## 4 Research Advances of Plant-Mimicking Camouflage Materials
### Bionic Materials Simulating Solar Reflection Characteristics of Leaves
The multi-layer bionic camouflage material designed by [PERSON] et al. encapsulates chlorophyll in PVA films (Figure 6), and the encapsulated chlorophyll is also prone to degradation, causing changes in the reflection curve of the bionic leaves [92]. [PERSON] et al. developed urea formaldehyde polymer microcapsules coated with chlorophyll and water through encapsulation, which simulated the spectral characteristics of leaves (Figure 7) [13]. The chlorophyll encapsulated in the microcapsule was also prone to degradation, making it impossible to achieve precise simulation. After chlorophyll leaves the leaf, it will decompose within 1 day under light and has poor stability, which limit its application in the field of camouflage. [PERSON] et al. proposed a biomimetic material by using trans-polyurethane foam, super absorbent resin particles, and oil-soluble chlorophyll to simulate the mesophyll, water, and chloroplasts of green leaves, respectively (Figure 8) [90]. [PERSON] et al. developed a polyurethane-based biomimetic material-bionic porous spectrum simulation material (BPSSM) embedded with water and vat dyes (or organic pigments) (Figure 9) [93]. However, the development of these materials and technologies lags far behind the developing speed of hyperspectral reconnaissance and detection technology, and the gap is increasingly wider. Figure 8: Structure of biomimetic camouflage materials imitating plant-leaf organs [90]. Figure 6: I Structure of bionic composite material [92]. Figure 7: Schematic view of procedure for preparing poly(urea-formaldehyde) microcapsules via in situ polymerization [13]. In modern high-tech warfare, troop operations often feature large combat areas, rapid maneuvers, seasonal changes, and large background changes. Facing battlefield environments with diverse and differentiated environmental backgrounds, the existing bionic camouflage materials that only simulate green vegetation backgrounds do not have adaptive camouflage capability. The traditional stealth technology is easily ineffective and difficult to maneuver. How to make camouflaged targets better adapt to complex background environments has become the focus of research in recent years. Bionic materials in previous studies only presented static colors [16; 17; 94]. In nature, however, leaves are not always green, and their colors change in time and space (Figure 10) [95; 96; 97]. For example, the colors of leaves in autumn change in a latitude-dependent way, showing obvious changes from high to low latitudes [98]. [PERSON] et al. developed a microencapsulated colorant with a chloroplast-like structure and chlorophyll-like absorptivity and designed a universal double-layer coating to provide changing high spectral similarity with leaves at different growth stages and seasons, as well as leaves of different species [99]. Moreover, this colorant was superior to the state-of-the-art spectral modeling materials. [PERSON] et al. studied tree-leaf discoloration and developed a color-changing bionic material (CCBM) [100]. CCBM is very similar to the ultraviolet-visible-near-infrared and thermal infrared characteristics of both green and yellow leaves (Figure 11), and it can be used as a camouflage net in bionic camouflage. By reviewing the research, development, and technical improvement of hyperspectral camouflage materials that simulate vegetation backgrounds, we find no major innovations, as the traditional ideas of bionic camouflage have continued being researched. Therefore, it is urgent to use new bionic camouflage technology to fill in the gaps in hyperspectral imaging detection technology. Hyperspectral and thermal infrared detectors are often combined in modern battlefields, but multi-band camouflage using both the solar spectrum and the infrared spectrum is challenging. To address this challenge, [PERSON] et al. developed a bionic metamaterial inspired from natural leaves [101].