A cellulose round wafer material with chiral photonic structure and its preparation method and application

Abstract

The application relates to the technical field of photonic materials, in particular to a cellulose round crystal sheet material with a chiral photonic structure and a preparation method and application thereof. The preparation method comprises the following steps: S1, mixing a cellulose nanocrystal suspension after ultrasonic treatment with an aqueous polyurethane dispersion liquid in proportion to obtain a mixed liquid; S2, pouring the mixed liquid into a glass filter cup in which filter paper is laid, and continuously performing vacuum filtration for 4-10 hours to obtain a cellulose nanocrystal / aqueous polyurethane composite film; and S3, soaking the composite film in an ethanol solution and fully stirring to dissolve the aqueous polyurethane, and then performing centrifugal separation, washing and drying. The obtained cellulose round crystal sheet material perfectly inherits the chiral photonic structure, and can be redispersed in various solvents to form a "photonic ink" or a "photonic slurry". A structural color effect like a sequin can be obtained through direct coating, the angle dependence is strong, the cellulose round crystal sheet material has the functions of beauty and anti-counterfeiting, and excellent large-scale processing performance is shown.

Description

Technical Field

[0001] This invention relates to the field of photonic materials technology, and in particular to a cellulose wafer material with a chiral photonic structure, its preparation method, and its application. Background Technology

[0002] Cellulose nanocrystals are rod-shaped nanoparticles extracted from natural cellulose, possessing a unique ability to self-assemble into chiral nematic (cholesterol) liquid crystals. When the solvent slowly evaporates, cellulose nanocrystals can spontaneously arrange themselves into periodic helical structures, forming bright and permanently color-resistant structural color films. This optical effect, derived from physical structure rather than chemical pigments, has attracted significant attention in fields such as environmentally friendly coatings, anti-counterfeiting labels, and high-end decoration.

[0003] However, traditional cellulose nanocrystal self-assembly relies heavily on a slow solvent evaporation process to obtain a long-range ordered chiral nematic phase structure. This "slow solvent evaporation self-assembly" process typically takes several days, and the final product is a brittle film, making it difficult to re-disperse or directly process as a coating for large-scale applications. This presents a profound dilemma for cellulose nanocrystal photonic materials in practical applications, balancing "slow self-assembly" with "rapid macroscopic processing." How to rapidly and controllably construct cellulose nanocrystal photonic structures and endow them with good processability is a key challenge in bringing such biophotonic materials to practical applications. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned shortcomings of the prior art by proposing a cellulose wafer material with a chiral photonic structure, its preparation method, and its applications.

[0005] The first objective of this invention is to provide a method for preparing a cellulose wafer material with a chiral photonic structure, comprising the following steps: S1. The cellulose nanocrystal suspension is ultrasonically treated and then mixed with an aqueous polyurethane dispersion in a certain proportion to obtain a mixture; the aqueous polyurethane is a carboxylic acid anionic aqueous polyurethane.

[0006] S2. Pour the mixture into a glass filter cup lined with filter paper and continue vacuum filtration for 4-10 hours to obtain a cellulose nanocrystal / waterborne polyurethane composite film with a pearl-like crystalline photonic structure. Driven by vacuum filtration, water is rapidly removed, and the waterborne polyurethane particles gradually concentrate on the filter film surface and fuse in situ to form a three-dimensional network. This in-situ formed polyurethane three-dimensional network acts as an isolation and template, separating the cellulose nanocrystals into countless tiny regions, forcing them to rapidly assemble within a confined space, ultimately forming a crystalline photonic structure resembling the aragonite plates inside pearl oyster shells. This yields a flexible cellulose nanocrystal / waterborne polyurethane composite film. S3. The composite film is immersed in an ethanol solution and stirred thoroughly to dissolve the aqueous polyurethane. After centrifugation, washing, and drying, a cellulose wafer material with a chiral photonic structure is obtained. Cellulose nanocrystal suspension and aqueous polyurethane dispersion were mixed evenly at a solute mass ratio of 1:(1.0 ~ 5).

[0007] Furthermore, the concentration of the cellulose nanocrystal suspension is 1 to 2 wt%.

[0008] Further, in step S1, the cellulose nanocrystals have a diameter of 10-20 nm and a length of 100-200 nm.

[0009] Furthermore, the solid content of the waterborne polyurethane dispersion is 30-35%.

[0010] Further, in step S1, the cellulose nanocrystal suspension is ultrasonically treated at a power of 350~400 W for 15~60 seconds to obtain uniformly dispersed single nanocrystals.

[0011] Furthermore, the filter paper is a polytetrafluoroethylene filter paper with a pore size of 0.22 μm.

[0012] Further, in step S1, the preparation method of the cellulose nanocrystal suspension is as follows: the pulp board is treated with sulfuric acid at 40-50℃ with a mass percentage of 54%-74% for 40-50 min. After treatment, deionized water is added, and the resulting solution is allowed to stand for 24-48 h. After centrifugation of the suspension, excess sulfuric acid is removed by washing and dialysis with deionized water to obtain the cellulose nanocrystal suspension. The cellulose nanocrystal suspension is concentrated in a constant temperature water bath for 12-24 h to obtain a cellulose nanocrystal suspension with a concentration of 1-2 wt%.

[0013] A second objective of this invention is to provide a cellulose wafer material with a chiral photonic structure prepared by the method described above.

[0014] A third objective of this invention is to provide a structural color coating in which the aforementioned cellulose wafer material with a chiral photonic structure is dispersed in a solvent, and a bright, angle-dependent structural color coating is rapidly reconstructed on a substrate through simple coating, spraying, or printing methods.

[0015] The beneficial effects of this invention are: 1. This invention overcomes the contradiction between the self-assembly and macroscopic processing of cellulose nanocrystals. By driving the process through vacuum filtration and utilizing an in-situ formed three-dimensional polyurethane network for spatial confinement, cellulose nanocrystals can complete ordered assembly within hours. The resulting spherical wafer structure exhibits a mother-of-pearl-like "brick-and-mortar" arrangement and chiral nematic phase optical activity. This is entirely different from the traditional slow evaporation film formation method, enabling rapid, large-scale preparation of structural color units.

[0016] 2. The obtained cellulose wafer material with a chiral photonic structure perfectly inherits the chiral photonic structure and can be redispersed in various solvents to form "photonic ink" or "photonic paste". A shimmering structural color effect, similar to glitter, can be obtained through direct coating. It exhibits strong angle dependence, combining aesthetic appeal with anti-counterfeiting features, demonstrating excellent large-scale processing performance, and can be directly used as a novel environmentally friendly pigment.

[0017] 3. The entire process uses water as the processing medium, and both the cellulose nanocrystals and waterborne polyurethane used are environmentally friendly and low in toxicity. The final product contains only natural cellulose and is biodegradable. Photonic coatings formulated with this product are environmentally friendly, meeting multiple requirements for environmental protection, durability, and biodegradability, and have broad prospects in the fields of packaging and printing, cosmetics, green building materials, and anti-counterfeiting. Attached Figure Description

[0018] Figure 1 A diagram of the vacuum filtration apparatus for preparing cellulose wafers with chiral photonic structures according to the present invention; Figure 2 This is a scanning electron microscope image of the cellulose wafer material with a chiral photonic structure prepared in Example 4; Figure 3 The reflectance spectra of the cellulose wafer materials with chiral photonic structures prepared in Examples 4 and 5 after being dispersed in ethanol and coated into films. Figure 4 This is a scanning electron microscope image of the thin film in Comparative Example 1; Figure 5 A photograph of a structural color coating formed on a black substrate using a coating prepared with the cellulose wafer material having a chiral photonic structure obtained in Example 3; Figure 6 Digital photographs of Examples 1-4 and Comparative Examples 3-6; Figure 7 This is a scanning electron microscope image of the thin film prepared in Comparative Example 3; Figure 8 Scanning electron microscope images of the products prepared according to the literature and Example 1; Figure 9 This is a tomographic CT scan of the cellulose nanocrystal / waterborne polyurethane composite film prepared in Example 1. Detailed Implementation

[0019] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings to further illustrate the technical solutions of the present invention. However, the present invention is not limited to these embodiments.

[0020] The cellulose nanocrystal suspension used in the examples was prepared as follows: bleached softwood pulp board was treated with 64 wt% sulfuric acid at 45°C for 45 minutes, followed by the addition of a large amount of deionized water and standing for 24 hours. The suspension was then repeatedly centrifuged, washed, and dialyzed until pH neutral. Finally, it was concentrated to 2 wt% in a 50°C water bath. The resulting cellulose nanocrystals had a diameter of approximately 10–20 nm and a length of approximately 100–200 nm. The aqueous polyurethane used was a commercially available carboxylic acid anionic aqueous polyurethane (Aqueous Polyurethane 1926, F0402, Shenzhen Yoshida Chemical Co., Ltd.), with a solid content of 32 wt%.

[0021] Example 1 S1: Take 10 g of the above 2 wt% cellulose nanocrystal suspension (containing 0.2 g cellulose nanocrystals) and another 1.875 g of aqueous polyurethane dispersion with a solid content of 32 wt% (containing 0.6 g polyurethane), with a solute mass ratio of 1:3.

[0022] S2: Place the cellulose nanocrystal suspension in an ultrasonic cell disruptor and sonicate at 350 W for 30 seconds.

[0023] S3: Mix the ultrasonically purified suspension with water-based polyurethane and stir for 20 minutes until homogeneous.

[0024] S4: Install a filtration device (such as...) Figure 1 As shown in the figure, 0.22 μm hydrophilic PTFE filter paper was selected, and the vacuum degree was controlled at -0.09MPa.

[0025] S5: Pour in the mixture and continue filtration for 6 hours. During filtration, the polyurethane forms an isolated three-dimensional network in situ, confining the cellulose nanocrystals and assembling them into a wafer photonic structure, thus obtaining a yellow-green cellulose nanocrystal / waterborne polyurethane composite film.

[0026] S6: Immerse the membrane in 200 mL of anhydrous ethanol and stir for 1 hour, then centrifuge at 10,000 rpm for 10 minutes and discard the supernatant containing polyurethane. Wash the precipitate with ethanol and repeat centrifugation three times. Finally, vacuum dry at room temperature for 4 hours to obtain a yellow-green glitter powder, which is a cellulose wafer material with a chiral photonic structure.

[0027] Example 2 The mass ratio of cellulose nanocrystals to polyurethane was changed to 1:5 (0.2 g of cellulose nanocrystals combined with 1 g of polyurethane), and the filtration time was extended to 8 hours. The rest was the same as in Example 1.

[0028] The resulting cellulose nanocrystal / waterborne polyurethane composite film and the cellulose wafer material with chiral photonic structure are pale gold in color.

[0029] Example 3 The cellulose nanocrystals and aqueous polyurethane were mixed at a mass ratio of 1:1 (0.2 g of cellulose nanocrystals and 0.2 g of aqueous polyurethane), ultrasonicated at 400 W for 15 seconds, and filtered for 10 hours. The product still exhibited a distinct crystalline structure and structural color, but with slightly lower saturation.

[0030] The resulting cellulose nanocrystal / waterborne polyurethane composite film and the cellulose wafer material with chiral photonic structure are greenish-blue.

[0031] The powder was dispersed in ethanol and coated onto black cardboard, resulting in a color change from gold to green. Figure 5 As shown.

[0032] Example 4 The mass ratio of cellulose nanocrystals to aqueous polyurethane was 1:2 (0.2 g of cellulose nanocrystals and 0.4 g of aqueous polyurethane), and the rest was the same as in Example 1.

[0033] The resulting cellulose nanocrystal / waterborne polyurethane composite film and the cellulose wafer material with chiral photonic structure are cyan in color.

[0034] Figure 2 The image shown is a scanning electron microscope image of the cellulose wafer material with a chiral photonic structure prepared in Example 4. Figure 2 As shown, the powder particles exhibit an ordered stacked structure of circular wafers, displaying a typical imitation mother-of-pearl circular wafer morphology and chiral nematic fingerprint characteristics.

[0035] The powder was dispersed in ethanol and coated onto black cardboard. After drying, a bright green structural color coating was obtained, which changed color with the viewing angle.

[0036] Example 5 The mass ratio of cellulose nanocrystals to aqueous polyurethane was 1:2 (0.2g of cellulose nanocrystals and 0.4g of aqueous polyurethane), and the ultrasonic time in step S2 was modified to 15s, 45s, and 60s. Other steps were the same as in Example 1, and various cellulose wafer materials with chiral photonic structures were prepared. Figure 3 The images show the reflectance spectra of the cellulose wafer materials with chiral photonic structures prepared in Examples 4 and 5 after dispersion in ethanol and coating. Figure 3 As shown, by adjusting the ultrasonic treatment time to 15s, 30s, 45s and 60s respectively, cellulose wafer materials with chiral photonic structures exhibiting optical colors of blue, green, yellow and red can be prepared.

[0037] Comparative Example 1 Without adding aqueous polyurethane, a 10 g 2 wt% cellulose nanocrystal suspension was vacuum filtered under the same conditions. The resulting film was extremely brittle and could not be completely peeled off.

[0038] Figure 4 The image shown is a scanning electron microscope (SEM) image of the thin film in Comparative Example 1; as shown. Figure 4 As shown, no wafer-like assemblies were observed under scanning electron microscopy; only a layered aggregation of chiral nanocrystals was observed. This indicates that the three-dimensional network constructed by waterborne polyurethane is indispensable for achieving confined assembly and the formation of wafer-like photonic structures.

[0039] Comparative Example 2 In step S6, during the elution process, an equal volume of pure water was used instead of anhydrous ethanol to soak the composite film obtained in Example 1. After soaking for up to 72 hours and repeated water changes, the polyurethane could not be completely removed, and the powder severely agglomerated after drying, with no visible structural color. However, when using ethanol, effective elution was achieved in just 1 hour. This demonstrates that ethanol, as an elution solvent, can efficiently remove the polyurethane template and release the photonic structure, which is crucial for ensuring the optical activity of the final powder.

[0040] Comparative Example 3 Take the same mixture as in Example 1 (i.e., 10 g of 2 wt% cellulose nanocrystal suspension and 1.875 g of 32 wt% aqueous polyurethane dispersion mixed evenly), and instead of vacuum filtration, pour it into a polytetrafluoroethylene petri dish and allow it to evaporate naturally at room temperature (25°C) and relative humidity (50%). After about 72 hours, the mixture was completely dried and formed a film.

[0041] The film appears as an uneven grayish-white color.

[0042] Comparative Example 4 The mass ratio of cellulose nanocrystals to polyurethane was changed to 1:5, while other aspects remained the same as in Comparative Example 3.

[0043] The film exhibits an uneven grayish-white color, and cracks appear on its surface.

[0044] Comparative Example 5 The mass ratio of cellulose nanocrystals to waterborne polyurethane was 1:1, and other parameters were the same as in Comparative Example 3.

[0045] The film is dark blue with uneven blue distribution, and the surface of the film is uneven.

[0046] Comparative Example 6 The mass ratio of cellulose nanocrystals to waterborne polyurethane was 1:2, and other parameters were the same as in Comparative Example 3.

[0047] The film exhibits a light blue color with a grayish tint, and the color is uniform.

[0048] Figure 6 Digital photographs of Examples 1-4 and Comparative Examples 3-6; such as Figure 6 As shown, the film prepared in Comparative Example 3 appears only as a hazy pale blue to the naked eye, and its color vibrancy is significantly lower than that of the vacuum filtration film in Example 1. More importantly, the film obtained by the comparative example through natural evaporation by adjusting the ratio of cellulose nanocrystals to polyurethane (e.g., 1:1 to 1:5) has a very small range of structural color variation, and its color adjustability is far inferior to that of the vacuum filtration method. Examples 1 to 4 can achieve wide-range structural color control from blue-green to golden yellow by simply changing the ratio and ultrasonic treatment.

[0049] Figure 7 This is a scanning electron microscope image of the thin film prepared in Comparative Example 3. Figure 7 It can be seen that the cellulose nanocrystal assemblies with circular wafer structures inside the naturally evaporated film are arranged in a relatively discrete manner, without the highly regular stacked structure of circular wafers seen in Example 1. The film formation time is as long as 72 hours, which is more than 10 times that of the vacuum filtration method (6-10 hours). These comparisons fully demonstrate that the confined assembly driven by vacuum filtration has an overwhelming advantage over the traditional naturally evaporated induced assembly in terms of structural color brightness, color controllability, assembly structure regularity, and preparation efficiency.

[0050] Comparative Example 7 Keeping all other conditions unchanged in Example 1, the carboxylic acid anionic waterborne polyurethane in Example 1 was replaced with cationic waterborne polyurethane (W741922, McLean), nonionic waterborne polyurethane (AH-1704, Anhui Anda Huatai New Materials Co., Ltd.), and thermoplastic polyurethane (PS455-203, Huntsman Materials Company), respectively, and the mixtures were prepared at the same solid content mass ratio of 1:3 and then filtered.

[0051] Figure 8 In the image 'a', the scanning electron microscope image shows a film formed by drying a mixture of thermoplastic polyurethane and cellulose nanocrystals, as described in the literature. The cellulose nanocrystals are isolated by the polyurethane, but the isolated cellulose nanocrystals still have a chiral layered structure. Biomacromolecules 2024, 25, 6737-6747).

[0052] Figure 8 b in the figure is a scanning electron microscope image of the longitudinal section of the cellulose nanocrystal / aqueous polyurethane composite film prepared in Example 1. As can be seen from the figure, the cellulose wafer material with chiral photonic structure is isolated by polyurethane and embedded inside the film.

[0053] Figure 8In Figure 'c', the cross-sectional scanning electron microscope image of the cellulose nanocrystal / aqueous polyurethane composite film prepared in Example 1 is shown. As can be seen from the image, the cellulose wafer materials with chiral photonic structures are all wafer photonic structures.

[0054] Figure 9 The image shows a tomographic CT scan of the cellulose nanocrystal / waterborne polyurethane composite film prepared in Example 1. As can be seen from the image, the cellulose nanocrystal / waterborne polyurethane composite film is uniformly distributed and filled with a regular wafer photonic structure throughout the bulk phase.

[0055] The negative charge on the surface of cellulose nanocrystals determines the crucial role of different ionic types of aqueous polyurethane in the film-forming process of vacuum filtration. Only when anionic aqueous polyurethane is used does the film formed by vacuum filtration become uniform, flexible, and exhibit a vibrant iridescent color, without particle agglomeration. An electrostatic repulsion exists between the polyurethane and the negatively charged cellulose nanocrystals. This repulsion prevents excessive particle agglomeration and ensures the colloidal stability of the system during the dehydration process. As water is gradually removed, the polyurethane undergoes phase separation and self-organizes into a three-dimensional network template, within which the cellulose nanocrystals are confined and assembled, ultimately forming a regular chiral wafer-like photonic structure. In contrast, cationic aqueous polyurethane, due to its opposite charge to the cellulose nanocrystals, experiences strong electrostatic attraction, directly leading to the demulsification and flocculation of the polyurethane latex particles, preventing the assembly of cellulose nanocrystals. Nonionic aqueous polyurethane, lacking charge interaction, cannot establish an effective repulsive equilibrium with the cellulose nanocrystals, nor can it induce chiral confined arrangement of the cellulose nanocrystals during drying, thus also failing to obtain a wafer-like photonic structure. The film formed by vacuum filtration is translucent, uniform, and lacks iridescence. This demonstrates that only the carboxylic acid anionic aqueous polyurethane selected in this invention can accurately construct a suitable confined three-dimensional network template and efficiently induce cellulose nanocrystals to form a chiral wafer photonic structure; other common commercial polyurethane varieties cannot replace it.

[0056] Comparative Example 8 A pure cellulose nanocrystalline structurally colored film was prepared using a traditional method: 10 g of a 2 wt% cellulose nanocrystalline suspension was ultrasonically dispersed and poured into a polytetrafluoroethylene (PTFE) petri dish. The mixture was then naturally evaporated and dried at room temperature for approximately 48 hours to obtain a pure cellulose nanocrystalline film with structural color. The film was then ground into a mortar and pulverized through a 200-mesh sieve to obtain cellulose nanocrystalline powder. This powder was dispersed in ethanol to form a slurry, which was then coated onto a black cardboard surface.

[0057] Tests revealed that the resulting coating had almost no visible structural color, only a weak white scattering. The reasons for this are as follows: (1) Pure cellulose nanocrystal films are extremely brittle, and the grinding process inevitably shatters the ordered assembly structure of the chiral nematic phase inside, destroying the source of structural color; (2) The fragments obtained from grinding are irregular in shape, have a large specific surface area, and poor refractive index matching, making it difficult to form an effective photonic bandgap again; (3) There is a lack of interfacial bonding or driving force for ordered stacking between the fragments, resulting in a chaotic accumulation after coating, making it impossible to reconstruct a long-range ordered photonic structure. In addition, if an unassembled cellulose nanocrystal suspension is directly sprayed or scraped onto the surface of a substrate with a complex morphology, due to the lack of a mild and controllable solvent evaporation environment, as well as the influence of substrate wetting and leveling properties, the cellulose nanocrystals cannot complete the ordered assembly in situ during the wet film drying process, and the coating basically does not show structural color after drying.

[0058] In stark contrast, this invention pre-constructs a chiral wafer photonic structure through confined assembly, and then extracts the photonic nacre powder through ethanol elution. Each wafer of this powder is itself a complete and stable chiral photonic structural unit, and its photonic bandgap is determined by the fixed layered periodicity within the particle, independent of reassembly during subsequent coating processes. Therefore, regardless of the dispersion method or the complex substrate surface to which it is coated, a bright structural color can be rapidly reconstructed simply by solvent evaporation and the particles approaching and spreading together, fundamentally avoiding the risk of assembly failure.

[0059] For any points not covered above, existing technologies shall apply.

[0060] Although specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the direction of the invention or exceeding the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc., made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a cellulose wafer material with a chiral photonic structure, characterized in that, Includes the following steps: S1. The cellulose nanocrystal suspension is ultrasonically treated and then mixed with an aqueous polyurethane dispersion in a certain proportion to obtain a mixture; the aqueous polyurethane is a carboxylic acid anionic aqueous polyurethane; the solid content of the aqueous polyurethane dispersion is 30-35 wt%. S2. Pour the mixture into a glass filter cup lined with filter paper and continue vacuum filtration for 4 to 10 hours to obtain a cellulose nanocrystal / waterborne polyurethane composite film with a pearl-like crystalline wafer photonic structure. S3. The composite film is immersed in an ethanol solution and stirred thoroughly to dissolve the aqueous polyurethane. After centrifugation, washing, and drying, a cellulose wafer material with a chiral photonic structure is obtained. Cellulose nanocrystal suspension and aqueous polyurethane dispersion were mixed evenly at a solute mass ratio of 1:(1.0 ~ 5).

2. The preparation method according to claim 1, characterized in that, The concentration of the cellulose nanocrystal suspension is 1 to 2 wt%.

3. The preparation method according to claim 1, characterized in that, In step S1, the cellulose nanocrystals have a diameter of 10-20 nm and a length of 100-200 nm.

4. The preparation method according to claim 1, characterized in that, In step S1, the cellulose nanocrystal suspension is ultrasonically treated at a power of 350~400 W for 15~60 seconds to obtain uniformly dispersed single nanocrystals.

5. The preparation method according to claim 1, characterized in that, The filter paper is a polytetrafluoroethylene filter paper with a pore size of 0.22 μm.

6. The preparation method according to claim 1, characterized in that, In step S1, the preparation method of the cellulose nanocrystal suspension is as follows: the pulp board is treated with sulfuric acid at 40-50℃ with a mass percentage of 54%-74% for 40-50 minutes. After treatment, deionized water is added, and the resulting solution is allowed to stand for 24-48 hours. After centrifugation, the excess sulfuric acid is removed by washing with deionized water and dialysis to obtain the cellulose nanocrystal suspension. The cellulose nanocrystal suspension is concentrated in a constant temperature water bath for 12-24 hours to obtain a cellulose nanocrystal suspension with a concentration of 1-2 wt%.

7. A cellulose wafer material with a chiral photonic structure prepared by the preparation method according to any one of claims 1-6.

8. A structural color coating, characterized in that, The cellulose wafer material with chiral photonic structure described in claim 7 is dispersed in a solvent, and a bright, angle-dependent structural color coating is rapidly reconstructed on the substrate by coating, spraying, or printing.

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