Liquid crystal-based structural color actuators

Abstract

Animals can modify their body shape and/or color for protection, camouflage and communication. This adaptability has inspired fabrication of actuators with structural color changes to endow soft robots with additional functionalities. Using liquid crystal-based materials for actuators with structural color changes is a promising approach. In this review, we discuss the current state of liquid crystal-based actuators with structural color changes and the potential applications of these structural color actuators in soft robotic devices.

Fig. 1. Overview of photonic LC materials for making structural color actuators and the corresponding mechanisms of actuation and structural color change.

Left: LC with chiral photonic structure, including CLC, CNC and BPLC. The pitch/lattice spacing of photonic LCs increases when exposed to stimuli, causing dimensional and structural color changes. Middle: LC with opal/inverse opal structures; the spheres represent nanoparticles or air voids. Right: LC with alternating layers of two media with distinct refractive indexes. The disorder change of the LC when exposed to stimuli causes anisotropic deformation, leading to lattice spacing and structural color changes. Δλ_max represents the maximum structural color shift achieved in the reported literature

Fig. 2. CLC-droplet-dispersed PVA films.

a Schematic illustration of oblate-CLC-droplet-dispersed PVA films with a gradient distribution of oblate CLC droplets perpendicular to the film plane and thus a gradient thermal expansion perpendicular to the film plane when oblate CLC droplets were heated to the isotropic phase. b An “octopus” on a black rock reversibly changed color from blue to colorless and shape from flat to curved for camouflage upon heating and cooling. Reproduced with permission. Copyright 2021, Wiley-VCH

Fig. 3. Temperature responsive bilayer structural color actuators based on CLCEs.

a Molecular structures of the monomers used to prepare the CLCE layer. b An illustration of the lamination technique employed to prepare laminated CLCE/NLCE. c Topological director profile (left) and photographs (right) of the bilayer film during shape transformation exhibiting concurrent color changes. Reproduced with permission. Copyright 2019, Wiley-VCH

Fig. 4. Temperature responsive single layer structural color actuators based on CLCEs.

a Molecular structures of the monomers used to prepare the CLCE film using a two-stage polymerization. b Photographs of a flat CLCEs (left) and a beetle shaped CLCEs (right) at different temperatures. Reproduced with permission. Copyright 2021, Wiley-VCH. c Molecular structures of the monomers used to prepare the near-infrared light fueled CLCE film. d Photographs of a flat CLCE (top) and a cuttlefish shaped CLCE (bottom) when locally exposed to 780 nm NIR light (scale bars = 10 mm). Reproduced with permission. Copyright 2022, American Chemical Society

Fig. 5. 3D printed humidity responsive structural color actuators based on CLCEs.

a Molecular structures of the components used for synthesizing the cholesteric liquid crystal (CLC) oligomer ink. b Photographs of a 3D-printed water-responsive beetles at increasing, and then decreasing, relative humidity. c Photographs of the scallop-shaped structural color actuator with increasing, and then decreasing, relative humidity (14–95–30 RH%, scale bar = 5 mm). Reproduced with permission. Copyright 2022, Wiley-VCH

Fig. 6. Temperature responsive structural color actuators based on CLCEs with dynamic covalent bonds.

a Molecular structures of the monomers used to prepare the CLCEs (top). The allyl dithiol (3) gives the CLCE the capability of undergoing reversible AFT bond exchange reactions via the AFT exchange mechanism (bottom): a radical from the photoinitiator or a thiyl radical can induce a bond exchange. b Photographs of CLCE films programmed at 25 °C, strained to 100%, exposed with light at 320–390 nm with an intensity of 70 mW cm⁻² for varying amounts of time (20, 60, 120, 600 s), measured at 25 °C and 120 °C. c Transmittance spectra of the CLCE film measured during the programming, cycling between red and blue shifts. Reproduced with permission. Copyright 2020, Wiley-VCH. d Molecular structures of the monomers used to prepare the CLCEs (left) and schematic mechanism (right) of B-O bond exchanging in the boronic ester: i) thermo-activated B-O bond exchange; ii) water-assisted B-O bond exchange. e Demonstration of reprogrammable and thermo-actuation properties in a single CLCE film (scale bars = 5 mm). Reproduced with permission. Copyright 2022, Wiley-VCH

Fig. 7. Light responsive structural color actuators based on azobenzene-containing CLC networks.

a Molecular structures of the monomers used to prepare the CLC networks. b Actuation at 25 °C (below T_g): reflection spectra and photographs of the CLC film with 405 nm light irradiation (right), and after 532 nm light irradiation (left). c Actuation at 50 °C (above T_g): the CLC film exhibited a redshift of the structural color and bending upon 405 nm light irradiation at 50 °C. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry

Fig. 8. Electrically stretched structural color actuators based on CLCEs.

a Schematic illustrations showing the reflection wavelength tuning mechanisms for mechanically (top) and electrically stretchable (bottom) CLCEs. b Photographs and c reflection spectra showing the reflective color change in electrically stretched CLCEs as a function of the applied electric field. Reproduced with permission. Copyright 2021, De Gruyter

Fig. 9. Pixelated camouflage in an inflating CLCE.

a Schematic representation of the pixelated structural coloration platform, consisting of a base with air channels, a supporting layer, and a main-chain CLCE membrane which was reversibly pneumatically actuated with pressure (p). b Top: photos showing simultaneous displays of R, G and B coloration at the same pressure by varying the aspect ratios, t/w = 0.04, 0.06 and 0.10. Bottom: photos showing the reflection spectral shift from NIR to visible and UV. c Demonstration of camouflage to match a background with periodic color patterns. Reproduced with permission. Copyright 2021, Nature Publishing Group

Fig. 10. Structural color micro-actuators based on CLCs.

a Molecular structures of the monomers used to prepare the photonic 3D micro-actuators using direct laser writing. b Optical microscopy (top) and scanning electron microscopy (SEM) images (bottom) of a flower-shaped microstructure before base treatment. c Crossed polarized micrographs of the flower showing color changes with direct (humidity) and indirect (temperature) triggers. Reproduced with permission. Copyright 2020, American Chemical Society. d Molecular structures of the monomers used to prepare the micrometer-sized CLC particles. e Temperature (top) and light (bottom) responses of CLC particles exhibiting spot-like structural colored domains. Reproduced with permission. Copyright 2020, Wiley-VCH

Fig. 11. Humidity responsive structural color actuators based on BPLCs.

a Molecular structures of the monomers used to prepare the BPLC film. b Schematic of the mechanism of the responsive behavior of the BPLC film. The original BPLC film exhibited a green color corresponding to a medium lattice size (210.9 nm), which was then corroded by alkali and swelled by water. After swelling, the film turned a red color corresponding to a larger lattice spacing (258.5 nm); upon evaporation of the water, the film shrunk and appeared blue, corresponding to a smaller lattice spacing (186.5 nm). c A strip of a BPLC film in the shrunken and swollen states. Reproduced with permission. Copyright 2020, Wiley-VCH

Fig. 12. Water/humidity responsive structural color actuators based on CNCs.

a Chemical structure of CNCs. b A sandwiched structure with PA-6 embedded between two CNCs/PEGDA layers showing reversible bending and twisting as well as reflective color change when exposed to a humid environment. Reproduced with permission. Copyright 2016, Royal Society of Chemistry. c A CNC/ PF resin composite bilayer with different helical pitch showing curling and color change when exposed to water. Reproduced with permission. Copyright 2014, Wiley-VCH. d PF resin/GO-containing CNCs films showing reversible unbending and color change when dipped in water and drying. e PF resin/GO-containing CNCs film with predetermined shapes treated with formaldehyde at selective regions and their responses toward a wetting–drying cycle. Reproduced with permission. Copyright 2019, Royal Society of Chemistry

Fig. 13. Humidity/NIR light responsive structural color actuators based on CNCs.

a Schematic mechanism of the CNCs/PU bilayer composite film and its NIR light- and humidity-responsive actuation mechanisms. Photographs of a “mimosa” shaped CNCs/PU bilayer splaying and closing when exposed to (b) moisture and (c) NIR light. Reproduced with permission. Copyright 2021, Wiley-VCH

Fig. 14. Color and surface topography changes achieved by embossing CLC polymers.

a Photographs of an embossed CLC polymer upon increasing the temperature from 0 to 55 °C which shows a change in the color from blue to orange. Reproduced with permission. Copyright 2017, American Chemical Society. b Photographs (left) and surface topographies (right) of the deformed CLC polymer coating at 10 °C and heated to 40 °C. Reproduced with permission. Copyright 2019, Wiley-VCH. c Photographs (top) of the programmed CLC coating showing color and surface topography change (bottom) when heating. Reproduced with permission. Copyright 2020, Wiley-VCH

Fig. 15. Color and surface topography changes achieved by patterning CLC coatings with local swelling properties.

a Mechanism schematic of the patterned humidity/pH responsive IPN polymer coating. b Left: optical micrographs of a patterned IPN coating at a RH of 15% and 85%. Middle: surface topography of the patterned humidity responsive IPN coating at RH = 15%, 50%, and 80% (T = 20 °C). Right: optical micrographs of the patterned IPN polymer film at a pH 3 and 9. Reproduced with permission. Copyright 2015, Wiley-VCH. c Photographs of a sponge-written “TU/e” logo that appeared upon exposure to water but remained hidden in the dry state. d Height profile of the “TU/e” logo in wet state showed swelling for the slash and shrinkage for the letters with respect to the green background and nearly flat surface in the dry state. Reproduced with permission. Copyright 2018, American Chemical Society

Fig. 16. LCN with opal or inverse opal structures.

a Top: schematic illustration of the actuation and dynamic change of the lattice distance in the SiO₂ opal/LCN composite films. Bottom: photographs of the reversible actuation and color change of the SiO₂ opal/LCN composite film induced by temperature variation. Reproduced with permission. Copyright 2016, American Chemical Society. b Schematic of the dual phase LCN photonic films (left) and photographs of finger action of a bionic hand based on the LC photonic films (right). Reproduced with permission. Copyright 2018, Wiley-VCH

Fig. 17. LCs on hierarchical microstructures.

a Chemical structure of the photo-responsive LCP containing azobenzene mesogens. b Schematic showing the deformation of the bilayer LLCP-MBW. With the increase of UV intensity, the penetration of incident light gradually increased into the lamellas of the ridges, causing the deformation of microstructures and the gradual increase of the 397 nm reflection peak. c Surface profiles of the LLCP-MBW show the reversible deformation of the scales on the LLCP-MBW upon UV (365 nm, 10 mW cm⁻²) and visible light (white light from the microscope) irradiations. d The changes of reflectance spectra of LLCP-MBW after 365 nm UV irradiation with different intensities for 10 s and subsequent 530 nm visible light irradiation for 10 s. Reproduced with permission. Copyright 2019, Wiley-VCH