MXenes for Bioinspired Soft Actuators:Advancements in Angle-Independent Structural Colors and Beyond

Soft actuators have garnered substantial attention in current years in view of their potential appliances in diverse domains like robotics,biomedical devices,and biomimetic systems.These actuators mimic the natural movements of living organisms,aiming to attain enhanced flexibility,adaptability,and versatility.On the other hand,angle-independent structural color has been achieved through innovative design strategies and engineering approaches.By carefully controlling the size,shape,and arrangement of nanostructures,researchers have been able to create materials exhibiting consistent colors regardless of the viewing angle.One promising class of materials that holds great potential for bioinspired soft actuators is MXenes in view of their exceptional mechanical,electrical,and optical properties.The integration of MXenes for bioinspired soft actuators with angle-independent structural color offers exciting possibilities.Overcoming material compatibility issues,improving color reproducibility,scalability,durability,power supply efficiency,and cost-effectiveness will play vital roles in advancing these technologies.This perspective appraises the development of bioinspired MXene-centered soft actuators with angleindependent structural color in soft robotics.

Bioinspired MXene-Based Soft Actuators Exhibiting Angle-Independent Structural Color

In nature,many living organisms exhibiting unique structural coloration and soft-bodied actuation have inspired scientists to develop advanced structural colored soft actuators toward biomimetic soft robots.However,it is challenging to simultaneously biomimic the angle-independent structural color and shape-morphing capabilities found in the plum-throated cotinga flying bird.Herein,we report biomimetic MXene-based soft actuators with angle-independent structural color that are fabricated through controlled self-assembly of colloidal SiO_(2) nanoparticles onto highly aligned MXene films followed by vacuum-assisted infiltration of polyvinylidene fluoride into the interstices.The resulting soft actuators are found to exhibit brilliant,angle-independent structural color,as well as ultrafast actuation and recovery speeds(a maximum curvature of 0.52 mm−1 can be achieved within 1.16 s,and a recovery time of~0.24 s)in response to acetone vapor.As proof-of-concept illustrations,structural colored soft actuators are applied to demonstrate a blue gripper-like bird’s claw that can capture the target,artificial green tendrils that can twine around tree branches,and an artificial multicolored butterfly that can flutter its wings upon cyclic exposure to acetone vapor.The strategy is expected to offer new insights into the development of biomimetic multifunctional soft actuators for somatosensory soft robotics and next-generation intelligent machines.

Effect of Different Conditions on the Structural Color of Konjac Glucomannan Particles

In this paper,molecular dynamics simulation was applied to synthesize a layered structural color from Konjac glucomannan（KGM） and the effect of particle diameter and temperature were investigated. A series of methods such as high voltage electric field treatment,the transfer matrix method and the CIE standard colorimetric system were simulated to obtain the chromaticity coordinates and to analyze the color changes of KGM particles. The results revealed that as the particle diameter increases,the structural color of KGM particles deflects towards the yellow wavelength within the visible spectrum; and as the reaction temperature rises,the structural color deflects towards the blue and violet wavelengths within the visible spectrum.

Tunable structural color of anodic tantalum oxide films

Tantalum （Ta） oxide flhns with tunable structural color were fabricated easily using anodic oxidation. The structure, components, and surface valence states of the oxide films were investigated by using gazing incidence X-ray diffractometry, X-ray photoelectron microscopy, and surface analytical techniques. Their thickness and optical properties were studied by using spectroscopic ellipsometry and total reflectance spectrum. Color was accurately defined using L＊a＊b＊ scale. The thickness of compact Ta2O5 films was linearly dependent on anodizing voltage. The film color was tunable by adjusting the anodic voltage. The difference in color appearance resulted from the interference behavior between the interfaces of air-oxide and oxide metal.

Structural Color Modified Fabrics with Excellent Antibacterial Property

With the improvement of living standards, people are paying more and more attention to health problems. The antibacterial function of fabrics is therefore of great importance. The structural color(photonic crystal), which has been widely investigated and applied on fabric dyeing, contains a large number of hollow microstructure and functional groups, and is easy to be modified and functionalized. Therefore, an innovative way of endowing structural color dye on fabrics with antibacterial property was presented in this paper. The latex spheres and zinc pyrrolidone were co-assembled on polydopamine modified fabrics, antibacterial ion zinc pyrrolidone was therefore loaded into the pores of structural color dye, and brilliant antibacterial fabrics were successfully achieved. The existence of zinc pyrrolidone had little influence on the color saturation of brilliant structural color and meanwhile ensured the structural color dye excellent antibacterial effect. The antibacterial reduction rate of the antibacterial fabric reached 99.99%. Owing to the addition of polyurethane(PUA) coating on the surface of structural color, the fabric modified by the antibacterial structural color dye also presented good washing resistance, which showed great application possibility in functional textile and antibacterial fields.

Design and fabrication of structural color by local surface plasmonic meta-molecules

In this paper, we propose a new form of nanostructures with Al film deposited on a patterned dielectric material for generating structural color, which is induced by local surface plasmonic resonant(LSPR) absorption in sub-wavelengthindented hole/ring arrays. Unlike other reported results obtained by using focus ion beam(FIB) to create metallic nanostructures, the nano-sized hole/ring arrays in Al film in this work are replicated by high resolution electron beam lithography(EBL) combined with self-aligned metallization. Clear structural color is observed and systematically studied by numerical simulations as well as optical characterizations. The central color is strongly related to the geometric size, which provides us with good opportunities to dye the colorless Al surface by controlling the hole/ring dimensions(both diameter and radius), and to open up broad applications in display, jewelry decoration, green production of packing papers, security code,and counterfeits prevention.

Emerging interactively stretchable electronics with optical and electrical dual-signal feedbacks based on structural color materials

The booming development of wearable devices has aroused increasing interests in flexible and stretchable devices.With mechanosensory functionality,these devices are highly desirable on account of their wide range of applications in electronic skin,personal healthcare,human–machine interfaces and beyond.However,they are mostly limited by single electrical signal feedback,restricting their diverse applications in visualized mechanical sensing.Inspired by the mechanochromism of structural color materials,interactively stretchable electronics with optical and electrical dual-signal feedbacks are recently emerged as novel sensory platforms,by combining both of their sensing mechanisms and characteristics.Herein,recent studies on interactively stretchable electronics based on structural color materials are reviewed.Following a brief introduction of their basic components(i.e.,stretchable electronics and mechanochromic structural color materials),two types of interactively stretchable electronics with respect to the nanostructures of mechanochromic materials are outlined,focusing primarily on their design considerations and fabrication strategies.Finally,the main challenges and future perspectives of these emerging devices are discussed.

Recent advances in photonic crystal with unique structural colors:A review

Inspired by special color-forming organisms in nature,photonic crystal materials with structural color function have been developed significantly with great potential applications for displays,sensors,anti-counterfeiting inks,etc.This review aims to summarize the functions,self-assembly modes,and ap-plications of different kinds of photonic crystal materials.The preparation methods and characteristics of monodisperse inorganic nanoparticles,polymer nanoparticles,inorganic/organic core-shell nanoparti-cles,and MOFs are discussed.Subsequently,we summarize the method of assembling colloidal parti-cles into photonic crystals,which is a template induction method,inkjet printing method,drop coating method,etc.Moreover,the potential application of structural color is presented including humidity re-sponse and magnetic field response in sensors fields,as well as the advantages and disadvantages of anti-counterfeiting,fabric coloring,displays,smart windows,and Biomedical Applications.Finally,we present the development prospects and key problems of photonic crystals.

Multicolored structural coloration of carbon nanotube fibers

Carbon nanotube fibers(CNTFs)are endowed with excellent mechanical,electrical,and thermal properties and are considered promising candidates in numerous cutting-edge fields.However,the inherent black color of CNTFs hinders their practical application in fields with high aesthetic requirements such as wearable devices and smart textiles.Due to the smooth surface and chemical inertness,CNTFs are hard to be dyed by conventional chemical dyes or colorful inks.Herein,we realize a structural coloration of CNTFs by coating them with two metal oxide layers via atomic layer deposition.The three elements of color,that is,hue,saturation,and brightness,can be controlled by adjusting the types and thickness of each oxide layer.Colorful CNTFs with wide color gamut and high saturation are achieved through different combinations.A film interference model is also established to reveal the mechanism of the structural coloration,which is a comprehensive result of thin-film interference and surface roughness briefly.The calculated reflectance well fits the measured results by introducing surface roughness parameters.Moreover,the colored CNTFs are not iridescent because of retinal signal delay,which will further expand their applications.

Angle-independent responsive organogel retroreflective structural color film for colorimetric sensing of humidity and organic vapors

The angle dependence of photonic crystals(PCs)dramatically limits their practical applications in the colorimetrical sensing of humidity and volatile organic compound(VOC)vapors.In addition,it is challenging for inverse opal PCs to colorimetrically distinguish between vapors with similar refractive indices.Different from the mechanism of PC-based sensors,here,we report an angle-independent polyacrylamide(PAAm)organogel structural color film based on the mechanisms of retroreflection,total internal reflection(TIR)and interference with a shape similar to a single-sided“egg waffle”.During the process of responding to humidity and VOC vapors,the color of the film remains angle-independent in the normal angle range of 0°to 45°under coaxial illumination and observation conditions.At the same time,the film can colorimetrically distinguish between vapors with similar refractive indices,such as methanol and ethanol,which is mainly due to the differences in their polarity and solubility parameters.The film shows good stability,reversibility and selectivity when exposed to vapors.A colorimetric sensor with a new response mechanism is proposed and has the potential to effectively distinguish between vapors with similar refractive indices.Furthermore,this responsive retroreflective structural color film(RRSCF)provides a universal strategy to develop targeted angle-independent structural color sensors by selecting optimized materials.

Large area patterning of ultra-high thermal-stable structural colors in transparent solids

Printing stable color with a lithography-free and environment-friendly technique is in high demand for applications.We report a facile strategy of ultrafast laser direct writing(ULDW)to produce large-scale embedded structural colors inside transparent solids.The diffraction effect of gratings enables effective generation of structural colors across the entire visible spectrum.The structural colors inside the fused silica glass have been demonstrated to exhibit excellent thermal stability under high temperature up to 1200℃, which promises that the written information can be stable for long time even with unlimited lifetime at room temperature.The structural colors in the applications of coloring,anti-counterfeiting,and information storage are also demonstrated.Our studies indicate that the presented ULDW allows for fabricating large-scale and high thermal-stability structural colors with prospects of three-dimensional patterning,which will find various applications,especially under harsh conditions such as high temperature.

Responsive and self-healing structural color supramolecular hydrogel patch for diabetic wound treatment

The treatment of diabetic wounds remains a great challenge for medical community.Here,we present a novel structural color supramolecular hydrogel patch for diabetic wound treatment.This hydrogel patch was created by using N-acryloyl glycinamide(NAGA)and 1-vinyl-1,2,4-triazole(VTZ)mixed supramolecular hydrogel as the inverse opal scaffold,and temperature responsive poly(N-isopropylacrylamide)(PNIPAM)hydrogel loaded with vascular endothelial cell growth factor(VEGF)as a filler.Supramolecular hydrogel renders hydrogel patch with superior mechanical properties,in which NAGA and VTZ also provide self-healing and antibacterial properties,respectively.Besides,as the existence of PNIPAM,the hydrogel patch was endowed with thermal-responsiveness property,which could release actives in response to temperature stimulus.Given these excellent performances,we have demonstrated that the supramolecular hydrogel patch could significantly enhance the wound healing process in diabetes rats by downregulating the expression of inflammatory factors,promoting collagen deposition and angiogenesis.Attractively,due to responsive optical property of inverse opal scaffold,the hydrogel patch could display color-sensing behavior that was suitable for the wound monitoring and management as well as guidance of clinical treatment.These distinctive features indicate that the presented hydrogel patches have huge potential values in biomedical fields.

Bioinspired structural color particles with multi-layer graphene oxide encapsulated nanoparticle components

Because of the unique features of spherical symmetry,angle-independency,good monodispersity,controllable components and morphologies,structural color particles(SCPs)have found great significances in various fields such as sensing,monitoring,biological assays,etc.Here,inspired by the melanosome-derived bright structural colors and the self-adhesivity of mussels,we present a kind of bioinspired SCPs assembled from polydopamine(PDA)-adhered multi-layer graphene oxide(GO)encapsulated silica nanoparticles(SNs).It is demonstrated that compared with traditional SCPs,the designed particles possess brighter and more vibrant structural colors,and no complicated modification is required during the functionalization process due to the abundant inherent functional groups of GO.The resultant SCPs are verified to be capable for direct hybridization chain reaction and multiplexed nucleic acid assays.These properties indicate the promising prospects of our designed SCPs.

Algorithm of Micro-Grooving and Imaging Processing for the Generation of High-Resolution Structural Color Images

The use of submicron structures for structural coloration of surfaces has broad applications for color filters,projection displays,virtual reality,and anti-counterfeiting.Currently,structural color images lack high resolution due to low manufacturing accuracy.In this study,the axial-feed fly cutting(AFC)method is proposed to fabricate submicron grooves for the diffraction of visible light to create structural color images.We establish the relationship between the color information in the pixels of the original image and the parameters of the array units corresponding to the pixels.An algorithm to determine groove spacing and the tool path is established,and array units with the desired groove spacing are machined to reproduce the structural color images.The submicron grooves fabricated by AFC have high quality and good consistency.Due to the excellent diffraction performance of the machined grooves,images with high saturation and resolution can be reproduced.It is verified that images with various colors can be efficiently fabricated using the proposed method and algorithm.

High-color-purity, high-brightness and angle-insensitive red structural color

We propose a simple five-layer structure for creating red structural color, which has high color purity and high brightness.The design is based on the superposition of a silver substrate and multilayer silicon material. Absorption at the shorter wavelengths of the structure is effectively guaranteed, and reflection at the longer wavelengths is well enhanced. The red structural color has a peak reflectivity of 91% and a colorimetric purity of 0.9. Moreover, the designed structure displays angle-invariant performance up to 60°. This kind of structure scheme is environmentally friendly with low fabrication cost,and it can play an important role in a variety of fields, such as color displays and image sensors.

Scalable, ultra-resistant structural colors based on network metamaterials

Structural colors have drawn wide attention for their potential as a future printing technology for various applications,ranging from biomimetic tissues to adaptive camouflage materials.However,an efficient approach to realize robust colors with a scalable fabrication technique is still lacking,hampering the realization of practical applications with this platform.Here,we develop a new approach based on large-scale network metamaterials that combine dealloyed subwavelength structures at the nanoscale with lossless,ultra-thin dielectric coatings.By using theory and experiments,we show how subwavelength dielectric coatings control a mechanism of resonant light coupling with epsilon-near-zero regions generated in the metallic network,generating the formation of saturated structural colors that cover a wide portion of the spectrum.Ellipsometry measurements support the efficient observation of these colors,even at angles of 70°.The network-like architecture of these nanomaterials allows for high mechanical resistance,which is quantified in a series of nano-scratch tests.With such remarkable properties,these metastructures represent a robust design technology for real-world,large-scale commercial applications.

Linear Liquid Responses of Morpho Butterfly Structural Color： Experiment and Modeling

This paper investigates the selective liquid response for Morpho didius butterfly wing scales and propose an optical model to explain the effect of different components on the liquid response. It is found out that the reason of the selective response is that the liquid media forms nanometre-thick films between ridge-lamellae nanostructures and changes the constructive interference wavelength. There is linear relation between the structural color of ridge-lamellae structure and index of liquid background media. The reason of vapor＇s responses is that the nanometre-thick liquid fi lms on ridge-lamellae nanostructures change the constructive interference wavelength. These liquid films are formed due to vapor adsorption. Therefore,the selective linear liquid response can be applied to design nano-engineered photonic liquid and vapor sensors.

Ionically Imprinting-Based Copper(Ⅱ)Label-Free Detection for Preventing Hearing Loss

Copper is a microelement with important physiological functions in the body.However,the excess copper ion(Cu^(2+))may cause severe health problems,such as hair cell apoptosis and the resultant hearing loss.Therefore,the assay of Cu^(2+)is important.We integrate ionic imprinting technology(IIT)and structurally colored hydrogel beads to prepare chitosan-based ionically imprinted hydrogel beads(IIHBs)as a low-cost and high-specificity platform for Cu^(2+)detection.The IIHBs have a macroporous microstructure,uniform size,vivid structural color,and magnetic responsiveness.When incubated in solution,IIHBs recognize Cu^(2+)and exhibit a reflective peak change,thereby achieving label-free detection.In addition,benefiting from the IIT,the IIHBs display good specificity and selectivity and have an imprinting factor of 19.14 at 100μmol·L^(-1).These features indicated that the developed IIHBs are promising candidates for Cu^(2+)detection,particularly for the prevention of hearing loss.

Application of color structured light pattern to measurement of large out-of-plane deformation

Measurement of out-of-plane deformation is significant to understanding of the deflection mechanisms of the plate and tube structures.In this study,a new surface contouring technique with color structured light is applied to measure the out-of-plane deformation of structures with one-shot projection.Through color fringe recognizing,decoding and triangulation processing for the captured images corresponding to each deformation state,the feasibility of the method is testified by the measurement of elastic deflections of a flexible square plate,showing good agreement with those from the calibrated displacement driver.The plastic deformation of two alloy aluminum rectangular tubes is measured to show the technique application to surface topographic evaluation of the buckling structures with large displacements.

Generating micro/nanostructures on magnesium alloy surface using ultraprecision diamond surface texturing process

The lightness and high strength-to-weight ratio of the magnesium alloy have attracted more interest in various applications.However,micro/nanostructure generation on their surfaces remains a challenge due to the flammability and ignition.Motivated by this,this study proposed a machining process,named the ultraprecision diamond surface texturing process,to machine the micro/nanostructures on magnesium alloy surfaces.Experimental results showed the various microstructures and sawtooth-shaped nanostructures were successfully generated on the AZ31B magnesium alloy surfaces,demonstrating the effectiveness of this proposed machining process.Furthermore,sawtooth-shaped nanostructures had the function of inducing the optical effect and generating different colors on workpiece surfaces.The colorful letter and colorful flower image were clearly viewed on magnesium alloy surfaces.The corresponding cutting force,chip morphology,and tool wear were systematically investigated to understand the machining mechanism of micro/nanostructures on magnesium alloy surfaces.The proposed machining process can further improve the performances of the magnesium alloy and extend its functions to other fields,such as optics.