Mechanical metamaterials are artificial structures with mechanical properties derived from the unit cell geometry instead of the base materials. Moreover, counterintuitive mechanical properties like high strength, lightweight and negative Poisson’s ratio can be realized by adopting lattices and shells. Although mechanical metamaterials can be fabricated via additive manufacturing techniques with high fidelity, efficiency is often lost. Lately, the art of paper cutting (known as kirigami) has emerged as an alternative solution for designing and fabricating mechanical metamaterials. Generally, kirigami is an extended version of origami (paper folding).
Over the years, kirigami and origami techniques have become attractive ways of designing lightweight materials and deployable structures. They enable low-cost and high-efficiency fabrication of metamaterials, including complex three-dimensional geometries. However, most of the existing kirigami designs focus on kinematic patterns where the function is highly influenced by the motion in the folded state. However, practical applications like biomedical implants and industrial weight reduction require lightweight and strong metamaterials to meet the low-mass and high-strength needs. Unfortunately, research on lightweight and strong origami/kirigami metamaterials is limited.
Kinematic origami patterns generally produce increased stiffness due to the interlocking folds. However, these patterns require constraints or external frames to maintain their folded configuration. Despite remarkable progress in developing kirigami techniques, a comprehensive study of kirigami designs for shape morphing as well as their characterization and mechanical application under loaded conditions, is still lacking. Although sealing the collocate cuts in folded configuration could achieve high payload capability, the ability to sustain compression loads is yet to be verified.
The design of robust kirigami metamaterial that can be transformed into arbitrary complex objects faces three crucial challenges: difficulty in inventing kirigami pattern of unit cells to achieve lightweight and strong metamaterials, developing an effective algorithm for automated transformation and designing gluing connector to ensure high strength. To overcome these limitations, Dr. Hongying Zhang from the National University of Singapore and led by Professor Jamie Paik from Reconfigurable Robotics Lab at EPFL (École Polytechnique Fédérale de Lausanne) in Switzerland developed a novel kirigami-based scheme for designing and modeling strong and lightweight metamaterials. Their work is currently published in the research journal, Advanced Functional Materials.
In their approach, the authors proposed a checkerboard kirigami metamaterial with mechanical connectors. The shape-transforming and mechanical strength of checkerboard kirigami were discussed. The kirigami patterns were created by creasing the designed folds and cuts on a planar sheet and folding it on checkerboard patterns with pop-down and pop-up blocks. A computational algorithm involving the deployment of the discretized 3D objects on planar sheets was adopted for automatic shape-transformation.
The authors conducted standard compression tests and demonstrated that a kirigami metamaterial weighing 12.05 g could carry 346.4 N payloads. The improved robustness and high strength were attributed to the design of a glue-free mechanical connector capable of locking the collocated cuts in the folded configuration. Based on this design, six curved surfaces were prototyped to validate the automatic shape transforming capability of the proposed metamaterial. The folding and cutting patterns for the flat and curved surfaces were automatically achieved using the computational model.
In summary, Dr. Hongying Zhang and Professor Jamie Paik reported a novel design of kirigami metamaterials capable of withstanding high loads. The proposed kirigami metamaterial design and modeling could effectively sustain high payloads and fortify flat and curved shapes and topologies, making up complex three-dimensional objects in various applications. The results implied that the adopted design could be extended to other materials to achieve optimized weight, strength and functionality. In a statement to Advances in Engineering, Professor Jamie Paik, the corresponding and lead author stated that their findings would pave the way for widespread adoption of kirigami metamaterials in diverse industrial applications requiring high strength and weight reduction.
Zhang, H., & Paik, J. (2022). Kirigami Design and Modeling for Strong, Lightweight Metamaterials. Advanced Functional Materials, 32(21), 2107401.