Bioprocessing-inspired Design And Synthesis Of Nanocomposites

Natural organisms can efficiently and accurately synthesize biological structuralmaterials at environment temperature to provide special functions such as support and protection.These biological structural materials show unique hierarchical structures and excellent mechanical properties,resulting from billions of years of evolution and natural selection.In the past decades,good results have been achieved by imitating the structure or function similar to biomaterials.More fundamentally,we focus on that these fantastic natural structures are formed by specific biological processes.Studying the relationship between biological manufacturing process or natural manufacturing/biological structure reveals the regulation mechanism of organisms on material composition and structure formation.It gets inspiration to guide"bioprocessing-inspired fabrication".One of the significant features of the formation process of structural materials is that insoluble organic matrix is acted as the organic framework while soluble organic matrix and inorganic ions are performed as the regulation.Teeth and shells are the most widely studied natural minerals.Their multi-level ordered organic/inorganic composite structure endows their excellent mechanical strength and toughness.For example,natural shell nacre contains 95%aragonite calcium carbonate and 5%organic matrix,but its strength and toughness are 3000 times that of natural aragonite minerals.The tooth can withstand about 4000 chews a day and maintain its structural integrity.There are many difficulties in preparing artificial tooth-like or shell-like structural materials.To solve these problems,starting with the formation process of teeth and shells,by analyzing the key regulatory factors,FAP/PVP/PAA enamel-like organic/inorganic nanocomposites and Ca CO₃/chitosan nacre-like organic/inorganic nanocomposites were successfully synthesized in vitro at room temperature and showed excellent mechanical properties.The study also revealed a non-classical single crystal growth mechanism based on the aggregation and rearrangement of nanoparticles,which led to the specific distribution of magnesium ions,so as to optimize the microstructure and improve the mechanical properties of nanocomposites.The research results are as follows:First,a two-step mineralization method inspired by the biological process wasproposed.The fluorapatite（FAP）nanorod arrays were synthesized in vitro at room temperature,and the FAP/PVP/PAA composites were prepared by the layer-by-layer mineralization method.The composite is a multilayer columnar structure microscopically formed by the gap and interlayer of fluorapatite nanorod array filled with PVP and PAA.We also investigated the effect of Mg²⁺on the microstructure of nanorod arrays.It is confirmed that Mg²⁺can reduce the size of a single nanorod and greatly improve the order and compactness of the array.The obtained Mg²⁺regulated fluorapatite nanoarray（FAP-M）has excellent mechanical strength and toughness.The hardness is 2.42±0.05 GPa and Young’s modulus is 81.5±2.7 GPa.The viscoelastic test shows that its loss modulus is 0.65±0.05 GPa,which is equivalent to that of natural enamel.Second,on the basis of the previous research,continue to further study the regulation mechanism of Mg²⁺on fluorapatite array.With the help of Atom Probe Tomography（APT）,the specific distribution of Mg²⁺in FAP-M arrays and single nanorods was analyzed at an atomic scale.It is confirmed that most Mg²⁺will preferentially gather at amorphous intergranular regions to form amorphous intergranular phase reinforced structures which acts like glue connecting the nanorods.Secondly,by exploring the growth mechanism of single crystal nanorods in this process,a non-classical single crystal growth mechanism is revealed:driven by the inherent surface pressure,nanoparticles contact and fuse with each other,and then crystallize and rearrange from inside to outside to form a complete nanorod single crystal.In this process,Mg²⁺will gradually segregate to the edge of the nanorod to form a Mg-rich amorphous layer.The amorphous layer at the edge hinders the lateral growth of nanorods and ultimately limits the size of nanorods.In the process of competitive growth,the amorphous phases at the edge of nanorods will fuse with each other to form amorphous intergranular phase,which serving like glue connects the nanorods to form a reinforced array.Third,using the synthesis method of in-situ mineralization,under the control of PAA,it attracts the positively charged groups on the surface of chitosan to form a polyelectrolyte complex,which can also attract Ca²⁺in the solution.Finally,a calcite calcium carbonate layer composed of nanoparticles was grown on the surface of chitosan film.Organic/inorganic multilayer composite materials with high mechanical properties were successfully synthesized by layer-by-layer mineralization,which also has excellent mechanical properties.Finally,inspired by the whole structure of tooth,FAP-M/Ca CO₃double-layer heterostructure nanocomposites with tooth like structure were successfully prepared by a three-step mineralization method at room temperature.Using amorphous fluorapatite nanoparticles as the transition layer,columnar FAP-M nanoarrays were successfully mineralized on the layered calcium carbonate calcite layer.The two layers are closely bonded,and the composites show good mechanical properties.The smooth calcium carbonate layer controlled by magnesium ions provides conditions for the subsequent mineralization of heterostructure composites.