# A Novel Bionic Structure Inspired by Lufta Sponge and Its Cushion Properties
## 1 Introduction  
Over millions of years, some natural structures have achieved more carrying capacity with lower energy consumption [1; 2; 3]. With the rapid development of the aerospace and transportation industries, high-strength, lightweight structures are in great need [4; 5]. The energy-absorbing structure with minimum weight and higher strength has becomes the focus of several studies [1; 6; 7]. Existing bionic objects include durian, palm, beetle Coleoptera, mother-of-pearl, fish scales, insect's forepaws, etc. [8; 9; 10; 11]. For example, the honeycomb structure is widely used in the aerospace industry and heavy packaging. Inspired by the unique microstructure of pomelo peel, [PERSON] et al. [11] researched a new type of hierarchical honeycomb structure, and studied the compressive and energy absorption properties of this structural material. Combining theory and finite element analysis (FEA) research, the energy absorption properties of the pomelo peel can be improved by increasing the structural hierarchy. The research results of pomelo peel bionic honeycomb materials provide a new perspective for the excellent mechanical properties of natural honeycomb materials and provide new ideas for the application of bionic engineering materials. [PERSON] et al. [9] used durian shell material as an alternative green material and imitated its structural characteristics to design a bionic packaging structure. The peel layer and spines in durian are important components of energy absorption because they protect the pulp during the durian landing process. Therefore, many researchers have been inspired by nature to obtain a structure with excellent mechanical performance. Luffa sponge is a natural fibrous network structure obtained by removing the outer skin and seeds of mature luffa fruits. Luffa sponge consists of four parts, which are O (out layer), M (middle layer), I (inner layer) and C (core layer) respectively, including the three main directions in Figure 1.  
Luffa sponge has excellent mechanical properties. [PERSON] et al. [12] were inspired by the porous model in the central part of the luffa cylinder. The foamed aluminum material was improved by inserting the carbon fiber composite thin-walled tube into the foamed aluminum. The experimental results show that the improved material has stronger energy absorption performance. [PERSON] et al. [13] tested the mechanical properties of luffa sponge samples for the first time. The results show that the stiffness, strength and energy absorption properties of luffa sponge can be compared with metals such as aluminum foam and Ni-P microcrystalline alloys at the same density. [PERSON] et al. [14] researched quasi-static compression test on luffa sponge, and the results showed that there were three deformation stages for the stress-strain curve of the luffa sponge, the elastic stage, the plateau stress stage, and the densification stage. It has a long plateau stress stage which means excellent buffer material. [PERSON] et al. [15] further investigated the dynamic mechanical properties of the luffa columnar samples. It was found that the compressive strength, plateau stress and specific absorption energy of luffa sponge showed obvious strain rate effects. In a word, luffa sponge is an environmentally friendly lightweight porous engineering material with great development potential. Nowadays, fractal structures have been studied and enhance mechanical properties. The essence of the fractal theory is to describe some grossly irregular and fragmented facets of nature [16]. It is more convenient than other methods to describe some natural growing plants and can be used to design a bionic structure. At present, the fractal theory has achieved some success in guiding the design of bionic structures. [PERSON] et al. [17] observed the structure of spider webs, and used fractal theory to design bionic thin-walled tubular structures. Using the regular hexagon of the spider web as the basic structural unit, small hexagonal structures are placed at the apex positions of each hexagon.