Article pubs.acs.org/cm
Cite This: Chem. Mater. 2018, 30, 1593−1601
Multistimuli-Responsive Camouflage Swimmers Emil Karshalev, Rajan Kumar, Itthipon Jeerapan, Roxanne Castillo, Isaac Campos, and Joseph Wang* Department of Nanoengineering, University of California, San Diego, La Jolla, California 92093, United States S Supporting Information *
ABSTRACT: Multistimuli-responsive camouflaging and autonomously propelled swimmers are presented. These bioinspired artificial swimmers are capable of color changing in response to a variety of environmental stimuli such as temperature, pH, and light. By multiplexing different stimuliresponsive materials on a single swimmer, a vast library of independently addressable colors can be achieved. Unlike other color-changing robots, our color-changing swimmers can move autonomously in solution and are decoupled from tethered supports. The utilized leuco dyes, pH indicators, and phosphorescent powders display excellent reversibility and prolonged color retention. Finally, we design camouflage patterns which render the swimmers invisible against virtually any background, while revealing the swimmer upon changing the environmental conditions. By mimicking animal camouflage strategies these artificial swimmers present a significant step toward realizing multienvironmental stimuli-responsive color-changing strategies for the next generation of smart robotics. Such capabilities can be further enhanced by coupling color-changing ability with stimuli-triggered speed or shape change.
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luminescent elastic skin17 and thin film interference proteinbased camouflage stickers.18−20 However, there are only a few examples of autonomously moving soft robots with a dedicated dynamic camouflage strategy and there are considerable needs for an improvement of this technology. Current examples of man-made soft camouflage materials have been inspired by the natural world. One such case is the X-shaped, pneumatically actuated soft robot of Morin et al. which can camouflage against different backgrounds based on colored dyes pumped through its hollow channeled interior.21 Other examples include a large-scale artificial chameleon based on tunable photonic structures22 and a soft pneumatically operated transparent hydrogel fish.23 A common set of problems plagues all of these devices: they require large-scale tethered electronics, pumps, and direct operator involvement in order to complete their tasks and change color. Furthermore, attempting to make these robots more independent of human interaction will require the ability to sense and autonomously respond to the environment.24,25 Autonomous color-changing behavior, without direct human involvement or cumbersome feedback loops, will rely on triggers by environmental stimuli, such as changes in temperature, pH, redox activity, or light.26−28 Combining the ability to respond to multiple environmental stimuli in a single mobile platform could serve to realize a truly autonomous color-changing machine. Here we present multistimuli-responsive color-changing environmentally adaptive swimmers, striving to imitate the functionality of cephalopods to adaptively change their
INTRODUCTION Evolution perfected an astounding diversity of intricate camouflage capabilities in the animal kingdom for concealment and disguise.1−6 Camouflage in nature is realized by the introduction of a rich variety of colors, vivid geometrical and random patterns, and morphological structures which assist the animal to avoid detection from predators. The three main mechanisms by which concealment is accomplished are matching the color and texture of the background, breaking the natural lines of one’s silhouette (disruptive camouflage), and blending in with downwelling light (countershading).2 In marine animals, these strategies can be easily seen in sandcolored bottom-dwelling fish (e.g., stonefish), brightly colored striped reef fish (e.g., Moorish idol), and dark- and light-colored great white sharks. Other species feature active, highly adaptable, and dynamic camouflage based on the three mechanisms outline above. Examples include the two spot goby, African cichlid, and the “masters of camouflage”, namely, squid, cuttlefish, and octopuses, all of whom are capable of responding to multiple environmental cues to rapidly change their coloration.4−6 Robots are permeating every aspect of life from household helpers and flexible prostheses to autonomous recreational or military drones. Especially, new fields such as soft robotics have pushed the development of electronic skin, artificial muscle, environmentally aware machines and autonomous locomotion which have significant implications in human-machine interfacing, sensing, national security, and environmental preservation.7−13 Color-changing and color-matching technologies have been employed for soft materials such as a strain\electric field activated spiropyran-elastomer cross-linked composite,14,15 black-and-white pixel array conformable composite,16 electro© 2018 American Chemical Society
Received: November 14, 2017 Revised: February 12, 2018 Published: February 14, 2018 1593
DOI: 10.1021/acs.chemmater.7b04792 Chem. Mater. 2018, 30, 1593−1601
Article
Chemistry of Materials
Swimming solutions were prepared by combining Milli-Q water and hydrogen peroxide (H2O2) solution (30%, Fisher Scientific, Hampton, NH) to produce 15% H2O2 by volume. For pH-responsive swimmers 15% H2O2 solutions with low or high pH were prepared with hydrochloric acid (HCl), 37% (EMD Chemicals, Billerica, MA), and anhydrous sodium hydroxide pellets (NaOH) (Fisher Scientific, Hampton, NH). Milli-Q water was used for all experiments. Fabrication of Stimuli-Responsive Swimmers. The fabrication process was comprised of printing various functional inks. The components of the swimmer body and tail were designed in AutoCAD (Autodesk, San Rafael, CA) and used as patterns in the 12″ × 12″ stainless steel stencils (Metal Etch Services, San Marcos, CA) shown in Figure S1A. A temporary tattoo paper sheet, precoated with a watersoluble adhesive, served as the substrate for the printing process. A typical temperature-responsive swimmer included the printing of at least three layers. First, the rigid conductive layer from graphite ink (E3449) was printed in the shape of the tail and dried at 60 °C in an oven. This conductive layer enabled subsequent electrodeposition of Pt on the tail. Then the process was repeated for the body partially overlapping the tail. Next, a white acrylic layer was printed to serve as a white background, making the visualization of colors easier. Lastly, a temperature-responsive ink was used to give the fish the final color pattern (see Figure S1B). Temperature-responsive leuco dyes were used due to their fast and reversible color change and stability. Leuco dyes are commonly protected via microencapsulation (see Figure S2), essentially filling a soft plastic bead with the color changing molecule, a weak acid which serves as a proton donor, and a polar organic solvent with a melting point around the desired transition temperature.31−34 Thus, the interaction between the solid/liquid solvent, the weak acid “developer”, and the dye molecule produces tautomerization, resulting in highly reversible, fast color changes which remain stable due to the microencapsulation shielding. Leuco dyes originate from a few families of organic molecules, but lactones and furans are the most prominent.31−34 Depending on the desired color, amounts of the temperature-responsive ink, acrylic paint, adhesive binder, and water were mixed in a dual asymmetric centrifugal mixer (Flacktek Speedmixer, DAC 150.1 kV−K, Landrum, SC) for 5 min at 3000 rpm. To demonstrate camouflaging ability over a number of differently colored backgrounds, an image of a single color background or a realistic environment was printed and placed under the Petri dish. The combinations of dyes were optimized by trial and error to produce two states: one which would be camouflaged over the background and one which will provide enough contrast to be easily seen. One example formulation which turns from orange to yellow above 33 °C features 1 part vermillion red chromicolor dye, 5 parts lemon yellow acrylic dye, 6 parts binder, and 1.75 parts water. Whereas the full coverage and striped designs are printed using a stencil, the camouflaged swimmers in Figure 5 are painted on by hand to give randomness to the design. For a pH-sensitive swimmer, pH indicator strips were utilized. These indicators display color change due to a protonation/deprotonation and changes in the resulting resonance structures which are very sensitive to H+/OH− concentrations.35 In short, indicator dye (4 mg), dissolved in THF (2 mL) with aliquot 336 (4 mg), NPOE (44 mg), and PVC (18.3 mg) was dropcast onto 20 μm pore size filter paper. Small strips were incised from it and attached to the swimmers via a double-sided adhesive. A third class of color-changing responsive materials are members of the spiropyran and azobenzene families where a cis/trans isomerism occurs upon irradiation with UV light.36−38 However, these organic molecules are known to be quite unstable and require constant irradiation which can be obtrusive and prevents truly autonomous behavior. Instead, light-responsive swimmers were fabricated as the temperature-responsive swimmers; however, the temperature-responsive ink was replaced with a glow-inthe-dark powder formulation, and two subsequent layers of the phosphorescent ink were used. An example formulation features 7.5 parts phosphorescent powder, 20 parts binder, and 1 part water. After the required layers are printed and dried, the tattoo paper substrate with the swimmers were put in water and left for 1 min to allow the dissolution of the adhesive used in the tattoo paper. The swimmer was then easily removed from the paper by sliding it off the edge. All
coloration upon taking cues from various environmental stimuli. These multistimuli-sensitive mesoscale catalytic swimmers move autonomously by the Pt-catalyzed decomposition of hydrogen peroxide (H2O2) fuel in their tails while responding dynamically to changes in the environment to cloak or decloak themselves. The color-changing processes of our untethered swimmers are carried out autonomously in response to environmental cues, rather than direct human involvement, and are dominated primarily by the surroundings of the swimmer. These artificial swimmers can thus transform their diverse surface color patterns “on-the-fly” to match their surrounding background. Localized changes in the temperature, pH, and light, or a combination of these, are thus used to alter and control the swimmer’s coloration in connection to thermochromic (temperature responsive), halochromic (pH responsive) dyes, and phosphorescent (light responsive) pigments. Leuco dyes, well-known pH-indicating dyes, and phosphorescent inorganic pigments have been employed to achieve a fast, efficient, and reversible coloration change, allowing for facile incorporation and high repeatability. This color-changing swimmer approach utilizes a high-throughput printing technique, used recently to prepare catalytic swimmers,29,30 enabling the simple fabrication of diverse color-changing patterns and swimmer layouts, and realization of fast, efficient, and reversible coloration change. Such swimmer systems, responding to numerous environmental stimuli, offer considerable promise for potential in situ sensing applications and security and surveillance missions. In a simple example, one can easily imagine that our autonomous swimmers are released into a target location and allowed to sense it, revealing the state of the environment through their coloration. Hazardous conditions or large changes in environmental cues, such as temperature and pH in wastewater runoff, industrial reactors, hydrothermal vents, or after acid rain, could thus be rapidly detected to serve as an early warning system. Furthermore, this inherent color-changing ability can also be utilized for dynamic camouflage against both humans and animals for security or intelligence gathering missions. Such successful realization of autonomous, multistimuli-responsive color-changing bioinspired swimmers leads one step closer to mimicking the functionalities of color-changing camouflage masters.
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MATERIALS AND METHODS
Reagents and Solutions. For the three essential layers of each type of swimmer we utilized E3349 graphite ink (Ercon, Inc., Wareham, MA), white acrylic paint (Liquitex, London, UK), RTP platinum plating solution (Technic Inc., Anaheim, CA), and temporary tattoo paper (Papilio, HPS LLC, Rhome, TX) as the printing substrate. To make temperature-sensitive inks we used Chromicolor Fast Blue and Vermillion Red (Matsui International Company, Gardena, CA) based on encapsulated CVL and Scarlet TF-R2 leuco dyes, respectively, black thermochromic capsule powders based on encapsulated ODB-2 leuco dye (HALI Industrial Co., Ltd., Changzhou, China), adhesive binder (Aleene’s, CA), acrylic paints (Nicole Industries, Moorestown, NJ), and Milli-Q water. For the fabrication of the pH-sensitive layers, we utilized filter paper with average pore size ∼20 μm (Whatman, Maidstone, UK), tetrahydrofuran (THF) (EMD Chemicals, Billerica, MA), bromocresol green (BG), bromothymol blue (BB), cresol red (CR), methyl red (MR), Aliquot 336 (Sigma-Aldrich, St. Louis, MO), nitrophenyl octyl ether (NPOE) (Fluka, Mexico City, Mexico), and polyvinyl chloride (PVC) with average molecular weight ∼233 kDa (Sigma-Aldrich, St. Louis, MO). Glow-in-the-dark swimmers utilized phosphorescent purple and green powders (encapsulated, 10 (a) to yellow, orange and yellow at pH < 3, respectively (b). Indicator molecules responsible for the pH-induced color change in (H): cresol red (CR, left stripe) (I), methyl red (MR, middle stripe) (J), and bromothymol blue (BB, right stripe) (K). Scale bars, 1 cm.
swimmer with pH-responsive coloration exhibits a bright blue coloration at pH > 10 (a) which transitions to a bright yellow color at pH 3 (b) (see Movie S3). The molecule responsible for the change in Figure 3F is bromocresol green (BG) whose structure is presented in Figure 3G. With this behavior in mind, multipatch swimmers can be fabricated, as illustrated in Figure 3H. Here, the swimmer consists of three different pHindicating stripes which exhibit dark purple, yellow, and blue coloration at pH > 10 (a). Upon decreasing the pH to 8 and pH < 3 over 5 cycles showing a blue to yellow transition characteristic of bromocresol green. (D) A different pH-responsive swimmer exhibiting reversibility over 5 cycles and a change between purple, yellow, and blue-green to yellow, orange, and yellow, characteristic of cresol red, methyl red, and bromothymol blue, respectively, under the same conditions as (C). Scale bars: 1 cm.
polymers can be precisely tailored for the exact application in terms of sensitivity, output color, and transition point. Furthermore, as was mentioned in the Introduction, many camouflage strategies found in nature employ a physical deformation or morphological change to aid in the concealment process, such as the puckering of cephalopod skin to resemble coral or kelp.3 The incorporation of stimuli-responsive shapechanging materials into our color-changing swimmers presents a next step in this direction. Shape memory alloys (SMAs) for robotics or biomedical devices and shape memory polymers (SMPs) with four or more arbitrary deformation states utilized as smart hinges or grippers can be integrated into the design of our swimmers and combined with the color-changing ability to further enhance their camouflage ability or modify motion.51−56 In addition, the printing of the color dyes can be coupled with stress-enduring inks57 to impart the stretchability necessary for such shape-deformed swimmers. Additionally, the potential of multistimuli-responsive swimmers can be recognized in automatic identification and data capture (AIDC) for identification and information relay purposes. Even a simple barcode pattern can yield a large number of unique codes in accordance with nm − 1, where n is the number of stripe widths and m is the number of distinguishable (color) components.58 Swimmers can thus be easily fabricated to become mobile barcodes with an expanded library code since not only can their geometrical pattern be controlled but also the trigger which changes their coloration and even their speed, leading to a vast number of unique configurations. Despite the potential advantages of these swimmers outlined above, a critical look at the limitations and remaining challenges must be undertaken. Currently, the propulsion is random and not directed which can hinder practical applications. The incorporation of navigation strategies, such as proposed magnetic elements for magnetic guidance field direction or regulating the bubble tail will be a topic of further exploration. Second, a transition to less toxic and more environmentally
below the transition temperature, while the pH-responsive stripe (middle) is purple at pH > 10 (a). Heating the system while keeping the pH constant leads to a color switching of the temperature-sensitive stripes to yellow (b). Finally, the pHsensitive stripe also changes color to yellow after the pH is lowered to 10 (left) and pH < 3 (right). The top row represents a swimmer with 3 patches of bromocresol green indicator, while the bottom row represents a swimmer with a patch of cresol red, methyl red, and bromothymol blue from tail to head, respectively (AVI) Movement of multistimuli-responsive swimmers below the transition temperature and at pH > 10 (left), above the transition temperature and pH > 10 (middle) and above the transition temperature and pH < 3 (right). The top and bottom rows represent different three patch (2 temperature responsive and 1 pH responsive) multistimuli-responsive swimmers which have been stimulated in a step-wise manner (AVI) Effect of deposition times of Pt on the speed of temperature-responsive swimmers below (top) and above (bottom) the transition temperature. The green
Figure 5. Multiplexed multistimuli-responsive swimmers. (A) Schematic of a multistimuli-responsive swimmer showing the arbitrary color change of individual patches induced by changes in temperature and subsequently pH. (B) Multistimuli-responsive swimmer with purple, blue, and black stripes at 23 °C and pH > 10 (a), light blue, blue, and light blue stripes after a temperature increase to 32 °C (b), and finally light blue, yellow, and light blue stripes after a change to pH < 3 (c). (C) Another multistimuli-responsive swimmer with green, purple, and orange patches at 24 °C and pH > 10 (a), yellow, purple, and yellow patches after temperature increase to 34 °C (b), and finally all yellow patches after a change to pH < 3 (c). (D) Trajectories of two temperature-sensitive swimmers during a 2 s motion in 15% H2O2 solution at 18 °C (a) and at 36 °C (b). Pt has been deposited for 30 or 60 s for the green and orange swimmers, respectively. (E) Distribution of speeds for each of the two Pt-based catalytic swimmers at an average temperature of 15 °C (a) and average temperature of 35 °C (b). Scale bars: 1 cm.
available fuels will be necessary to reduce the dependence on H2O2 fuel. Next, while motion enables coverage of large areas and interaction with large sample volumes, the swimmer will propel until it runs out of fuel, thus necessitating adaptive operations with switching between motion and rest states based on the specific environment or task. Finally, as these swimmers are expected to mimic their biological counterparts, a more advanced “sense and respond” closed-loop system should be devised.
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CONCLUSIONS In summary, we presented a multistimuli-responsive camouflage swimmer strategy which features unique advantages and enables these swimmers to sense and respond to their environments. First, our color-changing swimmers can move autonomously in solution and are decoupled from tethered supports which plague most color-changing soft robots. The movement enables swimmers to sense a large portion of the 1599
DOI: 10.1021/acs.chemmater.7b04792 Chem. Mater. 2018, 30, 1593−1601
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Chemistry of Materials
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swimmer has been plated for 30 s with Pt while the orange for 60 s. Both swimmers show increased speed at elevated temperatures (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Joseph Wang: 0000-0002-4921-9674 Author Contributions
E.K., R. K., and I.J. designed the experiments; E.K., R.C., and I.C. performed the fabrication, motion study, triggered colorchanging and camouflage experiments, and analyzed the data. E.K., R.K., I.J., R.C., I.C., and J.W. wrote the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank F. Soto and A. Garcia for fruitful discussions and revision of our manuscript. This project was supported by the Defense Threat Reduction Agency Joint Science and Technology Office for Chemical and Biological Defense (Grant Nos. HDTRA1−14−1−0064). E.K. acknowledges the Charles Lee Powell Foundation and UCSD for financial support. R.K. acknowledges the National Science Foundation Graduate Research Fellowship under Grant No. (DGE-1144086). I.J. recognizes Thai Development and Promotion of Science and Technology Talents Project (DPST). I.C. and R.C. acknowledge the Center for Investigations of Health and Education Disparities (CIHED).
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DOI: 10.1021/acs.chemmater.7b04792 Chem. Mater. 2018, 30, 1593−1601
