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Biomimetic Chameleon Soft Robot with Artificial Crypsis and Disruptive

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ARTICLE https://doi.org/10.1038/s41467-021-24916-w OPEN Biomimetic chameleon soft robot with artificial crypsis and disruptive coloration skin ✉ ✉ Hyeonseok Kim1,4, Joonhwa Choi1,4, Kyun Kyu Kim1, Phillip Won1, Sukjoon Hong 2 & Seung Hwan Ko 1,3 Development of an artificial camouflage at a complete device level remains a vastly chal- lenging task, especially under the aim of achieving more advanced and natural camouflage characteristics via high-resolution camouflage patterns. Our strategy is to integrate a ther- 1234567890():,; mochromic liquid crystal layer with the vertically stacked, patterned silver nanowire heaters in a multilayer structure to overcome the limitations of the conventional lateral pixelated scheme through the superposition of the heater-induced temperature profiles. At the same time, the weaknesses of thermochromic camouflage schemes are resolved in this study by utilizing the temperature-dependent resistance of the silver nanowire network as the process variable of the active control system. Combined with the active control system and sensing units, the complete device chameleon model successfully retrieves the local background color and matches its surface color instantaneously with natural transition characteristics to be a competent option for a next-generation artificial camouflage. 1 Applied Nano and Thermal Science Lab, Department of Mechanical Engineering, Seoul National University, Seoul, Korea. 2 Optical Nanoprocessing Lab, Department of Mechanical Engineering, BK21 FOUR ERICA-ACE Center, Hanyang University, Ansan, Gyeonggi-do, Korea. 3 Institute of Advanced Machinery and Design (SNU-IAMD)/Institute of Engineering Research, Seoul National University, Seoul, Korea. 4These authors contributed equally: Hyeonseok Kim, ✉ Joonhwa Choi. email: [email protected]; [email protected] NATURE COMMUNICATIONS | (2021) 12:4658 | https://doi.org/10.1038/s41467-021-24916-w | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-24916-w rtificial camouflage is the functional mimicry of the nat- change. The susceptible response towards temperature is indeed ural camouflage that can be observed in a wide range of an unfavorable feature for the thermal stability against changes in A 1–3 species . Especially, since the 1800s, there were a lot of the external environment, but also enables compact input range interesting studies on camouflage technology for military pur- and low power consumption during the operation once the poses which increases survivability and identification of an temperature is accurately controlled. anonymous object as belonging to a specific military force4,5. The selection of an appropriate heater together with a proper Along with previous studies on camouflage technology and nat- control circuit is therefore critical for the development of a TLC- ural camouflage, artificial camouflage is becoming an important based artificial camouflage device, and we conclude that Ag NW subject for recently evolving technologies such as advanced soft heater enables not only accurate temperature manipulation but robotics1,6–8 electronic skin in particular9–12. Background also a different route to express the fine patterns without asso- matching and disruptive coloration are generally claimed to be ciating the lateral pixelation scheme mentioned earlier. Owing to the underlying principles of camouflage covering many detailed its superior electrical and mechanical stability, Ag NW has been a subprinciples13, and these necessitate not only simple coloration promising material for flexible39,40 and stretchable heaters41,42. but also a selective expression of various disruptive patterns Also, comparing with other materials such as gold (Au) NW, according to the background. While the active camouflage found Copper (Cu) NW43, and hybrid materials (Cu-Ni NW44, Ag-Au in nature mostly relies on the mechanical action of the muscle NW45, and Ag NW-CNT46, Ag NW-PEDOT:PSS47), Ag NW has cells14–16, artificial camouflage is free from matching the actual excellent electrical conductivity and oxidation resistance as a anatomies of the color-changing animals and therefore incorpo- single material and has a cost-effective feature to be applied for rates much more diverse strategies17–22, but the dominant tech- large-area applications through a simple synthesis process. While nology for the practical artificial camouflage at visible regime at the same time, we noticed that the temperature coefficient of (400–700 nm wavelength), especially RGB domain, is not fully resistance (TCR) of the Ag NW network is sufficiently large, established so far. Since the most familiar and direct camouflage linear, and non-hysteric. These properties led us to use the strategy is to exhibit a similar color to the background23–25,a resistance of Ag NW network as the process variable of a negative prerequisite of an artificial camouflage at a unit device level is to feedback control system to maintain the target temperature under convey a wide range of the visible spectrum that can be controlled external environment fluctuations. The active control of the heat and changed as occasion demands26–28. At the same time, the flux also permits a further reduction in the response time even to corresponding unit should be flexible and mechanically robust, be comparable to the physiological color change found in especially for wearable purposes, to easily cover the target body as animals48. attachable patches without interrupting the internal structures, Meanwhile, the evolution of polymorphism in specific species49 while being compatible with the ambient conditions and the suggests that the display of an arbitrary image, which is only associated movements of the wearer29,30. conceivable via high resolution, individually addressable lateral System integration of the unit device into a complete artificial pixels, is superfluous for many camouflage applications that are camouflage device, on the other hand, brings several additional subject to the limited number of habitats31,32,50. In this regard, issues to consider apart from the preceding requirements. Firstly, instead of constructing innumerable miniaturized lateral heaters, the complexity of the unit device is anticipated to be increased as Ag NW heaters are firstly laser-patterned to the selected habitats the sensor and the control circuit, which are required for the and then piled vertically in a multilayer configuration. By stacking autonomous retrieval and implementation of the adjacent color, the Ag NW heaters composed of largely void and negligible are integrated into a multiplexed configuration. Simultaneously, thickness, the temperature profiles generated by the distinct for nontrivial body size, the concealment will be no longer heaters are superposed at the outermost TLC layer to allow the effective with a single unit unless the background consists of a matching of the background color and the expression of the monotone. As a simple solution to this problem, unit devices are microhabitat at the same time. The corresponding strategy allows often laterally pixelated12,18 to achieve spatial variation in the a great reduction in the overall system complexity compared to coloration. Since its resolution is determined by the numbers of the previous approaches together with more natural camouflage the pixelated units and their sizes, the conception of a high- characteristics by eliminating the dead zone between pixels and resolution artificial camouflage device that incorporates densely assisting acute but continuous transition in the coloration. At last, packed arrays of individually addressable multiplexed units leads by integrating the proposed Ag NW and TLC-based Artificial to an explosive increase in the system complexity. While on the Chameleon Skin (ATACS) with color sensors and feedback other hand, solely from the perspective of camouflage perfor- control systems, adaptive artificial camouflage at a complete mance, the delivery of high spatial frequency information is version of the device which is capable of detecting the local important for more natural concealment by articulating the tex- background color and matching its coloration in real-time has ture and the patterns of the surface to mimic the microhabitats of been accomplished on a chameleon model. Large-area, the nat- the living environments31,32. As a result, the development of ural and rapid coloration of the moving chameleon according to autonomous and adaptive artificial camouflage at a complete the underlying habitat grants the potential of the proposed device level with natural camouflage characteristics becomes an scheme as a scalable and practical next-level camouflage exceptionally challenging task. technology. Our strategy is to combine thermochromic liquid crystal (TLC) ink with the vertically stacked multilayer silver (Ag) nanowire (NW) heaters to tackle the obstacles raised from the earlier Results concept and develop more practical, scalable, and high- Vertically stacked ATACS consisted of Ag NW heaters and performance artificial camouflage at a complete device level. TLC. TLC ink layer and an Ag NW heater, or multiple Ag NW The tunable coloration of TLC, whose reflective spectrum can be heaters, are the core elements of the proposed multi-layered controlled over a wide range of the visible spectrum within the ATACS. For a typical ATACS, the vertically stacked patterned narrow range of temperature33,34, has been acknowledged as a heater layers of the multi-layered ATACS are observable from the potential candidate for artificial
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