Camouflage and Display for Soft Machines

Mechanical Chameleon A wide range of animals can adapt their color patterns as a means of camouflage or otherwise changing their appearance. This is accomplished through changes in coloration, contrast, patterning, or shape. Morin et al. (p. 828) show at a basic level that some of these features can be added as microfluidic layers attached to mobile, flexible, soft machines. By pumping different fluids through the channels, the robots were able to change their coloration or overall contrast and could thus blend into the background of the surface they were lying upon. Conversely, by pumping through fluids of different temperature, the infrared profile of the robot could be changed without changing its visible coloration. Soft robots with microfluidic channels in a skin layer show camouflaging abilities. Synthetic systems cannot easily mimic the color-changing abilities of animals such as cephalopods. Soft machines—machines fabricated from soft polymers and flexible reinforcing sheets—are rapidly increasing in functionality. This manuscript describes simple microfluidic networks that can change the color, contrast, pattern, apparent shape, luminescence, and surface temperature of soft machines for camouflage and display. The color of these microfluidic networks can be changed simultaneously in the visible and infrared—a capability that organisms do not have. These strategies begin to imitate the functions, although not the anatomies, of color-changing animals.

Synthetic systems cannot easily mimic the color-changing abilities of animals such as cephalopods. Soft machines-machines fabricated from soft polymers and flexible reinforcing sheets-are rapidly increasing in functionality. This manuscript describes simple microfluidic networks that can change the color, contrast, pattern, apparent shape, luminescence, and surface temperature of soft machines for camouflage and display. The color of these microfluidic networks can be changed simultaneously in the visible and infrared-a capability that organisms do not have. These strategies begin to imitate the functions, although not the anatomies, of color-changing animals.
C ephalopods (such as squid and cuttlefish) have amazing control over their appearance (color, contrast, pattern, and shape) (1,2). These animals use dynamic body patterns for disguise, for protection, and for warning. Other animals (such as chameleons and many insects) can also actively change their coloration for camouflage or display (3,4). Still others (such as jellyfish and fireflies) use bioluminescence to communicate (5). The color-changing capabilities of these animals have not been replicated by using soft synthetic systems, but such systems could enhance the function of certain machines (such as robots or prosthetics). This paper describes our initial approaches to change the color, contrast, pattern, apparent shape, luminescence, and infrared (IR) emission (that is, surface temperature) of soft machines fabricated from elastomers and flexible reinforcing sheets (6)(7)(8) by changing the color and pattern of microfluidic networks. These systems are first steps toward imitating the functions, al-though not the anatomies, of cephalopods (9,10) and other color-changing animals (4). These animals typically change color using specialized cells, such as chromatophores or iridophores (4,9,10), not simple microchannels. The nearperfect matching of environments used by colorchanging organisms with highly developed nervous systems is not required for camouflage to be effective.
Nature offers countless examples of camouflage and display (3,11,12). Although specific demonstrations of camouflage vary among species, the strategies used have common themes: background matching, disruptive coloration, and disguise (3,11,12). In background matching, animals use colors and patterns similar to their habitats (3,(11)(12)(13), such as counter/obliterative shading (for example, rabbits with white underbellies and brown backs) (11,12). Disruptive coloration breaks up the silhouette of an animal using contrasting patterns that do not follow its shape (3). Disguise is a strategy in which an organism adopts the shape and color of objects or animals in the environment (such as stick insects and phasmida) (3). Display strategies are used for communication, mating, and hunting and can be just as critical for survival as camouflage (3). Bioluminescence is commonly used for display in low-or no-light conditions (5,14); when lighting is strong, animals sometimes advertise themselves by combining vibrant contrasting colors with distinct shapes (15)(16)(17). For example, jellyfish use bioluminescence to warn would-be predators (5), and peacocks use bright plumage to attract mates (18).
Although most strategies of camouflage and display (3,12) emphasize the visible spectrum that most animals (especially mammals) can see (19)(20)(21), there are animals that can see (or otherwise sense) ultraviolet (UV) or IR light and use these spectral regions for signaling or hunting. For example, most birds, many arthropods, and some fish have UV vision (22), and many snakes, such as pit vipers, can sense IR light using specialized organs (23). Semiconductor technology has expanded our ability to see into the IR (24), and we explored techniques for camouflage or display in the IR.
Our system combines microfluidics, pattern, and color to provide both camouflage/display and movement of the soft machines. Camouflage or display results from pumping colored or temperature-controlled fluids through a network of microfluidic channels; mechanical actuation results from pneumatic pressurization and inflation of an independent network of microchannels (pneu-nets) embedded in highly extensible elastomers (6,7). The microfluidic networks used for camouflage/display are contained in thin silicone sheets [Ecoflex (Smooth-On, Easton, PA), 1 to 2 mm thick] referred to as color layers ( Fig. 1 and  fig. S1). Although there are technologies, such as electrowetting (25) and electrofluidics (26), that use microfluidics to tune color, they rely on electric fields to move fluid and are not immediately compatible with our compressed-air power source.
The color layers are easily fabricated (27), can cover large areas, and can transport a variety of fluids (both liquids and gases) using similar controls (28). The liquids can be colored with dyes or pigments and heated/cooled to change the color of the microfluidic networks in the visible and IR spectrum; the possible combinations of colorants at different temperatures provide greater flexibility in spectral tailoring than can be achieved by using other technologies: Thermoelectrics can change the IR signature, and electronic displays can change the visible color, but neither technology has control over both IR and visible coloration. Further, once filled, the color layers require no power, have low requirements for volume of fluid (~30 mL/cm 2 of surface), and are lightweight (130 mg/cm 2 of surface). We used closely packed microfluidic channels ( Fig. 1) or combined microfluidic channels with wider (millimeters to centimeters) channels (fig. S1D) to create features that are indistinguishable from continuous color in the far field. Ecoflex, unlike the materials used in commercially available flexible electronic displays, has a Young's modulus that is compatible with the flexible bodies and motions of soft machines and is also mechanically robust and inexpensive. The translucency of Ecoflex also helped the machines blend in with their surroundings.
We used a soft robotic quadrupedal "walker" (7) to demonstrate that the color layers are light and flexible enough to be compatible with a mobile and stretchy soft machine. We took advantage of the regular and symmetrical shape of the quadruped to illustrate camouflage strategies that can break up recognizable silhouettes.
We attached the color layers to the top of soft quadrupeds (Fig. 1, D to E), which were fabricated as described previously (6,7). Using a pumping rate of 2.25 mL/min and neglecting the time necessary to fill the tether, a change in color required 30 s (Fig. 1D) [color-change was also monitored with reflectance spectroscopy ( fig.  S6)]. The weight and flexibility of the color layer did not substantially slow the robot's locomotion (the velocity was 0.6 times the velocity of the same robot without the color layer) (Fig. 1E) (7). Illustrated in Fig. 2 are the operation of camouflage in a rock bed (movie S1) and a leafcovered concrete slab using two different color layers ( Fig. 1 and fig. S1D) filled with two different sets of colored solutions. In all demonstrations, the colored solutions were prepared manually by the operator. Both designs created large, disjointed patches of contrasting brightness similar to the environment but not to the shape of the robot; this disjuncture in shape [disruptive coloration (3)] helped conceal the robot. Simple image analysis performed on the camouflaged and uncamouflaged robot (Figs. 2B and 3 and figs. S4 and S5) demonstrated that the brightness of the camouflaged robot was significantly closer to the brightness (within the deviation defined by root mean square contrast) ( fig. S4) of the environment than that of the uncamouflaged robot (27,29). Additionally, we found the similarity in pattern between the background and the camouflaged robot to be favorable ( fig. S5). This brightness/contrast resemblance and similarity in pattern was important to this demonstration.
We also investigated the camouflage of soft robots [using background matching (3,(11)(12)(13)] in a patterned environment similar to a tiled floor (Fig. 2, E to F, and fig. S2) by creating a color layer with periodic microfluidic channels filled with colors matched to that environment. Although not perfectly camouflaged, the colored robots are less obvious and demonstrate the potential of this system. Dynamic camouflage would be useful for applications in which soft machines must do their job without standing out (for example, soft robots performing maintenance operations).
Patterns and contrasting coloration schemes that make the soft machine stand out against a background (display), although less technically demanding than camouflage, are just as important and useful (an example would be to aid location and recovery of soft machines from poorly lit or cluttered environments). We changed the camouflage/display modes of the color layers simply by changing the fluid within them (Fig. 1C, bottom). This ability of a single color layer to switch between modes of coloration is an advantage. We further illustrate display by filling dense, shape-matching color layers (fig. S1B) with fluorescent (Fig. 2, G to H, and movie S2), black and white (Fig. 2J), or chemiluminescent ( Fig. 2I and movie S3) solutions that revealed the location of the robots in light or dark. We also demonstrated camouflage and display in the IR spectrum ( Fig. 4 and movie S4) using warm (70°C) or cool (2°C) colored solutions. Pumping these temperature-controlled solutions through the color layer created contrasting IR color patterns that make the system stand out against its thermal environment. This strat-egy makes it possible simultaneously to change the visible and IR colors. As an illustration, we camouflaged a color layer within the visible spectrum while simultaneously displaying it in the IR spectrum (Fig. 4, F to G). At a pumping rate of 2.25 mL/min, we achieved IR color change in 80 s (movie S4). The rate of IR color change was limited by the thermal conductivity of Ecoflex [0.16 Wm −1 K −1 (27)] and not the pumping rate.
Simple systems can approach some of the functionality of dynamic coloration that many animals use to control their appearance (4). Microfluidic networks that make up a color layer that is  (Fig. 1).
independent of other functions can effectivelyalbeit crudely when compared with animalscamouflage or display soft machines by altering their apparent shape, color, contrast, luminescence, temperature, and pattern. The ability of color layers simultaneously to change color in the visible and the IR is a capability not used by organisms and not easily replicated by using other technologies: Animals are limited in their ability to control their temperature; soft machines fabricated in silicone elastomers are not.
These devices confirm that microfluidic channels can serve multiple functions in soft machines: actuation, camouflage, display, fluid transport, and temperature regulation. Combining different functional microfluidic systems simplifies the design and increases the functionality of soft machines. Complex/specialized microfluidic networks can take the place of these relatively simple color layers to bring more capabilities (such as reagent handling and storage) to soft machines.
We believe that the simple design, fabrication, and operation of these color-changing soft machines make these systems accessible and potentially useful to many different scientific fields. Biologists studying camouflage/display could use specially designed soft machines to observe how dynamic color, temperature, and pattern influence  animal behavior (such as predator/prey relationships) (13,30). In anatomy, our devices could simulate fluid vessels and muscle motion for realistic modeling or training. Although we have focused on soft robots, our display systems can also interface with hard robots and present new opportunities for modifying the appearance of these devices. These applications are approachable by using tethered microfluidic systems such as those demonstrated here, but some applications will demand more technically advanced autonomous systems. The challenges associated with autonomy will be addressed initially by using larger-bodied machines or robots that can carry power sources, pumps, electronics, and fluids. D iesel engines are attractive because of their higher fuel efficiency than gasoline engines (1). However, their lean exhaust with high oxygen (O 2 ) content requires specialized devices to reduce the engine-generated nitrogen oxide (NO x ) pollutants, mostly nitric oxide (NO), to environmentally benign nitrogen (N 2 ). One popular strategy is the use of selective catalytic reduction (SCR) of NO x with ammonia (NH 3 ) through the reaction: 2NH 3 + NO + NO 2 →2N 2 + 3H 2 O. The rate of this reaction is maximized (fast SCR) when the NO:NO 2 ratio approaches unity (2,3). A lean NO x trap (LNT) adsorbs NO x under lean conditions and reduces NO x to N 2 under fuel-rich regeneration cycles. NO oxidation products such as nitrogen dioxide (NO 2 ) have a higher trapping efficiency on LNT materials (4). In addition to the NO x -based pollutants, diesel engines also generate black soot with polynuclear aromatic content. One process to remove carbon soot is a continuous regenerable trap (CRT), which uses NO 2 as an oxidant (2NO 2 + C→CO 2 + 2NO, NO 2 + C→CO + NO). Oxidation of engine-generated NO to NO 2 is critical in diesel engine emission control for reducing NO x and carbon soot from the tail-pipe exhaust.
To date, no thermally stable material has matched platinum's catalytic performance under diesel exhaust temperatures, despite a substantial amount of work that has been undertaken in search for catalysts based on metal oxides (5, 6).
Here, we report a class of hydrothermally stable, mixed-phase oxides based on Mn-mullite, (Sm, Gd)Mn 2 O 5 . This catalyst demonstrates activity at temperatures as low as 120°C and has a~64% increase in NO oxidation catalytic performance over 2% Pt on g-Al 2 O 3 at 300°C. The catalytic behavior is mainly linked to the Mn-Mn dimers on the mullite surface based on characterization methods and quantum-mechanical simulations.
Mixed-phase oxide catalysts were prepared by a coprecipitation method (7). Transition metal, alkaline earth, and rare earth precursors were dissolved in water with a non-ionic surfactant. The pH was adjusted to between 9 and 10 with tetramethylammonium hydroxide, followed by the addition of oxalic acid and hydrogen peroxide. The precipitate was filtered, dried, calcined, and hydrothermally aged (materials and methods S-I).
To evaluate catalytic performance, we exposed the aged sample Mn 7 CeSmSrO 14.83 , labeled MnCe-7:1, to the reactant gas mixture (450 parts per million NO and 10% O 2 ) while ramping the temperature up to 350°C to test for NO conversion (materials and methods S-II). †These authors contributed equally to this work.