Bioinspired Multi-Stimuli Responsive Actuators with Synergistic Color- and Morphing-Change Abilities

Abstract

The combination of complex perception, defense, and camouflage mechanisms is a pivotal instinctive ability that equips organisms with survival advantages. The simulations of such fascinating multi-stimuli responsiveness, including thigmotropism, bioluminescence, color-changing ability, and so on, are of great significance for scientists to develop novel biomimetic smart materials. However, most biomimetic color-changing or luminescence materials can only realize a single stimulus-response, hence the design and fabrication of multi-stimuli responsive materials with synergistic color-changing are still on the way. Here, a bioinspired multi-stimuli responsive actuator with color- and morphing-change abilities is developed by taking advantage of the assembled cellulose nanocrystals-based cholesteric liquid crystal structure and its water/temperature response behaviors. The actuator exhibits superfast, reversible bi-directional humidity and near-infrared (NIR) light actuating ability (humidity: 9 s; NIR light: 16 s), accompanying with synergistic iridescent appearance which provides a visual cue for the movement of actuators. This work paves the way for biomimetic multi-stimuli responsive materials and will have a wide range of applications such as optical anti-counterfeiting devices, information storage materials, and smart soft robots.

Keywords: actuators; cellulose nanocrystals; color-changing; infrared/humidity sensitive; multi-stimuli responses.

Figure 1

a) Design of the bio‐inspired bilayer composite film based on CNCs and its NIR light‐ and humidity‐responsive actuating principles. b) Schematic of the glucose‐assisted self‐assembly process of CNCs/glucose suspension and the POM image of a typical fingerprint pattern of the cholesteric phase.

Figure 2

Optical properties of the CNCs@glucose layer. a) SEM cross‐section image of the film showing evident cholesteric structure of CNCs. b) POM‐λ images of the film showing a periodic color change from blue to orange following the clockwise rotation. c) The angle‐related structural colors of an artificial “Morpho Helena”. d) CIE 1931 color space chromaticity diagrams for the LC layer during the color‐changing process, coordinate values of which were obtained for every 5° increment in the view angle.

Figure 3

NIR‐ and humidity‐actuating performance of the material. a) Schematic illustration of a bilayer smart gripper bending on exposure to NIR/humidity. b) Optical images of PU/CNC bilayer film and PU/CNC@glucose bilayer film before and after bending for 5000 times. c) UV‐visible spectrum and d) Laser confocal Raman microspectroscopy of the material. Photographs of an actuator reversibly bending when e) humidity and g) NIR light is alternately switched on and off. Bending angle and actuation force of the film with f) humidity and h) NIR light alternately turned on and off.

Figure 4

The comparison of the two actuating methods (humidity and NIR) about actuation features including a) speed and b) force. Digital images of c) a smart “rangoon creeper flower” blooming and closing. d) A “palm” grasps and transfers an object to the destination. e) A “palm” changes its color while bending its fingers and the temperature variation during the process. Scale bars, 1 cm.

Figure 5

The mechanism of structural color changing of CNCs during the a) humidity and b) NIR laser light actuating process. c) Digital pictures and d) CIE 1931 color space chromaticity diagrams show the relationship between color changing and actuating angle. Photographs of a smart “mimosa” splaying and closing when exposed to e) moisture and f) NIR light. Scale bars, 1 cm.