All-Optical Reversible Azo-Based Wrinkling Patterns with High Aspect Ratio and Polarization-Independent Orientation for Light-Responsive Soft Photonics

One-step rapid formation of wrinkled fractal antibiofouling coatings from environmentally friendly, waste-derived terpenes

Wrinkled coatings are a potential drug-free method for mitigating bacterial attachment and biofilm formation on materials such as medical and food grade steel. However, their fabrication typically requires multiple steps and often the use of a stimulus to induce wrinkle formation. Here, we report a facile plasma-based method for rapid fabrication of thin (<250 nm) polymer coatings from a single environmentally friendly precursor, where wrinkle formation and fractal pattern development are controlled solely by varying the deposition time from 3 s to 60 s. We propose a mechanism behind the observed in situ development of wrinkles in plasma, as well as demonstrate how introducing specific topographical features on the surface of the substrata can result int the formation of even more complex, ordered wrinkle patterns arising from the non-uniformity of plasma when in contact with structured surfaces. Thus-fabricated wrinkled surfaces show good adhesion to substrate and an antifouling activity that is not observed in the equivalent smooth coatings and hence is attributed to the specific pattern of wrinkles.

Deformation-Induced Photoprogrammable Pattern of Polyurethane Elastomers Based on Poisson Effect

Elastomers with high aspect ratio surface patterns are a promising class of materials for designing soft machines in the future. Here, a facile method for fabricating surface patterns on polyurethane elastomer by subtly utilizing the Poisson effect and gradient photocrosslinking is demonstrated. By applying uniaxial tensile strains, the aspect ratio of the surface patterns can be optionally manipulated. At prestretched state, the pattern on the polyurethane elastomer can be readily constructed through compressive stress, resulting from the gradient photocrosslinking via selective photodimerization of an anthracene-functionalized polyurethane elastomer (referred to as ANPU). The macromolecular aggregation structures during stretching deformation significantly contribute to the fabrication of high aspect ratio surface patterns. The insightful finite element analysis well demonstrates that the magnitude and distribution of internal stress in the ANPU elastomer can be regulated by selectively gradient crosslinking, leading to polymer chains migrate from the exposed region to the unexposed region, thereby generating a diverse array of surface patterns. Additionally, the periodic surface patterns exhibit tunable structural color according to the different stretching states and are fully reversible over multiple cycles, opening up avenues for diverse applications such as smart displays, stretchable strain sensors, and anticounterfeiting devices.

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.

Adaptive 2D and Pseudo-2D Systems: Molecular, Polymeric, and Colloidal Building Blocks for Tailored Complexity

Two-dimensional and pseudo-2D systems come in various forms. Membranes separating protocells from the environment were necessary for life to occur. Later, compartmentalization allowed for the development of more complex cellular structures. Nowadays, 2D materials (e.g., graphene, molybdenum disulfide) are revolutionizing the smart materials industry. Surface engineering allows for novel functionalities, as only a limited number of bulk materials have the desired surface properties. This is realized via physical treatment (e.g., plasma treatment, rubbing), chemical modifications, thin film deposition (using both chemical and physical methods), doping and formulation of composites, or coating. However, artificial systems are usually static. Nature creates dynamic and responsive structures, which facilitates the formation of complex systems. The challenge of nanotechnology, physical chemistry, and materials science is to develop artificial adaptive systems. Dynamic 2D and pseudo-2D designs are needed for future developments of life-like materials and networked chemical systems in which the sequences of the stimuli would control the consecutive stages of the given process. This is crucial to achieving versatility, improved performance, energy efficiency, and sustainability. Here, we review the advancements in studies on adaptive, responsive, dynamic, and out-of-equilibrium 2D and pseudo-2D systems composed of molecules, polymers, and nano/microparticles.

Flexible Azo-Polyimide-Based Smart Surface with Photoregulatable Surface Micropatterns: Toward Rewritable Information Storage and Wrinkle-Free Device Fabrication

Stimulus-sensitive materials are of great fascination in surface and interface science owing to their dynamically tunable surface properties and/or morphologies. Herein, we have synthesized an azobenzene-containing polyimide (azo-PI) with enhanced chain flexibility for the fabrication of photosensitive surface patterns on a film/substrate wrinkle system or wrinkle-free devices. The phototriggered cis-trans isomerization kinetics of azobenzene groups in the novel azo-PI with various chain structures were systematically investigated. On the basis of the characteristics of stress relaxation that azobenzene reversible cis-trans isomerization induces in the wrinkled azo-PI film/substrate system, a variety of rewritable visual surface patterns with high resolution and a long legibility time (>30 days) could be easily constructed via visible-light irradiation, enabling the wrinkled azo-PI surfaces to be used as rewritable information storage media. Meanwhile, because of the visible-light irradiation strategy, these photoresponsive surfaces could find potential application in the fabrication of wrinkle-free flexible devices. This study not only sheds light on the influence of the azo-polymer chain structure on its photoresponsive behavior but also provides a versatile strategy for realizing tailor-made smart surface patterns on multilayer functional devices.

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.

Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties—Thermoresponsive and Non-Thermoresponsive Gels

The unique photomotion of azo materials under irradiation has been in the focus of research for decades and has been expanded to different classes of solids such as polymeric glasses, liquid crystalline materials, and elastomers. In this communication, azo dye-containing gels are obtained by photocrosslinking of non-thermoresponsive and lower critical solution temperature type thermoresponsive copolymers. These are analysed with light microscopy regarding their actuation behaviour under laser irradiation. The influences of the cloud-point temperature and of the laser power are investigated in a series of comparative experiments. The thermoresponsive hydrogels show more intense photoactuation when the cloud-point temperature of the non-crosslinked polymer is above, but closer to, room temperature, while higher laser powers lead to stronger motion, indicating a photothermal mechanism. In non-thermoresponsive gels, considerably weaker photoactuation occurs, signifying a secondary mechanism that is a direct consequence of the optical field-azo dye interaction.

Light-assisted anti-wrinkling on azobenzene-containing polyblend films

Undesired surface wrinkling is a persistent issue far from being resolved. Here, we report a simple light-assisted strategy to prevent surface wrinkling on azobenzene-containing polyblend films, which is based on the unique photo-responsive behaviors of azobenzene moieties. Upon visible light irradiation, the mechanical strain-induced surface wrinkling of the azo-based polyblend film attached on a pre-strained compliant substrate can be effectively suppressed. The influence of light irradiation conditions and polyblend composition on the wrinkling resistance has been systematically investigated. Notably, empirical scaling laws that can quantify the connection of the critical wrinkling conditions with external and internal factors are derived. This spatiotemporal light-assisted strategy combined with the simple universal blending method would provide a general guideline for the anti-wrinkling purpose in diverse functional material systems/devices.

Light-Induced Surface Wrinkling on Azo-Based Composite Films

Here we report a simple micro/nano patterning strategy based on light-induced surface wrinkling. Namely, we fabricated a film/substrate system composed of polydimethylsiloxane (PDMS) as a soft substrate and non-photosensitive polymer polystyrene (PS) mixed with azo-polymer (polydisperse orange 3, PDO3) as a stiff film. Taking advantage of the photo-thermal effect and photo-softening effect of PDO3, we fabricated various microstructured wrinkling morphologies by a simple light illumination. We investigated the influence of two exposure modes (i.e., static selective exposure and dynamic moving exposure), the illumination conditions, the composition of the blended film, and the film thickness on the resulting wrinkling patterns. It is highly expected that this azo-based photosensitive wrinkling system will be extended to functional systems for the realization of light-induced surface micro/nanopatterning.

Dynamic multifunctional devices enabled by ultrathin metal nanocoatings with optical/photothermal and morphological versatility

Smart devices characterized by micro-/nanotopographies, such as cracks, wrinkles, folds, etc., have been fabricated for widespread application. Here, with the combination of multiscale hierarchical architecture, ultrathin metal nanocoatings with high optical/photothermal tunability and morphological versatility, and surface/interface engineering, a set of multifunctional devices with multistimuli responsiveness was fabricated. These devices can adapt to external stimuli with reversible and instantaneous responses in optical signals, which include strain-regulated light-scattering properties, photothermal-responsive wrinkled surface coupled with moisture-responsive structural color, and mechanically controllable light-shielding properties. The structural designs that rationally overlay micro-/nanostructured ultrathin nanocoatings with other elements are the key to realize this advanced system, which provides avenues for designing versatile, tunable, and adaptable multifunctional devices.

Inspired by the intriguing adaptivity of natural life, such as squids and flowers, we propose a series of dynamic and responsive multifunctional devices based on multiscale structural design, which contain metal nanocoating layers overlaid with other micro-/nanoscale soft or rigid layers. Since the optical/photothermal properties of a metal nanocoating are thickness dependent, metal nanocoatings with different thicknesses were chosen to integrate with other structural design elements to achieve dynamic multistimuli responses. The resultant devices demonstrate 1) strain-regulated cracked and/or wrinkled topography with tunable light-scattering properties, 2) moisture/photothermal-responsive structural color coupled with wrinkled surface, and 3) mechanically controllable light-shielding properties attributed to the strain-dependent crack width of the nanocoating. These devices can adapt external stimuli, such as mechanical strain, moisture, light, and/or heat, into corresponding changes of optical signals, such as transparency, reflectance, and/or coloration. Therefore, these devices can be applied as multistimuli-responsive encryption devices, smart windows, moisture/photothermal-responsive dynamic optics, and smartphone app–assisted pressure-mapping sensors. All the devices exhibit high reversibility and rapid responsiveness. Thus, this hybrid system containing ultrathin metal nanocoatings holds a unique design flexibility and adaptivity and is promising for developing next-generation multifunctional devices with widespread application.

Synergistic Stain Removal Achieved by Controlling the Fractions of Light and Thermo Responsive Components in the Dual-Responsive Copolymer Immobilized on Cotton Fabrics by Cross-Linker

Enhanced synergistic stain removal is realized by tailoring the comonomer fractions of a light- and thermo-dual responsive copolymer, which is immobilized on cotton fabrics by a cross-linker. The copolymer poly(acrylamide azobenzene-co-ethylene glycol methacrylate-co-triethylene glycol methyl ether methacrylate), denoted P(AAAB₁-co-EGMA₂-co-MEO₃MA₁₇), is prepared by the ATRP polymerization method. The present molar ratio for these monomers is 1:2:17. Because of the existence of the light-responsive AAAB unit, the transition temperature of its aqueous solution under UV radiation is shifted to 39 °C, which is 2 °C higher than that in ambient conditions. This increase is caused by the trans-cis isomerization from the azobenzene groups, indicating an increased hydrophilicity of P(AAAB₁-co-EGMA₂-co-MEO₃MA₁₇) under UV radiation. After being immobilized onto cotton fabrics by a cross-linker, they are also dual-responsive. The equilibrium swelling ratio (ESR) of the cotton fabrics is further increased after UV radiation. Compared to our former investigation, the reduction of the AAAB molar fraction from 0.1 to 0.05 causes an increase of the ESR value by 10%. Moreover, the stain removal efficiency of the cotton fabrics immobilized with P(AAAB₁-co-EGMA₂-co-MEO₃MA₁₇) by cross-linker is also significantly improved under UV radiation. The hydrophilicity of the copolymer mainly from the MEO₃MA units is crucial to the cleaning capability. Additionally lowering the attachment between stain and the copolymer coating on the cotton fabrics by trans-cis isomerization in those AAAB units also favors the cleaning. Hence, the stain removal is strongly improved by optimizing the fraction of light- versus thermo-responsive components in the copolymer, which can profoundly reduce the consumption of chemical detergents and energy during laundry.

Heteroditopic Macrobicyclic Molecular Vessels for Single Step Aerial Oxidative Transformation of Primary Alcohol Appended Cross Azobenzenes

A series of oxy-ether tris-amino heteroditopic macrobicycles (L1-L4) with various cavity dimensions have been synthesized and explored for their Cu(II) catalyzed selective single step aerial oxidative cross-coupling of primary alcohol based anilines with several aromatic amines toward the formation of primary alcohol appended cross azobenzenes (POCABs). The beauty of this transformation is that the easily oxidizable benzyl/primary alcohol group remains unhampered during the course of this oxidation due to the protective oxy-ether pocket of this series of macrobicyclic vessels. Various dimensionalities of the molecular vessels have shown specific size complementary selection for substrates toward efficient syntheses of regioselective POCAB products. To establish the requirement of the three-dimensional cavity based additives, a particular catalytic reaction has been examined in the presence of macrobicycles (L2 and L3) versus macrocycles (MC1 and MC2) and tripodal acyclic (AC1 and AC2) analogous components, respectively. Subsequently, L1-L4 have been extensively utilized toward the syntheses of as many as 44 POCABs and are characterized by different spectroscopic techniques and single crystal X-ray diffraction studies.

Abnormal fast dehydration and rehydration of light- and thermo-dual-responsive copolymer films triggered by UV radiation

Abnormal fast dehydration and rehydration of light- and thermo-dual-responsive copolymer films of poly(oligo(ethylene glycol) methyl ether methacrylate-co-6-(4-phenylazophenoxy)hexyl acrylate), abbreviated as P(OEGMA300-co-PAHA), are triggered by UV radiation. Both rapid kinetic processes are probed by in situ neutron reflectivity (NR). The transition temperatures (TTs) of P(OEGMA300-co-PAHA) are 53.0 (ambient conditions) and 52.5 °C (UV radiation, λ = 365 nm). Thin P(OEGMA300-co-PAHA) films show a random distribution of OEGMA300 and PAHA segments. They swell in a D2O vapor atmosphere at 23 °C (below TT) to a swelling ratio d/das-prep of 1.61 ± 0.01 and exhibit a D2O volume fraction φ(D2O) of 39.3 ± 0.5%. After being exposed to UV radiation for only 60 s, d/das-prep and φ(D2O) significantly decrease to 1.00 ± 0.01 and 13.4 ± 0.5%, respectively. Although the UV-induced trans-cis isomerization of the azobenzene in PAHA induces increased hydrophilicity, the configuration change causes a breaking of the intermolecular hydrogen bonds between OEGMA300 and D2O molecules and unexpected film shrinkage. As compared to thermal stimulus-induced dehydration, the present dehydration rate is 100 times faster. Removal of the UV radiation causes immediate rehydration. After 200 s, d/das-prep and φ(D2O) recover to their hydrated states, which is also 30 times faster than the initial hydration. At 60 °C (above TT), thin P(OEGMA300-co-PAHA) films switch to their collapsed state and are insensitive to UV radiation. Thus, the UV-induced fast dehydration and rehydration depend on the existence of hydrogen bonds.

Light-Associated Surface Wrinkling-Based Metrology for the Photosoftening Characterization in Azobenzene-Polymer Supramolecular Complexes

As an intriguing characteristic of azobenzene-containing materials (azo-materials), photoinduced changes in mechanical properties (e.g., photosoftening) have stimulated many efforts both theoretically and experimentally. Here a simple yet powerful tool (i.e., a light-associated surface wrinkling-based method) to study the photosoftening effect in azobenzene-polymer (azo-polymer) supramolecular complexes is reported. The photo-induced modulus decrease of supramolecular complex films is deduced by analyzing the change of critical wrinkle wavelength of strain-induced surface wrinkling, in the case of varying experiment parameters. In particular, thanks to the facile modular tunability of the supramolecular system, the photosoftening effect has been systematically investigated as a function of azo-moiety content and the molecular weight of the host polymer. Notably, a photosoftening coefficient that is related to the chemical composition/structure of azo-polymers is introduced, and a simple formula that can quantify the connection of the photosoftening with external irradiation conditions and internal chemical factors of azo-polymers is derived for the first time. The obtained results are of great importance not only to enhance understanding of the photosoftening mechanism, but also to thoroughly apply it in diverse smart fields.

Laser-induced formation of "craters" and "hills" in azobenzene-containing polymethacrylate films

Functional organic polymer materials with an ability to change their surface topography in response to external contactless stimuli, like light irradiation, have attracted considerable attention. This work is devoted to the study of contactless control of the surface topography and the formation of the surface features in the amorphousized and liquid crystalline films of two azobenzene-containing polymers. The investigated polymers are side-chain polymethacrylates containing azobenzene chromophores with two lateral methyl substituents in ortho-positions and differing in the length of flexible spacer with six and ten methylene units. Two lateral methyl substituents at the azobenzene chromophore ensure high photoresponses of these polymeric samples in the whole visible spectral range. Irradiation of the polymethacrylate films by focused polarized light of green (532 nm) and red (633 nm) lasers induces a specific photodeformation of the film surface. In the case of the green light formation of circular "craters" with anisotropic borders was found, whereas for the red light highly asymmetric "hills" were observed. The possible mechanisms of the surface topography formation and their features are discussed.

Light‐Boosting Highly Sensitive Pressure Sensors Based on Bioinspired Multiscale Surface Structures

Pressure sensors have attracted tremendous attention because of their potential applications in the fields of health monitoring, human–machine interfaces, artificial intelligence, and so on. Improving pressure‐sensing performances, especially the sensitivity and the detection limit, is of great importance to expand the related applications, however it is still an enormous challenge so far. Herein, highly sensitive piezoresistive pressure sensors are reported with novel light‐boosting sensing performances. Rose petal–templated positive multiscale millimeter/micro/nanostructures combined with surface wrinkling nanopatterns endow the assembled pressure sensors with outstanding pressure sensing performance, e.g. an ultrahigh sensitivity (70 KPa−1, <0.5 KPa), an ultralow detection limit (0.88 Pa), a wide pressure detect ion range (from 0.88 Pa to 32 KPa), and a fast response time (30 ms). Remarkably, simple light illumination further enhances the sensitivity to 120 KPa−1 (<0.5 KPa) and lowers the detection limit to 0.41 Pa. Furthermore, the flexible light illumination offers unprecedented capabilities to spatiotemporally control any target in multiplexed pressure sensors for optically enhanced/tailorable sensing performances. This light‐control strategy coupled with the introduction of bioinspired multiscale structures is expected to help design next generation advanced wearable electronic devices for unprecedented smart applications.

Modeling of Stripe Patterns in Photosensitive Azopolymers

Placed at interfaces, azobenzene-containing materials show extraordinary phenomena when subjected to external light sources. Here we model the surface changes induced by one-dimensional Gaussian light fields in thin azopolymer films. Such fields can be produced in a quickly moving film irradiated with a strongly focused laser beam or illuminating the sample through a cylindrical lens. To explain the appearance of stripe patterns, we first calculate the unbalanced mechanical stresses induced by one-dimensional Gaussian fields in the interior of the film. In accordance with our orientation approach, the light-induced stress originates from the reorientation of azobenzenes that causes orientation of rigid backbone segments along the light polarization. The resulting volume forces have different signs and amplitude for light polarization directed perpendicular and parallel to the moving direction. Accordingly, the grooves are produced by the stretching forces and elongated protrusions by the compressive forces. Implementation into a viscoplastic model in a finite element software predicts a considerably weaker effect for the light polarized along the moving direction, in accordance with the experimental observations. The maximum value in the distribution of light-induced stresses becomes in this case very close to the yield stress which results in smaller surface deformations of the glassy azopolymer.

Path-Guided Hierarchical Surface Relief Gratings on Azo-Films Induced by Polarized Light Illumination through Surface-Wrinkling Phase Mask

Surface relief gratings (SRGs) with hierarchical microstructures are highly needed owing to their diverse applications in various fields. Here, we introduce surface-wrinkling templates as novel nonlithographic phase masks to direct the generation of hierarchical well-prescribed SRGs on nonconformally contacted azo-films by a simple single-beam illumination. The light-induced SRGs have controlled microstructures including single/double/triple wavelengths and single/double orientations as well as their organizations. These microstructures can be well tailored by the wavelength of the surface-wrinkling phase masks and the polarization direction of incident light relative to the wrinkling patterns in the phase masks. Interestingly, we find that the larger wavelength is induced prior to the smaller ones, offering another new strategy to tailor the microstructures of SRGs through simple manipulation of the illumination duration. In particular, path-guided SRGs with unprecedented well-organized hierarchical microstructures have been available in the case of controlled moving of the light illumination through the surface-wrinkling phase mask. As demonstrated, the obtained hierarchical SRGs with the capability of multiple optical inscription/erasure have great application potentials in fields such as confidential information (or pattern) records and encryption/decryption.