avoiding stress concentration,and therefore,combine high strength and high toughness.In addition,the composition and structure of the hydrogel can be controlled,thus the functional regions of the ordered structure can be designed by ion-doped ultralong hydroxyapatite nanowires and programmed injection process for multifunctional hydrogels with the Janus structure.The abundant ordered water channels inside the hydrogel have good potential for the adsorption of small molecules and heavy metal ions as well as the directed transport of ions.