Reasearch On Biomimetic 3D Printing System Towards Complex Composition Gradient And Its 3D/4D Printing Application

Gradients are widely common seen in biological materials.These materials utilize complex gradient design with spatially non-linearly varying properties to obtain superior performance beyond the artificial materials,such as stiffness but toughness,lightness but strength,self-adaptation and multi-functionality.It is very important to understand the construction principle of complex gradients in biological materials.Designing and manufacturing biomimetic gradient materials is one of the vital development direction for the next generation of advanced materials.However,despite extensive works have been done,the manufacture of functionally graded materials?FGMs?stillremainssignificantchallenges,especiallyfor three-dimensional nonlinear graded materials.The emerging additive manufacturing?3D printing?technology is a digitally-driven,material-additive forming technology with potential control of material composition and structure,making it the most promising method for fabricating gradient materials.However,due to technical limitations,the manufacture of gradient materials with continuously varying composition is difficult for current 3D printing technologies.Therefore,in order to realize the spatial nonlinear distribution of materials,it is urgent to develop a new gradient 3D printing technology.Then,the new properties brought by the material gradient can be explored to meet various engineering needs.4D printing refers to that the physical properties,functions and shapes of 3D printed materials and structures can be spontaneously and programmably evolved with time when they are exposed to a predetermined stimulus,such as heat,water,light,pH,etc.,which has the capability to create intelligent material systems that combine sensing,control and response functions.4D printing has the advantages of High volume efficiency and quality efficiency,high reliability,self-driving capability and multi-functionality over traditional automation systems,making it has broad application prospects in many fields,such as soft robots,biomedical devices,drug delivery and aerospace intelligent structures,etc.It is difficult to design the control logic and print it in the material for 4D printing,which is the core of 4D printing.That is,how to integrate material and structure design and embed the nonlinear control logic into the material by 4D print to produce the predetermined nonlinear response behavior.The self-deforming behavior of some plants can provide inspiration for the design of 4D printing.Plants utilize the gradient distribution of active tissue to produce controlled deformation.By analyzing the deformation characteristics of plants and dissecting the relationship between their material distribution and their deformation behavior,biomimetic design methods for 4D printing can be established.Herein,based on complex composition gradient accessible by biological materials,a new 3D/4D printing method capable of realizing continuous three-dimensional nonlinear complex gradient,termed as‘biomimetic gradient 3D/4D printing',was developed.This work presented a biomimetic gradient 3D printing system,which handled a number of key technologies,and established a full-process hardware and software environment for implementing gradient 3D printing.The usability of the biomimetic gradient printing system and the wide material compatibility were verified by printing 1D,2D and 3D gradient materials with different material properties.Biomimetic gradient 4D printing research was carried out by combining biomimetic design,responsive materials and gradient printing technology.The effects of material structure configuration,compositional change and external stimuli on shape deformation were systematically explored,and the principle of non-uniform responsive deformation and 4D printing methods programmed by external stimuli were revealed.The deformation effects of gradient distribution of the active material and the programming results of external stimuli were verified by 4D printing of various gradient heterogeneous structures,demonstrating various complex 4D deformation functions.This paper has achieved the following main research results:?1?A biomimetic gradient 3D printing system based on material extrusion technology was developed to realize the fabrication of 3D nonlinear complex gradient materials.This system integrates the necessary functions of gradient printing,including gradient modelling,data conversion,digital feeding,dynamic mixing,and material extrusion.The software and hardware system were established for completing gradient printing.Using voxel-based and grayscale-based algorithms,function-based gradient modeling software was developed to create print models containing both material composition information and geometric information.The software used for controlling gradient printing was developed to convert the geometry and material information contained in gradient models into the commands of material transport and 3D motion.The material digital feeding and 3D motion platform were built,which could print materials containing pre-designed components in the corresponding geometric position according to the specified ratio.?2?The manufacturing capabilities of the biomimetic gradient 3D printing system were verified by formulating different printing materials with different properties and designing various gradient distribution models,confirming the practicality of the printing system and material suitability.The design and print of a variety of color gradient samples demonstrate the printing system's ability to control material gradient distribution.The spatial distribution of particles in the printed 3D solid with nano-Al₂O₃ particles concentrate gradient was verified by CT scanning,proving the capability of 3D gradient printing of this system.The printed samples with gradiently distributed mechanical properties by reactive 3D printing polyurethane characterizes the effect of the gradient on the mechanical properties of the material,and demonstrates the effect of the material gradient distribution on alleviate the stress concentration analogous to biological materials.?3?Combining the biomimetic gradient 3D printing method and the active elastomer material with water-swelling property,a biomimetic gradient 4D printing strategy is proposed,which provides a general strategy for realizing programmable non-uniform site-specific deformation.By designing bilayer structures with differential expansion properties to simulate the non-uniform distribution of the active tissue in botany,the continuous gradient change of the water-swelling ratio of the material is achieved by using the gradient 3D printing technology.The difference of internal strain between two layers is generated duo to the different swelling ratio of the two layers,causing the smooth and continuous change in the degree of bending deformation.The mathematical relationship between the deformation and the difference of water-swelling ratios between the two layer materials was analyzed,and a mathematical theoretical model describing the gradient and deformation shape is established.The finite element?FE?analysis is also used to simulate the shape change of the printed structures,which verify the feasibility of the biomimetic gradient 4D printing technology.The strategy of"theoretical calculation-simulation analysis-experimental verification"for 4D printing is proposed,which has the advantages of accuracy and efficiency compared with the commonly used"trial and error"research method.It is a universally applicable 4D printing forward design method.?4?Combining the proposed biomimetic gradient 4D printing strategy and a SMP material with thermal contraction deformation,a dual temperature-and humidity-responsive bilayer structure was designed and printed.This bilayer structure can not only generate non-uniform bending deformation in response to humidity alone,or receive temperature stimuli alone to generate uniform bending or uniform spiral deformation,but also generate non-uniform spiral deformation under the superposition of temperature and humidity.By designing the geometric parameters and material distribution modes of the bilayer structure,a variety of dual-responsive heterogeneous bilayer structures with non-uniformly distributed materials were designed and printed out utilizing the relationship between the shrinkage degree of SMP material and temperature,and the relationship between the degree of spiral deformation and the printing angle.The shape change of this composite structure under different stimuli proves the feasibility of using external stimuli to programme and control 4D printing.Finally,as a proof-of-concept,a soft robot that can grab objects and bear 80 times its own mass was designed and printed out.The non-uniform spiral deformation caused by the combined excitation of temperature and humidity endows the robot more gripping force.