4D Printing Of Multistable Soft Materials Based On Mechanical Assembly

Multistable structure is a deformable structure with shapes that can be changed between different configurations.In addition,multistable structures do not need to consume additional energy to maintain the deformed shape after triggering,thus their excellent performance has broad application prospects in many fields,such as aerospace,soft robotics,flexible electronics,biomedical engineering.Among them,flexible multistable structures developed using soft materials（such as polymers,cells,biological soft tissues,and hydrogels）have better scalability,adaptability,and human-computer interaction than rigid multistable structures.Although the research about multistable soft materials has made great progress,the research on the preparation of multistable soft materials by 4D printing is relatively lacking.Furthermore,the study of multistable soft materials with complex Gaussian curvature is challenging.Finally,for multistable structures with n bistable units,which contain at most 2ⁿ stable configurations,multistable structures containing more stable configurations in simple structures have not yet been reported.Based on the above challenges,we prepared a bistable soft material with an elastic substrate with prestretched strain as the middle layer and two stress-free elastic sheets as the upper and lower layers by a 4D printing method based on mechanical assembly.We study the mechanics,structural design and application of bistable structures through experiments and finite element simulations.The main content of this paper includes the following three parts:（1）A method for bistable structures inspired by plant growth is proposed,and bistable structures with saddle and twisting configurations were fabricated based on 4D printing of mechanical assembly.The mechanical mechanism of the bistable structure was studied through finite element simulation.The buckling and post-buckling analysis results indicate that with increasing prestrain levels,the three-layer system buckles into a saddle or twisting configuration,depending on the geometrical parameters.The analysis of the bistable characteristics shows that during the snap-through process of the bistable structure,two strain energy minima correspond to two stable configurations.Finally,we propose 4D printing of multistable structures,with multiple stable configurations.（2）A method for 4D printing multistable structures with bending and folding deformations is presented.For the uniaxial prestretched three-layer structure,when the length of the middle layer is equal to the length of the upper and lower layers,the three-layer system is a bistable structure;when the length of the middle layer is greater than the length of the upper and lower layers,the three-layer system is an octa-stable structure.Finally,bending and folding deformation multistable structures are fabricated by assembling basic stable units.For a bending deformation multistable structure,n basic units contain 2³⁹)stable configurations.A multistable structure with 10 basic units was experimentally prepared,containing about 1 billion stable configurations.（3）Utilize the developed multistable structure,the flexible grippers and multi-mode switches are developed,and the application prospects of multi-stable soft materials in the fields of soft robots and flexible electronic devices are shown.