| After a long period of survival of the fittest,biomaterials have evolved many excellent properties that surpass those of artificial materials,such as light and strong,strong and tough,and self-adaptive,etc.The core characteristic of biomaterials is of high efficiency,achieving ultra-high specific strength and optimal functionality with minimal material consumption.To achieve efficient enhancement,biomaterials are constructed using a strategy of selective(region-specific,direction-specific,and indexspecific)rather than uniform enhancement,thereby reducing weight and increasing efficiency.Unlike man-made materials that rely on the inherent properties of their components,biomaterials usually utilize internal microstructures to achieve modulation of their functions.The anisotropy and selective enhancement are generated using structural design,thus producing an almost infinite number of variations to meet a diversity of functional needs.To develop efficient biomimetic materials,we need to learn the strategies of biomaterials for the selective enhancement of material properties based on working conditions and the region-selective enhancement using material structures.Biomaterials are structured by using various reinforcing units(polysaccharide fibers,mineral lamellae)to form complex spatial arrangements,such as herringbone structure with spiral arrangement of fibers,and brick and clay structures with accumulation of lamellae.Traditional manufacturing methods mainly focus on shaping the geometries of materials,and lack the ability to control the internal organization of materials,making it difficult to create biomimetic materials with complicated internal structures.In addition to the ability to prepare complex geometries of the materials,3D printing technology has the potential to control the internal organization due to its inherent property of bottom-up and point-by-point accumulation of materials.The properties of3 D printed material are formed during printing,which gives us the opportunity to control the internal organization of the material to obtain the desired properties through the design of the process parameters.4D printing is 3D printing of smart material that can produce pre-designed geometry or physicochemical changes under external stimuli(e.g.,temperature,light,humidity,etc.).How to design the changes is the key to 4D printing.Borrowing the biological strategies of controlling shape-morphing,e.g.the pinecones utilize oriented fibers to open and close their scales,is an essential tool for4 D printing.This work combines field-assisted technologies with 3D printing technology to manufacturing biomimetic oriented structural materials,including magnetic-fieldassisted 3D printing and flow-field-assisted 3D printing based on light curing and powder bed technique,and flow-field-assisted 3D printing based on direct writing technique.The effects of printing parameters and material properties on the orientation of the reinforcing fibers are investigated,and the optimized parameters and material formulations are obtained.Employing the developed printing technology and biomimetic design,we printed biomimetic mechanical reinforced materials and biomimetic shape-morphing materials.We examine the influence of the internal organization and printing parameters on the mechanical properties and shape-morphing characteristics of the biomimetic structures,which provides reliable support for the preparation of biomimetic materials.It also facilitates the further exploration and verification of structure-property relationships of biomaterials with valid experimental results.The content of this paper includes the following four aspects:(1)For different material systems and target structures,light-curing-based magneticfield-assisted 3D printing and flow-field-assisted 3D printing,and extrusion-based flow-field-assisted 3D printing process were developed to explore the structureproperty relationship of biological materials.In addition,their manufacturing principle,system construction and processing flow were introduced in detail.The magnetic fieldassisted 4D printing process has a larger orientation freedom and higher orientation accuracy,but it is only applicable to magnetic-sensitive fibers.Under the dynamic induction of the magnetic field,the fibers form adjustable orientation degrees and orientation angles in response to the moving magnetic stripes.The relationship between the magnetic field intensity,the layer thickness and the fiber orientation degree was quantitatively characterized.The powder bed flow-field-assisted 3D printing process is aiming at viscous materials with high solid content,where the enhanced fibers can be oriented along the motion direction of the scraper with the mechanical induction.The effect of layer thickness and fiber content on the degree of fiber orientation was quantitatively characterized.The flow-field-assisted 3D printing process employs the shear-induced force on the inner wall of the needle to make the fibers highly oriented and precisely controlled with no need to be functionalized.During the extrusion process,the polymer crystal domains are transformed from random to aligned state.The experiments explored the effects of print speed,print pressure and needle diameter on the filament diameter,and quantitatively characterized the relationship between needle diameter and fiber orientation degree.(2)Utilizing the powder bed flow-field-assisted 3D printing process,the biomimetic structural materials were prepared,and the relationship between mechanical properties and biomimetic structural designs was investigated.The technique can adjust the local microstructure through the dynamic induction,which has the advantages of high efficiency and wide range of material selection..The particles do not require functionalization(magnetization,polarization,etc.),and can print highly viscous materials with high solids content.A mathematical model was used to analyze the mechanisms in the alignment process.The influence of parameters such as layer thickness,fiber aspect ratio and fiber content on the degree of fiber orientation was clarified.Inspired from the sinusoidal and herringbone structure of the mantis shrimp clamp,biomimetic fiber-reinforced materials were manufactured.The effects of fiber content,fiber aspect ratio and aligned structure on the compression properties and impact resistance were investigated,and the relationship between fiber orientation and the mechanical properties was revealed.This research proposed a biomimetic design method for structural materials.(3)Utilizing the magnetic-field-assisted 4D printing process,the biomimetic shapemorphing materials were prepared,and the relationship between the biomimetic structural designs and shape-morphing characteristics was investigated.This technique can adjust the local microstructure through the dynamic magnetic induction,which has the superiority of large orientation degree,high orientation accuracy,controllable alignment direction and adjustable orientation degree.The associations between the height of the magnetic stripe,the movement path and the orientation degree,the orientation angle were discussed and validated with mathematical models and experimental results,providing a theoretical framework for the design of locally anisotropic biomimetic materials.Inspired by biological prototypes such as pinecones scales,wheat awns,and soft wood branches that can dynamically change by environmental stimuli,a shape memory composite with biomimetic structures was fabricated using the magnetic-field-assisted 4D printing process and smart materials.The effects of fiber alignment angle and fiber alignment degree on the stress-driven deformation and shape memory properties were investigated,and the relationship between alignment structures and shape memory properties was established.By applying the relationship between fiber structure and deformation properties,the selffolding flowers,sequential unfolding gripper and gradient unfolding tentacles were printed,providing theoretical basis for precise control of soft robots,sequential release of drug carriers,and intelligent unfolding of satellites,etc.(4)Utilizing the flow-field-assisted 4D printing process,the biomimetic shapemorphing materials were prepared,and the relationship between the biomimetic structural designs,processing parameters and shape-morphing characteristics was investigated.The parameter-driven 4D printing is proposed,i.e.,the material properties can be modified by printing parameters on the fly.The method enables varying material properties using only a single material and can yield gradient structures and properties.The influences of the process parameters,the biomimetic structures on the orientation degrees,the deformation characteristics were investigated.It was found that print speed significantly influenced the orientation degree,where higher speeds resulted in larger orientation degree.The print path has a significant impact on the deformation pattern,and bending,twisting and curling deformations can be realized through the design of the print path.In addition,two theories were used to interpret the intrinsic mechanism of print speed-regulated deformation.By adjusting the distribution of print speed in specific areas and the design of the print path,various forms of deformation,such as snap-through,self-assembly and self-oscillation behaviors,have been achieved.The gradient shape-morphing,such as serpentine curling,was achieved using gradient print speed.The experimental results of these deformations were further verified using a combination of experiments and simulations.Parameter-driven 4D printing unleashes the potential of the printing process and expands the programming space of 4D printing. |
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