## 4 Research Advances of Plant-Mimicking Camouflage Materials
### Bionic Materials Simulating Solar Reflection Characteristics of Leaves  
Achieving hyperspectral consistency with leaves is a crucial requirement for modern camouflage materials to combat hyperspectral reconnaissance [3, 77]. In nature, rod-like insects can spectrally simulate leaves [78, 79] by taking in carotenoids and chlorophyll from their plant-based diet and sequestering these pigments in their exoskeleton, forming a pattern consistent with leaves [80]. They complete camouflage by adapting their colors and spectra to match the leaves of tree species at their growth stage, season, and habitats. Therefore, many scholars have considered that pigments are the key influencing factors of the solar reflection characteristics of plant leaves and have developed dyed fabrics using dyes with similar spectral characteristics as green leaves [12, 81, 82, 83, 84, 85]. Two types of commercial inorganic green pigments, viz. Cr\({}_{2}\)O\({}_{3}\) and CoCr\({}_{2}\)O\({}_{4}\), are widely used in camouflage applications, but their spectra still differ from those of leaf surfaces. [PERSON] et al. used polyvinyl alcohol (PVA, a hydrophilic polymer) as the film-forming main body and green pigment Cr\({}_{2}\)O\({}_{3}\) and strong hygroscopic agent LiCl as additives, and they prepared biomimetic materials using a solution casting method (Figure 4) [11, 15, 86]. [PERSON] et al. used a hydrothermal method to successfully insert organic pigment anions acid green (AG) and acid yellow (AY) into the layers of magnesium and aluminum layered double hydroxides (LDHs) and thereby prepared optical materials with the visible light and near-infrared characteristics of natural green plants, a color closely related to the spectral reflectance in visible light wavebands, and colors that change with the peak position and reflectance of the visible light spectrum (Figure 5) [14], and the materials were combined with film-forming resin to prepare a bionic coating, and the colored pigment Cr\({}_{2}\)O\({}_{3}\) was also used. The reflectivity of Cr\({}_{2}\)O\({}_{3}\) shows a peak at 540 nm and an additional peak around 400 nm, although this can be compensated for by doping [87]. However, the slope of reflectance  
Figure 3: 1 Classic camouflage pattern. (**A**) The tiger-stripe camouflage of US military, (**B**) the leaf camouflage of Germany and former Soviet Union, and (**C**) the frog-shaped camouflage of US military. increase in the red-edge region is much smaller, which limits its spectral similarity with leaves. The slope of CoCr\({}_{2}\)O\({}_{4}\) in this region is close to the red edge, but its blue shading and strong absorption of tetrahedrally coordinated cobalt in the near-infrared region are undesirable [88]. In addition, several commercial vat dyes and disperse dyes were applied to match with leaf reflectance [12,89]. However, the spectral differences between these dyes and in vivo pigments may be amplified by the environmental influence of the dye molecules, making it difficult to simultaneously achieve high spectral similarity, precise red-edge reflectance, and controllable color difference. Moreover, fabricating these dyes is challenging. Although bionic camouflage based on inorganic dyes can achieve the same color, the key dyes used cannot completely have the same spectrum as green vegetation. To accurately simulate the spectral characteristics of green vegetation, one of the simplest and most effective ways is to directly use chlorophyll, which is the exact same coloring dye as that found in green leaves. Therefore, researchers have proposed an  
Figure 4: Schematic diagram of the preparation processes of the biomimetic materials [11]. Figure 5: | VIS-NIR diffuse reflectance spectrum of samples and AG pigment. (**A**) Different concentrations of acid green (**B**,**C**) different ratios of acid green and acid yellow [14]. other technical approach to bionic camouflage materials, which is to simulate the color of green vegetation by extracting chlorophyll from green leaves [13; 90; 91].