Recent Progress in Artificial Structural Colors and their Applications in Fibers and Textiles

Abstract

Structural colors have been regarded as an ideal alternative to pigments because of the advantages of environmental friendliness, resistance to fading, especially in the fields of textile dyeing and printing, which is highly polluted in traditional ways and needs to be upgraded. They are generated by the interaction between the micro/nano structure of the material surface and the incident light, which is closely related to the sizes and periods of the structures. Based on different nano arrangements, the structures can be roughly divided into photonic crystals and non-photonic crystals. In this review, we summarized recent research progress in structural coloration of fibers and textiles. Learning from nature, researchers proposed various methods to fabricate biomimetic structural colors, and the applications of structural colored fibers and textiles were extended. The challenges and perspectives were also presented. Hopefully, reference for the design and preparation of structural color-based textiles can be inspired.

Introduction

From animals such as chameleons, butterflies, and beetles to natural phenomena like rainbow, and soap bubbles, colors make the world lively and beautiful. As early as the Stone Age, colorful ores were ground into colored powders, and pigments were extracted from natural plants. Afterwards, ever since the development of chemical industry, chemical synthetic pigments have been widely used to color fabrics and textiles. The mechanism of chemical pigments is that photons with certain energy is released by electron transition and colors exhibit when received by eyes.^[1] However, the chemical dyes have many disadvantages such as environmental pollution, chemical disability, and gradual optical degrading over time. On the one hand, chemical industry is usually accompanied with serious pollution, which is not environmentally friendly. On the other hand, colors will fade with time especially in acid or alkali environment, and the color cannot be erased or changed once painted. Besides, some organic dyes are also faced with production problems such as complicated preparation process and high energy consumption.^[2] As chemical dyes become more and more difficult to meet the requirements, a new dyeing method is urgently needed. While admiring the miracle of nature, people are also learning from it. It is found that colors in nature include not only pigment colors obtained through absorption or reflection of light by dyes but also structural colors obtained through scattering, diffraction and interference of light.^[3] Compared with chemical dyes or pigments, structural color is a brand-new color expression. Many materials have periodic micro/nano structures.^[3a,4] Due to the resonance characteristics of micro/nano structures, their resonant wavelengths are affected by the size and period of the structure.^[5] Under the illumination of white light, light of specific color can be scattered on the surface of materials.^[6]

The unique principal of structural colors brings unique characteristics. Firstly, structural colors are usually bright and highly saturated. High-purity colors can be scattered as required and all the colors can be achieved. In addition, displaying specific colors by controlling material surface structure is completely environment-friendly and the structural colors will never fade. What's more, the light waves scattered by the materials’ surfaces can be flexibly controlled by adjusting the micro units through external forces, and electromechanical controls, etc.^[7] Moreover, different from chemical colors, there are no restrictions for structural colors as long as specific micro/nano structures can be built on the surfaces of materials, which is of vital importance for fibers with high curvature surfaces.

As a clean and ecological method to obtain colors with unique characteristics, structural colors are often employed for coloring fibers and textiles. With the increasing requirements of environmental protection, structural coloration is considered as one of the most promising ways to replace traditional printing and dyeing methods. In this review, we summarized recent research progress in structural coloration of fibers and textiles (Figure 1). Basic types, preparation methods, and applications of structural colored fibers and textiles were presented and discussed. We hope that this review can provide valuable reference for the design and preparation of structural color-based textiles that may offer an alternative option to traditional chemical dyeing and helps to solve the problems of high energy consumption and high pollution in traditional printing and dyeing industry.

Figure 1. Structural coloration of fibers and textiles.

Basic Types of Structural Colors in Nature

Almost all the structural colors in nature were generated by the interaction between structures and light, such as interference, scattering and diffraction. Generally, based on the difference of nano arrangements, the structures can be roughly divided into photonic crystals and non-photonic crystals. Photonic crystals show a long range ordered regular periodic arrangement while other non-photonic crystal structures are mostly ordered in short range instead of long range. The non-photonic crystal structures are represented by thin-film interference and diffraction gratings.

Photonic crystals

Photonic crystals can be further divided into one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D), which correspond to the refractive index changes in spatial dimension (Figure 2a, 2d, 2g). The colors of jewel beetles are caused by a stack of layers with very small layer spacings which are typical 1D photonic crystals and able to diffract light (Figure 2b, c).^[8] The coloration in peacock feathers takes the advantages of partial photonic bandgaps of 2D photonic-crystal structures in the cortexes (Figure 2e, f). Through the variation of the lattice constant or the number of periods, colors were produced in peacock feathers. The midgap frequency of the partial photonic bandgaps was shifted varying the lattice constant.^[9] 3D photonic crystals are the most widely studied and also the most complicated photonic crystals. The colors of natural opals are produced by the physical interaction between the 3D photonic crystal structures composed of silica microspheres and light. The neotropical diamond weevils are marked by rows of brilliant spots on the overall black elytra, which is a typical 3D photonic-crystal structure (Figure 2h-j).^[10]

Figure 2. (a) 1D, (d) 2D, (g) 3D photonic crystal structures. (b) Photograph of a Japanese jewel beetle. (c) Transmission electron microscopy (TEM) images of the cuticular surface of a Japanese jewel beetle. Reprinted from Ref. [8], Copyright (2013) with permission from IOP Science. (e) A photograph of peacock feathers. (f) Schematic illustration and a scanning electron microscopy (SEM) image of peacock feathers. Reprinted with permission from Ref. [9], Copyright (2003) with permission from Elsevier. (h) A photograph of the diamond weevil. (i) A single scale with a few differently colored domains. (j) A SEM image of the cuticular surface of a diamond weevil. Reprinted from Ref. [10], Copyright (2012) with permission from The Royal Society of Chemistry.

Thin-film interference

Thin-film interference-based structural colors are attributed to different refractive indexes of films, which are common in nature and has been widely studied. One of the most common phenomena is colorful soap bubbles under the sun (Figure 3a). The light waves are partially reflected by the upper and lower interfaces of the film respectively, and new light waves are formed due to interference. Due to different film thicknesses and viewing angles, different colors are reflected from the internal and external surfaces, which makes them look colorful. Both experiments and theories have proved that interference fringes can only be generated when two columns of light waves have certain relations, which are called coherence conditions. There are three requirements for coherence conditions of films. First, two light waves have the same frequency. Second, the vibration direction of the beam wave is the same. Third, the phase difference between the two light waves remains constant. The optical path difference (ΔL) can be calculated by [Eq. 1]: 1 $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr \Delta L=2d\sqrt{{n}_{2}^{2}-{n}_{1}^{2}{{\rm s}{\rm i}{\rm n}}^{2}\theta }{\rm \char44 }\hfill\cr}}$$$

Figure 3. (a) A photograph of a soap bubble showing iridescent colors. (b) Optical paths of single layer thin-film interference. (c) A photograph of CDs. (d) SEM images of the grating structures of CDs. Reprinted with permission from Ref. [12], Copyright (2009) with permission from American Association for the Advancement of Science.

where d represents the film thickness, θ represents the incident angle and n represents the refractive index of the film (n₂) and the surrounding medium (n₁, n₃).

When n₁<n₂>n₃ or n₁>n₂<n₃, half-wave loss needs to be considered. The optical path difference is calculated by [Eq. 2]: 2 $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr \Delta L=2d\sqrt{{n}_{2}^{2}-{n}_{1}^{2}{{\rm s}{\rm i}{\rm n}}^{2}\theta }+{{\lambda }\over{2}}\hfill\cr}}$$$

Diffraction grating

Diffraction gratings are periodic structures that split and diffract light into several beams travelling in different directions, thus creating structural colors, which depend on the spacings between the structures, as well as the directions of observations.^[11] Typical examples of grating structures are compact discs (CDs) (Figure 3c), which are composed of a large number of parallel slits with equal widths and spacings (Figure 3d).^[12] When light is incident to the gratings, light of different frequencies will be reflected in different directions due to the effect of diffraction, showing iridescent colors. In fact, the grating usually presents iridescent when the grating period is close to the wavelength of visible light.

Others

In recent years, new types of structural colors were also widely reported. Plasmonic color is an emergent property of the interaction between light and metallic surfaces.^[13] Wang et al.^[13c] used tandem nanodisk arrays to realize vivid red-green-blue (RGB) colors in reflection and cyan-magenta-yellow (CMY) colors in transmission (Figure 4a). The fabrication of short wavelength colors with relatively large nanostructures was enabled by this method (Figure 4b, c). Li et al.^[13d] proposed a facile inject printing method that can print all the colors with a transparent ink (Figure 4d). The structural color was generated by total internal reflection. Kumar et al.^[13a] achieved bright-field color prints with resolutions up to the optical diffraction limit. Color information was encoded in the dimensional parameters of metal nanostructures, so that tuning their plasmon resonance determined the colors of the individual pixels. Colors achieved using plasmonic resonance mentioned before have very interesting and important features. For example, they usually have high resolution, near-permanent lifetime, and material simplicity. The resolution may be higher than 10⁵ dpi, which exceeds the diffraction limit of light. Nevertheless, the method also has shortcomings such as low throughput, high patterning costs, and elaborating color tuning mechanisms, which hinders its commercial development.

Figure 4. (a) Schematic drawing of the tandem nanodisk square array. (b, c) Measured (b) reflection and (c) transmission image structural colors. Reprinted with permission from Ref. [13c], Copyright (2017) with permission from American Chemical Society. (d). Inkjet printing method to generate structural color with a single transparent ink. Reprinted with permission from Ref. [13d], Copyright (2021) with permission from American Association for the Advancement of Science.

Thanks to the vivid examples in nature, researchers presented various methods to fabricate and product structural colored fibers and textiles. The dyeing process is green and environmentally friendly, which has significant ecological values.

Structural Coloration of Fibers and Textiles

Researchers presented a few methods to fabricate structural color-based fibers and textiles. Different fabrication methods and progress are summarized here to help understand and guide the design of optimized structural colorations of fibers and textile.

Thin-film deposition

Thin-film interference is a widespread method to fabricate structural coloration in nature. Materials with certain refractive index should be carefully selected, and the thickness of each layer should be in an optimum range. Each layer can produce a constructive interference of light and reflect a particularly bright color of a certain wavelength meeting the above requirements. Techniques include atomic layer deposition, magnetron sputtering deposition, etc., which are commonly used to construct multilayers.

Magnetron sputtering technology is commonly used. Metal or non-metal films can be deposited on textiles such as polyester, cotton, linen, etc. as long as the appropriate sputtering process is selected.^[14] The main structures of the textile substrates include woven fabrics, knitted fabrics, and non-woven fabrics. Fabrics can not only be endowed with single or compound functions such as electromagnetic shielding,^[15] UV protection,^[16] antibacterial,^[17] conductive,^[18] or waterproof properties,^[19] etc. More importantly, structural colors can be obtained through interference and diffraction characters of the nano-films. Therefore, the magnetron sputtering coating nano-films technology used on fabrics has been widely studied and used.^[20] Composite films were successfully deposited on the polyester fabric substrates with magnetron sputtering by Yuan et al.^[21] Ning et al.^[22] fabricated Ag and Ag₂O films which were deposited on a polyester fabric in sequence (Figure 5a). In addition, the thickness of the coatings was only nanometer-scale on the fiber surfaces instead of the whole fabric, leaving the gap of the fabric open, which retained most of the properties of the original fabric substrate, and had almost no adverse effect on the wear resistance of the colored fabrics.

Figure 5. (a) “RINTT” pattern of different color. Reprinted with permission from Ref. [22], Copyright (2020) with permission from Elsevier. (b) Schematic illustration of the fabrication process of structural colored CF yarns and fabrics. Reprinted with permission from Ref. [25], Copyright (2019) with permission from American Chemical Society. (c) Schematic illustration and optical images of fabrication structural colored CNTFs via ALD. Reprinted with permission from Ref. [26], Copyright (2022) with permission from American Association for the Advancement of Science.

Atomic layer deposition (ALD) is a more precise method to control the film thickness compared to magnetron sputtering which was proposed by Tuomo Suntola to fabricate electroluminescent (EL) flat panel displays in Finland for the first time.^[23] With the rapid development of the semiconductor industry, ALD developed very fast since 1990s as a necessary process. In most cases, two self-limited surface reactions occur and binary compound films with atomic level control are deposited during ALD processes.^[24] Due to the high efficiency, accurate and uncomplicated thickness control, and good conformability to various surfaces, ALD is recognized as one of the most effective and promising methods to generate structural colors till now. Niu et al.^[25] reported new photonic crystal carbon fiber (CF) yarns and fabrics with tunable structural colors and wonderful mechanical properties. A densified 1D photonic crystal structure was adopted to produce brilliant structural colors which was grown on the surfaces of CFs coaxially (Figure 5b). Chen et al.^[26] fabricated structural colored carbon nanotube fibers (CNTFs) (Figure 5c) and CF fabrics^[3c] by depositing a TiO₂ layer on them. CNTFs were regarded as the darkest materials around the world before. In addition, the color category could be easily controlled by adjusting the thickness of TiO₂ layer.

Self-assembly

Self-assembly is the process of realizing structural sequences of different scales without any direct external effects.^[27] Various building blocks can be used to create periodic nanostructures such as block copolymers,^[28] liquid crystals,^[29] and colloids.^[30] In many cases, this phenomenon is quite common in nature that optimal colors and multifunctional materials are created. Therefore, to replicate them efficiently, it is of great importance to understand the structure and coloring mechanism of creatures in nature in order to create faultless, structurally colored materials.^[6] Many colloidal self-assembly methods, such as gravitational sedimentation,^[31] vertical deposition,^[32] and electrophoretic deposition^[33] were proposed and developed in recent years.

Under gravity, colloidal microspheres can be deposited on the surfaces of substrates, forming periodic photonic crystal structures spontaneously after the volatilization of solution, which is called gravitational sedimentation method.^[34] There is no need for the introduction of extra energy with the help of gravity, which is quite energy-saving. Zhang et al.^[35] drew bare fibers from colloidal suspensions and fabricated structural colored fibers in this simple way (Figure 6a, b). The nanospheres were assembled into photonic crystal structures, which displayed brilliant colors under visible light. Kohri et al.^[36] developed PDA-based 3D colloidal photonic balls and fibers. The structural color was bright for the shell of PDA could absorb light, which was beneficial for the development of structural colored inks. Kim et al.^[37] applied microfluidic jetting method in the production of mechanochromic fibers. Xu et al.^[38] demonstrated that CFs had intelligent properties with color responsiveness besides wonderful mechanical robustness (Figure 6c). 2D graphene oxide (GO) sheets were coated on CF surfaces, where they stacked together in order and self-assembled as a lamellar stacked structure. Such self-assembled GO films showed structural colors resulted from the similarity of the stacked structure and 1D photonic crystals (Figure 6d).

Figure 6. (a) Schematic illustration for the drawing method, and (b) microscope images of 5 different structural colored fibers with 5 different sized PS nanospheres. Reprinted with permission from Ref. [35], Copyright (2019) with permission from American Chemical Society. (c) Coloring mechanism of GO pseudo-photonic crystal films, and (d) Optical images of CF and colored-CF. Reprinted with permission from Ref. [38], Copyright (2022) with permission from Elsevier. (e) Images of all the iridescent cotton fabrics and corresponding color images. (f) Images with different water evaporation time. Reprinted with permission from Ref. [39], Copyright (2022) with permission from Elsevier.

The self-assembly of cellulose nanocrystals (CNCs) into structurally colored films has aroused considerable interest by researchers and has been regarded as a potential candidate for the production of more sustainable photonic pigments. Wei et al.^[39] reported an iridescent cotton fabric with reversible multiple stimulus response. CNC could self-assemble into nanostructures with structural color on cotton fabrics. Gradient structural colors were produced by changing the self-assembly way of the CNCs onto CF surfaces, which was innovative and environmentally friendly for there was no need to add superfluous additives (Figure 6e, f).

Electrophoretic deposition can also be used for the production of structural colored fabrics as a common electrochemical method.^[40] One pole of the electrophoresis cell was the group deposited, and the counter electrode was usually a platinum sheet. Zhang et al.^[33] fabricated structural colored core−shell colloidal fibers by this method. The building blocks were colloidal spheres with diameters of 100∼300 nm. They assembled onto CFs electrophoretically such that structural colors were obtained (Figure 7a–c). As Figure 7d shows, dye-free electrospun fibers were fabricated via electrospinning successfully.^[41] Individual colloidal fibers with several micrometers in diameter were obtained, and they were non-iridescent with tunable colors. Many colloidal spheres were attached on the fibers, which were shortrange ordered (Figure 7e). The fiber was non-iridescent with homogeneous colors, which was originated from the reflectance of photonic band gaps and Mie scattering. Full color display could be achieved by adjusting the sizes of the colloidal spheres, and most of the colors could be obtained using only three sizes of nanospheres. The method presented an eco-friendly conscious alternative to the polluted dyeing practices that were currently used in the textile industry. This novel coloration method was able to scaled up, making dye-free textile coloration feasible.

Figure 7. (a) Fabrication process of the structural colored fibers by the EPD process. (b) Microscope images of red, green and blue fibers. (c) SEM images of a green fiber. Reprinted with permission from Ref. [33], Copyright (2013) with permission from American Chemical Society. (d) Schematic illustration of the colloidal electrospinning process and preparation of structural colored fibers. (e) SEM images of colloidal fibers composed of 280 nm spheres. Reprinted with permission from Ref. [41], Copyright (2015) with permission from American Chemical Society.

Printing

Printing methods are mainly through spray guns or atomizers. With the help of pressure or centrifugal forces, colloidal microsphere suspension dispersed into uniform and fine droplets and coated on the surface of the substrate coating.^[42] One of the distinguished advantages of printing methods is that they are non-contact coating methods, which can also be adapted for curved surfaces and other irregular surfaces, realizing the assembly of photonic crystals on the surfaces of objects with arbitrary shapes, and obtaining the effect of structural color patterns combined with the mask methods.^[43] Compared with methods mentioned above including gravitational sedimentation, electrophoretic deposition, vertical deposition, self-assembly, etc., printing is a convenient and energy-efficient method to assemble structural colored photonic crystals. For instance, Liu et al.^[44] used a soap-free emulsion polymerization method to prepare four different sized P(St-MAA) colloidal microspheres, showing bright colors on the surfaces of CFs.

The screen-printing technique is an easy and convenient way to produce flexible printed electronics,^[45] microfluidic dielectrophoresis chip,^[46] and patterned cotton fabrics,^[47] which is widely adopted around the world. Similar to the screen-printing, Zhang et al.^[48] reported atomization deposition of colloidal silica nanoparticles to fabricate large-area homogeneous APSs with brilliant non-iridescent structural colors. The fabrication process of the APS coatings could be controlled, and adjusting the coating thickness was effortless, as shown in Figure 8a–c. Based on this, different colors could be realized by stacking APS coatings. Tang et al.^[49] realized vivid non-iridescent structural colors with good mechanical stability on white fabric by a rapid screen-printing method. Large-scale production was not complicated for screen printing. Moreover, multicolor pattern output was feasible as long as the printing process was repeated. Colors obtained were highly visible on different substrates. Furthermore, laundering and rubbing tests proved that the structural and mechanical stability were enhanced to a great extent because of the addition of polyacrylate (PA). The creative and eco-friendly dyeing method showed great potential in textile industry (Figure 8d, e).

Figure 8. (a) Schematic illustration of the fabrication of large area APSs with non-iridescent colors on different substrates (flexible and rigid) by atomization deposition method. (b) Structural colored rose pattern on silk fabric composed of five non-iridescent colors. (c) Stretching the patterned silk fabric in (b) with ∼15 % elongation. Reprinted with permission from Ref. [48], Copyright (2018) with permission from American Chemical Society. (d, e) Optical images of the leaf pattern (d) and butterfly pattern (e) coated fabrics with the addition of CB. Reprinted with permission from Ref. [49], Copyright (2020) with permission from Elsevier.

Moreover, the atomization method was omnidirectional, which made 3D conformal coating on objects with irregular or curved surfaces was feasible in few steps or even one step. The fabrication of highly crystalline photonic crystal-modified saturated colored fabrics with comfortable wearing experience, such as breathability and softness, was still a great challenge.

Micro-nano structuring

Adding micro-nano rough surfaces to fibers or having a porous amorphous structure on or inside them were commonly used to create micro-nano structures. These two methods control the physical interaction between periodic structures and light by combining absorption, reflection, refraction, transmission, interference, scattering and diffraction to obtain the structural colors. Up to now, surface patterning of materials at micro and nanoscales has been used in diffraction gratings,^[50] surface wetting,^[51] plasmonic metasurfaces,^[52] surface enhanced Raman scattering (SERS),^[53] and triboelectricity.^[54] V Various techniques have been developed, including molding and embossing, mask deposition, optical lithography, electron beam lithography, and scanning probe technology, for patterning on nanometer to millimeter length scale. There was also work about thermal fiber drawing to texture fibers.^[55] The fibers should be flexible and uniform to functionalize textiles, which was also important to be produced at high throughput. A thermal drawing method was proposed to achieve fiber surface gratings on a rectangular section, exhibiting directional wetting properties and grating-based structural colors.^[56] The patterning technology brought huge opportunities in micro/nanofluidics, plasmonic metasurfaces, smart surfaces, organic photonics, biosensors, etc. The synergistic effect of fiber surface patterning and fiber internal functional structure is able to create more advanced functions, paving the way for the development of more functional multi-functional fiber equipment and intelligent textile platform.

Zhang et al.^[57] used a two-phase printing method to prepare fabrics. Multiple colors output could be obtained on the fabrics at the same time, so that fascinating patterns could be printed. The colors of the fabrics could be adjusted by changing the microspheres diameters. Due to the adhesive capacity of PA, 3D colloidal crystals were closely combined with yarn, and even after washing and friction for many times, the coloring film still had good integrity. In addition, the addition of CB was convenient because of the continuous production technology, which improved the color saturation, (Figure 9a-c). The dyeing method was proved to have good prospect for industry. Li et al.^[58] reported a clean textile coloration method consisting of two steps, as shown in Figure 9d–f. The first step was to immobilize functionalized CB nanoparticles onto cotton fabric through a darkness system. Besides, an RGB color control system by immobilizing cobalt blue, cobalt green, iron oxide red nanoparticles onto cotton fabric that could color the cotton fabric with many different colors besides the primary RGB colors. The color of the cotton fabric could be adjusted accurately by varying the degree of grafting of the nanoparticles. The release of nanoparticles during usage and the color remained intact even after intense laundering because of the strong interaction of nanoparticles and cotton fabrics.

Figure 9. (a) Schematic illustration of the two-phase self-assembly technique to print 3D photonic crystals colored fabrics. (b) The process of colorful peacock feather pattern output via the transfer printing process on fabics. (c) SEM images of the ordered photonic crystals. Reprinted with permission from Ref. [57], Copyright (2017) with permission from American Chemical Society. (d) Schematic illustration of the fixing of CB nanoparticles onto the surface of cotton fabric. (e) Standard colourimetric test of colored cotton fabric with PANTONE card by mixed different materials. (f) SEM images of pristine cotton-g-CB fabric. Reprinted with permission from Ref. [58], Copyright (2019) with permission from The Royal Society of Chemistry.

Among various bionic materials, the inverse opal hydrogel material is copied from the self-assembled photonic crystal template. Because its spatially ordered lattice shows bright structural color, it has attracted great research interest. Inverse opal hydrogel microspheres exhibited non-iridescent structural colors because of the spherical symmetry. Particularly, when the volume or shape of materials varied under different stimuli, their structural colors or photonic bandgap (PBG) spectra would shift. Zhao et al. used a co-flow capillary microfluidic device with multiple injection channels to continuously generate jellyfish tentacle like hydrogel microfibers and implant discontinuous discrete structural color microsphere units.^[59] It was found that when a certain force (such as pressure or tension) was applied to the microspheres, the PBG would move synchronously, thus the structural color changed back. Based on this feature, it was demonstrated that these structural color microspheres implanted microfibers were with excellent capabilities of sensitive and rapid response to motion stimulations, according to the localization of finger bending experiments.

Microfluidic refers to the science and technology involved in the system of using micropipes to process or manipulate micro fluids.^[60] With the help of these unique fluid phenomena, microfluidic can realize a series of micromachining and micromanipulation which are hard to achieve by conventional methods. Zhao et al.^[61] prepared microfibers by microfluidic spinning method that were injected programmed. When the cultivated cardiomyocytes recovered autonomous beating cycles, the structural color part would be stretched and showed synchronous stretch cycles with dynamic color change and wavelength shifts, which converted the forces generated by microscopic cells into macroscopic signals. What's more, the single-cell-level mechanics detecting platform could be achieved by adjusting the sizes and diameters of the heterogeneous structural color microfibers. These characteristics help heterostructure color microfiber to become an ideal platform in the biomedical field.

Large-scale fabrication

In general, the colloidal self-assembly process is the most common method in the preparation of structured colored fabrics, which involves many complex steps, such as translational diffusion, crystal nucleation and crystal growth.^[62] But this process is efficient and often takes hours to days. In order to improve the assembly efficiency of photonic crystal structures, researchers have done a lot of work in the direction of large-scale manufacturing. Yin et al.^[63] designed a preparation strategy for high-performance liquid colloidal crystals (LCCs) based on shear induced assembly pre-crystallization, which can generate large-scale photonic crystal films on a flexible substrate for structural coloring. The fluid properties of pre-crystallized LCC and its rapid reconstruction ability are important characteristics of large-scale preparation. These properties allow easy diffusion to textured substrates under shear forces and rapid assembly into bright, highly crystalline photonic crystal films. The minimal amount of solvent in the LCC dispersion is also a key factor in the efficient formation of the structural color film, which evaporates quickly and produces high-quality meter-scale solid photonic crystal films within minutes. This strategy provides a method for large-scale and rapid preparation of structure-colored photonic crystal films on textured fabrics and paper substrates, which is conducive to the commercial development of structure-colored textiles.

Vignolini et al.^[64] used continuous roll-to-roll coating technology to optimize the self-assembly of CNC suspension into photonic film, as shown in Figure 10. which demonstrate excellent optical properties. In addition, the particles are produces by film ground after further heat treatment that can be used in pigments. Importantly, these particles have ultra-high durability and still retain their optical properties after a year without fading. They successfully combined a self-assembled biomaterials with high-throughput fabrication techniques, such as roll-to-roll coatings, to produce cellulosic films with large-scale structural colors.

Figure 10. Overview of the roll-to-roll processing of a CNC suspension into photonic films and particles. Reprinted with permission from Ref. [64], Copyright (2022) with permission from Nature Publishing Group.

Applications of Structural Colored Fibers and Textiles

Based on the above methods, researchers have made abundant efforts to explore the applications of structural colored fibers and textiles, such as anti-counterfeiting, wearable electronics, and large-scale fabrication. In this section, we summarized three main applications of structural colors and discussed their potential applications in the future.

Anti-counterfeiting

Anticounterfeiting is more and more important for authentication, currency, and security. Structural colored anticounterfeiting tags have the merit of high efficiency, low cost, easy to manufacture.^[65] Invisible ink is a traditional magic trick, usually done by adding alcohol and lemonade to the document. The invisible ink is controlled by chemical reactions that change the color of the pigment under certain conditions.^[66] However, the ink on the paper fades over time and is not conducive to long-term storage of critical information. On the contrary, photonic crystals exhibit vibrant structural colors through multiple scattering and interference due to the periodic arrangement of dielectric materials. Thus, they have been extensively used as colorants for image displays^[67] and sensors,^[68] and are called “photonic inks” (P-inks). If the color image of this structure shows the same color as the surrounding paper, the hidden image can be selectively displayed under appropriate conditions, such as chemicals substances,^[69] magnetic field,^[70] or mechanical strains.^[71] Since these photonic displays are composed of structural colors rather than pigment molecules, no photobleaching or degradation occurs and the information can be safely stored on the photonic paper for a long time. This type of photonic material is therefore ideal for use in anti-counterfeiting devices or the storage of secret information.

For example, Gu et al.^[72] designed an intelligent color material with non-uniform stripe pattern based on the non-synchronous process of colloid assembly in capillaries and the descent of the solid-liquid-gas interface. The width and spacing of the structural color stripe pattern can be achieved by adjusting the closed colloid self-assembly parameters such as the diameter of the glass capillary tube and the concentration of colloid nanoparticles. Besides, by self-assembly of nanoparticles of different sizes, stripe patterns with different structural colors can be generated. The graphene hydrogel was also integrated into the stripe pattern to make it NIR-light-controlled reversible bending behavior.

Ding et al.^[73] prepared polymer opal films (POFs) with invisible patterns that could be displayed by mechanical stretching or chemical expansion (Figure 11a). The patterned areas have a ground mask that is cross-linked with UV light, and they are more resistant to mechanical stretching and chemical swelling than the areas that are not cross-linked. When the POFs is stretched, both the cross-linked and non-cross-linked regions have obvious color changes and strong color contrast, which can be clearly seen on the POFs. Alternatively, swelling may cause the patterned and unpatterned areas to expand to different degrees, resulting in a different redshift effect, with sharp contrast, again revealing invisible patterns. Kim et al.^[74] successfully prepared a highly stretchable film heater with a thermochromic display. The SFH exhibits excellent electrical conductivity, high mechanical tensile properties and excellent reliability, with no significant changes after 10,000 tensile cycles. Lee et al.^[75] prepare K photonic crystals by colloidal self-assembly and photoetching. Each small K crystal shows bright green light reflected under normal incident light, but as the incident light Angle gradually increases, the reflected color shifts blue, as shown in Figure 11b. At different angles of incidence, the film will show a bright reflective color. For higher incidence angles, blue shift occurs. Based on the above characteristics, this photonic crystal pattern is expected to be used for anti-counterfeiting or optical identification codes.

Figure 11. (a) Optical photo of POFs. Images of different proportions of EtOH and H2O (i)-(vii). Scale bar is 0.5 cm. (vii) Wavelength in the irradiated region (blue line) and the wavelength in the non-irradiated region (red line) when in a mixture of EtOH and H2O with different volume fractions. Reprinted with permission from Ref. [73], Copyright (2015) with permission from American Chemical Society. (b) (i) Optical images of colloidal crystal patterns consisting of small K at different incident angles, and (ii) optical images of individual films with K patterns. (iii) The color of patterned photonic crystal films on Korean banknotes observed at different incident angles. Reprinted with permission from Ref. [75], Copyright (2013) with permission from American Chemical Society.

Taking inspiration from the butterfly, Zhao et al.^[76] synthesized colloidal crystal arrays (CCAs) films of structural colors (Figure 12a). Graphene-labeled polymer components were integrated into CCA films to generate hydrogel films with inverse opal structure in order to obtain better properties. The results showed that the synthetic hydrogel obtained the color transfer property which was not affected by the Angle under certain stimulation conditions. In addition, poly (N-isopropylacrylamide) (PNIPAM) hydrogel is heat sensitive, and the transition of hydrophilic and hydrophobic state is affected by temperature. When heated at 0~50°C, the color of the chameleon pattern gradually changed from red to blue with the shrinkage of the hydrogel volume, and finally disappeared, as shown in Figure 12b. Moreover, a dove-patterned security label was also prepared, with only one color when viewed from the front. But depending on the Angle and the near-infrared and ultraviolet light, the color will be different sharply, as shown in Figure 12c, d.

Figure 12. (a) Schematic diagram ofof preparation of chameleon pattern hydrogel. (b) Optical images of the color hydrogel heating process. (c) Pigeon pattern anti-counterfeiting label production diagram. (d) Optical image observed after rotation or NIR. The scale bar is 2 mm. Reprinted with permission from Ref. [76], Copyright (2019) with permission from Springer.

Wearable electronics

Wearable systems that monitor physical activity, store data and collect physiological information are the next hot direction for personalized medicine.^[77] However, some demanding technical problems followed. For example, the manufacture of high-performance and energy-efficient sensors, and memory modules in close contact with the body, these technologies limit the application of wearable systems. Generally speaking, wearable electronic products should be at least stretchable and lightweight, and structural color textiles are designed to meet these requirements. In recent years, structural color wearable electronics have become particularly attractive due to the characteristics of responsive color shift readings.

The skin is an important window for the body to communicate with the environment, sensing a variety of different stimuli and encoding them as electrical signals to the nervous system to obtain information.^[78] To mimic the properties of human skin, various artificial skins have been researched, including electronic skins^[79] and ionic skins (I-skins).^[80] I-skins have inherent advantages in human skin ion transduction mechanism, excellent optical transparency and flexible mechanical properties, so they exhibit great potential in wearable intelligent devices^[81] and soft robotics.^[82] The preparation methods and materials of I-skin are various, but the ion gel composed of polymer network and encapsulated ionic liquid (ILs) is considered to be one of the most promising material candidates for wearable systems due to its wide operating temperature range, high ionic conductivity, non-flammability and excellent stability.^[83] Inspired by chameleon skins, Sun et al.^[84] synthesized a new bionic color-changing material to prepare photonic ion skin (PI-skin), which can output collaborative photoelectric signals under strain and has good adhesion, stability and elasticity. The ionic conductive organohydrogel which was prepared by Qiu et al.^[85] as I-skin was affixed to different parts of the human body to simulate the actions of various body bruises, successfully demonstrating the perception of mechanical stimulation and optical visualization. Under large deformation, the optical changes from light yellow to bruise like blue purple were observed. It is a strain sensor with the function of visualization of mechanical impact damage and damage warning. Niu et al.^[86] have developed a series of new interactive MET sensors that integrate a mechanochromic supramolecular photonic PTIP elastomer with a conductive AZO polyester fabric to give the sensor a fast and durable photoelectric response and demonstrate its visual application in stretchable electronic devices. Color change information can be directly captured by the human eye and quantified by optical spectrum, while the readings of electrical signals can be accurately recorded by electronic instruments, thus realizing direct human-computer interaction visualization (Figure 13a, b). Compared to other types of sensors, the MET sensor can be easily woven, sewn onto clothing or attached directly to human skin to track human movement in an electro-optical dual signal response. This could lead to interesting developments in the emerging world of interactive electronics.

Figure 13. (a) Schematic diagram of the MET sensor as a visual interactive device for detecting human motion. (b) Relative resistance change and sensor color change and reflection spectrum corresponding to different movements of human body. Reprinted with permission from Ref. [86], Copyright (2021) with permission from Elsevier.

Summary and Outlook

Inspired from nature's masterpieces, structural color related materials were widely investigated in the past few years. Researchers made great efforts in understanding the fundamental mechanism of light interaction, production of artificial structural colored fabrics, and extending their applications in numerous fields. Nature has always continued to enlighten researchers for endowing structural color materials with refreshing and remarkable functions such as anti-counterfeiting, autonomous regulation, shape memory, etc. These break-through developments revolutionized the field of structural color materials by endowing them with not only attracting visual effects but also dynamic and adaptive properties.

Different from the traditional textile coloring technology, the production of structural color does not need to use dyes or pigments and other chemical colorants. It is a comprehensive effect of nano structures and light interference, diffraction and scattering, usually with features of bright, flexible and never fade. Colorful clothes decorate people‘s life, but in the pursuit of color, we should also consider the harmonious development of industry and environment. The development of structural color technology and its application in textiles are expected to meet the increasingly strict economical, aesthetic, and environmental protection requirements, and will have a good prospect for development.

Nevertheless, at the same time, there are still several problems about the application of structural colors in textiles that need to be further explored and studied. Firstly, technological process of the preparation of structural colors should be simplified. As we know, the generation of structural color relies on micro/nano patterns on the surfaces, which are usually complicated to manufacture. Therefore, to achieve complete replacement of traditional dyes, the manufacturing process needs to be simplified. “One-step method” is a direction of efforts as too many procedures will affect the coloration speed and efficiency to a great extent. Meanwhile, reducing cost to the level of traditional dyes and meeting the requirements of mass continuous production are also of great importance, which is closely related to the complexity of production. In addition, the color fastness of structural color needs to be improved, so that the structural colored fibers and textiles can resist damage caused by daily activities. As clothes for daily wear, they need to resist thousands of times of washing without fading. This is quite significant for compatibility with commonly used fibers and fabrics such as cotton, silk, linen, etc., instead of newly prepared fibers. In a word, simplifying fabrication process, reducing cost, increasing efficiency and enhancing performance are four vital directions for both research and practical application of structural colored fibers and textiles.

Overall, there are still challenges to be overcome about for their wide applications. It is believed that structural colors will play an important role with the continuous development of fundamental science and advanced micro nanoscale manufacturing techniques, which will definitely achieve outstanding performance and dramatically promote the potential applications in information, health, energy, environmental resources, and other related fields in the near future.

Acknowledgments

This work is supported by National Natural Science Foundation of China (Grant No. 22075163 and 51872156) and National Key Research and Development Program (2020YFC2201103, 2020YFA0210702).

Conflict of interest

The authors declare no conflict of interest.

Data Availability Statement

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

Biographical Information

Run Li received her B.E. degree from Tianjin University in 2020. She then joined Prof. Rufan Zhang's group at Tsinghua University to pursue her PhD in the area of functionalization of CNT-based materials.

Biographical Information

Zhang Shiliang is an M.D. candidate in the School of Materials Science and Engineering, Hubei Unversity, China. He received her bachelor's degree at Qufu Normal University in 2020. He research interests are carbon nanomaterials, energy-saving, metafabric etc.

Biographical Information

Prof. Rufan Zhang received his bachelor‘s degree in Chemical Engineering and Technology from the China University of Petroleum (Beijing) in 2009 and his Ph.D. degree in Chemical Engineering and Technology from Tsinghua University in 2014. From Nov. 2014 to Dec. 2017, he worked as a postdoctoral researcher in the Department of Materials Science and Engineering at Stanford University (USA). Now Prof. Zhang is an associate professor in the Department of Chemical Engineering, Tsinghua University. His research interests focus on the synthesis and property study of carbon materials and functional materials.