Bioinspired reflective display based on photonic crystals

Reflective displays have the advantages of energy efficiency, high brightness, eye protection, and good readability, making them an attractive display technology. Photonic crystal (PhC) structural color is highly regarded as an ideal choice for reflective displays for its ecofriendliness, colorfastness, and adjustability. In this review, we introduce the fundamental classification and manufacturing methods of PhC reflective displays. We systematically summarize the display principles of PhC‐based displays driven by various stimuli. Furthermore, we present the latest research advancements in PhC displays based on smart actuators. Additionally, we offer a detailed overview of the current research status and application prospects of liquid crystal structural color displays and three‐dimensional PhC displays. Finally, we discuss the challenges faced by PhC displays and provide insights into their prospects.

Moreover, reflective displays show superior readability under outdoor ambient light.Emissive displays are greatly limited by insufficient brightness when used outdoors.To address this issue, emissive displays usually increase their luminance, which further increases energy consumption.In contrast, the brightness of the reflective display increases under bright ambient light outdoors, making it suited for high-brightness outdoor environments. [3]urrently, reflective displays have made significant research progress and some commercialized products.Among them, the most widely commercialized is the electrophoretic display. [4,5]Electrophoretic displays rely on an electric field to drive charged dye particles to gather in an orderly manner to achieve display.Initially, electrophoretic displays only had black and white particles for two-color display.Now, electrophoretic displays have successfully achieved colorful displays by using red, yellow, blue, white, and black dye particles.However, the biggest issue with electrophoretic displays is their slow refresh rate, making them mainly used for static displays or e-papers, and not suitable for high-frequency dynamic displays.Reflective LCD (RLCD) is also a significant advancement in reflective display technology. [6]Compared with traditional emissive LCDs, RLCDs replace the emissive layer used for emitting light with a highreflective layer.This eliminates the need for a backlight and allows the display to rely on reflected ambient light for visibility.RLCD has a color range and refresh rate comparable to that of emissive LCD, making them advantageous for dynamic displays.However, the polarizing filter in LCDs blocks a substantial portion of ambient light, which leads to a reduction in display brightness.Therefore, RLCD cannot be used in environments with insufficient ambient light.Other reflective displays such as electrowetting displays, [7,8] electrochromic displays, [9] and iMOD displays based on thin-film interference [10] are still in the experimental research stage due to high power consumption, low contrast ratio, or short lifespan.
Photonic crystals (PhCs) are periodic structures formed by alternating materials with different dielectric constants, generally possessing photonic band gaps.The light whose frequency overlaps the photonic band gap cannot propagate in PhCs and is reflected to form structural colors. [11,12]The structural color of PhCs has the advantages of high color saturation, high brightness, and no photobleaching, making them more suitable for display than pigment colors and other structural colors, such as thin-film interference and total internal reflection.[15][16][17] PhCs made of smart, responsive materials change their structural and material parameters when subjected to external stimulation, resulting in a color change.This characteristic is perfectly suited for the field of displays.
PhC displays pave a new path in the field of reflective displays.In this review, we will discuss recent developments of PhC displays underlying different regulatory mechanisms.In the following sections, we first outline the PhC structures in nature that can serve as inspiration for artificial PhC displays.Second, we summarize the research progress of PhC displays based on changes in structural and material parameters driven by different stimuli.Then, we review the latest progress in PhC displays based on smart actuators.This review also discusses the advancements of structural color LCDs and three-dimensional (3D) PhC displays.Finally, we summarize and prospect the remaining challenges of this research field.

| PHOTONIC CRYSTAL IN NATURAL CREATURES
In nature, many organisms' colors can adapt to their surrounding environments.[20][21][22] The formation mechanism of colors can be divided into two categories: pigment color and structural color.Pigment color refers to the color exhibited by a substance when it absorbs or emits light of specific wavelengths. [23]Pigment color is essentially produced by electron transition in molecular orbitals, so it is easy to fade under environmental influence.Structural color mainly comes from the interaction between light and fine micro/nanostructure. [24,25] Compared with pigment color, structural color has obvious advantages in terms of color brightness, durability, and tunability.
During the process of biological evolution, organisms have developed various PhC structures and present brilliant colors.For instance, jewel beetles' elytra [26] possess onedimensional (1D) PhCs composed of alternating assemblies of melanin and cellulose (Figure 1A), and feather tips of peacocks [27,28] have 3D PhC structures with ordered arrays of melanin particles (Figure 1B).Pachyrhynchus argus [29] has a metallic coloration that is visible from any direction owing to a PhC structure analogous to that of opal (Figure 1C).The scarlet macaw [30] blue feather barbs display a vivid noniridescent blue color because of an amorphous diamond-structured PhC with only short-range order (Figure 1D).In addition to these simple optical structures, some organisms form composite structures to exhibit structural colors with some advanced properties, such as higher brightness, saturation, or chirality.Morpho butterflies, [31][32][33][34][35][36] for example, have composite structures on their wing scales (Figure 1E).The multilayer reflection structure on the scales generates bright colors through interference, while different ridges with multilayer structures form nanometer-scale diffraction gratings that produce a wide perspective range of structural colors.Marble berry Pollia condensate fruits, [37] for instance, exhibit spiral stacking of cellulose fibers on their fruit surface, resulting in chirality-responsive structural colors (Figure 1F).
Besides static structural color, dynamic structural color also exists widely in organisms.These organisms with dynamic structural color can reversibly change their color to achieve efficient camouflage and information communication.For example, the structural color of Charidotella egregia [38] is mainly produced by the stacking of the melanin and cellulose on the shell (Figure 2A).The thickness of the stacked structure is tuned when water vapor reversibly penetrates or exits, resulting in quickly color changing.Another example is the chameleon, [39] which has an ordered array of guanine platelets in its skin that can produce bright structural colors (Figure 2B).When the external environment changes, the guanine platelet array in its skin is stimulated and the particle spacing increases, causing the color to change from green to yellow.Unlike these two methods of changing lattice constant for color changes, Paracheirodon innesi [40] has two parallel rows of guanine crystal platelets that can cause multilayer interference in its cells below the blue stripes (Figure 2C).This creates a 1D periodic Bragg reflector stack that changes orientation by rotating around the base.This effectively reduces the gap between adjacent platelets around the interface and changes the structural color.The Anna hummingbird [41,42] (Figure 2D) can utilize the contraction and relaxation of its erector muscle to change the orientation of its feathers, thereby altering the angle of the photonic structure composed of keratin and melanin particles in the feathers.This changes the direction of diffracted light in its feather structure and results in different colors. 6][47][48][49][50][51]

| ELECTRIC DRIVING PhC DISPLAY
Electrically driven PhC displays show significant potential for practical applications due to the following reasons.First, the voltage applied to the device can be easily and accurately The stunning metallic green sheen of the "jewel beetle" is produced by one-dimensional (1D) PhC in the form of alternating assemblies of melanin and cellulose.Reproduced with permission. [26]Copyright 2005, John Wiley and Sons.(B) 3D PhC structures with ordered arrays of melanin particles in the feather tips of peacocks.Reproduced under terms of the CC-BY license.Reproduced with permission. [27]Copyright 2002, Forma.(C) The beetle's metallic coloration is visible from every direction owing to the optical properties of its scales, which act as a photonic crystal and have a structure analogous to that of opal.Reproduced with permission. [29]Copyright 2003, Springer Nature.(D) The scarlet macaw blue feather barbs display a vivid noniridescent blue color because of the photonic structure.Reproduced under terms of the CC-BY license.Reproduced with permission. [30]Copyright 2012, National Academy of Science.(E) Composite structures on morpho butterfly wing scales, where the multilayer reflection structure on the scales can produce a wide perspective range of structural colors.Reproduced with permission. [26]Copyright 2002, John Wiley and Sons.(F) Marble berry Pollia condensate fruits undergo spiral stacking of cellulose fibers, producing chirality-responsive structural colors.Reproduced under terms of the CC-BY license.Reproduced with permission. [37]Copyright 2012, National Academy of Science.PhC, photonic crystal.adjusted, allowing for precise control of the color of PhC.Second, the preparation of electrically responsive units is simplified through advanced processing methods in electronics.At the same time, the easy processing of microelectrodes and patterned electrodes facilitates the pixelated integration of displays.54][55][56][57] Ozin et al. fabricated a 3D PhC containing SiO 2 colloidal microspheres and crosslinked network polyferrocenylsilane (PFS).By utilizing the expansion and contraction of PFS under electrochemical reactions, they could change the distance between SiO 2 spheres and thus change the color.This PhCs display can change color in the entire visible light range under different voltages.However, this opal structure-based PhC has a slow response speed (about 10 s).To improve the response speed of the structure, they etched away SiO 2 to obtain an inverse opal structure-based PhC structure (Figure 3A).The response speed of the inverse opal structure-based PhC is about 1 s, which is faster than the opal structure-based PhC, but the color recovery of the inverse opal structure-based PhC is slow, and its lifetime is short.Electrowetting is another technique for PhC displays.The electrowetting PhC display relies on changing the affinity of colloidal surfaces to solvents.Solvents can penetrate colloidal crystals under voltage.Thus, the effective refractive index of the PhC is changed, resulting in its color changing. [58,59]iu et al. (Figure 3B) prepared a PhC structure based on a metal-organic inverse opal structure Pb(NO 3 ) 2 -poly (styrenemethyl methacrylate-acrylic acid).Electrowetting induces partial dissolution of Pb(NO 3 ) 2 -poly(styrenemethyl methacrylate-acrylic acid) assembly at the skeleton interface, leading to the transformation from an inverse opal porous to separate hollow spheres.The blue shift of the photonic bandgap can be observed after the transformation.Using this method, different patterns can be constructed by moving liquid along a predictable path.However, this method has a slow pattern-forming speed (90 s) and is difficult to reproduce. [60]To meet the requirements of dynamic display for color-change speed, many scientists have focused on dielectric elastomers that F I G U R E 2 Dynamic structural colors in nature.(A) Charidotella egregia switches its structural color triggered by water infiltration into multilayer structures.Reproduced with permission. [38]Copyright 2007, American Physical Society.(B) The color change of chameleon skin is caused by altering the spacing between guanine particles.Reproduced under terms of the CC-BY license.Reproduced with permission. [39]opyright 2015, Springer Nature.(C) The Paracheirodon innesi changes its structural color by rotating its one-dimensional diffraction grating composed of guanine.Reproduced with permission. [40]Copyright 2015, John Wiley and Sons.(D) Anna's hummingbird can change its color by using the pectoral muscles to alter feather orientation.Reproduced with permission. [43]Copyright 2022, John Wiley and Sons.
[63][64] By compounding a PhC structure on a dielectric elastomer or directly preparing a PhC from a dielectric elastomer, the expansion and contraction of the dielectric elastomer under voltage action lead to the changes in the lattice spacing of the PhC.These PhC displays based on dielectric elastomers can achieve fast color switching within 1 s (Figure 3C). [65]owever, dielectric elastomers themselves require kilovoltlevel high voltage drive and the color-change process is difficult to precisely control, which limits their application.
Currently, electrically driven PhC displays closest to application are electrophoretic displays.The charged particles disperse in solvents and can assemble into a regular structure to produce structural colors under an external electric field.With the increase of voltage, the lattice constant of the PhC becomes denser, which leads to the blue shift of color. [67]Fu et al. (Figure 3D) used a mixture of propylene carbonate, polyethylene glycol, and ethylene glycol as dispersants and negatively charged SiO 2 colloidal particles as building blocks.Under an electric field, SiO 2 microspheres are assembled to produce bright structural colors.Compared with previous electrophoretic display technologies, by introducing high-viscosity dispersants, the color of PhCs can be maintained even after removing the voltage.The color disappears only when a reverse voltage is applied.This design further reduces the energy consumption of electrophoretic displays and improves the response speed of colloidal particles.The authors also presented pixelated displays based on electrophoretic technology in their article, demonstrating the potential application of this method in the display field. [66]he main challenge for electrically driven PhC displays is their low response speed, which is insufficient to meet the demands of commercial dynamic displays.Therefore, introducing new materials or principles to accelerate the response speed of electrically driven PhC displays is an important direction for their development.
F I G U R E 3 Electrically driven dynamic PhC display.(A) Electrochemically driven dynamic display of inverse opal PhC based on redox reactions.Reproduced with permission. [58]Copyright 2009, John Wiley and Sons.(B) Patterned deformation of PhC driven by electric wetting.Reproduced with permission. [60]Copyright 2016, John Wiley and Sons.(C) Driven by an electric field, the dielectric elastomer undergoes in-plane expansion, causing an increase in the lattice constant of the PhC and resulting in the color change from red to green.Reproduced with permission. [65]Copyright 2017, John Wiley and Sons.(D) Bistable electrophoretic PhC display.Reproduced under terms of the CC-BY license.Reproduced with permission. [66]Copyright 2022, Springer Nature.PFS, polyferrocenylsilane; DEA, dielectric elastomer actuator; HF, hydrogen fluoride; MOIO, metal-organic inverse opals; PhC, photonic crystal.DISPLAY Magnetic stimulation has the advantages of fast response, easy integration, and high control precision, and has been widely used in the field of dynamic PhC display.70][71][72][73] Yin's group [74] developed magnetically responsive PhC, which contained 1D chains composed of superparamagnetic Fe 3 O 4 @SiO 2 core/shell particles (Figure 4A).The orientation of 1D chains in such photonic microspheres can be changed according to the magnetic field, thus tuning the diffractive colors of photonic microspheres.This magnetically responsive PhC display presents fast response, wide color range, and excellent reversibility.
F I G U R E 4 Magnetic field-driven dynamic PhC display.(A) Magnetic field-driven color-changing display using orientation change of one-dimensional magnetic nanochains.Reproduced under terms of the CC-BY license.Reproduced with permission. [74]Copyright 2009, American Chemical Society.(B) Magnetic field-driven color-changing display using orientation change of Janus PhCs microspheres.Reproduced with permission. [75]Copyright 2012, John Wiley and Sons.(C) Magnetic field-driven color-changing PVP-modified magnetic microspheres by changing the lattice spacing.Reproduced with permission. [76]Copyright 2023, John Wiley and Sons.(D) Patterned solidstate magnetic field-driven PhCs based on 3D printing technology.Reproduced with permission. [77]Copyright 2021, Royal Society of Chemistry.3D, three-dimensional; CNC, cellulose nanocrystal; EMF, enternal magnetic field; MP, the interface tension between methylsilicone oil and the aqueous solution of polystyrene microsphere; PhC, photonic crystal; PS, polystyrene; PVP, polyvinylpyrrolidone; TA, tannic acid; TMPTA, trimethyolpropane ethoxylate triacrylate; UV, ultraviolet.
Besides, it is also common to change the orientation of PhC by rotating the Janus PhC microspheres. [78]Yu et al. synthesized magnetic-induced Janus microspheres using a three-phase droplet microfluidic technique (Figure 4B).The Janus microspheres consist of magnetic hemispheres and PhC hemispheres.When the PhC hemisphere is on top, the microsphere exhibits structural color, while when the magnetic hemisphere is on top, the microsphere appears black.The orientations of these Janus microspheres can be adjusted by a magnetic field achieving the switch between ON(light) and OFF(dark) state.In this way, they achieved repeatable photonic patterns.with high brightness and fast response speed. [75]][81] By grafting more charges or surface-active substances on the surface of magnetic particles, colloidal stability can be improved.Liu et al. used polyvinylpyrrolidone molecules grafted on the surface of magnetic nanoparticles to prepare magnetic microspheres that can be stably dispersed in solvents (Figure 4C).Under the magnetic field, magnetic microspheres can achieve orderly assembly.As the external magnetic field strengthens, the spacing between the orderly assembled microspheres will decrease, leading to the blue shift of the color of PhC. [76]lthough dispersing microspheres in solvents can improve their response speed and color-change range, the liquid environments inevitably restrict the usability of magnetic responsive PhC displays.Many scientists have attempted to transfer magnetic microspheres to solid media such as hydrogels for color display.Fang et al. combined magnetic responsive PhCs with 3D printing technology and encapsulated Fe 3 O 4 magnetic nanocluster suspensions in an elastomeric outer phase (Figure 4D). [77]Through this method, 3D patterns can be assembled under external magnetic fields.Under enhanced external magnetic fields, the color of the sample shifts from red to blue gradually with the gradual shortening of interparticle distances.
Although we often draw analogies between magnetic fields and electric fields, there are still many challenges that need to be overcome in magnetically driven displays, such as achieving highly integrated pixelation and mitigating control interference between adjacent pixels.

| OTHER DRIVING MODES
In addition to the aforementioned electric-and magneticdriven displays, other external stimuli, such as stress, gas, and solvents can also be used to drive the PhC display.[84][85][86][87][88][89] One example is the stress-induced color-changing PhC display developed by Miller et al. [90] They combined 1D elastomeric PhCs with digital light processing (DLP) technology to create a stress-responsive and easily controllable color-changing pattern display (Figure 5A).The elastomeric photopolymer used in the display could bond with silicone, resulting in a structured silicone substrate with mechanical tunability and support.The spatial color distribution of the projected image determined the reflection structural color at each point on the film.The deformability of the elastomeric photopolymer allowed for predictable, repeatable, and reversible color changes.Zhong et al. [91] developed a solvent and vapordriven PhC display (Figure 5B).They utilized a hydrophilic-hydrophobic treatment to fabricate patterned hydrophilic regions on the PhC film.When water vapor or other solvent vapor passes through the PhC film, the hydrophilic regions become wetted and undergo a refractive index change, leading to a color change.
In addition to external driving methods, PhCs can undergo color changes through biomimetic self-growth.Xue et al. [92] developed a PhC composite with controllable structural color (Figure 5C).The system consisted of a living polymer matrix and embedded SiO 2 microspheres.The growth of the polymer matrix was mainly achieved by uniform expansion of the crosslinked polymer matrix through photopolymerization using a nutrient solution composed of monomers, crosslinkers, photoinitiators, and catalysts.This resulted in a dual network structure with elongated original crosslinked networks and increased spacing between SiO 2 microspheres, causing a color change from purple to red.This approach allowed for programmable fabrication of patterned PhC films.
Wu et al. [89] assembled PhC films using thermoresponsive poly(ethylene glycol)-acrylic acid copolymers and ZnS@SiO 2 microspheres (Figure 5D).These copolymers had a lower critical dissolution temperature, and their response temperature could be adjusted by varying the copolymer ratio.This allowed different regions to have different response temperatures within the range of 5-55°C, enabling a controlled programmable display of complex patterns with multiple colors during temperature changes.This programmable thermochromic patterned PhC film design provides a new direction for encryption anticounterfeiting technology, allowing for richer information content and more complex and precise encryption.
The PhC displays driven by stress, temperature, gas, or solvents exhibited the ability to visually respond to various external stimuli, which presents immense application potential in areas, such as visual detection and flexible wearable devices.

| AMORPHOUS PHCS DISPLAY
The angle-dependent of PhC structural colors used in displays limits the viewing angle of the display.[95] Amorphous PhCs consist of short-range-ordered colloidal crystals. [30,96,97]They only have a nonzero photonic density of states photonic pseudogap.100][101][102][103][104] These PhC structures can change color by varying lattice spacing and refractive index under external stimuli.Lee et al. developed angle-independent photonic pixels that can change with voltage (Figure 6A). [53]When the bias voltage increases from 1.0 to 4.0 V, the reflection peak of the photonic pixel reversibly shifts from 655 to 490 nm, changing the structural color from red to blue, and covering the entire visible light spectrum.Xia et al. reported a hydrogel film consisting of an amorphous arrangement of microgels that exhibits dual responsiveness to temperature and pH (Figure 6B). [105]The volume phase transition temperature of poly(N-isopropylacrylamide) in water provides the thermochromic property of the hydrogel film, while methacrylic acid provides pH responsiveness to the hydrogel film.Ge et al. reported an intelligent display window composed of an array of silica particles embedded in a poly(dimethylsiloxane) (PDMS) elastomer (Figure 6C). [106]Due to the matching refractive indices of the silica particles and PDMS, the elastic film is initially highly transparent.When stretched and strained beyond 40%, microwrinkles and gaps are formed, resulting in a decrease in transmittance to 30% and a noniridescent structural color appearance.Zhang et al. synthesized a photonic gel with noniridescent structural colors, high processability, and Reproduced with permission. [90]Copyright 2022, Springer Nature.(B) Selective modification of structural color film through masks to obtain structural color patterns.Patterns can exhibit different colors in different solvents.Reproduced with permission. [91]Copyright 2018, John Wiley and Sons.(C) Polymer self-growing driven color-changing PhC display.Reproduced under terms of the CC-BY license.Reproduced with permission. [92]Copyright 2009, Springer Nature.(D) Programmable temperature-driven patterned PhC.Reproduced with permission. [89]Copyright 2022, John Wiley and Sons.3D, three-dimensional; CPC, colloidal photonic crystal; DLP, digital light processing; EG, ethylene glycol; KUL, Katholieke Universiteit Leuven; PhC, photonic crystal; RI, refractive index.self-healing capability (Figure 6D). [107]By adjusting the mechanical properties and the size of the internally short-range-ordered silica particles in the photonic gel, bright multicolored patterns can be created.

| PHC DISPLAY BASED ON SMART ACTUATOR
The color change of traditional PhC display is usually based on the change in the micro-nano-structure of the PhC, such as the expansion or contraction of the lattice constant, or modifications in the material properties.Therefore, these PhC displays are hindered by poor repeatability or a long response time.In nature, many creatures, such as Anna's hummingbird and butterfly, can achieve color switching through movements of their scales or feathers, rather than changing the fine micro-nano-structure and materials of PhC.In these creatures, the structural color variation is related to a change in angle arising from the movement of the animal's scales or feathers. [43,108,109]spired by this mechanism, Lai et al. [110] fabricated the PhC display by combining the PhCs with a kirigami structure and proposed a new strategy to achieve color change by adjusting the angle of PhCs through topological deformation of kirigami (Figure 6A).The rectangle flap gates of this kirigami lift out of the plane under the uniaxial tension, leading to an angle change of PhC.The change of color is due to the view angle changing, which avoids the risk of damaging the delicate photonic structures by expansion and contraction.As shown in Figure 7A, with the increase of lifting angle of the PhC gate increased, the color of PhC gradually changed from red to blue.They demonstrated that the chamfer on the incision and the thickness of the sheet can influence the stretchinginduced out-of-plane deformation of the PDMS kirigami.Therefore, by endowing PDMS kirigami with different thicknesses according to different patterns, the PhC gates at the patterned area can be lifted at an angle different from the background area, giving rise to variable colorful patterns through stretching.This PhC-PDMS kirigami display realized a rapid, repeatable, and wide-range color change.However, the lifting effect leads to a 3D F I G U R E 6 Amorphous photonic crystals (PhCs) display based on different external stimuli.(A) Electrically driven dynamic amorphous PhC display.Reproduced with permission. [53]Copyright 2010, John Wiley and Sons.(B) Temperature-responsive short-range-ordered PhC display based on PNIPAAM.Reproduced under terms of the CC-BY license.Reproduced with permission. [105]Copyright 2021, American Chemical Society.(C) Stress-driven amorphous PhCs display.Reproduced with permission. [106]Copyright 2015, John Wiley and Sons.(D) Isotropic colorful PhC patterns.Reproduced with permission. [107]Copyright 2021, John Wiley and Sons.NP, nanoparticle; PDMS, poly (dimethylsiloxane); PNIPAAM, poly(N-isopropylacrylamide).
deformation, which limits its exploitation for thin display.To solve this issue, they further developed a kirigami grating sheet that can achieve color change in the twodimensional (2D) plane (Figure 7B). [111]In this work, they utilize the topological space deformation in a 2D plane of a kirigami structure to achieve predictable and precise adjustment of pixelated 1D grating orientations.Grating diffraction generates structural colors that possess orientationality, only visible when viewed at an angle perpendicular to the direction of the grating.By designing gratings with different initial azimuth angles on different square units, a programmable change of structural color can be realized, and different patterns can be observed under different strains.This display presents rapid response time (80 and 30 ms for on-and off-switching, respectively) and excellent cycling life (cycles >15,000 times).
Researchers combined other smart actuators with PhCs to create a PhC display.Utilizing smart actuators to induce bending deformation under external stimuli, the angle of the PhC can change, resulting in a color change.Fu et al. [112] (Figure 8A) prepared a 3D inverse opal structure of hydrogel PhCs and used a template method to create regular grooves on the back of the hydrogel for cultivating cardiac cells, allowing the cardiac cells F I G U R E 7 PhC display based on kirigami.(A) Bioinspired robust PhC kirigami display.The bioinspired PhC-PDMS kirigami demonstrates color switching between red and blue reversibly under uniaxial tension.Reproduced with permission. [110]Copyright 2021, John Wiley and Sons.(B) The kirigami grating display can achieve reversibly color-switching by utilizing the topological space deformation of kirigami structures to manipulate the orientations of 1D gratings.Reproduced with permission. [111]Copyright 2023, Elsevier.
to grow along the grooves.The external culture medium flows over the back of the photonic chip.When the external culture medium flows over the back of the photonic chip, the cardiac cells contract, bending the PhC film and exhibiting color changes.This enables visual monitoring of the activity of cardiac cells.Mu et al. [113] (Figure 8B) used polyethylene terephthalate film (PTF) as a substrate and assembled a monolayer of closely packed SiO 2 spheres on its surface.When one side of the PTF is stimulated by water vapor, the side in contact with water expands and causes the PTF to bend, resulting in color changes on another side of the PhC.
For continuous dynamic display, precise control of each pixel unit is required.Li et al. [114] (Figure 8C) used the photoresponsive gel as the driving component and nearinfrared (NIR) light as the energy source.Through the precise design of a hinge-like structure, the lifting angle of the PhC film showed a strict relationship with the irradiation time of NIR light.The lifted PhC film can revert to a flat state when exposed to water vapor.This approach can realize precise control of each pixel unit.After processing the pixel units into an array, dynamic patterns formed along the scanning path of NIR light can be obtained.This driving method makes it possible to dynamically display variable colorful patterns on a single screen.However, it still has disadvantages, such as slow response speed and complex operation.To accelerate the response speed of pixel units, Ma et al. [115] (Figure 8D) combined dielectric elastomer actuators with PhC films, utilizing the dielectric elastomer actuator bending under an electric field, to change the angle of PhC.This greatly improves the response speed and makes overall pixel integration easier.However, due to the requirement of high voltages in the kilovolt range for dielectric elastomer driving, its further application is limited.Additionally, dielectric elastomers still have limitations in terms of control accuracy.
The utilization of smart actuators to alter the angle of the PhC does not affect its microstructure, thus prolonging the lifespan of PhC displays.Moreover, the color-changing speed of the PhC is solely dependent on the response speed of the chosen smart actuator, allowing for an expanded range of display applications through careful actuator selection.However, this approach, which relies on angledependent properties of PhCs for display, is limited in viewing angle and requires further advancements in technology to overcome this limitation.
F I G U R E 8 PhC displays based on smart actuators.(A) Self-regulating structural color hydrogels can change their shape and structural color because of the contraction or relaxation of cardiomyocytes.Reproduced with permission. [112]Copyright 2018, The American Association for the Advancement of Science.(B) The stimuli-responsive structural color actuator can change its shape and color depending on the humidity.Reproduced under terms of the CC-BY license.Reproduced with permission. [113]Copyright 2018, Springer Nature.(C) Pixelated display based on photothermal responsive gel.Reproduced under terms of the CC-BY license.Reproduced with permission. [114]Copyright 2022, Elsevier.(D) PhC reflective display based on dielectric elastomer actuator.Reproduced with permission. [115]opyright 2022, Royal Society of Chemistry.DEA, dielectric elastomer actuator; NIR, near-infrared; PhC, photonic crystal.STRUCTURAL COLOR DISPLAY Liquid crystals (LCs) are essentially macroscopically ordered anisotropic materials, with the average orientation direction referred to as director.Cholesteric liquid crystals (CLCs), composed of a periodic arrangement of twisted layers made up of rod-like molecules or aligned nanoparticles, are important PhC structural color materials.The pitch (P) of the helix determines the structural color at the wavelength λ o = (n o + n e )P/2, where n o and n e are the ordinary and extraordinary refractive indices of the CLC, respectively.118][119][120][121][122][123][124][125] Zhang et al. [126] proposed a new type of cellulose CLCs with additional molecular interactions and dynamic regulation as a biomimetic prototype for structural coloring (Figure 9A).They mixed cellulose molecules with acrylamide monomers, and the abundant hydrogen bonding between them facilitated the self-assembly of cellulose LCs, resulting in bright structural colors.These cellulose CLC mixtures exhibited more extensive thermal and humidity response behaviors compared to pure CLCs.By controlling the irradiation time, varying degrees of crosslinking can be achieved in different parts of the polymer.Upon exposure to humidity or temperature, polyacrylamide with distinct crosslinking degrees demonstrates diverse response capabilities, leading to the display of distinct colorful patterns in different regions of the LC.Intelligent responsive LCs have been used to create specific pattern displays and intelligent feedback-adjustable skins.Kim et al. [127] utilized thermoresponsive LCs to create a multilayered intelligent feedbackadjustable skin (Figure 9B).The silver nanowires embedded under the LC layer can generate heat under the electric current, resulting in the color change of the thermoresponsive LC at the top.By designing an intelligent feedbackadjustment system, the skin can dynamically display different colors according to external environmental conditions, achieving camouflage.Wang et al. [128] prepared cholesteric phase LCs with chiral halogen bonds (Figure 9C).The halogen bond switch, acting as a chiral dopant, had high helical twisting power.When exposed to ultraviolet (UV) light, the pitch of the LC significantly increases due to the twisting force exerted by the halogen bonds.The alteration of the interlayer spacing of the LC leads to changes in color.Using this method, reversible color patterns identical to the UV light irradiation path could be formed on the LC film.Kim et al. [129] used highly stretchable cholesteric columnar LCs with large stretch ratios as substrates to create pixelated LCs that change color pneumatically (Figure 9D).They fabricated a highly stretchable and height-stable single-domain mainchain chiral nematic liquid crystalline elastomers (MCLCEs) film with a large Poisson ratio.When gas entered the MCLCE film through channels, the film expanded in the vertical plane direction, causing the LC film to become thinner and the interlayer spacing to decrease, resulting in a color change from red to purple.Each gas channel could serve as a display pixel, allowing the display device to show desired digital patterns on demand.
Expanding on the use of external stimuli to alter the pitch of LCs, Cui et al. [130] introduced the reconfiguration of helical axes into the LCs structural color (Figure 9E).They used a polymer-induced phase separation technique to combine photoresponsive cholesteric LCs with polymer-dispersed liquid crystals (PDLCs) to develop a photonic paper with handwriting, electrical erasing, and light color adjustment functions.These functions depend on the PDLC layer inside the photonic paper, which consists of photoresponsive cholesteric LC microdroplets and a polymer network.When writing on photonic paper with a pen, the pressure causes the helical axis of the cholesteric LC to restructure from a scattered light's disordered focal cone state to an ordered planar state of reflected light, and the pattern is written.By using light stimulation to control the change in helical pitch of the cholesteric LC, the written pattern transitions from a monochromatic to multicolor state.Applying an electric field reconstructs the helical axis into a focal cone state, erasing the pattern.It is worth noting that our designed new photonic paper functions have been upgraded from simple "write-erase" to "write-color-adjust-erase" and "color-adjust-write-erase" working modes.This new photonic paper has potential applications in recording, programming, and storing complex multicolor information for user-interactive display technology.
Currently, research on structural color displays for LCs is primarily focused on changing the pitch of the LCs using external stimuli to achieve color variation.However, this approach has the disadvantage of slow response speed.Exploring new color-changing principles is essential for the future development of structural color displays using LCs.Furthermore, utilizing the inherent ability of LC structures to modulate light polarization is also an important direction for advancements in the display field.
Additionally, the spiral structure of LC also endows it with the ability to regulate the polarization of light, leading to more functional LC structural color displays, such as holographic and encrypted displays.Chen et al. [131] efficiently reconstructed high-quality directionally related images in tunable broadband by combining photosensitive chiral molecules with asymmetric light pattern boundaries, preparing visible light-activated hybrid multiplex holography based on chiral reversible LC structures (Figure 10A).The reversible switching between phototriggered holographic images (e.g., smiling or crying faces) exhibits high sensitivity to circularly polarized light.Cellulose nanocrystals (CNCs) can form helical periodic structures that produce structural colors.Anusuyadevi et al. [63] combined CNCs with LCs to achieve chiral light reflection (Figure 10B).By forming an LC array on the CNC/glucose film, a birefringent layer was created to distort the incident light polarization before interacting with the chiral cellulose nanocomposite material.Using photoresponsive LCs, this effect can be further suppressed by exposure to UV light, thereby switching the nematic LC to a nonbirefringent isotropic phase.CLCs are becoming increasingly popular due to their unique chiral structural colors.He et al. [132] prepared monodisperse CLC particles by dispersion polymerization F I G U R E 9 Display based on responsive liquid crystal (LC) structural color.(A) Humidity-responsive cellulose-liquid crystal structural color display.Reproduced with permission. [126]Copyright 2021, John Wiley and Sons.(B) Self-feedback intelligent skin based on thermoresponsive LC.Reproduced under terms of the CC-BY license.Reproduced with permission. [127]Copyright 2021, Springer Nature.(C) Photoresponsive liquid crystal display containing chiral halogen bonds.Reproduced with permission. [128]Copyright 2018, John Wiley and Sons.(D) A pressure-responsive display based on mainchain chiral nematic liquid crystalline elastomer membrane.Reproduced with permission. [129]Copyright 2022, Springer Nature.(E) Multi-erasure-write structural color display based on polymer liquid crystal.Reproduced with permission. [130]Copyright 2022, John Wiley and Sons.FC-M, focal conic-multiple pitch length; FC-S, focal conic-single pitch length; KSU, Kent State University; LCI, liquid crystal institute; NIR, near-infrared; TLC, thermochromic liquid crystal.
(Figure 10C).By producing CLCs into micrometer-sized monodisperse spheres, the pitch of the CLCs can be altered according to their particle size, allowing control over the structural color of CLC particles by influencing the spacing of Bragg reflection and photonic bandgap.This structure can exhibit different structural colors under different circularly polarized light.As shown in Figure 10D, Park et al. [133] achieved a chiral PhC film using the CLC and the helical nanofiber (HNF), which can self-assemble into chiral 1D photonic structures.This periodic 1D chiral structure leads to polarizationdependent transmission color variation due to optical rotation.Placing a micrometer-scale mask with a "K" pattern on top of the HNF-PhC film, the transmission mode with two polarizers was studied.Unique color changes were observed by altering the polarization angle.
[139][140] Liu et al. [141] used two-photon polymerization lithography printing techniques to prepare 3D PhC models, and then obtained crosslinked 3D photonic structures using thermal shrinkage (Figure 11A).The precision of this structure can reach 280 nm, which is the same as the optical structure scale of butterflies in nature.This method can be used to print 3D structures of various colors, such as 3D colored Eiffel Tower.Liao et al. [142] and Liu et al. [143] combined a printable colloidal PhC ink with a 3D printing system based on DLP to manufacture 3D PhC models (Figure 11B).In addition to precise structural design, by introducing components with response capabilities into the ink, responsive 3D PhC models can be obtained.For example, after adding NIPAAM, a component that undergoes volume shrinkage when the temperature changes, to the printing ink, the composite 3D PhC model can change color as the temperature changes.
Zhang et al. [144] used DLP technology and 3D printing to combine hydrogen bond-induced colloid particles with stable dispersion and continuous solidification induced by capillary force in the UV-curing system during printing (Figure 11C).They achieved macroscopic printing and microscopic particle assembly simultaneously.The resulting 3D models have bright structural colors, and as the solvent evaporates and the volume shrinks during the printing process, the color gradually changes from red to purple.This method can one-step mold many complex twisted structures, greatly expanding the application range of PhC displays.In addition to innovative printing methods, variable color models of 3D structure assembly can also be obtained by constructing PhC substrates of different materials and structures.
Siegwardt and Gallei [145] proposed a scalable polymer core-shell particle design and printing method (Figure 11D).The core-shell particles are composed of a hard polystyrene core and a relatively soft polyalkyl acrylate-based shell.The core-shell particles were prepared by emulsion polymerization using a slurry-feed mode.Since the structural color depends on the size of the underlying particles, the obtained color can be easily adjusted by adjusting the applied synthesis parameters.Under mechanical deformation, the PhC lattice and color change, realizing mechanical color-changing sensing with the printed object.

| OUTLOOK
PhC reflective displays have attracted widespread attention due to their high brightness, wide color gamut, and high tunability.This review takes inspiration from the structural color of PhCs in nature and systematically outlines the research progress of PhC reflective displays.
PhC reflective displays hold enormous potential for various applications, particularly in wearable devices, flexible displays, and visual sensing.
Although much progress has been achieved in recent years, there is still a long way from practical applications of PhC reflective displays.First, it is a challenge to achieve the reproducible mass production of PhC with a stable periodic structure, uniform response characteristics, and acceptable costs.Second, the response speed of PhC displays still has a significant gap compared to the response speed of emissive displays.Finally, the angledependent properties of PhCs themselves limit the viewing angle of PhC displays.To address these issues, finding suitable materials and smart actuators is an effective approach.By drawing inspiration from natural structural color materials like butterfly wings and hummingbird feathers, researchers can explore innovative designs for photonic structures and driving mechanisms that are better suited for display applications.Copyright 2019, Springer Nature.(B) 3D printed temperature-responsive PhC display based on digital light processing technology.Reproduced with permission. [142]opyright 2022, Elsevier.(C) 3D printed PhC controlled by hydrogen bonding and capillary force.Reproduced under terms of the CC-BY license.Reproduced with permission. [144]Copyright 2022, Springer Nature.(D) 3D printed force-responsive PhC based on polymer core-shell microspheres.Reproduced under terms of the CC-BY license.Reproduced with permission. [145]Copyright 2023, John Wiley and Sons.3D, three-dimensional; HENP, highly charged elastic nanoparticles; PhC, photonic crystal; PNIPAAM, poly(N-isopropylacrylamide); UV, ultraviolet.

F I G U R E 5
PhC display based on different external stimuli.(A) Stress-driven color-changing DLP patterned composite films.

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I G U R E 11 Color-changing 3D printed PhC.(A) 3D model of the Eiffel Tower with different colors by two-photon polymerization lithography printing techniques.Reproduced under terms of the CC-BY license.Reproduced with permission.