Biomimética, camuflaje

Development of an artificial camouflage at a complete device level remains a vastly challenging task, especially under the aim of achieving more advanced and natural camouflage characteristics via high-resolution camouflage patterns. Our strategy is to integrate a thermochromic 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.

Artificial camouflage is the functional mimicry of the natural camouflage that can be observed in a wide range of species^1,2,3. Especially, since the 1800s, there were a lot of interesting studies on camouflage technology for military purposes which increases survivability and identification of an anonymous object as belonging to a specific military force^4,5. Along with previous studies on camouflage technology and natural camouflage, artificial camouflage is becoming an important subject for recently evolving technologies such as advanced soft robotics^1,6,7,8 electronic skin in particular^9,10,11,12. Background matching and disruptive coloration are generally claimed to be the underlying principles of camouflage covering many detailed subprinciples¹³, and these necessitate not only simple coloration but also a selective expression of various disruptive patterns according to the background. While the active camouflage found in nature mostly relies on the mechanical action of the muscle cells^14,15,16, artificial camouflage is free from matching the actual anatomies of the color-changing animals and therefore incorporates much more diverse strategies^{17,18,19,20,21,22}, but the dominant technology for the practical artificial camouflage at visible regime (400–700 nm wavelength), especially RGB domain, is not fully established so far. Since the most familiar and direct camouflage strategy is to exhibit a similar color to the background^23,24,25, a prerequisite of an artificial camouflage at a unit device level is to convey a wide range of the visible spectrum that can be controlled and changed as occasion demands^26,27,28. At the same time, the corresponding unit should be flexible and mechanically robust, especially for wearable purposes, to easily cover the target body as attachable patches without interrupting the internal structures, while being compatible with the ambient conditions and the associated movements of the wearer^29,30.

System integration of the unit device into a complete artificial camouflage device, on the other hand, brings several additional issues to consider apart from the preceding requirements. Firstly, the complexity of the unit device is anticipated to be increased as the sensor and the control circuit, which are required for the autonomous retrieval and implementation of the adjacent color, are integrated into a multiplexed configuration. Simultaneously, for nontrivial body size, the concealment will be no longer effective with a single unit unless the background consists of a monotone. As a simple solution to this problem, unit devices are often laterally pixelated^12,18 to achieve spatial variation in the coloration. Since its resolution is determined by the numbers of the pixelated units and their sizes, the conception of a high-resolution artificial camouflage device that incorporates densely packed arrays of individually addressable multiplexed units leads to an explosive increase in the system complexity. While on the other hand, solely from the perspective of camouflage performance, the delivery of high spatial frequency information is important for more natural concealment by articulating the texture and the patterns of the surface to mimic the microhabitats of the living environments^31,32. As a result, the development of autonomous and adaptive artificial camouflage at a complete device level with natural camouflage characteristics becomes an exceptionally challenging task.

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 concept and develop more practical, scalable, and high-performance artificial camouflage at a complete device level. The tunable coloration of TLC, whose reflective spectrum can be controlled over a wide range of the visible spectrum within the narrow range of temperature^33,34, has been acknowledged as a potential candidate for artificial camouflage applications before^21,34, but its usage has been more focused on temperature measurement^35,36,37,38 owing to its high sensitivity to the temperature change. The susceptible response towards temperature is indeed an unfavorable feature for the thermal stability against changes in the external environment, but also enables compact input range and low power consumption during the operation once the temperature is accurately controlled.

The selection of an appropriate heater together with a proper control circuit is therefore critical for the development of a TLC-based artificial camouflage device, and we conclude that Ag NW heater enables not only accurate temperature manipulation but also a different route to express the fine patterns without associating the lateral pixelation scheme mentioned earlier. Owing to its superior electrical and mechanical stability, Ag NW has been a promising material for flexible^39,40 and stretchable heaters^41,42. Also, comparing with other materials such as gold (Au) NW, Copper (Cu) NW⁴³, and hybrid materials (Cu-Ni NW⁴⁴, Ag-Au NW⁴⁵, and Ag NW-CNT⁴⁶, Ag NW-PEDOT:PSS⁴⁷), Ag NW has excellent electrical conductivity and oxidation resistance as a single material and has a cost-effective feature to be applied for large-area applications through a simple synthesis process. While at the same time, we noticed that the temperature coefficient of resistance (TCR) of the Ag NW network is sufficiently large, linear, and non-hysteric. These properties led us to use the resistance of Ag NW network as the process variable of a negative feedback control system to maintain the target temperature under external environment fluctuations. The active control of the heat flux also permits a further reduction in the response time even to be comparable to the physiological color change found in animals⁴⁸.

Meanwhile, the evolution of polymorphism in specific species⁴⁹ suggests that the display of an arbitrary image, which is only conceivable via high resolution, individually addressable lateral pixels, is superfluous for many camouflage applications that are subject to the limited number of habitats^31,32,50. In this regard, instead of constructing innumerable miniaturized lateral heaters, Ag NW heaters are firstly laser-patterned to the selected habitats and then piled vertically in a multilayer configuration. By stacking the Ag NW heaters composed of largely void and negligible thickness, the temperature profiles generated by the distinct heaters are superposed at the outermost TLC layer to allow the matching of the background color and the expression of the microhabitat at the same time. The corresponding strategy allows a great reduction in the overall system complexity compared to the previous approaches together with more natural camouflage characteristics by eliminating the dead zone between pixels and assisting acute but continuous transition in the coloration. At last, by integrating the proposed Ag NW and TLC-based Artificial Chameleon Skin (ATACS) with color sensors and feedback control systems, adaptive artificial camouflage at a complete version of the device which is capable of detecting the local background color and matching its coloration in real-time has been accomplished on a chameleon model. Large-area, the natural and rapid coloration of the moving chameleon according to the underlying habitat grants the potential of the proposed scheme as a scalable and practical next-level camouflage technology.