Diphylleia grayi-Inspired Stretchable Hydrochromics with Large Optical Modulation in the Visible-Near-Infrared Region

Water-Rewritable Structural Color Textiles with Fast Response Speed and Antispreading Capability

The stimuli-responsive textiles, especially water-responsive textiles, have garnered attention owing to their environmental compatibility. Inspired by the hydrochromic behavior of Diphylleia grayi, water-rewritable structural color (WRSC) textiles exhibiting fast response speed and antispreading capability were fabricated by spraying hollow SiO2 (H-SiO2) microspheres and poly(trifluoroethyl methacrylate-butyl acrylate) [P(TFEMA-BA)]. The water-written textiles exhibited structural color changes in 0.6 s via an increase in the refractive index, driven by water penetrating the gaps between H-SiO2 microspheres. The structural color was restored to the initial state after the water evaporated, allowing multiple cycles of the write-erase-write mode. The hydrophobic P(TFEMA-BA) adhesive was used to construct a stable chromogenic array and endow WRSC textiles with antispreading properties, thereby improving structural stability and achieving clear writing patterns. The prepared WRSC textiles demonstrated high flexibility, structural stability, and water-rewritable properties, providing advanced bionic inspiration and valuable design ideas for rewritable materials and smart textiles.

Bio-inspired micropatterned thermochromic hydrogel for concurrent smart solar transmission and rapid visible-light stealth at all-working temperatures

Thermochromic hydrogels exhibit a smart capacity for regulating solar spectrum transmission, enabling automatically change their transmissivity in response to the ambient temperature change. This has great importance for energy conservation purposes. Military and civilian emergency thermochromic applications require rapid visible-light stealth (VLS); however, concurrent smart solar transmission and rapid VLS is yet to be realized. Inspired by squid-skin, we propose a micropatterned thermochromic hydrogel (MTH) to realize the concurrent control of smart solar transmittance and rapid VLS at all-working temperatures. The MTH possesses two optical regulation mechanisms: optical property regulation and optical scattering, controlled by temperature and pressure, respectively. The introduced surface micropattern strategy can arbitrarily switch between normal and diffuse transmission, and the VLS response time is within 1 s compared with previous ~180 s. The MTH also has a high solar-transmission regulation range of 61%. Further, the MTH preparation method is scalable and cost-effective. This novel regulation mechanism opens a new pathway towards applications with multifunctional optical requirements.

Hydrochromic film for dynamic information storage using cellulose nanofibers and silica nanoparticles

A hydrochromic composite film was fabricated by incorporating silica nanoparticles (SiNPs) with cellulose nanofibers (CNFs). The CNF/SiNP composite film underwent a reversible change in transparency in response to external moisture variation. The CNFs improved the dimensional stability of the CNF/SiNP composite film and induced morphological differences in SiNP agglomerates, which control the water vapor condensation in a porous film. The condensed water in the pores reduced the difference in refractive index over the CNF/SiNP film, enhancing its transparency. The selective transparency of the composite film was challenged by printing CNF/SiNP inks at different composition ratios. The differing susceptibility of the printed patterns to moisture provided selective transparency at specific patterns, which can store dynamic information such as QR or numerical codes by simple water vapor adsorption and desorption.

Controllable hydrophilic/superhydrophobic patterned coatings for optical information encryption/decryption based on water-triggered opaque to translucent transition

Anti-counterfeiting technologies are crucial for securing the authenticity and proof of commodities, in which the optical information encryption/decryption has attracted extensive attention for its overriding advantages of visibility and convenience. Inspired by the unique transparency transformation phenomenon of Diphylleia grayi petals, a controllable hydrophilic/superhydrophobic patterned coating with water-triggered opaque to translucent transition is proposed through the construction of a superhydrophobic coating, subsequent air plasma etching under a mask, and final hydrophilic modification to introduce stable invisible patterns. The superhydrophobic region exhibits great water repellency with a water contact angle (WCA) at 157°, while the hydrophilic region quickly absorbs water with a WCA at 61°. The patterned coating presents an opaque state for the serious light scattering induced by the rough microstructure and large refractive index difference between the coating and air, while the hydrophilic patterns on the coating transform to translucent after water infiltration for the reduced roughness and close refractive indexes of the coating and water. The information revealing is rapid and reversible, and demonstrates heat and long-term stability and great reusability. The findings conceivably stand out as a new methodology to fabricate controllable superwettable coatings with optical information encryption/decryption capability for application in anti-counterfeiting.

Synthesis of temperature- and humidity-induced dual stimulation film PU-PNIPAm n and its independent film formation as a smart window application

Dynamic windows, which switch between transparent and opaque states as the temperature changes, can be applied in buildings to reduce building energy consumption. Poly(N-isopropylacrylamide) (PNIPAm) is the most studied thermochromic hydrogel for climate-resilient smart window applications. In addition to its poor mechanical properties and low reaction rate, the PNIPAm hydrogel must be sandwiched between two pieces of glass to form an interlayer in practical applications. Here, durable PU-PNIPAm _n copolymers for smart windows were synthesized by reacting the synthesized poly-NIPAm diols with isocyanate (-NCO) monomer, which greatly improved the mechanical properties of the hydrogel and it was able to form a film alone. These temperature-sensitive films can switch between transparent (>80% transmittance) and opaque (<5% transmittance) states in less than 10 minutes, with no degradation in optical contrast, switching speed, or uniformity after at least 100 switching cycles.

Novel Green Reversible Humidity-Responsive Hemiaminal Dynamic Covalent Network for Smart Window

Recently, smart windows have attracted widespread attention on account of their unique features, yet traditional smart windows still rely on external energy support to accomplish dynamic reversible switching, which not only confines usage but also causes waste of energy. For this purpose, we have prepared hemiaminal dynamic covalent network (HDCN) film with outstanding flexibility and strength by a simple and low-cost method, in which the modulus is 206.28 MPa and the elongation at break is 39.02%. Additionally, the transition from a transparent to an opaque state is achieved when the film is stimulated by humidity, and the dynamic transformation of the film to different phases of transparency is obtained when the film is exposed to different relative humidities (60-99%). Most importantly, HDCN film fulfills the modern green requirements and enables complete dissolution in a certain mildly acidic solution, avoiding environmental pollution when the material is discarded due to loss of function. The dynamic tunability of HDCN film demonstrates great advantages and potential in smart windows and anticounterfeiting.

Azobenzene/Acid Binary Systems for Colorimetric Humidity Sensing with Reversibility, High Sensitivity, and Tunable Colors

Humidity sensors are important for humidity detection in many storage and manufacturing processes. Issues like sensibility, response rate, controllability, and material and preparation process costs need to be taken into consideration for practical applications. Herein, we report an investigation on a series of azobenzene/acid binary systems using easily accessible compounds, whose thin films display reversible and widely tunable color changes in response to humidity stimulation, with high sensitivity, fast color change, and recovery speed. The interesting properties for colorimetric humidity sensing are showcased with potential applications in dynamic art painting, smart windows, and respiration monitoring.

Light-driven dynamic surface wrinkles for adaptive visible camouflage

Camouflage is widespread in nature, engineering, and the military. Dynamic surface wrinkles enable a material the on-demand control of the reflected optical signal and may provide an alternative to achieve adaptive camouflage. Here, we demonstrate a feasible strategy for adaptive visible camouflage based on light-driven dynamic surface wrinkles using a bilayer system comprising an anthracene-containing copolymer (PAN) and pigment-containing poly (dimethylsiloxane) (pigment-PDMS). In this system, the photothermal effect-induced thermal expansion of pigment-PDMS could eliminate the wrinkles. The multiwavelength light-driven dynamic surface wrinkles could tune the scattering of light and the visibility of the PAN film interference color. Consequently, the color captured by the observer could switch between the exposure state that is distinguished from the background and the camouflage state that is similar to the surroundings. The bilayer wrinkling system toward adaptive visible camouflage is simple to configure, easy to operate, versatile, and exhibits in situ dynamic characteristics without any external sensors and extra stimuli.

Humidity-Driven Switch in the Transparency of a Nanofiber Film for a Smart Window

Traditional smart windows use electrical signals to transform transparency. However, this electric transmission mode greatly limits their uses. Here, we have prepared a transparent PVA-co-PE/CA composite film, which can realize the reversible transformation of transparency under the stimulation of humidity. The preparation method of the composite film included simple immersion and a thermal curing process, showing high optical transparency (96.61%) and an excellent tensile strain at break of 536.34%. Once exposed to moisture stimulation, the rapid hygroscopic swelling of the composite film led to the increase in the difference in the refractive index between the citric acid filling phase and the nanofibers, which directly led to the sharp decrease in the composite film's transparency. Moreover, the composite film can be arbitrarily attached to the surface of the transparent substrate and designed as some special visualization devices or smart windows, which have a promising future in information encryption and intelligent homes.

Wetting-Enhanced Structural Color for Convenient and Reversible Encryption of Optical Information

Encrypted storage of optical information has attracted increasing interest for anticounterfeiting, information transmission, and military applications. In this study, an inverse opal-structured titanium dioxide/heptadecafluorodecyltrimethoxysilane (IOS-T/F) panel is developed. Based on a unique wetting-enhanced mechanism of structural color vision derived from a reduced light scattering and strengthened effective refractive index, this panel is capable of reversible writing/erasing and encryption/decryption of optical information. Multiple levels of information can be compiled, concealed, and erased simply using controlled ultraviolet irradiation to form patterned hydrophilic/hydrophobic differences, and the process of revealing or concealing the information only requires a few drops of water or evaporation, respectively. Importantly, the functions of the IOS-T/F panel can be well maintained under harsh conditions, including strongly acidic/alkaline environments or extreme temperatures (from -40 to 80 °C), as well as can be recovered after staining by various pollutants. This system provides simple encryption, rapid decryption, and the ability to store multiple sets of information under diverse application scenarios, which represents a novel material design strategy for security-related applications and smart optical systems.

Nature-inspired robust hydrochromic film for dual anticounterfeiting

Nature-inspired materials have been actively developed for anticounterfeiting applications. Among a variety of stimuli-responsive anticounterfeiting strategies, hydrochromic materials exhibit reversible color change in response to moisture or water and have the advantage of being easy to authenticate. However, the security level of current hydrochromic anticounterfeiting materials is not sufficient for practical applications since they only exhibit a single anticounterfeiting function, where the information switches between visible and invisible. To improve the security level and efficiency of hydrochromic anticounterfeiting materials, here we developed a robust dual hydrochromic material via the self-assembly of polyurethane (PU)-polyelectrolytes colloids with which the desired information can not only switch between visible and invisible but also transform from one pattern to another within 3 s without the need of any external instruments. The bio-inspiration, material design and demonstrated hydrochromic properties might have profound implications for using colloidal complexes to make advanced anticounterfeiting materials.

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Micro-structures are formed by self-assembly of polyurethane-polyelectrolyte colloids

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Information changes from one pattern to another within 3 s when exposed to water

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The hydrochromic films are mechanically robust in both dry and wet state

Diphylleia grayi-Inspired Intelligent Hydrochromic Adhesive Film

Diphylleia grayi-inspired hydrochromic nano/microstructured films have received much attention for its promising smart hydrochromic applications owing to their simple and low-cost but energy-effective strategy. A new type of water-switchable glazing film patterned with various nano/micro air-hole inverse opal arrays is introduced by selectively removing nano/microsphere polystyrene arrays embedded in the surface of polydimethylsiloxane (PDMS) films. Using the significant contrast ratio of the bleaching and the scattering states, we have optimized the switching properties of Mie scattered patterns. As a result, we obtained a single inverse opal layer-embedded PDMS adhesive film with hexagonally close-packed 1 μm air-hole arrays as an optimum scattered film. The differences of diffusive transmittance and optical haze values between the dry and the wet states of the best scattered film reached 44.93% (ΔT_D.T = 59.11-14.18%) and 54.88% (ΔH = 69.42-14.54%), respectively. In addition, using the best-optimized inverse opal layer-embedded PDMS film, we fabricated a perfectly imitated Diphylleia grayi structure for camouflage application and an intelligent hydrochromic window device. The dynamic water modulation of the scattered opaque and nonscattered transparent state of the inverse opal-patterned PDMS adhesive film can provide an advanced platform structure in the area of hydrochromic technology for smart windows, camouflage, and clear umbrellas for rainy days.

Transparent Metal-Organic Framework-Based Gel Electrolytes for Generalized Assembly of Quasi-Solid-State Electrochromic Devices

Metal-organic framework (MOF)-based electrolytes under gel/solid states have been widely used for electrochemical devices recently due to their designable metal centers/ligands and diffusion channels in the porous structures. Therefore, it is always desired to apply the MOF-based electrolytes in electrochromic (EC) fields. Yet, challenges exist in realizing their high optical transparency to satisfy the unique optical requirements of EC devices. Herein, a transparent MOF-based gel electrolyte (MGE) is demonstrated through the incorporation of 2-methylimidazole among MOF nanocrystals to prevent the strong light scattering of MOF nanocrystals. As a result, the gel electrolyte showed an improved average transmittance of ca. 82.2% compared with the MOF electrolytes without 2-methylimidazole (ca. 59.2%). In addition, because of the designed large channels in the porous MOF structure, the gel electrolyte exhibited a high ionic conductivity of 2.66 × 10^-3 S cm^-1. At last, we used the transparent MGEs to assemble two types (rigid and flexible) of quasi-solid-state EC devices based on inorganic WO₃ and organic poly(3,4-ethylenedioxythiophene) (PEDOT), respectively. Both devices showed great EC performances, and the flexible devices exhibited high mechanical stability under the bending state or even after being cut and punched, advancing the general applications of our transparent MGEs in EC fields.

Towards full-colour tunability of inorganic electrochromic devices using ultracompact fabry-perot nanocavities

Intercalation-based inorganic materials that change their colours upon ion insertion/extraction lay an important foundation for existing electrochromic technology. However, using only such inorganic electrochromic materials, it is very difficult to achieve the utmost goal of full-colour tunability for future electrochromic technology mainly due to the absence of structural flexibility. Herein, we demonstrate an ultracompact asymmetric Fabry-Perot (F-P) nanocavity-type electrochromic device formed by using partially reflective metal tungsten as the current collector and reflector layer simultaneously; this approach enables fairly close matching of the reflections at both interfaces of the WO3 thin layer in device form, inducing a strong interference. Such an interference-enhanced device that is optically manipulated at the nanoscale displays various structural colours before coloration and, further, can change to other colours including blue, red, and yellow by changing the optical indexes (n, k) of the tungsten oxide layer through ion insertion.