Creating Chameleon-like Smart Actuators

Abstract

Inspired by how chameleons change their skin color, Du and co-workers have developed a general biomimetic strategy for preparing smart photonic crystal actuators with the fast and reversible colorimetric response and programmable shape transformation. Inspired by how chameleons change their skin color, Du and co-workers have developed a general biomimetic strategy for preparing smart photonic crystal actuators with the fast and reversible colorimetric response and programmable shape transformation. Can artificial microactuators or microrobots be as smart as the Earth’s natural creatures? This is a fundamental yet exciting question for scientists working on soft robotics and microactuators. A team led by Du from China has provided positive hints to this question by developing an innovative strategy to the creation of microactuators that can display both biomimetic color changes and programmable shape transformations in response to solvent vapor exposure.1Wang Y. Cui H. Zhao Q. Du X. Chameleon-inspired structural color actuators.Matter. 2019; 1 (this issue): 626-638Abstract Full Text Full Text PDF Scopus (142) Google Scholar Smart actuators that can respond actively to external stimuli have attracted long-term attention in many research fields.2Camacho-Lopez M. Finkelmann H. Palffy-Muhoray P. Shelley M. Fast liquid-crystal elastomer swims into the dark.Nat. Mater. 2004; 3: 307-310Crossref PubMed Scopus (784) Google Scholar, 3Zhao Q. Wang Y. Cui H. Du X. Biol.-inspired sensing and actuating materials.J. Mater. Chem. C Mater. Opt. Electron. Devices. 2019; 7: 6493-6511Crossref Google Scholar The tailorable modulation of their chemical and physical properties has potential applications in drug delivery, environmental sciences, healthcare, microrobots, and responsive mass transportation.4Fu F. Shang L. Chen Z. Yu Y. Zhao Y. Bioinspired living structural color hydrogels.Science Robotics. 2018; 3: eaar8580Crossref PubMed Google Scholar Learning from nature has been recognized as an effective strategy to develop new concepts and principles for designing novel actuators, and it has shown tremendous success especially when combined with the employment of functional materials of tailored properties and dimensions. However, most of these successes have been limited to achieving a single function, and developing soft actuating systems capable of multiple capabilities remains a significant challenge, for example, realizing changes in both shape and color in response to one external stimulus.5Zhao Z. Wang H. Shang L. Yu Y. Fu F. Zhao Y. Gu Z. Bioinspired heterogeneous structural color stripes from capillaries.Adv. Mater. 2017; 29: 1704569Crossref Scopus (102) Google Scholar In fact, nature has mastered highly effective means to drive such complex actuating actions.6Gur D. Palmer B.A. Leshem B. Oron D. Fratzl P. Weiner S. Addadi L. The mechanism of color change in the neon tetra fish: a light-induced tunable photonic crystal array.Angew. Chem. Int. Ed. 2015; 54: 12426-12430Crossref PubMed Scopus (108) Google Scholar, 7Teyssier J. Saenko S.V. van der Marel D. Milinkovitch M.C. Photonic crystals cause active colour change in chameleons.Nat. Commun. 2015; 6: 6368Crossref PubMed Scopus (644) Google Scholar For example, chameleons exhibit reversible color changes during social interactions through active tuning of a lattice of guanine nanocrystals within the superficial skin layer of dermal iridophores. By exciting this layer, they can increase the periodicity between the crystals and thereby reflect the light of longer wavelength. Since the early research report of the coloration mechanism in 2015, scientists have made a lot of research efforts in fabricating chameleon-like actuators that are capable of changing colors on demand,7Teyssier J. Saenko S.V. van der Marel D. Milinkovitch M.C. Photonic crystals cause active colour change in chameleons.Nat. Commun. 2015; 6: 6368Crossref PubMed Scopus (644) Google Scholar, 8Chou H.-H. Nguyen A. Chortos A. To J.W. Lu C. Mei J. Kurosawa T. Bae W.-G. Tok J.B.-H. Bao Z. A chameleon-inspired stretchable electronic skin with interactive colour changing controlled by tactile sensing.Nat. Commun. 2015; 6: 8011Crossref PubMed Scopus (636) Google Scholar typically by incorporating color elements such as dyes, phosphors, and photonic crystals into the actuating systems. Compared with other coloration mechanisms, photonic crystals promise greater potential in the preparation of chameleon-like actuators thanks to their order-dependent and persistent structural colors.9Ge J. Yin Y. Responsive photonic crystals.Angew. Chem. Int. Ed. 2011; 50: 1492-1522Crossref PubMed Scopus (882) Google Scholar Unfortunately, although many strategies are now available to control photonic band gaps in the visible spectrum,10Li Z. Yin Y. Stimuli-responsive optical nanomaterials.Adv. Mater. 2019; 31: e1807061Crossref PubMed Scopus (132) Google Scholar there are few integrated actuation mechanisms that can respond to external stimuli with simultaneous and reversible color change and mechanical motion, which many creatures like chameleons can do easily. The research team led by Du reports a bioinspired strategy to fabricate inverse opal actuators that are capable of displaying simultaneous and instantaneous shape deformation and color change (Figure 1). Their strategy involves first the self-assembly of silica nanoparticles into colloidal crystal arrays, the infiltration of UV-curable poly(trimethylolpropane triacrylate) (PTMPTA), and finally, chemical etching of silica particles to form inverse opal-structured PTMPTA films. The as-prepared PTMPTA films exhibited brilliant structural colors due to the diffraction of light at a specific wavelength. In addition, upon exposure to organic vapors, the films swell because of the diffusion of organic molecules, leading to an increase in lattice spacing and thereby redshift of the diffraction wavelength and corresponding change of the perceived colors. One of the advantages of the highly porous structures is the fast diffusion of solvent vapors, which enables instantaneous color changes (less than 1 s). To achieve programmable locomotion and deformations, the researchers prepared micro-patterned PTMPTA films by modeling against a polymer stripe template, which was then cut into specific shapes along pre-designed angles relative to the one-dimensional (1D) stripes. Interestingly, the films were able to perform complex mechanical actions such as tube curling, twisting, and rolling for the relative angles of 0°, 45°, and 90°, respectively, with simultaneous color changes from green to bright orange. This reversible process could last for more than 110 cycles without any fatigue, demonstrating the excellent mechanical properties of the inverse opal films. Further, biomimetic actuators were prepared with pre-designed color and shape changes. For example, a flower-shaped actuator could mimic the simultaneous color and shape changes of natural flowers when closing or blooming upon exposure to or removal of acetone vapor. By carefully engineering the friction of the inverse opal films, they reported a chameleon-like walking actuator. Their strategy involved a heavy polytetrafluoroethylene leg of a rectangular geometry that was attached to the front of the actuator. This simple practice would induce asymmetric friction with the substrate: stronger in the front and weaker in the rear, resulting in a slipping forward of the actuators powered by acetone vapors. The artificial walker could simultaneously exhibit green-red-green color changes due to the dynamic shift in photonic band gaps of the inverse opals. By repeating the on-off switching of acetone vapors, the artificial walker crawled continuously at a speed of 0.16 cm/s. In this work, Wang et al. have developed several inspiring prototypes of smart actuators that feature synergistic shape morphing and color changes in response to solvent vapor exposure. They leverage both the high porosity and periodicity of inverse opals and the vapor-responsive polymer stent to achieve the fast colorimetric and mechanical response and excellent cycling performance of the structural-colored actuators. Therefore, these smart actuators are expected to have promising applications in communication, environmental science, healthcare, sensing, and anti-counterfeiting. However, the current system needs solvent vapors to drive the color and mechanical responses, which limit its use to very particular conditions, as it is challenging to design sophisticated actions with high temporal and spatial resolution. The cost of solvents and their impact on the environment and health are additional concerns. For most real-world applications, we shall recognize that much work still needs to be done to go beyond the proof-of-concept phase by developing integrated actuator systems that can effectively respond to various stimuli that exist in natural settings and can be perceived by creatures, including temperature, light, color, smell, taste, electrical and magnetic fields, sound, vibration, mechanical force, etc. Nevertheless, this work proposes a promising bioinspired route toward smart soft robotics that can mimic natural creatures by responding to a stimulus with multiple types of reactions. One may, therefore, expect that this exciting work will inspire a broad spectrum of future researches to create next-generation smart actuators for many fascinating applications. Chameleon-Inspired Structural-Color ActuatorsWang et al.MatterJuly 31, 2019In BriefStructural-color films that feature unprecedented capabilities of both sensitive vapochromic and robust vapomechanical responses have been successfully developed. Based on these films' extraordinary properties, various structural-color actuators have been further fabricated and demonstrated, for example, a rotating pinwheel, an artificial blooming flower, and a worm-like walker with accompanying dynamic color alterations. These actuators exhibit fast, stable, and sharp color alterations together with non-fatigued and programmable motions, which can be used for sensing, communication, and disguise in soft robotics. Full-Text PDF Open Archive