Hierarchical 3D Patterns with Dynamic Wrinkles Produced by a Photocontrolled Diels-Alder Reaction on the Surface

Visualizing the Sliding Motion of Dynamic Rotaxanes by Surface Wrinkles

Visualizing the sliding dynamics of a topological network can provide critical insight into determining the design and properties of mechanically interlocked materials. Although several auxiliary techniques have been proposed to infer the microscopic motion of rotaxanes, employing intuitive and convenient methods to explore the microscopic dynamics of a mechanically interlocked polymer remains a significant challenge. Herein, this work introduces a mechanically interlocked network (MIN) into the patterned surfaces for visualizing and regulating the sliding process of [2]rotaxane units through the evolution of surface wrinkles. Upon the photodimerization of the anthracene-functionalized polymer chain, the surface wrinkle can be formed after thermal treatment and subsequent cooling to room temperature. Specifically, the cross-linked films exhibit visible changes in wrinkle topography through the disruption of host-guest recognition by alkaline stimuli. Moreover, by leveraging the unique mechanical properties of surface wrinkles, we prolonged and amplified the originally extremely transient and difficult-to-detect sliding motion of rotaxane units in terms of time scale. Through statistical analysis of the changes in wrinkle morphology, we were able to correspondingly deconstruct the three processes of the rotaxane sliding motion: (I) unrestricted rapid sliding following host-guest dissociation; (II) restricted sliding; and (III) termination of sliding. The novel approach we propose opens a new avenue for studying the microscopic molecular motion of mechanically interlocked materials, facilitating the advancement and application of mechanically interlocked structures. In addition to using macroscopic surface patterns to visualize and explore microscopic molecular motion, the motion of microscopic molecules can also be used to regulate macroscopic surface patterns.

High-Sensitivity and Wide-Range Flexible Pressure Sensor Based on Gradient-Wrinkle Structures and AgNW-Coated PDMS

Flexible pressure sensors have garnered significant attention due to their wide range of applications in human motion monitoring and smart wearable devices. However, the fabrication of pressure sensors that offer both high sensitivity and a wide detection range remains a challenging task. In this paper, we propose an AgNW-coated PDMS flexible piezoresistive sensor based on a gradient-wrinkle structure. By modifying the microstructure of PDMS, the sensor demonstrates varying sensitivities and pressure responses across different pressure ranges. The wrinkle microstructure contributes to high sensitivity (0.947 kPa-1) at low pressures, while the PDMS film with a gradient contact height ensures a continuous change in the contact area through the gradual activation of the contact wrinkles, resulting in a wide detection range (10-50 kPa). This paper also investigates the contact state of gradient-wrinkle films under different pressures to further elaborate on the sensor's sensing mechanism. The sensor's excellent performance in real-time response to touch behavior, joint motion, swallowing behavior recognition, and grasping behavior detection highlights its broad application prospects in human-computer interaction, human motion monitoring, and intelligent robotics.

Diverse reactivity of maleimides in polymer science and beyond

Maleimides are remarkably versatile functional groups, capable of participating in homo- and copolymerizations, Diels-Alder and (photo)cycloadditions, Michael additions, and other reactions. Their reactivity has afforded materials ranging from polyimides with high upper service temperatures to hydrogels for regenerative medicine applications. Moreover, maleimides have proven to be an enabling chemistry for pharmaceutical development and bioconjugation via straightforward modification of cysteine residues. To exert spatiotemporal control over reactions with maleimides, multiple approaches have been developed to photocage nucleophiles, dienes, and dipoles. Additionally, further substitution of the maleimide alkene (e.g., mono- and di-halo-, thio-, amino-, and methyl-maleimides, among other substituents) confers tunable reactivity and dynamicity, as well as responsive mechanical and optical properties. In this mini-review, we highlight the diverse functionality of maleimides, underscoring their notable impact in polymer science. This moiety and related heterocycles will play an important role in future innovations in chemistry, biomedical, and materials research.

Fabricating Microstructures on Freeform Surfaces via Flexible Hydrogel Micromolds

Fabricating microstructures on curved surfaces is of great importance in several engineering fields, and it is an ongoing challenge to develop technologies that do not require complicated devices and cumbersome procedures. A novel approach is presented for fabricating microstructures with different topographies on curved surfaces using flexible microstructured hydrogels as molds. The hydrogel film is first microstructured via spatially controlled ultraviolet illumination utilizing its photo-responsive property. The microstructured hydrogel film is then transferred to a desired surface. Due to its low modulus, the hydrogel film can be adapted to the curved surfaces including semi-cylindrical and semi-spherical surfaces as well as freeform surfaces with both convex and concave features. The final freeform surfaces with microstructures are obtained by polydimethylsiloxane (PDMS) casting. The resulting microstructures show high uniformity and high surface smoothness with an average roughness (Ra) ≈2.7 nm and a root mean square roughness (Rq) ≈4.1 nm. The ability to fabricate microstructures on large-area curved surfaces of ≈4.5 cm by 6.5 cm is demonstrated. Finally, it is shown that the fabricated microstructures allow good imaging performance. This method utilizes the hydrogel's photo-responsive ability to construct flexible molds and flexibility to adapt to freeform surfaces, thereby allowing significantly simplified fabrication processes.

Self-Wrinkling-Induced Mechanically Adaptive Patterned Surface of Photocuring Coating for Abrasion Resistance

Providing mechanically adaptive performance to surfaces is significant in preserving materials from damage in variable environments, however, it has rarely been studied. Inspired by the mechanically adaptive behaviors of the surface microstructure on the carapace of desert scorpions and bark of desert tamarisks, a self-wrinkled mechanically adaptive patterned surface (SWMAPS) using one-step UV-curing and self-wrinkling technique is reported. Because of the fluorinated polyurethane photo-initiator formed by self-assembly at the top surface, UV-induced photo-crosslinking can spontaneously generate a gradient-crosslinked structure and wrinkled patterns with different morphology. With mechanically adaptive behavior originating from self-assembled fluorinated polyurethane photo-initiators, gradient-crosslinked structures, and self-wrinkled patterns, the SWMAPS remains intact under 600 cycles of reciprocating friction with little variation in the coefficient of friction and water contact angle. The SWMAPS prepared by programmable UV irradiation maintains integral under 1800 cycles of reciprocating friction with a stable friction coefficient. Furthermore, the SWMAPS is fabricated with high efficiency, regulated morphology, good surface mechanical properties, and self-recovery performance. This strategy establishes a new field of mechanically adaptive patterned surfaces, which significantly improves durability and prolongs the service life of materials in variable environments.

Phase-transition-induced dynamic surface wrinkle pattern on gradient photo-crosslinking liquid crystal elastomer

Liquid crystal elastomers (LCEs) with various deformation properties based on phase transition were widely used as actuators and provided potential to fabricate functional surfaces with tunable microstructure. Herein, we demonstrate a strategy to fabricate dynamic micro wrinkles on LCE surfaces based on LC phase transition. Stable micron-sized surface wrinkles on the anthracene-containing LCE film (AnLCE) are fabricated by ultraviolet exposure induced gradient cross-linking and subsequently stretching-releasing (UV-SR). The surface wrinkle is stabilized by the orientation of liquid crystal mesogens in the crosslinked top layer, while it can be erased by heating due to the isotropic phase-transition and recovered by stretching-releasing again. The dynamic natures cooperated with multi display modes under natural light, UV light and polarized light enable wrinkled AnLCE as a dynamic and multi-mode display platform. This strategy provide a path for modifying LCEs and regulating surface polarized images via wrinkling, which may be potential in soft sensors and optics, smart windows and anti-counterfeiting.

Photochemical Control of Network Topology in PEG Hydrogels

Hydrogels are often synthesized through photoinitiated step-, chain-, and mixed-mode polymerizations, generating diverse network topologies and resultant material properties that depend on the underlying network connectivity. While many photocrosslinking reactions are available, few afford controllable connectivity of the hydrogel network. Herein, a versatile photochemical strategy is introduced for tuning the structure of poly(ethylene glycol) (PEG) hydrogels using macromolecular monomers functionalized with maleimide and styrene moieties. Hydrogels are prepared along a gradient of topologies by varying the ratio of step-growth (maleimide dimerization) to chain-growth (maleimide-styrene alternating copolymerization) network-forming reactions. The initial PEG content and final network physical properties (e.g., modulus, swelling, diffusivity) are tailored in an independent manner, highlighting configurable gel mechanics and reactivity. These photochemical reactions allow high-fidelity photopatterning and 3D printing and are compatible with 2D and 3D cell culture. Ultimately, this photopolymer chemistry allows facile control over network connectivity to achieve adjustable material properties for broad applications.

Smart Wrinkled Interfaces: Patterning, Morphing, and Coding of Polymer Surfaces by Dynamic Anisotropic Wrinkling

In contrast to traditional static surfaces, smart patterned surfaces with periodical and reversible morphologies offer limitless opportunities for encoding surface functions and properties on demand, facilitating their widespread application as functional building blocks in various devices. Advances in intelligently controlling the macroscopic properties of these smart surfaces have been accomplished through various techniques (such as three-dimensional printing, imprint lithography and femtosecond laser) and responsive materials. In contrast to the sophisticated techniques above, dynamic anisotropic wrinkling, taking advantage of dynamic programmable manipulation of surface wrinkling and its orientation, offers a powerful alternative for fabricating dynamic periodical patterns due to its spontaneous formation, versatility, convenient scale-up fabrication, and sensitivity to various stimuli. This review comprehensively summarizes recent advances in smart patterned surfaces with dynamic oriented wrinkles, covering design principles, fabrication techniques, representative types of physical and chemical stimuli, as well as fine-tuning of wrinkle dimensions and orientation. Finally, advanced applications of these smart patterned surfaces are presented, along with a discussion of current challenges and future prospects in this rapidly evolving field. This review would offer some insights and guidelines for designing and engineering novel stimuli-responsive smart wrinkled surfaces, thereby facilitating their sustainable development and progressing toward commercialization.

Physical Unclonable Functions with Hyperspectral Imaging System for Ultrafast Storage and Authentication Enabled by Random Structural Color Domains

Physical unclonable function (PUF) is attractive in modern encryption technologies. Addressing the disadvantage of slow data storage/authentication in optical PUF is paramount for practical applications but remains an on-going challenge. Here, a highly efficient PUF strategy based on random structural color domains (SCDs) of cellulose nanocrystal (CNC) is proposed for the first time, combing with hyperspectral imaging system (HIS) for ultrafast storage and authentication. By controlling the growth and fusion behavior of the tactoids of CNC, the SCDs display an irregular and random distribution of colors, shapes, sizes, and reflectance spectra, which grant unique and inherent fingerprint-like characteristics that are non-duplicated. Based on images and spectra, these fingerprint features are used to develop two sets of PUF key generation methods, which can be respectively authenticated at the user-end and the manufacturer-front-end that achieving a high coding capacity of at least 22304. Notably, the use of HIS greatly shortens the time of key reading and generation (≈5 s for recording, 0.5-0.7 s for authentication). This new optical PUF labels can not only solve slow data storage and complicated authentication in optical PUF, but also impulse the development of CNC in industrial applications by reducing color uniformity requirement.

Stimuli-responsive mechanically interlocked polymer wrinkles

Artificial wrinkles, especially those with responsive erasure/regeneration behaviors have gained extensive interest due to their potential in smart applications. However, current wrinkle modulation methods primarily rely on network rearrangement, causing bottlenecks in in situ wrinkle regeneration. Herein, we report a dually cross-linked network wherein [2]rotaxane cross-link can dissipate stress within the wrinkles through its sliding motion without disrupting the network, and quadruple H-bonding cross-link comparatively highlight the advantages of [2]rotaxane modulation. Acid stimulation dissociates quadruple H-bonding and destructs network, swiftly eliminating the wrinkles. However, the regeneration process necessitates network rearrangement, making in situ recovery unfeasible. By contrast, alkaline stimulation disrupts host-guest recognition, and subsequent intramolecular motion of [2]rotaxane dissipate energy to eliminate wrinkles gradually. The always intact network allows for the in situ recovery of surface microstructures. The responsive behaviors of quadruple H-bonding and mechanical bond are orthogonal, and their combination leads to wrinkles with multiple but accurate responsiveness.

Generation of precision microstructures based on reconfigurable photoresponsive hydrogels for high-resolution polymer replication and microoptics

Microstructured molds are essential for fabricating various components ranging from precision optics and microstructured surfaces to microfluidics. However, conventional fabrication technology such as photolithography requires expensive equipment and a large number of processing steps. Here, we report a facile method to fabricate micromolds based on a reusable photoresponsive hydrogel: Uniform micropatterns are engraved into the hydrogel surface using photo masks under UV irradiation within a few minutes. Patterns are replicated using polydimethylsiloxane with minimum feature size of 40 μm and smoothness of Rq ~ 3.4 nm. After replication, the patterns can be fully erased by light thus allowing for reuse as a new mold without notable loss in performance. Utilizing greyscale lithography, patterns with different height levels can be produced within the same exposure step. We demonstrate the versatility of this method by fabricating diffractive optical elements devices and a microlens array and microfluidic device with 100 µm wide channels.

Photochemical Design for Diverse Controllable Patterns in Self-Wrinkling Films

Harnessing the spontaneous surface instability of pliable substances to create intricate, well-ordered, and on-demand controlled surface patterns holds great potential for advancing applications in optical, electrical, and biological processes. However, the current limitations stem from challenges in modulating multidirectional stress fields and diverse boundary environments. Herein, this work proposes a universal strategy to achieve arbitrarily controllable wrinkle patterns via the spatiotemporal photochemical boundaries. Utilizing constraints and inductive effects of the photochemical boundaries, the multiple coupling relationship is accomplished among the light fields, stress fields, and morphology of wrinkles in photosensitive polyurethane (PSPU) film. Moreover, employing sequential light-irradiation with photomask enables the attainment of a diverse array of controllable patterns, ranging from highly ordered 2D patterns to periodic or intricate designs. The fundamental mechanics of underlying buckling and the formation of surface features are comprehensively elucidated through theoretical stimulation and finite element analysis. The results reveal the evolution laws of wrinkles under photochemical boundaries and represent a new effective toolkit for fabricating intricate and captivating patterns in single-layer films.

Viscoelasticity‐Controlled Relaxation in Wrinkling Surface for Multistage Time‐Resolved Optical Information Encryption

As counterfeit techniques continue to evolve, ensuring the security of conventional "static" encryption methods becomes increasingly challenging. Here, the viscoelasticity-controlled relaxation is introduced for the first time in a bilayer wrinkling system by regulating the density of hydrogen bond networks in polymer to construct a "dynamic" encryption material. The wrinkling surface can manipulate light during the dynamic relaxation process, exhibiting three stages with frosted glass, structural color, and mirror reflection. By regulating the viscoelasticity of skin layer through UV irradiation, the wavelength and the relaxation rate of the wrinkles can be controlled. As a result, dynamic wrinkling anti-counterfeiting patterns and time-resolved multistage information encryption are achieved. Crucially, the encryption material is developed as an anti-counterfeiting label for packing boxes in daily applications, allowing the encrypted information to be activated manually and identified by naked eyes, surpassing the existing time-resolved encryption materials in utilization potential. Besides, the dynamic hydrogen bond networks are extended to various dynamic interaction networks, demonstrating the versatility of the dynamic encryption strategy. This work not only provides an additional dimension for dynamic information encryption in daily practical use, but also offers theoretical guidance for the development of advanced optical anti-counterfeiting and smart display materials in the future.

The Recent Advances of Polymer-POSS Nanocomposites With Low Dielectric Constant

This study provides a comprehensive overview of the preparation methods for polyhedral oligomeric silsesquioxane (POSS) monomers and polymer/POSS nanocomposites. It focuses on the latest advancements in using POSS to design polymer nanocomposites with reduced dielectric constants. The study emphasizes exploring the potential of POSS, either alone or in combination with other materials, to decrease the dielectric constant and dielectric loss of various polymers, including polyimides, bismaleimide resins, poly(aryl ether)s, polybenzoxazines, benzocyclobutene resins, polyolefins, cyanate ester resins, and epoxy resins. In addition, the research investigates the impact of incorporating POSS on improving the thermal properties, mechanical properties, surface properties, and other aspects of these polymers. The entire study is divided into two parts, discussing systematically the role of POSS in reducing dielectric constants during the preparation of POSS composites using both physical blending and chemical synthesis methods. The goal of this research is to provide valuable strategies for designing a new generation of low dielectric constant materials suitable for large-scale integrated circuits in the semiconductor materials domain.

Biomimetic growth in polymer gels

By modeling gels growing in confined environments, we uncover a biomimetic feedback mechanism between the evolving gel and confining walls that enables significant control over the properties of the grown gel. Our new model describes the monomer adsorption, polymerization and cross-linking involved in forming new networks and the resultant morphology and mechanical behavior of the grown gel. Confined between two hard walls, a thin, flat "parent" gel undergoes buckling; removal of the walls returns the gel to the flat structure. Polymerization and cross-linking in the confined parent generates the next stage of growth, forming a random copolymer network (RCN). When the walls are removed, the RCN remains in the buckled state, simultaneously "locking in" these patterns and increasing the Young's modulus by two orders of magnitude. Confinement of thicker gels between harder or softer 3D walls leads to controllable mechanical heterogeneities, where the Young's modulus between specific domains can differ by three orders of magnitude. These systems effectively replicate the feedback between mechanics and morphology in biological growth, where mechanical forces guide the structure formation throughout stages of growth. The findings provide new guidelines for shaping "growing materials" and introducing new approaches to matching form and function in synthetic systems.

Biomimetic 3D Color-Changing Hydrogel Actuators Constructed Based on Soft Permeable Photonic Crystals

The integration of photonic crystals and self-shaping actuators is a promising method for constructing powerful biomimetic color-changing actuators. The major barrier is that common photonic crystals generally block the transfer/orientation of monomers/fillers and hence hinder the formation of heterogeneous structures for programmed 3D deformations as well as degrade the deformation capacity and mechanical properties of actuators. Herein, we present the construction of complex and strong 3D color-changing hydrogel actuators by asymmetric photolithography based on soft, permeable photonic crystals. The soft permeable photonic crystals are assembled by hydrogel microspheres with an ultralow volume fraction. During the asymmetric photolithography, the monomers in precursor solutions can thus transfer freely to generate heterogeneous microstructures, spatially patterned internal stresses, and interpenetrating networks for programming the deformation trajectories and initial 3D configurations and enhancing mechanical properties of actuators. Various 3D color-changing hydrogel actuators (e.g., flower and scroll painting) are constructed for applications such as information encryption and display.

Wrinkled Interfaces: Taking Advantage of Anisotropic Wrinkling to Periodically Pattern Polymer Surfaces

Periodically patterned surfaces can cause special surface properties and are employed as functional building blocks in many devices, yet remaining challenges in fabrication. Advancements in fabricating structured polymer surfaces for obtaining periodic patterns are accomplished by adopting "top-down" strategies based on self-assembly or physico-chemical growth of atoms, molecules, or particles or "bottom-up" strategies ranging from traditional micromolding (embossing) or micro/nanoimprinting to novel laser-induced periodic surface structure, soft lithography, or direct laser interference patterning among others. Thus, technological advances directly promote higher resolution capabilities. Contrasted with the above techniques requiring highly sophisticated tools, surface instabilities taking advantage of the intrinsic properties of polymers induce surface wrinkling in order to fabricate periodically oriented wrinkled patterns. Such abundant and elaborate patterns are obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate through conventional patterning techniques. Focusing on oriented wrinkles, this review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine-tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already in use or may be prospective in the near future are overviewed.

Smart Polymer Surfaces with Complex Wrinkled Patterns: Reversible, Non-Planar, Gradient, and Hierarchical Structures

This review summarizes the relevant developments in preparing wrinkled structures with variable characteristics. These include the formation of smart interfaces with reversible wrinkle formation, the construction of wrinkles in non-planar supports, or, more interestingly, the development of complex hierarchically structured wrinkled patterns. Smart wrinkled surfaces obtained using light-responsive, pH-responsive, temperature-responsive, and electromagnetic-responsive polymers are thoroughly described. These systems control the formation of wrinkles in particular surface positions and the reversible construction of planar-wrinkled surfaces. This know-how of non-planar substrates has been recently extended to other structures, thus forming wrinkled patterns on solid, hollow spheres, cylinders, and cylindrical tubes. Finally, this bibliographic analysis also presents some illustrative examples of the potential of wrinkle formation to create more complex patterns, including gradient structures and hierarchically multiscale-ordered wrinkles. The orientation and the wrinkle characteristics (amplitude and period) can also be modulated according to the requested application.

Self-growing photonic composites with programmable colors and mechanical properties

Many organisms produce stunning optical displays based on structural color instead of pigmentation. This structural or photonic color is achieved through the interaction of light with intricate micro-/nano-structures, which are "grown" from strong, sustainable biological materials such as chitin, keratin, and cellulose. In contrast, current synthetic structural colored materials are usually brittle, inert, and produced via energy-intensive processes, posing significant challenges to their practical uses. Inspired by the brilliantly colored peacock feathers which selectively grow keratin-based photonic structures with different photonic bandgaps, we develop a self-growing photonic composite system in which the photonic bandgaps and hence the coloration can be easily tuned. This is achieved via the selective growth of the polymer matrix with polymerizable compounds as feeding materials in a silica nanosphere-polymer composite system, thus effectively modulating the photonic bandgaps without compromising nanostructural order. Such strategy not only allows the material system to continuously vary its colors and patterns in an on-demand manner, but also endows it with many appealing properties, including flexibility, toughness, self-healing ability, and reshaping capability. As this innovative self-growing method is simple, inexpensive, versatile, and scalable, we foresee its significant potential in meeting many emerging requirements for various applications of structural color materials.

Photo-induced stress relaxation in reconfigurable disulfide-crosslinked supramolecular films visualized by dynamic wrinkling

Stress relaxation in reconfigurable supramolecular polymer networks is strongly related to intermolecular behavior. However, the relationship between molecular motion and macroscopic mechanics is usually vague, and the visualization of internal stress reflecting precise regulation of molecules remains challenging. Here, we present a strategy for visualizing photo-driven stress relaxation induced by infinitesimal perturbations in the intermolecular exchange reaction via reprogrammable wrinkle patterns. The supramolecular films exhibit visible changes in microscopic wrinkle topography through ultraviolet (UV)-induced dynamic disulfide exchange reaction. In accordance with the trans-scale theoretical models, which quantitatively evaluate the chemical-dependent mechanical stresses in the supramolecular network, the unexposed disordered wrinkles evolved into highly oriented patterns and underwent subsequent mutations after thermal treatment. The stress-sensitive wrinkle macro-patterns can be repetitively written/erased through network topology rearrangement using different stimuli. This strategy provides an approach for visualizing and understanding the molecular behavior from dynamic chemistry to mechanical changes, and directly programming wrinkle patterns with regulated structures.

Photo-induced spatial gradient network for shape memory polymer with pattern-memorizing surface

Incorporating a pattern-memorizing surface into a multi-functional shape memory polymer (SMP) offers various extraordinary opportunities for their engineering applications. However, current memory-patterned approaches prepared by artificial loading are at the cost of initial balance, whose potential is greatly limited by the internal relationship between thermodynamic equilibrium and the entropy-driven pattern-memorizing cycle. Here, a robust yet effective strategy is presented for fabricating a spontaneous pattern on a poly(styrene-block-butadiene-block-styrene) (SBS)-based SMP with a gradient crosslinking network via molecular diffusion for equilibrium. Benefiting from the photo-induced diffusion of maleimide, the resulting steady-state pattern as a permanent shape ensures the recovery of morphology, and the gradient network formed by the diffusion-regulated spatial Diels-Alder (D-A) crosslinking reaction makes the pattern memory cycle from existence to elimination possible. Furthermore, taking advantage of an uneven structural network, the shape reconfigurations from 2D patterned sheets to 3D configurations with a patterned surface can be achieved conveniently through a shape memory effect, simplifying programming setups. In addition, this type of 3D shape also can shift back to a 2D patterned film via an inverse D-A decrosslinking reaction upon thermal treatment. This straightforward approach for fabricating a pattern of a single layer on an SMP surface with a spatial gradient network opens a new avenue for functional smart materials, which expands the technological perspectives in many fields of flexible electronics, smart actuators, switching sensors and soft robotics.

Strong and tough self-wrinkling polyelectrolyte hydrogels constructed via a diffusion-complexation strategy

Self-wrinkling hydrogels enable various engineering and biomedical applications. The major challenge is to couple the self-wrinkling technologies and enhancement strategies, so as to get rid of the poor mechanical properties of existing self-wrinkling gels. Herein we present a facile diffusion-complexation strategy for constructing strong and ultratough self-wrinkling polyelectrolyte hydrogels with programmable wrinkled structures and customizable 3D configurations. Driven by the diffusion of low-molecular-weight chitosan polycations into the polyanion hydrogels, the high-modulus polyelectrolyte complexation shells can form directly on the hydrogel surface. Meanwhile, the polyanion hydrogels deswell/shrink due to the low osmotic pressure, which applies an isotropous surface compressive stress for inducing the formation of polygonal wrinkled structures. When the diffusion-complexation reaction occurs on a pre-stretched hydrogel sheet, the long-range ordered wrinkled structures can form during the springback/recovery of the hydrogel matrix. Moreover, through controlling the regions of diffusion-complexation reaction on the pre-stretched hydrogels, they can be spontaneously transformed into various 3D configurations with ordered wrinkled structures. Notably, because of the introduction of plenty of electrostatic binding (i.e., sacrificial bonds), the as-prepared self-wrinkling gels possess outstanding mechanical properties, far superior to the reported ones. This diffusion-complexation strategy paves the way for the on-demand design of high-performance self-wrinkling hydrogels.

Dynamic Surface Wrinkles for In Situ Light-Driven Dynamic Gratings

Dynamic diffraction gratings (DDGs) are considered as one of the most promising technologies for application in smart optical devices because of their in situ dynamic regulation of light propagation on demand; however, it is still a challenge to fabricate dynamic periodic micro/nanostructures due to limited materials and processes. Here, a facile and feasible strategy to construct a near-infrared (NIR) radiation-driven DDG is developed based on a double-sided surface pattern, which is fabricated by dynamic wrinkles and/or soft-imprinted static wrinkles. Poly(dimethylsiloxane) (PDMS) containing carbon nanotubes (CNTs) serves as the substrate, and wrinkles are formed on both sides. The resulting double-sided wrinkle pattern can be used as a DDG to generate various adjustable two-dimensional (2D) diffraction patterns driven by NIR light. Furthermore, with various combinations of wrinkles, we demonstrated a single-sided responsive DDG and a double-sided responsive DDG to realize the evolution of diffraction patterns from 2D to one-dimensional (1D) and 2D to zero-dimensional (0D), respectively. The results provide an alternative for DDGs that will have wide applications in smart display, sensing, and imaging systems.

Tailoring Ordered Wrinkle Arrays for Tunable Surface Performances by Template-Modulated Gradient Films

Complex wrinkled microstructures are ubiquitous in natural systems and living bodies. Although homogeneous wrinkles in film-substrate bilayers have been extensively investigated in the past 2 decades, tailoring heterogeneous wrinkles by a facile method is still a challenge. Here, we report on the controllable heterogeneous wrinkles in template-modulated thickness-gradient metal films sputter-deposited on polydimethylsiloxane substrates. It is found that the stress of the gradient film is strongly position-dependent and the wrinkles are always restricted in thinner film regions. The morphological characteristic and formation mechanism of the heterogeneous wrinkles are analyzed and discussed in detail based on the stress theory. Ordered wrinkle arrays are achieved by adjusting the deposition time, copper grid period, template shape, and lifting height. The surface performances (e.g., the friction property) are well controlled by the wrinkle arrays. This work could promote better understanding of the spontaneously heterogeneous wrinkles in template-modulated gradient films and controllable fabrication of various wrinkle arrays by independently tuning film deposition conditions and template parameters.

Cephalopod-Inspired Design of Photomechanically Modulated Display Systems for On-Demand Fluorescent Patterning

Cephalopods can display variable body color/patterns upon environmental stimulation via bioelectricity-controlled muscle contraction/expansion of skin chromatophores. However, it remains challenging to produce artificial display analogs that exhibit reversible and rapid switching between multiple expected luminescent patterns, although such systems are very appealing for many practical uses (e.g., data encryption). Inspired by the bioelectromechanical display tactic of cephalopods, in this work, a conceptually new photomechanically modulated fluorescent system that enables on-demand display of fluorescent patterns via a cascading stimulation-mechanical movement-optical output conduction mechanism is presented. Specifically, this biomimetic system comprises a customizable hollow display panel and a bottom-tethered photothermally responsive fluorescent actuator. Under NIR light, the photomechanically bending movements of the fluorescent actuator will immediately cover the hollow window of the display panel and synchronously manifest as the display of fluorescent patterns. Owing to its desirable time- and light-power-dependent actuating behaviors, diverse fluorescent patterns/information can be dynamically and reversibly displayed by facilely controlling this single remote NIR signal. This bioinspired strategy is universal and promising for fabricating on-demand fluorescent display platforms that combine a wide choice of fluorophores, remote control with high spatial/temporal precision, and especially single-input multiple-output features.

Highly photosensitive furan acrylate derivatives and their solid-state photopolymerization

Highly photosensitive multifunctional furan acrylate monomers synthesized from biomass furfural can be photopolymerized into polyesters without photoinitiators and solvents.

Diels–Alder Cycloadditions of Bio-Derived Furans with Maleimides as a Sustainable «Click» Approach towards Molecular, Macromolecular and Hybrid Systems

This mini-review highlights the recent research trends in designing organic or organic-inorganic hybrid molecular, biomolecular and macromolecular systems employing intermolecular Diels–Alder cycloadditions of biobased, furan-containing substrates and maleimide dienophiles. The furan/maleimide Diels–Alder reaction is a well-known process that may proceed with high efficiency under non-catalytic and solvent-free conditions. Due to the simplicity, 100% atom economy and biobased nature of many furanic substrates, this type of [4+2]-cycloaddition may be recognized as a sustainable “click” approach with high potential for application in many fields, such as fine organic synthesis, bioorganic chemistry, material sciences and smart polymers development.

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.

Micropatterns Fabricated by Photodimerization-Induced Diffusion

Pattern technology plays an important role in the generation of microstructures with different functionalities and morphologies. In this report, a straightforward and versatile strategy is presented for spatially regulating the growth of a microstructure on a surface by the photodimerization of maleimide (MI). Upon exposure of ultraviolet (UV) light, photodimerization of MI in a film comprising furan-grafted polymer and bismaleimide (BMI) produces a chemical gradient, which can drive the diffusion of BMI from the unexposed to the exposed region and from the bottom to the surface, resulting in the growth of micropatterns. Sequential crosslinking induced by the Diels-Alder reaction between MI and furan maintains the stability of pattern shape. Theoretical modeling with reaction-diffusion equations reveal that as photodimerization moves the system far from thermodynamic equilibrium, the formation of a chemical potential gradient requires the redistribution of matter, resulting in the formation of topographies. Directional molecular motion induced by UV light can generate complex morphology, and produce materials with unique optical functions, such as charming-ordered gratings. This straightforward method of fabricating micropatterns by photodimerization-induced diffusion is successfully applied to patterned curved surfaces, microfluidic channels and encapsulation of integrated light emitting diode chips.

Robust Scalable-Manufactured Smart Fabric Surfaces Based on Azobenzene-Containing Maleimide Copolymers for Rewritable Information Storage and Hydrogen Fluoride Visual Sensor

Functionalized materials with reversible color switching are highly attractive in many application fields, especially as rewritable media for information storage. It is critical yet challenging to develop a cost-effective strategy for the fabrication of stimulus-responsive chromogenic systems. Herein, we present a versatile dip-coating approach to fabricate robust smart textile with acid/base-driven chromotropic capability. Owing to the introduction of novel maleimide-based copolymers bearing azobenzene derivative moieties, smart textiles possess rapid color switching between yellow and orange-red, which is triggered by acid-base stimulations with the resulting reversible protonation/deprotonation of maleimide moieties. As a proof of concept of the application of the smart textile for high-performance rewritable media, various rewritable elaborate patterns can be fast trifluoroacetic acid-printed/triethylamine-erased (within 20 s) with excellent cycling stability and long legible duration (>30 days). Meanwhile, the smart textile can be employed as a visual sensor for the detection of hydrogen fluoride gas leakage. It is highlighted that the as-prepared robust smart textiles with superhydrophobic surfaces have excellent antifouling properties and chemical/mechanical stabilities, which can tolerate harsh environmental conditions and repetitive mechanical deformation. The robust smart textiles with simple low-cost large-scale production may find more advanced potential applications besides information storage and sensors demonstrated.

A Hierarchically Tailored Wrinkled Three-Dimensional Foam for Enhanced Elastic Supercapacitor Electrodes

Recently, three-dimensional (3D) porous foams have been studied, but further improvement in nanoscale surface area and stretchability is required for electronic and energy applications. Herein, a general strategy is reported to form a tailored wrinkling structure on strut surfaces inside a 3D polydimethylsiloxane (PDMS) polymeric foam. Controlled wrinkles are created on the struts of 3D foam through an oxygen plasma treatment to form a bilayer surface of PDMS on uniaxially prestretched 3D PDMS foam, followed by relaxation. After plasma treatment for 1 h and prestretching of 40%, the wrinkled 3D foam greatly improves specific surface area and stretchability by over 60% and 75%, respectively, compared with the pristine 3D PDMS foam. To prove its applicability with improved performances, supercapacitors are prepared by coating a conductive material on the wrinkled 3D foam. The resulting supercapacitors exhibit an increased storage capacity (8.3 times larger), maintaining storage capacity well under stretching up to 50%.

Spatially and Reversibly Actuating Soft Gel Structure by Harnessing Multimode Elastic Instabilities

Autonomous shape transformation is key in developing high-performance soft robotics technology; the search for pronounced actuation mechanisms is an ongoing mission. Here, we present the programmable shape morphing of a three-dimensional (3D) curved gel structure by harnessing multimode mechanical instabilities during free swelling. First of all, the coupling of buckling and creasing occurs at the dedicated region of the gel structure, which is attributed to the edge and surface instabilities resulted from structure-defined spatial nonuniformity of swelling. The subsequent developments of post-buckling morphologies and crease patterns collaboratively drive the structural transformation of the gel part from the "open" state to the "closed" state, thus realizing the function of gripping. By utilizing the multi-stimuli-responsive nature of the hydrogel, we recover the swollen gel structure to its initial state, enabling reproducible and cyclic shape evolution. The described soft gel structure capable of shape transformation brings a variety of advantages, such as easy to fabricate, large strain transformation, efficient actuation, and high strength-to-weight ratio, and is anticipated to provide guidance for future applications in soft robotics, flexible electronics, offshore engineering, and healthcare products.

Photo-Oxidation-Controlled Surface Pattern with Responsive Wrinkled Topography and Fluorescence

Wrinkles and photo-oxidation reactions are widely found in soft materials, which are intimately associated with the failure of materials and structures. It is expected that the photo-oxidation process could also have a positive effect on the material and its surface. Here, we report the photo-oxidation of 2-(4-dietheylaminophenyl)-4,5-bis(4-methoxyphenyl) imidazole (DEA-TAI) into a wrinkled bilayer system to control surface wrinkle and fluorescent patterns, in which a supramolecular polymer network composed of carboxylic acid-containing copolymer (PS-BA-AA; PS=poly(styrene), BA=butyl acrylate; AA=acrylic acid) and DEA-TAI were used as the skin layer. Ultraviolet (UV) irradiation can induce photo-oxidation of the imidazole ring of DEA-TAI to weaken the intermolecular hydrogen bonding between PS-BA-AA and DEA-TAI, resulting in the release of stress in the bilayer system. The wrinkled morphology and fluorescence of the surface can be simultaneously regulated by photo-oxidation of DEA-TAI under UV light, and the resulting wrinkles are extremely sensitive to the pH value, which can be quickly and reversibly erased by NH₃ gas. Smart surfaces with specific hierarchical wrinkles and fluorescence can be achieved by selective irradiation with photomasks, which may find potential applications in smart displays and multi-code information storage.

Transparency Change Mechanochromism Based on a Robust PDMS-Hydrogel Bilayer Structure

Hydrogels and polydimethylsiloxane (PDMS) are complementary to each other, since the hydrophobic PDMS provides a more stable and rigid substrate, while the water-rich hydrogel possesses remarkable hydrophilicity, biocompatibility, and similarity to biological tissues. Herein a transparent and stretchable covalently bonded PDMS-hydrogel bilayer (PHB) structure is prepared via in situ free radical copolymerization of acrylamide and allylamine-exfoliated-ZrP (AA-e-ZrP) on a functionalized PDMS surface. The AA-e-ZrP serves as cross-linking nano-patches in the polymer gel network. The covalently bonded structure is constructed through the addition reaction of vinyl groups of PDMS surface and monomers, obtaining a strong interfacial adhesion between the PDMS and the hydrogel. A mechanical-responsive wrinkle surface, which exhibs transparency change mechanochromism, is created via introducing a cross-linked polyvinyl alcohol film atop the PHB structure. A finite element model is implemented to simulate the wrinkle formation process. The implication of the present finding for the interfacial design of the PHB and PDMS-hydrogel-PVA trilayer (PHPT) structures is discussed.

Bioinspired Multiscale Wrinkling Patterns on Curved Substrates: An Overview

The surface wrinkling of biological tissues is ubiquitous in nature. Accumulating evidence suggests that the mechanical force plays a significant role in shaping the biological morphologies. Controlled wrinkling has been demonstrated to be able to spontaneously form rich multiscale patterns, on either planar or curved surfaces. The surface wrinkling on planar substrates has been investigated thoroughly during the past decades. However, most wrinkling morphologies in nature are based on the curved biological surfaces and the research of controllable patterning on curved substrates still remains weak. The study of wrinkling on curved substrates is critical for understanding the biological growth, developing three-dimensional (3D) or four-dimensional (4D) fabrication techniques, and creating novel topographic patterns. In this review, fundamental wrinkling mechanics and recent advances in both fabrications and applications of the wrinkling patterns on curved substrates are summarized. The mechanics behind the wrinkles is compared between the planar and the curved cases. Beyond the film thickness, modulus ratio, and mismatch strain, the substrate curvature is one more significant parameter controlling the surface wrinkling. Curved substrates can be both solid and hollow with various 3D geometries across multiple length scales. Up to date, the wrinkling morphologies on solid/hollow core-shell spheres and cylinders have been simulated and selectively produced. Emerging applications of the curved topographic patterns have been found in smart wetting surfaces, cell culture interfaces, healthcare materials, and actuators, which may accelerate the development of artificial organs, stimuli-responsive devices, and micro/nano fabrications with higher dimensions.