Bioinspired Bilayer Structural Color Hydrogel Actuator with Multienvironment Responsiveness and Survivability

Abstract

Inspired by the actuator mechanism of Venus flytrap leaves, presented is a bilayer structural color hydrogel actuator by employing a hybrid inverse opal scaffold to join the poly(acrylic acid‐co‐acrylamide) layer and the poly(N‐isopropylacrylamide) layer together. The nanostructure of the inverse opal scaffold imparts the bilayer hydrogel with brilliant structural color, which can show a reversible coloration switch during the structural change. An internal water distribution occurs in the composite bilayer hydrogel during the heating or cooling process because of the opposite thermo‐responsiveness of the two hydrogels. Thus, this intelligent soft material can work in various environments and break the limitation of aqueous media of most structural color hydrogels. In addition, the water transference of the bilayer hydrogel enables the material with bending/unbending deformations and hence a series of complex motions like screwing, catching, and releasing can be achieved. Attractively, with the integrating of graphene oxide, the hydrogel systems can even be imparted with near‐infrared remote responsiveness to both of their color change and 3D deformation. These features of the bioinspired bilayer structural color hydrogel indicate its potential values in a variety of intelligent soft material applications.

Hydrogel‐Based Artificial Muscles: Overview and Recent Progress

“…A unimorph system is most simple one that contains a responsive layer and nonresponsive layer, called active and passive layers, respectively. If the active layer contracts, bending motion occurs toward active layer, and conversely, if active layer expand, bending motion occurs toward passive layer (Figure B) . When an active layer, which can contract and expand in response to multiple stimuli, is adopted, bending motion can occur in both directions .…”

“…C) Example of bending motion of hydrogel‐based artificial muscle with bimorph bilayer structure. B) Adapted with permission [90]. Copyright 2019, Wiley‐VCH.…”

Intelligent Polymer‐Based Bioinspired Actuators: From Monofunction to Multifunction

“…However, reliable and long‐term stable sensing and reporting functions of the bioinspired color‐shifting actuators based on the pigmentary colorations are sometimes restricted by their inherent drawbacks of photobleaching. Structural colorations, arisen from the optical interference, diffraction, and reflection by unique micro‐/nanostructures [ 76 ] and showing highly stable colors, [ 77 ] have therefore been introduced for the design and fabrication of the bioinspired color‐shifting actuators with enhanced sensing and reporting functions. Through combining stimuli–responsive polymers with periodically ordered structures, various bioinspired actuators enabling color shifting in response to different stimuli (e.g., humidity, [ 78 ] organic vapors, [ 79 ] temperature, [ 80 ] light, [ 81 ] and magnetic field [ 82 ] ) have been prepared based on the adjustable structural colorations, showing great versatility in sensing and reporting different environmental signals.…”

Bioinspired Design of Graphene‐Based Materials

“…[111] Additional functionality in the form of structural color (like in the periodic photonic nanostructures on creatures such as beetles, butterflies, chameleons, and peacocks) can be added by introducing a second hydrogel layer with opposite thermoresponsiveness on an inverse opal scaffold that switches color with the opposite volume changes of the hydrogels. [112] More complex devices, such as a soft robot with gecko-inspired feet and a graphene-PDMS composite muscle that walks (even on difficult terrain and in harsh conditions) by expansion of the muscle under irradiation, can also be produced by taking advantage of this effect (Figure 5c). [113] In a particularly innovative approach, complex bioinspired compound eyes (BCE) with a panoramic field of view (like insect eyes) and vari-focal ability (like mammal eyes) have been reported, consisting of arrays of lenslets containing graphene nanosheets that heat the surrounding fluid under local laser irradiation, causing them to expand and individually focus ( Figure 5d).…”