Smart Patterned Surface with Dynamic Wrinkles - Accounts of

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Smart Patterned Surface with Dynamic Wrinkles Honghao Hou, Jie Yin, and Xuesong Jiang*

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School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China CONSPECTUS: Patterned surfaces are fundamentally important to physics, chemistry, materials, and biology science, endowing significant functions and thus bearing broad and fantastic applications whether in natural or man-made events. Among the various methods for patterning surfaces, wrinkling or buckling offers a powerful alternative to prepare surface patterns because of its spontaneous nature, versatility, easy preparation in large-scale, and capability to be responsive to various stimuli. In particular, patterned surfaces with dynamic wrinkles can tailor the encoded surface properties on demand and can provide a promising alternative for smart surfaces, which has potential for wide applications in enhanced and tunable optical or photoelectric devices, responsive microstructures, switchable wettability, smart adhesion and friction, and so on. The concept of smart patterned surfaces based on dynamic wrinkles is fundamental and versatile, and it is expected that there will be extensive future work based on this concept in generalizing this work to other smart materials and systems, and in using dynamic pattern systems to tune not only morphology but also functional properties encoded in the system’s topography. In this Account, we present recent progress on smart surfaces with dynamic wrinkle patterns, including their design, preparation, and potential applications. First, we provide a brief introduction of a basic concept for mechanical instability induced wrinkle patterns and outline the general strategies and mechanics for dynamic wrinkles. Then, we discuss how the wrinkling and dewrinkling processes of a rigid skin bound to a soft substrate in bilayers or gradient layer systems occur by controlling the mechanical properties and geometric characteristics of the top and bulk layers, thereby paving the way for a smart patterned surface using chemical and physical approaches. Next, we discuss various chemical and physical stimuli, including light, temperature, pH, and chemicals, which can be harnessed into an extensive library of complex dynamic wrinkles. We highlight recent advances in preparing multiresponsive dynamic wrinkling patterns by adjusting the intrinsic properties of the skin layers (i.e., Young’s modulus and cross-linking density) via dynamic chemistry, such as the Diels−Alder reaction, photodimerization, and supramolecular chemistry. Then, we outline how functional inclusions, such as photothermic or photoelectric additives and magnetic nanoparticles, can enable the composite elastic substrate as a dynamic platform for various functional top-layers to fabricate a smart surface for a desired function. In particular, photothermally reconfigurable wrinkle systems were investigated, where the carbon nanotube (CNT) served to efficiently convert its absorbed light energy into heat, hence actuating a real-time response of a near-infrared light (NIR)-sensitive wrinkle pattern and providing access to the development of advanced optoelectronic devices. In addition, based on their unique characteristics, applications of dynamic wrinkle patterns for smart displays, memory, flexible electronics, dynamic gratings, tunable adhesion, friction, and wettability are presented. Finally, we conclude by offering our perspective on future developments of this rapidly evolving field.

1. INTRODUCTION

responsive materials have been adopted to intelligently control the macroscopic properties of these smart surfaces. However, the development of dynamic patterns remains a significant challenge. Among various approaches to fabricate surface patterns with different morphologies, the wrinkling of thin films is a powerful alternative to generate dynamic patterns due to spontaneous formation, versatility, easy preparation in largearea, and sensitivity in response to various stimuli.1,14−19 In this Accounts, we review the involved materials and principle in the focused field of smart patterned surfaces with dynamic

Surface patterns have gained extensive interest in both fundamental science and industrial applications since micro/ nanoscale patterned structures with spatially periodical or aperiodic topographies endow surfaces with distinctive acoustic, electric, photic, mechanical, and biotic properties.1−6 In particular, dynamic surface patterns with tunable morphologies can enable the possibility for on-demand control of the encoded surface functions and properties, providing a compelling method for realizing smart surfaces for enhanced and tunable optical devices,7,8 smart electronic devices,9,10 responsive microfluidic channels,11 switchable wettability,12 and tunable adhesion.12,13 To this end, various surface patterning technologies have been harnessed, and the involved © XXXX American Chemical Society

Received: December 7, 2018

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DOI: 10.1021/acs.accounts.8b00623 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Schematic representation of the dynamic wrinkle (a) through dynamic chemistry strategy to tune the modulus and cross-linking density of the skin layer and (b) through the physical strategy to in situ regulate the compress stress. (c) Dynamic chemistry can be adopted to prepare the dynamic cross-linked polymer network for reversible wrinkles. (d) Some typical stimuli can be applied for dynamic wrinkle patterns.

challenge but also opens up a number of interesting applications. On the one hand, if the parameters ε, hf, and E̅ s are set to be constant for a given system, E̅ f is the only dominant factor for wrinkle formation. The modulus of polymeric materials is usually subjected to the cross-linking density; this makes it possible to regulate the generation and erasure of the wrinkle pattern by reversibly tuning the modulus and cross-linking density of the skin layers through a dynamic cross-linked polymer network (Figure 1a). As illustrated in Figure 1c, diverse and versatile dynamic chemistry, such as a thermalreversible Diels−Alder (D-A) reaction, photoreversible cycloaddition, light-reversible photoisomerization, a pH-sensitive aldehyde-amine covalent bond, a glucose-sensitive boronic ester bond, and multiresponsive supramolecular chemistry, can be adopted to the dynamical cross-linking polymer network and related systems to achieve a rapid, reversible, and robust change of the wrinkle pattern. Various factors, such as temperature, light, pH, and supramolecular interaction, can be considered as external stimuli for the dynamic chemistry; therefore, dynamic cross-linking polymer networks that respond to these individual stimuli or multistimuli, that is, control for the dynamic wrinkle patterns, are of great significance but are underexplored. Another key point is to control the applied strain ε. Although a great deal has been achieved, an ex situ tunable wrinkled surface with well-performing properties can only be obtained via an artificially physical intervention, such as the wetting/dewetting of the solvent and the release/stretch of prestrain. These typical methods, which take advantage of the externally imposed stimulation, have been restricted to discrete, special-purpose manual operation or even need to alter the material’s fundamental properties; hence, it placed obstacles and challenges for the real-time, in situ dynamic control of the pattern. As shown in Figure 1b, a facile and feasible strategy was presented to prepare smart patterned surfaces with dynamic (i.e., photothermally reconfigurable) wrinkles via regulating the thermal expansion difference and consequent applied strain of a bilayer system using a photothermic/photoelectric or ferromagnetic nanoparticle containing a composite elastic substrate. One or several kinds of additives (i.e., photothermally responsive CNT) can be introduced and the responsive composite soft substrate can

wrinkles and highlight recent advances via unique chemical and physical strategies.

2. BASIC CONCEPT AND GENERAL STRATEGIES FOR DYNAMIC WRINKLE PATTERN As a kind of surface mechanical instability, wrinkling is ubiquitous and has been investigated for a long time.1,2,16,20−22 This behavior can be harnessed to fabricate surface microstructures with various geometries and dimensions, and has been widely applied in biology,23,24 metrology,25 optics,7,26 and electronics.15,27−29 Wrinkles occur in layered systems to minimize the total energy of the system by releasing localized stress between layers21,30 when the compressive strain (ε), originating from gradients31−35 or mismatches36,37 in stress or physical properties between the rigid skin layer and the soft substrate, exceeds the critical threshold (εc). The characteristic amplitude (A) of the wrinkle pattern can be predicted by the typical linear buckling theory as ln(A2 + hf 2) =

y i 2 2 ln Ef̅ + jjj2ln hf + ln4ε − ln3Es̅ zzz 3 3 { k

(1)

where the subscripts f and s represent the skin layer film and substrate, respectively, hf is the thickness of the skin layer, and E̅ f and E̅ s refer to the plane strain moduli of the skin layer film and substrate, respectively. The thermoinduced compressive stress is given by ε = (∂ s − ∂f )ΔT

(2)

Here ∂s and ∂f refer to the thermal expansion coefficients of the skin layer film and substrate, respectively, and ΔT is the temperature variation of the system. According to eqs 1 and 2, if hf and E̅ s are constant values in a given wrinkle system, E̅ f and ε can be regarded as the two determining parameters for the generation of wrinkle pattern, which provides unique opportunities to explore the possibility for dynamical control the wrinkle pattern. Obviously, for the dynamic wrinkle, it is crucial to harness the precise and reversible regulation of the system-matched stress through desirable and reasonable approaches. Controlling these surface mechanical instabilities to achieve a dynamic wrinkle pattern not only represents a significant experimental and theoretical B


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Figure 2. (a) Strategy for a dynamic Diels−Alder reaction induced reversible wrinkle pattern, and (b) AFM images showing the cyclic evolution of the dynamic wrinkle on the surface of PFA/BMI-coated PDMS sheet heated at different temperatures. (c, d) AFM images and corresponding amplitude and surface Young’s modulus demonstrating the generation of the wrinkle pattern, which is dependent on the D-A reaction time. (e) 3D AFM images and corresponding line analysis showing the self-healing of damaged wrinkle pattern based on the dynamic D-A reaction. (f) Highly reversible evolution of the wrinkle morphology with the sequential, cyclic D-A reaction. Reproduced with permission from ref 37. Copyright 2016 Wiley-VCH.

the surface modulus can be tuned by reversible cross-linking37 (Figure 2a). With the D-A reaction at 70 °C for 4 h, the soft skin layer gradually became cross-linked and rigid with a higher modulus, leading to a considerable mismatch in the modulus between the stiff cross-linked skin layer and the soft PDMS substrate. The mismatch in modulus and thermal expansion ration between the top layer and substrate can cause the compressive stress ε. Consequently, a wrinkle pattern can be easily obtained after cooling to room temperature, as a result of releasing the localized compressive stress. Furthermore, the wrinkle can be erased with the retro-D-A reaction a the higher temperature of 120 °C, which causes the de-cross-linking of the cross-linked skin layer, rendering it unable to accumulate compressive stress (Figure 2b). The Young’s modulus of the skin layer and the morphology of the wrinkle pattern evolved dependent on the D-A reaction time, showing that the generation of the wrinkle pattern is strictly governed by the DA reaction (Figure 2c,d). The damage and destruction of the patterned surface is fatal and irrecoverable for practical applications, while the nature of the dynamic wrinkle pattern offers the chance to repair and regenerate the damaged pattern in situ, allowing the self-healing of the wrinkled surface without altering the bulk properties37,40 (Figure 2e). The highly cyclic evolution of the pattern morphology and Young’s modulus on the as-prepared wrinkled surface with multiple cycles of alternate heating at different temperatures manifests the remarkable reversibility and good feasibility of the dynamic wrinkle pattern based on the reversible D-A reaction (Figure 2f). The switchable surface wrinkle morphology inherently endows the tunable adhesion, wettability, and friction properties.

be obtained. This approach can dynamically tune the thermal expansion and the resulted thermal-induced stress ε via the heat energy converted from the absorbed NIR light under NIR irradiation according to eq 2, thereby actuating a real-time response of a near-infrared light (NIR)-sensitive wrinkle pattern and providing access to the development of advanced optoelectronic devices. NIR light is a noninvasive and spatially resolved stimulation by a noncontact and on/off switchable approach, making it ideal to dynamically tune the wrinkle pattern with unique advantages of an ultrasensitive response, facile accessibility, and region-selectivity. The composite elastic substrate with functional inclusions, such as photothermic or photoelectric additives and magnetic nanoparticles, enables the dynamic wrinkle system to act as real-time platforms with various functional top-layers to fabricate a smart patterned surface for the desired function and demand, which is promisingly potential for smart surfaces and advanced optoelectronic devices.

3. CHEMICAL REACTION DRIVEN DYNAMIC WRINKLE A recent boom in the research of dynamic chemistry and its reversible nature offers the possibility of preparing dynamic wrinkle patterns based on dynamic cross-linking polymer networks.38,39 In this context, the chemical strategy, via tunably tailoring the modulus and cross-linking density of the skin layer based on the dynamic chemistry, enables a general and robust approach to fabricate smart, reversible, and multifunctional patterned surfaces due to the versatile and multiresponsive dynamic bonding. 3.1. Thermally Reversible Wrinkle Based on D-A Dynamic Reaction

3.2. Light Reversible Wrinkle Based on Dynamic Photoreaction

The thermal-responsive D-A dynamic reaction was first chosen by our group to demonstrate the ability for reversible wrinkle patterns owing to its excellent reversibility and facile operability. A soft film composed of a furan-grafted polymer (PFA) and cross-linker bismaleimide (BMI) acts as the skin layer on the poly(dimethylsiloxane) (PDMS) elastomer, where

Photocontrol offers the inherent advantages of high sensitivity, good feasibility, and unique opportunity to spatiotemporally tune the surface pattern with site-special and hierarchical features. In this design, we envisaged the possibility of C


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Figure 3. (a) Strategy and (b−e) photographs of the light-responsive hierarchical wrinkle pattern induced by the dynamic photodimerization of PAN undergoing reversible generation and erasure using selective exposure with different wavelengths or heating. Reproduced with permission from ref 40. Copyright 2017 Royal Society of Chemistry. (f) Strategy and involved chemical structures and (g) AFM and optical images of the dualpattern with reversible fluorescence and wrinkles prepared by PAN-NDI-BA/PDMS bilayers. Reproduced with permission from ref 42. Copyright 2018 American Chemical Society. (h) Schematic illustration and involved chemical structures for the light reversible micropattern based on the photoisomerization of P(F-AZO8-MMA). (i−k) Reversible surface morphology of micropatterned P(F-AZO8-MMA) films with cyclic, alternant irradiation under 365 nm UV light, and sequential 450 nm visible light. Reproduced with permission from ref 45. Copyright 2016 Wiley-VCH.

promising for the dual or multiplex control of surface morphology (Figure 3g). Another star photoresponsive molecule is azobenzene (AZO), which can take place with trans-to-cis photoisomerization along with an obvious change of molecular volume. Seki et al.43 and Lu et al.44 demonstrated light-erasable wrinkles where the trans-to-cis photoisomerization of AZO moieties in the AZO-containing polymer skin layer could disturb the compressive stress filed upon light irradiation. Our group45 introduced the fluorinated AZO-containing polymer P(F-AZO8-MMA) into the UV-curing self-assembled system and obtained a photoreversible micropattern through a biomimetic bottom-up strategy, in which the interfacial selfassembly of P(F-AZO8-MMA) and the photoisomerization induced mismatch of the volume change between gradient layers contribute to the growth and elimination of a reversible micropattern (Figure 3h). A cyclic change of the surface morphology of the film can be achieved reversibly from a wrinkled to a smooth state under sequential 365 nm UV and 450 nm visible light exposure (Figure 3i−k). The underlying mechanism and growth kinetics of the pattern were further studied, indicating that the photoreversible isomerization of

achieving a change in the pattern topography and corresponding functions in response to light. For this purpose, different categories of photosensitive molecules are adapted into the polymeric chains to develop dynamic cross-linking polymer networks and wrinkle patterns capable of responding to light. Recently, our group has adopted the anthracene (AN)containing polymer (PAN) as the photoresponsive skin layer onto the soft PDMS substrate to fabricate a series of dynamic wrinkles with controllable size features and functions based on the reversible photodimerization of AN40,41 (Figure 3a). In addition, compatible with photolithography technology, various hierarchical patterns with different topographies and dimension features were created and sequentially erased by means of alternative UV exposure at different wavelengths after a thermal stimulus (Figure 3b−e). Additionally, a reversible dual-pattern with tunable fluorescence and wrinkle topography was demonstrated42 (Figure 3f), in which the cross-linking density of the system and charge-transfer (CT) interaction between an AN and naphthalene diimide (NDI) containing polymer (PAN-NDI-BA) in the skin layer can be simultaneously controlled by the reversible photodimerization of AN, D


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Figure 4. (a) Schematic illustration and the involved chemical structure of multiple-responsive wrinkles based on a P4VP-PS-PnBA/AN-COOH/ PDMS bilayer in response to UV light, acid and base gas. (b, c) Characteristic size dependent on the exposed time and (d−f) cyclic morphology evolution traced by AFM and optical microscope of wrinkle pattern during the generation/erasure process by UV light, ammonia and hydrogen chloride gas. Reproduced with permission from ref 41. Copyright 2017 American Chemical Society.

Figure 5. (a) Schematic illustration and (b) AFM images indicating the reversibility of the NIR-driven wrinkles. (c) Temperature variation of CNT-PDMS for a switchable NIR on/off and (d) highly reversible cycles of the NIR-driven dynamic wrinkle pattern. (e, f) Optical microscope images show the dynamic extinction/formation evolution processes of various hierarchical/wrinkle patterns via the NIR switchable on/off cycle. Reproduced with permission from ref 50. Copyright 2018 American Association for the Advancement of Science.

chloride gases41 (Figure 4a−c). By adjusting the reversible photodimerization, the feature size of the wrinkle pattern can be tuned to reach the requirements of reversibly switchable sensors to dynamically monitor environmental gases, such as trace acidic/basic noxious gases. The generation and erasure of wrinkle pattern can be reversibly achieved through controlling the photodimerization of AN and hydrogen bonding spatially and dynamically (Figure 4d−f). Based on this general strategy, miscellaneous supramolecular chemistries, such as ionic interaction and CT interaction, are expected to be adapted to obtain smart surfaces responsive to multiplex and multispecies stimuli, enabling the dynamic wrinkle pattern in situ and on-demand controlled by means of multi optional approaches.

AZO is responsible for the reversible surface pattern at the microscale, which is ascribed to the dynamic generation and release of compressive stress generated from the mismatch of the volume change between the gradient layers. 3.3. Multiple Responsive Wrinkle Based on Supramolecular Chemistry

A major challenge with the growing trend is to design a stimuli-responsive wrinkle pattern system integrated with multiple-responsive covalently or noncovalently bonded components that can response to diverse stimuli. One of the most interesting examples is a multiresponsive wrinkle pattern based on the supramolecular polymer network, which is comprised of a copolymer containing pyridine moieties (P4VP-PS-PnBA) and an anthracene-based carboxyl acid (AN-COOH). This network can be formed and tuned via dynamic cross-linking by the reversible photodimerization of AN and the hydrogen bond between AN-COOH and pyridine segments, consequently rendering a high sensitivity to multiplex stimuli, such as light, ammonia and hydrogen

4. PHYSICALLY DRIVEN DYNAMIC WRINKLE As physically mechanical instability-induced surface patterns, wrinkles are easily and deservedly produced and controlled by physical approaches. As noted earlier, by precisely regulating E


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Accounts of Chemical Research the applied stress ε, a variety of wrinkle patterns can be reversibly controlled by means of some externally imposed stimuli. For example, the groups of Sun,46 Zhao,23,47 and Yang26 developed a series of multifunctional wrinkle pattern systems tunable by the release and stretch of prestrain. Hayward and Crosby’s group48 reported a type of osmotically driven wrinkle pattern, where the wrinkle formed and then disappeared upon the wetting and evaporation of solvents. Sun and co-workers demonstrated moisture-sensitive wrinkles by designing moisture-dependent gradient changes of the crosslinking degree, modulus, and thus swelling degree in the poly(vinyl alcohol) top layer.49 In addition, the incorporation of photochromic molecules or magnetic nanoparticles provides an opportunity to develop a kind of reversible wrinkle pattern capable of stimuli-responsiveness to magnetic17 or electric fields.18 Most of the previous studies chiefly concerned the complicated fabrication of stimulus-responsive and functional skin layers to achieve a dynamic wrinkle pattern. In this case, the surface morphology is usually governed by the physically or chemically essential attribute of the skin layers; therefore, it is extremely difficult to independently regulate the surface morphology and material constitution from each other. Additionally, although the advantages of ex situ imposed stimulation can be taken for the tunable wrinkled surface, these methods involved in a manually physical intervention are constrained to discrete, special-purpose artificial behaviors or even requiring surface modification. To overcome these dilemmas, by real-time controlling the applied strain ε, a NIR-responsive in situ dynamic wrinkle pattern system was demonstrated,50 in which a small amount of photothermally single-walled CNT was incorporated into the soft PDMS for a responsive composite substrate to NIR light (Figure 5a). Consequently, according to eq 2, the thermal expansion of the CNT-PDMS composite substrate can be easily tuned by the heat converted from the absorbed light energy via NIR irradiation, resulting in the dynamic adjusting the thermal-induced stress ε to obtain real-time reversible wrinkle patterns (Figure 5b, c). The erasure/regeneration of the wrinkle pattern can be in situ and reversibly controlled by cyclic NIR on/off switching (Figure 5d, e). Furthermore, a series of NIR-driven dynamic hierarchical patterns can be produced using a light-sensitive PAN skin layer (Figure 5f). The excellent feasibility and high reversibility of this system enables the NIR-responsive dynamic wrinkles as a dynamic platform for various functional skin layers with desired properties. This strategy of dynamic wrinkle via introducing various photothermic and electromagnetic materials into substrates can be extended to a general method to achieve real-time wrinkle pattern systems based on various responsive composite substrates as platforms to in situ tune surface properties as needed.

5.1. Smart Window and Dynamic Display

Because of their unique optical characteristics, the wrinkled pattern can be used as a basic unit to fabricate display materials or devices. For instance, the groups of Cho7 and Yang26,55,56 developed a series of mechanically induced wrinkles, which are promising for smart windows with angle-independent structural color and reversible transparency switches through the dynamically precise control of the applied prestrain. However, in situ dynamic control of the wrinkle pattern with switchable transparency is still challenging but can enable this application to be more operable and facile. The thermal-responsive wrinkle can endow a reversible optical transparency switchable from a wrinkled to a plain state by facile temperature control37 (Figure 6a). The as-prepared wrinkled surface was opaque

Figure 6. (a) Tunable optical transmittance of the wrinkle pattern surface. Reproduced with permission from ref 37. Copyright 2016 Wiley-VCH. (b) Cyclic writing and erasing of the letters “SJ” through heating, UV light, or HCl gas. Reproduced with permission from ref 41. Copyright 2017 American Chemical Society. (c) Reversible “stripe”, “annulus”, and “SJTU” dual-patterns with fluorescence and wrinkle prepared with PAN-NDI-BA/PDMS bilayer using different photomasks. Reproduced with permission from ref 42. Copyright 2018 American Chemical Society. (d) Optical and SEM images showing the erasable Braille typography for the Braille characters “SJTU”. Reproduced with permission from ref 40. Copyright 2017 Royal Society of Chemistry.

because of the intense light scattering from the microstructures, and thus the overlapped “SJTU” letters could not be observed. Once the wrinkles were erased, the surface recovered to the originally transparent state and exhibited a legible transmitted image. Information such as letters, characters, special patterned stripes, and annulus could be selectively or fully written, erased, and even rewritten by different stimuli41 (Figure 6b, c). Here, the generation of the wrinkle pattern by selective exposure under 365 nm UV light to can be regarded as the process of “writing/storing” the information, and the subsequent 254 nm UV light blanket illumination or heating at 150 °C represents the editing or erasing program for various information. Moreover, the erased

5. APPLICATIONS The capabilities of reversibly altering the surface properties encoded in the system’s reconfigurable topography are fascinating for wide applications. Aside from tunable physical properties, such as wettability,7,51,52 adhesion,48,53 and friction,45 the dynamic wrinkled surface is also very useful in bioapplications,54 for instance, a microcarrier platform for facilitating cell growth and transfer. Herein, we highlight some original and promising applications to demonstrate the distinctive features of these dynamic patterned surfaces with reversibly switchable wrinkles. F


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Figure 7. (a) Writing and erasure of the letter “CT” with dual information on the fluorescence and visual wrinkle morphology under UV light and natural light. Photographs illustrating (b) “flower” and (c) “barcode” applications with reversible fluorescence and wrinkles for the recording and erasure of dual information. Reproduced with permission from ref 42. Copyright 2018 American Chemical Society.

Figure 8. (a) Scheme for the NIR-driven wrinkle pattern serving as a dynamic light grating. (b) Photographs showing the evolution of the light diffraction through the wrinkle pattern under the NIR on/off switching process. (c) Scheme for the NIR-responsive electronic devices. (d, e) Optical microscope images showing the erasure/generation cycle of the gold-coated PAN/CNT-PDMS wrinkled sample and the electrical performance with the NIR on/off switch. Reproduced with permission from ref 50. Copyright 2018 American Association for the Advancement of Science.

from unexposed flat regions. Therefore, this hierarchical pattern composed of a set of programmable wrinkled or wrinkle-free micropillar arrays at a prescribed position can compose and compile a targeted Braille character “SJTU” (Figure 6d). The light reversible wrinkle pattern enables the capability of modifying and recycling the Braille characters through alternating selective exposure of UV light to control the wrinkle on the special-site micropillars. Consequently, based on this controllable process of dynamic wrinkle and selective exposure controlled by light, we can repeatedly write, erase, and reproduce Braille text, potentially for a novel method of recycled Braille typography.

information can be recovered and redisplayed based on the reversible wrinkle pattern. This method exhibits great potential for smart no-ink display and erasable information storage. Interestingly, the dual-pattern display with multiplex information combining fluorescence and topography can be dynamically tuned on-demand by various stimuli based on the exclusive characteristic of the reversible wrinkle pattern42 (Figure 6c), suggesting the potential for responsive surfaces in smart displays and memory. Moreover, thanks to the inherent advantages of the excellent compatibility and generalizability, a photodimerization-induced reversible hierarchical wrinkle was demonstrated to act as a recycled platform for erasable Braille typography.40 The wrinkle surface exhibited an inherently rough tactile sense owing to the presence of wrinkle microstructures; thus, the as-prepared wrinkled micropillars array prepared by the photolithography can be easily identified

5.2. Anticounterfeiting

The effective strategies for multiplexing and dynamic anticounterfeiting are undoubtedly urgent because of the remaining risk that individual or stereotypical anticounterfeitG


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Figure 9. (a) Strategy and microscope images of the 2D ordered pattern. (b) Proposed model and simulated stress distribution for the gradient mechanical field in a photo-cross-linked LAP film. (c−f) Laser scanning confocal microscope images showing various continuous and discontinuous 2D ordered direct-written patterns with different morphologies. (g−j) Optical images demonstrating the application for erasable and lithographyfree 2D ordered pattern direct writing. Reproduced with permission from ref 59. Copyright 2018 Wiley-VCH.

facile-operation, versatility and good reversibility of the NIRdriven approach for the dynamic control of surface morphology, the NIR-responsive dynamic wrinkle pattern is well appropriate to act as a platform for dynamically controlling its physical properties in optical and electronic devices. The effect of the morphology and feature size of the wrinkles and the laser beam on the light diffraction patterns were investigated.50 These results show that the light diffraction can be reversibly controlled by NIR light for many cycles and this is the first sample of NIR-responsive dynamic light grating (Figure 8a, b). Furthermore, the dynamic wrinkle can be harnessed to prepare light-controllable electronics. Figure 8c presents a NIR-driven dynamic electronic device realized by gold-deposited PAN/CNTPDMS wrinkled bilayer on two pieces of separated ITO substrates.50 The gold-coated PAN/CNT-PDMS wrinkled sample can realize an erasure/generation cycle under the NIR on/off switching process (Figure 8d). The electrical performance of the proof-of-concept device can be tuned via switching the conformal contact between the conductive wrinkled sample and ITO glass via controlling the morphology of the surface wrinkles through NIR irradiation. After cycled more than 100 times by altering the NIR on/off state, the sample still retained its capacities, suggesting potential in the field of light-operated electronics (Figure 8e).

ing can be easily reproduced or screened. The wrinkle patterns with similar fingerprint-like topography have been demonstrated experimentally to act as a higher level of anticounterfeiting by Park and Kwon’s group57 due to the nondeterministic, randomness, sensitivity, and unpredictability of wrinkle formation. In addition, a theoretical simulation by Yin and Boyce confirmed the sensitivity of wrinkle patterns in films to geometrical imperfections, further proving that generating identical topography via wrinkling is very difficult.58 More interestingly, the potential of higher-level anticounterfeiting was demonstrated by applying a reversible dual-pattern by combining two kinds of information from the fluorescence and dynamic wrinkle topography,42 which remarkably improves both the capacity and complexity of the encoded information. The letters “CT” and “flower” patterns with dynamic fluorescence and wrinkles were direct-written on a PDMS sheet using a PAN-NDI-BA solution as ink; these patterns could be reversibly erased by heating at 150 °C or by irradiation under UV light of 254 nm (Figure 7a, b). The prepared barcode with wrinkles on a PAN-NDI-BA/PDMS bilayer can encode information regarding their specific structural color and spectroscopic information and enable simple and real-time identification taking advantage of the unique double-channel properties with fluorescence and wrinkle patterns. Moreover, based on the highly reversible nature of the system, barcodes with dual pattern information can be dynamically written and erased, which can avert the divulging problem and undoubtedly further improve the security level (Figure 7c).

5.4. Light-Direct Writing Lithography-Free Pattern Through 2D-Ordered Wrinkle

Despite much attention, it is very challenging to obtain twodimensionally (2D) ordered wrinkle patterns because stress relaxation is difficult to control in bilayer systems of wrinkles. By generating a continuous gradient field of compressive stresses in a film through the depth-dependent photo-crosslinking of light-responsive anthracene-containing polymer (LAP), we engineered the 2D, controllable release of stress

5.3. Optical and Electronic Devices

A great concern for light control is to dynamically and conveniently tune light diffraction. The existence of surface micropatterns can lead to intense light diffraction, making the wrinkled surface capable of an optical grating. Considering the H



■

across the layers, making it possible to achieve 2D ordered surface patterns59 (Figure 9a). It is experimentally and theoretically revealed that the key to accomplish 2D globally ordered patterns in our system is the generation of compressive stresses in a photo-cross-linking gradient LAP film that contains a continuous gradient in its mechanical properties, enabling a controllable release of stress across the layers, rather than being limited to in-plane stress relaxation (Figure 9b). This convenient, operable and reliable methodology without solvent development allows us to directly write various complex patterns with high fidelity and continuous and even discontinuous topological order across a global scale, which is comparable with conventional photolithography (Figure 9c−f). Moreover, the 2D ordered patterns can be repeatedly erased, and rewritten by UV exposure with different wavelengths (Figure 9g−j), promising for erasable and rewritable information storage, and reconfigurable materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xuesong Jiang: 0000-0002-8976-8491 Notes

The authors declare no competing financial interest. Biographies Honghao Hou received his B.S. degree from Anhui University in 2012. He is now a Ph.D. candidate majoring in polymer chemistry and physics at Shanghai Jiao Tong University. In 2017−2018, he worked as a visiting research scholar at Prof. Thomas P. Russell’s group at the Lawrence Berkeley National Laboratory. His recent interests focus on soft materials and integrated applications for health, energy, and sustainability. Jie Yin, born in 1964 in Shanghai, China, obtained his B.S. degree at the Shanghai Jiao Tong University in 1989 and his Ph.D. from Lancaster University in the UK (1989−1992). He joined the School of Chemistry and Chemical Engineering in 1992 and has been a full professor from 1997 at the Shanghai Jiao Tong University. He was the Vice President of Shanghai Jiao Tong University and Vice director of Shanghai Education Commission. Now he is the vice president, academic dean, and cofounder of Shanghai Tech University. His research focus on the functional polymer materials and their applications.

6. SUMMARY AND OUTLOOK In this Account, we illustrated the main strategies and highlighted the important recent progress to achieve dynamic wrinkle patterns for smart surfaces by facilely tuning the surface mechanical instability governing the wrinkling behavior of the systems and the ability to switchable alter the topography and encoded surface properties in situ. Smart surfaces with dynamic wrinkles can serve as tunable functional platforms to pattern, align, and order various functional materials, promising for broad and miscellaneous applications of microfabrication, biology, metrology, optics, and electronics. The research on dynamic wrinkle patterns is burgeoning, and there remains considerable challenges and fascinating opportunity to be developed. The key feature of these systems is the ease with which they can adapt and respond to changes in their external environment. The actuation is usually subjected to the intersystem mass/energy transport of responsive inclusions. So how to improve the response speed and high reversibility is a crucial challenge for dynamical and rapid change of the morphology and properties. In addition, as a transdisciplinary topic, it is anticipated that recent advances from various areas, such as the approach and variety of stimuli to actuate the transformation process, will be adopted to a more complex wrinkle systems to improve multifunctionality and to fulfill the intricate demands for smart surfaces. Above all, the topography switching of a dynamic wrinkle pattern should be integrated with a combination of outstanding operability, various patterning technology and molecular-level designability; this integration will endow the patterned surface with extraordinary functions and new applications considering that the wrinkle pattern inherently processes a relatively low aspect ratio and usually requires a bilayer or multilayer system.23 For example, a three-dimensional nano-microhierarchical wrinkle pattern is expected to allow for more controllable approaches to access the reversible actuation and patterning of materials.2,60,61 In summary, further development of these concepts requires a further fundamental understanding of the physics and the mechanism of surface instability and patterning technologies, as well as the development of new functional or responsive materials that draw inspiration from both synthetic chemistry and biology, enabling us to fully harness the potential of dynamic microstructured surface systems.

Xuesong Jiang received his B.S. and Master degree from East China University of Science and Technology. He became a faculty member at Shanghai Jiao Tong University, where he obtained his Ph.D. in 2005. In 2009−2010, he worked as a research scholar postdoc supported by Humboldt Foundation at Georg-August-Universität Göttingen. Since 2014, he was promoted to be a full professor of polymer department at Shanghai Jiao Tong University. He is currently leading the group working on responsive soft materials and smart patterned surface.

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ACKNOWLEDGMENTS The authors thank the National Nature Science Foundation of China (51773144 and 21704062) and Shanghai Municipal Government (17JC1400700) for their financial support.

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