### Abstract: Advances in Mechanical Metamaterial Design and Optimization by Tsinghua University's Aerospace Department **Introduction:** The concept of metamaterials, initially rooted in the electromagnetic field, has expanded to encompass various scientific and engineering domains, including acoustics, optics, and thermodynamics. Mechanical metamaterials, a subset of this broader category, are artificial materials designed with specific microstructures, meso-connections, and macro-layouts. Unlike natural materials, which are primarily defined by their chemical composition, mechanical metamaterials emphasize structural design, often referred to as mechanical metastructures. These materials are known for their unique properties and superior performance, making them highly promising for advanced engineering applications. **Recent Development:** Recently, a research team led by Professor Jianbin Du from the Department of Aerospace Engineering at Tsinghua University has made significant strides in the design and optimization of mechanical metamaterials, particularly focusing on shell-based structures. This area of research has been relatively underexplored, and the team's novel approach aims to fill this gap by introducing a multi-layer nested hybrid strategy based on minimal surfaces. This strategy broadens and diversifies the design space, offering greater flexibility and potential for enhanced material performance. The integration of topology optimization techniques further advances the design process, moving from a traditional parameter-based approach to a more flexible configuration space, which is crucial for unlocking the full potential of these materials. **Design and Optimization Techniques:** The research team combined the multi-layer strategy with topology optimization to develop a new design methodology for shell-based lattice metamaterials. Numerical simulations demonstrated that this approach allows for the automatic formation of complex structures, including beams, plates, and shells, from multi-layer shell models. The optimized single cells of the metamaterials exhibit stiffness approaching theoretical limits, and physical experiments confirmed the accuracy of these simulations. **Mechanical Performance:** The team's optimized lattice metamaterials showed remarkable improvements in energy absorption under impact loads. The high stiffness of the lattice ensures that the material reaches plastic deformation uniformly, maximizing its utilization. Compared to the original, non-optimized models, the optimized designs can achieve up to 136% higher energy absorption with the same mass. This enhancement is attributed to the controlled deformation and uniform stress distribution within the material. **Controllability and Versatility:** The combination of the multi-layer strategy and topology optimization introduces several controllable dimensions for lattice design, such as cell shape, thickness, material composition, area fraction, number of layers, and layer arrangement. These dimensions allow for the creation of isotropic and functionally graded materials, which are essential for a wide range of applications. For instance, isotropic materials can be used in structures requiring uniform properties in all directions, while functionally graded materials can be tailored to have varying properties across different sections of the material. **Applications:** The research also explored the potential of these optimized lattice metamaterials in multi-physics problems. In acoustics, the multi-layer strategy can create a series of cavities, enhancing the material's acoustic control capabilities. This is particularly significant for designing materials with noise absorption and sound reduction properties. In the context of electric field control, the multi-layer approach enables selective control or shielding of electric fields in different regions, which is crucial for the design of solid-state battery electrodes. Additionally, the multi-layer structure can alter fluid flow patterns, providing new methods for fluid field control, especially in the design of pneumatic devices such as soft robotic actuators and flexible pneumatic limbs. **Publication and Acknowledgments:** The findings of this research, titled "Ultrastiff Metamaterials Generated Through a Multilayer Strategy and Topology Optimization," were published in *Nature Communications* on April 6, 2023. The lead author of the paper is Yang Liu, a doctoral student at Tsinghua University's Department of Aerospace Engineering, with Professor Jianbin Du serving as the corresponding author. The study was a collaborative effort involving other researchers from Tsinghua University, Nanyang Technological University in Singapore, Guangdong University of Petrochemical Science, and Northwestern University in the United States. The research was supported by the National Natural Science Foundation of China and other funding sources. **Conclusion:** This groundbreaking work by Professor Du's team at Tsinghua University represents a significant advancement in the field of mechanical metamaterials. By combining multi-layer strategies with topology optimization, the researchers have not only expanded the design possibilities but also enhanced the mechanical performance and multifunctionality of these materials. The results hold promise for a wide range of applications, from structural engineering and noise reduction to battery technology and soft robotics, highlighting the potential of mechanical metamaterials to revolutionize various industries.