Article

Switching periodic membranes via pattern transformation and shape memory effect

Authors:
  • AMOLF & Eindhoven University of Technology
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Abstract

We exploited mechanical instability in shape memory polymer (SMP) membranes consisting of a hexagonal array of micron-sized circular holes and demonstrated dramatic color switching as a result of pattern transformation. When hot-pressed, the circular holes were deformed to an array of elliptical slits (with width of tens of nanometers), and further to a featureless surface with increasing applied strain, therefore, switching the membrane with diffraction color to a transparent film. The deformed pattern and the resulting color change can be fixed at room temperature, both of which could be recovered upon reheating. Using continuum mechanical analyses, we modeled the pattern transformation and recovery processes, including the deformation, the cooling step, and the complete recovery of the microstructure, which corroborated well with experimental observations. We find that the elastic energy is roughly two-orders of magnitude larger than the surface energy in our system, leading to autonomous recovery of the structural color upon reheating. Furthermore, we demonstrated two potential applications of the color switching in the SMP periodic membranes by (1) temporarily erasing a pre-fabricated “Penn” logo in the film via hot-pressing and (2) temporarily displaying a “Penn” logo by hot-pressing the film against a stamp. In both scenarios, the original color displays can be recovered.

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... This approach enables diverse applications including flexible electronics [7,[150][151][152][153], optical [154] and acoustic [62,155,156] switches, auxetic materials [14], surface pattern control [157], and soft robotics [9,158]. The instability-induced microstructure transformations discovered in the soft system with periodic voids [39,71] have led to the development of programmable mechanical metamaterials [48,49], switchable auxetic materials [19,45,46,159], color displayers [160], wave absorbers [6,20,68,69], and actuators [21]. To predict the onset of elastic instabilities and associated microstructure transformations in soft materials, the nonlinear elasticity framework of small perturbations superimposed on large deformations is employed [70]. ...
... Overvelde et al. [45] considered the effect of pore shape on the mechanical response; Shim et al. [19] systematically investigated the role of circular hole arrangement on the post-buckling behavior of the periodic porous structures. Remarkably, these reversible pattern transformations have been demonstrated to be instrumental to design tunable color displays [160,196], phononic [20,68,69] and photonic [154] switches. The design of the periodic elastomeric porous structures is based on various distribution of voids in a single phase matrix material. ...
... Within the framework, the onset of macroscopic or long wave instabilities can be predicted through the loss of ellipticity analysis based on the homogenized response of the material [13,[29][30][31]103,104]; while microscopic instabilities can be detected through the Bloch-Floquet analysis [11,12,25,37,68,107], which also allows detecting the long wave instabilities for a special limit. In this work, we employ the Bloch-Floquet technique to investigate the instability phenomenon and, then, post-buckling behavior in soft composite with various periodic distributions of voids and stiff inclusionsthese structures can exhibit cooperative and controllable collapse of the voids leading to sudden pattern transformations, which hold the potential for responsive and reconfigurable functional materials and devices, such as highly stretchable metamaterials [149], switchable auxetic materials [194], and elastic wave filters [6,14,20,68,69], color displayers [160], and actuators [21]. ...
... e recovery of the indented nanostructures to permanent smooth surface, thus, may not be unique for SMPs. Another widely reported manner for demonstrating SMEs at micro-/ nanoscale is to incorporate permanent micro-/nanostructures onto SMP substrates using molding replication as well as to deform them into either flat surface or other structures with different features [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Furthermore, free-standing SMP matrices with all dimensions at microscale are also of increasing interest [44][45][46][47][48]. ese freestanding particles or fibers may behave differently from indented and micro-/nanostructured SMP surfaces because the underlying bulk material may, to some extent, contribute to their shape-memory functionality. ...
... For the reported SMP-structured surfaces, 2D and 3D structures have a small AR (≤1) [32,33,[50][51][52][53][54][55], while 2.5D structures possess a relatively high AR (≥1) [29][30][31]56]. e main method to create permanent SMP-structured surface is to enable the replication of polymer or prepolymer surface from master molds ( Table 1). ...
... Permanent 2D and 3D structures with AR <1 are typically deformed into flat surfaces or other features via compression or imprinting for applications in the fields of [29,33,[50][51][52][53]. By compressing against a flat quartz substrate at 105°C for 5 min, 3D microprism arrays of crosslinked poly(ethylene-co-vinyl acetate) (cPEVA) were distorted during SMCP before the sample was cooled down to room temperature in the presence of the compression force [53]. ...
Article
Full-text available
Shape-memory polymers (SMPs) are one kind of smart polymers and can change their shapes in a predefined manner under stimuli. Shape-memory effect (SME) is not a unique ability for specific polymeric materials but results from the combination of a tailored shape-memory creation procedure (SMCP) and suitable molecular architecture that consists of netpoints and switching domains. In the last decade, the trend toward the exploration of SMPs to recover structures at micro-/nanoscale occurs with the development of SMPs. Here, the progress of the exploration in micro-/nanoscale structures, particles, and fibers of SMPs is reviewed. The preparation method, SMCP, characterization of SME, and applications of surface structures, free-standing particles, and fibers of SMPs at micro-/nanoscale are summarized.
... When excessive deformation is applied, they may eventually become unstable. Beyond the instability threshold, rapid and dramatic changes of the structural geometry occur, and a careful design of the initial architecture may lead to the formation of new and homogeneous periodic patterns [159][160][161][162]. Interestingly, it has been recently shown that such dramatic geometric rearrangements induced by instabilities can be exploited to rapidly tune the macroscopic response and functionalities of the structures [22,34,[163][164][165][166][167][168][169][170][171][172][173][174]. ...
... Therefore, they do not only result in the formation of complex patterns but can also be instrumental to design materials with new modes of functionality. Recently, instabilities in periodic structures have been exploited to design metamaterials with tunable negative Poisson's ratio [163,164] (see Fig. 10) and effective negative swelling ratio [165] (see Fig. 12(a)), reversible encapsulation systems [22] (see Fig. 11(d)), structures capable of switching between achiral and chiral configurations [166][167][168] (see Fig. 13(a)), soft actuators [169] (see Fig. 11(b)) and robots [170] (see Fig. 13(b)), materials with tunable optical properties [171,172] (see Fig. 13(c)), and as described later, metamaterials with tunable dynamic response [34,173,174]. ...
... (c) Buckling-induced pattern transformation in shape-memory polymer membranes comprising a hexagonal array of micron-sized circular holes results in dramatic color switching. Adapted from Ref. [171]. ...
Article
Instabilities in solids and structures are ubiquitous across all length and time scales, and engineering design principles have commonly aimed at preventing instability. However, over the past two decades, engineering mechanics has undergone a paradigm shift, away from avoiding instability and toward taking advantage thereof. At the core of all instabilities - both at the microstructural scale in materials and at the macroscopic, structural level - lies a nonconvex potential energy landscape which is responsible, e.g., for phase transitions and domain switching, localization, pattern formation, or structural buckling and snapping. Deliberately driving a system close to, into, and beyond the unstable regime has been exploited to create new materials systems with superior, interesting, or extreme physical properties. Here, we review the state-of-the-art in utilizing mechanical instabilities in solids and structures at the microstructural level in order to control macroscopic (meta)material performance. After a brief theoretical review, we discuss examples of utilizing material instabilities (from phase transitions and ferroelectric switching to extreme composites) as well as examples of exploiting structural instabilities in acoustic and mechanical metamaterials.
... Natural polymers, like alginate [93,94], chitosan [95][96][97] and collagen [98], have been used to fabricate porous scaffolds and applied in biomedical fields, for which they can recover to the initial shape with the stimuli of water. Synthetic polymers, like crosslinked poly(caprolactone) (PCL) [85,86,90,[99][100][101][102][103][104][105][106][107], poly(D, l-lactide) (PDLLA) [108][109][110][111], polyurethane (PU) [25,86,87,, poly(lacticco-glycolic acid) (PLGA) [138][139][140], epoxy polymers [141][142][143][144][145][146][147][148] and polyacrylate polymers [149][150][151], have been fabricated into micro/nano-patterns, fibers, porous scaffolds and microspheres. These polymers are crosslinked by chemical bonds or polymer chains possessing a high glass transition temperature (T g ), forming permanent network for SMPs. ...
... Many intelligent behaviors depend on the changes in the surface chemistry of astatic micro/nanostructure. Nevertheless, dynamic changed surface micro/nanostructures endow surfaces with some special performances, such as switchable adhesion [190,216], water directional spreading [217,218], water collection [219,220], and adjustable optics [148,189], which cannot be realized by only altering the surface chemistry, The application of SMPs makes the dynamically tunable micro/nanostructures possible in many fields. ...
... After cooling, patterned SMP surfaces can be obtained. Using different stamps or molds, micropatterns, including straight microgrooves [85,99,100,150,195,223], circular microgrooves [169,191], micropillar arrays [141][142][143][144]190,196,[224][225][226][227], microwells [148,168,170,191,193,194] and microwrinkles [145,147,151,228] have been fabricated on SMP surfaces. Nano-patterns such as straight nanogrooves [191,229], nanopillars [191], nanowells [191] and nanowrinkles [230] can also be fabricated using similar methods. ...
Article
Cell behaviors are influenced by the surrounding dynamic microenvironment where the extracellular matrix (ECM) composed of different micro/nano-structures plays a key role. Many micro/nano-structures have been developed to mimic the structure of ECM, but these structures are almost static, and fail to emulate the dynamicity and function of the ECM in vivo. Notably, certain micro/nano-structures based on shape-memory polymers (SMPs), including patterns, fibers, porous scaffolds and microspheres, have attracted increasing attention due to the unique spatiotemporal variations. The dynamical shifting of these biomimetic micro/nano-structures arising from the shape-memory effect endows the materials with unique functions, e.g., regulating cell behaviors and prompting tissue growth. Therefore, the dynamically tunable biomimetic micro/nano-structures based on SMPs can be an ideal platform to mimic dynamic changes in the ECM structure both in vitro and in vivo. This review summarizes the latest advances in the development of the methodologies for fabricating various SMP biomimetic micro/nano structures, spatiotemporal control of cell behaviors, and broad applications in biomedical engineering. Several crucial points for future research are presented.
... [14] This approach enables diverse applications including flexible electronics, [15][16][17][18][19] optical [20] and acoustic [21][22][23] switches, auxetic materials, [24] surface pattern control, [25] and soft robotics. [26,27] The instabilityinduced microstructure transformations discovered in the soft system with periodic voids [28,29] have led to the development of programmable mechanical metamaterials, [30,31] switchable auxetic materials, [32][33][34][35] color displayers, [36] wave absorbers, [37][38][39][40] ...
... [14] This approach enables diverse applications including flexible electronics, [15][16][17][18][19] optical [20] and acoustic [21][22][23] switches, auxetic materials, [24] surface pattern control, [25] and soft robotics. [26,27] The instabilityinduced microstructure transformations discovered in the soft system with periodic voids [28,29] have led to the development of programmable mechanical metamaterials, [30,31] switchable auxetic materials, [32][33][34][35] color displayers, [36] wave absorbers, [37][38][39][40] and the stiff circular inclusions were printed in the VeroWhite resin (shear modulus μ i = 600 MPa). We first examined the behavior of the soft composites with large spacing ratio and small periodicity aspect ratio, where the domain formations or long wave modes are predicted. ...
Article
Full-text available
Experimental observations of domain formations and pattern transitions in soft particulate composites under large deformations are reported herein. The system of stiff inclusions periodically distributed in a soft elastomeric matrix experiences dramatic microstructure changes upon the development of elastic instabilities. In the experiments, the formation of microstructures with antisymmetric domains and their geometrically tailored evolution into a variety of patterns of cooperative particle rearrangements are observed. Through experimental and numerical analyses, it is shown that these patterns can be tailored by tuning the initial microstructural periodicity and concentration of the inclusions. Thus, these fully determined new patterns can be achieved by fine tuning of the initial microstructure. Experimental observations of domain formations and pattern transitions in soft particulate composites under large deformations are reported. The formation of microstructures with antisymmetric domains and their geometrically tailored evolution into a variety of patterns of cooperative particle rearrangements are observed. It is demonstrated that these fully determined new patterns can be achieved by tuning of the initial microstructure.
... It is this reversible elastic instability that can trigger pattern changing. This pattern transformation can therefore be used to tailor material properties, such as tunable negative Poisson's ratio (switchable auxetics) [26,216,225,226], chiral patterns [74,152], phononic and photonic switches [224], and other reprogrammable colour displays [227,228]. It is worth mentioning that a reversible chiral symmetry-breaking mechanism, or rigid kagome folding lattice (Fig. 16C [82]), can be introduced to pattern transformation. ...
... Finally we introduce the comprehensive studies from Bertoldi's group [18,23,26,[80][81][82]135,216,217,220,226,228] in pattern transformation. Current results [229] demonstrate that a structure having holes of the same size cannot switch to another variant by using an external mechanical force. ...
Article
Mechanical metamaterials are man-made structures with counterintuitive mechanical properties that originate in the geometry of their unit cell instead of the properties of each component. The typical mechanical metamaterials are generally associated with the four elastic constants, the Young's modulus E, shear modulus G, bulk modulus K and Poisson's ratio υ the former three of which correspond to the stiffness, rigidity, and compressibility of a material from an engineering point of view. Here we review the important advancements in structural topology optimisation of the underlying design principles, coupled with experimental fabrication, thereby to obtain various counterintuitive mechanical properties. Further, a clear classification of mechanical metamaterials have been established based on the fundamental material mechanics. Consequently, mechanical metamaterials can be divide into strong-lightweight (E/ρ), pattern transformation with tunable stiffness, negative compressibility (−4G/3 < K < 0), Pentamode metamaterials (G ≪ K) and auxetic metamaterials (G ≫ K), simultaneously using topology optimisation to share various fancy but feasible mechanical properties, ultralight, ultra-stiffness, well-controllable stiffness, vanishing shear modulus, negative compressibility and negative Poisson's ratio. We provide here a broad overview of significant potential mechanical metamaterials together with the upcoming challenges in the intriguing and promising research field.
... Such instabilities lead to a sudden transformation of the circular holes into mutually orthogonal ellipses. This reversible morphological change has been harnessed to realize structures with unusual mechanical properties, such as negative Poisson's ratio [8] and negative swelling ratio [9] , which can function as phononic and photonic switches [10][11][12][13][14] , color displays [14] , and soft robots capable of grasping and walking [15] . ...
... Such instabilities lead to a sudden transformation of the circular holes into mutually orthogonal ellipses. This reversible morphological change has been harnessed to realize structures with unusual mechanical properties, such as negative Poisson's ratio [8] and negative swelling ratio [9] , which can function as phononic and photonic switches [10][11][12][13][14] , color displays [14] , and soft robots capable of grasping and walking [15] . ...
Article
Full-text available
Porous materials with well-defined periodicity are commonly encountered in biological and synthetic structures and exhibit a wide range of behaviors, ranging from negative Poisson’s ratios, to high energy absorption and acoustic damping. Recently, the response of these systems has been shown to be enhanced by mechanical instabilities that lead to sudden and reversible geometric transformations. Although buckling induces planar transformations in most of 2D porous metematerials, here we describe the emergence of 3D morphologies triggered by mechanical instabilities in an elastomeric block with tilted cylindrical holes. As a proof of concept, we demonstrate that these structures can be leveraged to tune surface properties including friction and light reflection, thus providing a new experimental platform for investigating deformation-dependent dynamics for tribological and optical applications.
... Mechanical modulation such as the opening and closing of blinds or curtains is already a common strategy to tune transparency in window applications. Mechanoresponsive, light-modulating materials have been designed for applications, including smart windows, [31,32,[189][190][191] color display, [192,193] and strain sensors. [120,194] Approaches based on periodic nano-and microstructured elastomers, including microhole arrays, [192] micro-and nanopillar arrays, [190,191,195] microprism arrays, [196] wrinkles, [193,197] and 3D photonic crystals, [198] as well as mechanochromic dye-containing elastomers, [29] have been extensively studied. ...
... Mechanoresponsive, light-modulating materials have been designed for applications, including smart windows, [31,32,[189][190][191] color display, [192,193] and strain sensors. [120,194] Approaches based on periodic nano-and microstructured elastomers, including microhole arrays, [192] micro-and nanopillar arrays, [190,191,195] microprism arrays, [196] wrinkles, [193,197] and 3D photonic crystals, [198] as well as mechanochromic dye-containing elastomers, [29] have been extensively studied. Often, these systems are opaque in the initial state attributed to the light scattering from the microand nanostructures, and their transparency increases mono-tonically under the mechanical stretching or compressing. ...
Article
Full-text available
Windows play significant roles in commercial and residential buildings and automobiles, which direct and control light illumination, thermal insulation, natural ventilation, and aesthetics. Various approaches are attempted to make windows “smart” by tailoring their transparency and thermal insulation in response to environmental changes. Hence, there has been much effort to develop smart windows that can dynamically modulate the transmission and reflectance of the visible light and solar radiance into buildings according to weather conditions or personal preferences. Development of smart window materials is also beneficial to applications including wearable sensors, energy harvesting and storage, and medical devices. By carefully matching the refractive indices of nanoparticle (NPs) and polymer matrix, surface chemistry, and their mechanical properties, particle‐embedded polymer composites can exhibit synergistic effects with improved chemical and mechanical stability, enhanced dispersion of NPs, and optimized and stimuli‐responsive optical properties. Here, an overview of recent progresses in the development of smart windows based on electro‐, thermo‐, and mechanoactuations is provided. Additional functionalities, e.g., flexibility, stretchability, and mechanical/chemical stability, can also be achieved by careful choices of NPs and polymers. Windows play significant roles in buildings, automobiles, and displays to direct and control light illumination, thermal insulation, natural ventilation, and aesthetics. Various approaches are attempted to tailor windows' transparency and thermal insulation in response to different environmental settings. Recent progress in the development of smart windows based on nanoparticle/polymer composites via electro‐, thermo‐, and mechanoactuations is reviewed.
... NS mechanical metamaterials (Hewage et al., 2016;Liu et al., 2018;Florijn et al., 2014) are constructed with periodically arranged NS element, and possess many exciting properties, such as multi-stable and snap-back behaviors. Harnessing the multi-stable property, the NS mechanical metamaterials can be applied to tune the propagation of various types of waves, like force Raney et al., 2016), sound (Jang et al., 2009), and light (Holmes and Crosby, 2007;Li et al., 2012). NS mechanical metamaterials, on the other hand, show a long stress plateau owing to the snap-back behavior and therefore are promising in impact protection and isolation. ...
Article
Full-text available
Negative stiffness mechanical metamaterial possesses many interesting properties but fairly low strength. Introducing filler appears to be an effective strategy to enhance the cellular materials' mechanical properties. In this work, we show how to improve the mechanical properties of the cylindrical negative stiffness structure via introducing filler. We first offer the principle of the negative stiffness material combined with an alternative mechanism and then further investigate the mechanical response of the filler-free structure and the filled structure with different filler. We demonstrate that the linear elastic filler can improve the energy absorption but degrade the energy dissipation performance. The visco-hyperelastic filler can enhance the energy dissipation of the structure whether the snap-back behavior exists or not. The true negative stiffness filler can improve the structure's energy dissipation and decrease the strength simultaneously. The presented methods can be seen as a reference to the design of other negative stiffness mechanical metamaterials.
... In the aerospace industry, SMPCs can be employed to fabricate deployable structures and morphing structures [10]. In the micro-device filed, SMPCs can be used to tune the surface morphology [8,129], and this properties can be further used to control the wettability [174], or the optical diffraction [175]. ...
Article
Full-text available
Shape memory polymers (SMPs) can be programmed to a temporary shape, and then recover its original shape by applying environmental stimuli when needed. To expands the application space of SMPs, the shape memory polymer composites (SMPCs) were fabricated either to improve the mechanical properties, or to incorporate more stimulus methods. With the deepening of research, the filler arrangement can also be used to reshape the composites from a two dimensional sheet to a three dimensional structure by a strain mismatch. Recently, SMPCs show more and more interesting behaviors. To gain systematic understanding, we briefly review the recent progress and summarize the challenges in SMPCs. We focus on the reinforcement methods and the composite properties. To look to the future, we review the bonding points with the advanced manufacturing technology and their potential applications.
... Yuan et.al.39 used the difference of dual materials' entropic elasticity to induce a spontaneous pattern switching, by which Zhao et.al.40 achieved the thermal tunable auxetics. Li et.al.41 exploited the materials' shape memory effects to memorize the buckled state of membranes with holes, which can change the diffraction color to a transparent film. ...
Article
Full-text available
Two-dimensional lattice structures with specific geometric features have been reported to have a negative Poisson’s ratio, termed as auxetic metamaterials, i.e., stretching induces expansion in the transversal direction. In this paper, we designed a novel auxetic metamaterial, which by utilizing the shape memory effect of the constituent materials, the in-plane moduli and Poisson's ratios can be continuously tailored. During deformation, the curved meshes ensure the rotation of the mesh joints to achieve auxetics. The rotations of these mesh joints are governed by the mesh curvature, which continuously changes during deformation. Due to the shape memory effect, the mesh curvature after printing can be programmed, which can be used to tune the rotation of the mesh joints and the mechanical properties of auxetic metamaterial structures, including the Poisson's ratios, moduli, and fracture strains. Using the finite element method, the deformation of these auxetic meshes was analyzed. Finally, we designed and fabricated gradient/digital patterns and cylindrical shells, and used the auxetics and shape memory effects to reshape the printed structures.
... Within the framework, the onset of macroscopic or long wave instabilities can be predicted through the loss of ellipticity analysis based on the homogenized response of the material [18][19][20][21][22][23]; while microscopic instabilities can be detected through the Bloch-Floquet analysis [24][25][26][27][28][29], which also allows to detect the long wave instabilities for a special limit. In this work, we employ the Bloch-Floquet technique to investigate the instability phenomenon and, then, post-buckling behavior in soft composite with various periodic distributions of voids and stiff inclusionsthese structures can exhibit cooperative and controllable collapse of the voids leading to sudden pattern transformations, which hold the potential for responsive and reconfigurable functional materials and devices, such as highly stretchable metamaterials [30], switchable auxetic materials [31], and elastic wave filters [25,[32][33][34][35], color displayers [36], and actuators [37]. ...
Article
We experimentally and numerically investigate instability-induced pattern transformations and switchable auxetic behavior in multiphase composites consisted of circular voids and stiff inclusions periodically distributed in a soft elastomer. We specifically focus on the role of inclusion distribution on the behavior of the soft transformative composites. The inclusions are distributed in either square or triangular periodic configurations, while the voids are distributed in the triangular periodic array – the configurations enabling cooperative buckling induced transformations of the unit cells. Through the survey of microstructure parameter, we show that tailored positioning of the stiff inclusions can be exploited to expand the set of admissible switchable patterns in multiphase composites. Thus, extreme values of negative Poisson's ratio can be attained through applied strains; moreover, the onset of instabilities, and the corresponding switches to extremely soft behavior are shown to be controlled by the inclusion arrangements and volume fractions. Furthermore, the dependence of the microstructure buckling and post-buckling behavior on loading direction is investigated, and the composite anisotropic properties depending on the microstructure parameters are discussed.
... Moreover, the effect of buckling on the propagation of elastic waves has also been experimentally verified in both phononic crystals (60) and locally resonant metamaterials (57). Finally, because elastic instabilities persist to the submicrometer scale (18,61), the changes in the architecture induced by the applied deformation can also be exploited to significantly alter the optical transmittance of photonic crystals (62). ...
Article
Mechanical instabilities are traditionally regarded as a route toward failure. However, they can also be exploited to design architected cellular materials with tunable functionality. In this review, we focus on three examples and show that mechanical instabilities in architected cellular materials can be harnessed (a) to design auxetic materials, (b) to control the propagation of elastic waves, and (c) to realize reusable energy-absorbing materials. Together, these examples highlight a new strategy to design tunable systems across a wide range of length scales.
... Recently, some research groups have used mechanical force to change the optical properties of polymer thin films with photonic crystals distributed on their surface, such as quasi-amorphous array of silica nanoparticles embedding poly(dimethylsiloxane) (PDMS), 12 nano-13 and micropillar arrays 14 on wrinkled PDMS, shape memory polymers composed of micro-optic components, 15 and periodic microhole arrays. 16 However, the modulated wavelength region is narrow (usually in the visible range) based on the periodic structure and specific distance between photonic crystals, as governed by the basic principles of the Bragg equation shown in eq 1 17−19 ...
... Some groups demonstrated multi-switchable patterns with a multi-shape memory effect of Nafion [30,31]. Li et al. [32] showed a switchable SMP membrane with color diffraction to a transparent film while harnessing the mechanical instability and shape memory effect. They all showed the novel design, demonstrating large deformations of cellular structures with SMP. ...
Article
Most thermally triggered reconfigurable mesostructures with shape memory polymers (SMPs) require direct mechanical training at high temperatures. In this work, we suggest a method to generate reconfiguration of mesostructures using preload and reverse stiffness combined with shape memory effects, producing programmable deformations at high temperatures. We analyze the transformation mechanism of the reconfigurable structures, providing a design guideline on the thermomechanical deformation for preloading and geometric conditions of multi-materials. Applying preload to a mechanical assembly, we demonstrate a temperature-triggered shape-change of mesostructures using a reverse stiffness effect at a temperature above its glass transition, followed by recovery with a shape memory effect. Using an analytical model verified by experiments and finite element (FE)-based simulations, we demonstrate the unconventional thermal transformation with recovery for three patterns: circle–triangle, circle–square, and circle–hexagon. This work shows that the strain energy conversion by reverse stiffness of two materials, which are designed with prestressing and triggered by temperature, can open a new field of the design of reconfigurable metamaterials.
... abundant pattern switch behaviors from nature have greatly inspired the emer gence in engineering fields ranging from tunable photonic [3][4][5] and phononic [6][7][8][9] crystals, energy absorption devices [10] to tunable color displays, [11] soft actuators, [12] and multistable structures. [13][14][15][16] The majority of the aforementioned applica tions employs elastic instability in periodic cellular structures [12,17,18] to achieve pat tern switching, where enormous transfor mation can be generated once the critical buckling load is reached. ...
Article
Full-text available
Pattern switching (or transformation) widely exists in the activities of various creatures and plays an important role in designing adaptive structures in modern materials. Utilizing the glass transition behavior in amorphous polymers, thermomechanically triggered two-stage pattern switching of 2D lattices is achieved, where components made of an amorphous polymer and a flexible elastomer are interconnected in predesigned layouts. Upon loading at room temperature, the elastomer is far more flexible than the amorphous polymer and the lattice switches into one pattern. With temperature increasing, the modulus of the amorphous polymer decreases due to glass transition. Under the proper choice of amorphous polymer whose storage modulus can decrease to below the modulus of the elastomer, a change in the relative stiffness can be achieved and can switch the overall pattern from one to another while maintaining the external load. Both the experimental and computational studies are carried out to investigate the switching mechanism. Several periodic structures are fabricated to demonstrate several switched patterns. Particularly, a proof-of-concept smart window design is fabricated to explore the potential engineering applications.
... 3 For example, pattern transformations induced by mechanical instabilities under compression can be used to design soft reconfigurable phononic crystals that can be reversibly tuned by applying a deformation; 4,5 similar pattern transformations can be exploited to obtain color switching in polymeric membranes. 6 The development of smart materials that can be programmed on demand would be useful in a number of applications, such as soft robotics, shape morphing or biomedical devices. 7 This paper focuses on reconfigurable architected materials made of unit cells with snap-through instabilities. ...
Article
Full-text available
When an architected material with snap-through instabilities is loaded, the unit cells of the architected material snap sequentially to a series of deformed configurations. In this paper, we propose the novel concept of multimaterial viscoelastic architected materials whose snapping sequence can be tuned using temperature as a control parameter. Because different polymers have different temperature-dependent properties, it is possible that one polymer that is stiffer than another polymer at one temperature becomes softer at a higher temperature. A 3D printing inverse molding process is used to fabricate soft multimaterial architected materials that consist of two different polymers. Using finite element simulations and experiments, we demonstrate that the snapping sequence of these multimaterial architected materials depends on temperature. The influence of the geometrical parameters of the design on the critical temperature at which the snapping sequence switches from one sequence to another sequence is systematically analyzed using simulations and experiments. Being able to tune the snapping sequence using temperature makes it possible to obtain a large number of distinct stable configurations in response to compressive loads. To illustrate a potential application, we demonstrate that these materials can be used as soft reconfigurable metamaterials with tunable stiffness.
... If an elastic sheet is perforated with a two-dimensional square array of circular holes, the sheet can exhibit pattern switching upon compression that internalizes the buckling: the circular holes deform into ellipses with adjacent holes elongated in orthogonal directions [5,6]. The resulting material properties of the sheet, including negative Poisson's ratio [7], have been applied to design of photonic [8,9] and phononic [10] cellular devices and have even been used in soft robotics [11]. Similar pattern switching also occurs in a column containing a line of equally spaced holes first studied by Pihler-Puzović et al. [12], in which the traditional lateral buckling of a column under compression can be preceded by an instability of the micro-structure between the holes (figure 1). ...
Article
Full-text available
We report the results of a numerical and theoretical study of buckling in elastic columns containing a line of holes. Buckling is a common failure mode of elastic columns under compression, found over scales ranging from metres in buildings and aircraft to tens of nanometers in DNA. This failure usually occurs through lateral buckling, described for slender columns by Euler’s theory. When the column is perforated with a regular line of holes, a new buckling mode arises, in which adjacent holes collapse in orthogonal directions. In this paper, we firstly elucidate how this alternate hole buckling mode coexists and interacts with classical Euler buckling modes, using finite-element numerical calculations with bifurcation tracking. We show how the preferred buckling mode is selected by the geometry, and discuss the roles of localized (hole-scale) and global (column-scale) buckling. Secondly, we develop a novel predictive model for the buckling of columns perforated with large holes. This model is derived without arbitrary fitting parameters, and quantitatively predicts the critical strain for buckling. We extend the model to sheets perforated with a regular array of circular holes and use it to provide quantitative predictions of their buckling.
... Then the permanent shape is restored due to the release of elastic entropy by the micro-Brownian motion of polymeric chains. By using this function, SMPs have been researched for many applications, such as anticounterfeiting, [26][27][28] 4D printing, 29 smart actuators, [30][31][32] biomedical applications, [33][34][35][36] and photonic crystals. 37,38 However, nanostructures fabricated with SMPs have rarely been employed in those areas, especially in optical applications, where optical transparency, a short recovery time, and good processability are required. ...
Article
Antireflection (AR) nanostructures mimicking Moth’s eye have been developed for optical devices to achieve high transmittance of light energy. However, mechanical vulnerability has always been pointed out as a drawback of the nanostructures. This research proposes a unique AR strategy which is to infuse shape recovery ability into nanopattern arrays for the high sustainability of the AR nanostructure. A shape memory polymer (SMP) was prepared through tri-copolymerization, where the transition temperature was modulated close to body temperature for facile shape recovery. Nanoscale shape recoverability of the patterns were explored at body temperature. Light transmittance was analyzed experimentally according to the shape state of the AR nanopatterns, and the shape recovery function restored 50% of the damaged antireflection performance. The underlying mechanism of the sustainable AR based on nanopatterns was suggested by calculating theoretical transmittance. Omnidirectional antireflectivity by the nanopatterns was examined by measuring oblique incident transmittances with their sustainability. The nanomechanical property and the sustainable self-cleaning effect induced by the smart nanostructures were also confirmed experimentally. We foresee that the approach suggested in this study can provide new insights into biomimetic optics.
... He et al. [23] discovered that the transformation phenomenon for the shape memory polymer (SMP) can be triggered even by the stress relaxation process, through numerical simulation. In addition, Jang et al. [24] and Li et al. [25] combined pattern instability and shape-memory hysteresis of these structural materials for photonic switching. ...
Article
Full-text available
It is well known that elastic instabilities induce pattern transformations when a soft cellular structure is compressed beyond critical limits. The nonlinear phenomena of pattern transformations make them a prime candidate for controlling macroscopic or microscopic deformation and auxetic properties of the material. In this present work, the novel mechanical properties of soft cellular structures and related hydrogel-elastomer composites are examined through experimental investigation and numerical simulations. We provide two reliable approaches for fabricating hydrogel-elastomer composites with rationally designed properties and transformed patterns, and demonstrate that different geometries of the repeat unit voids of the periodic pattern can be used to influence the global characteristics of the soft composite material. The experimental and numerical results indicate that the transformation event is dependent on the boundary conditions and material properties of matrix material for soft cellular structures; meanwhile, the deformation-triggered pattern of matrix material affects the pattern switching and mechanical properties of the hydrogel-elastomer material, thus providing future perspectives for optimal design, or serving as a fabrication suggestion of the new hydrogel-elastomer composite material.
... Overvelde et al. 23 considered the effect of pore shape on the mechanical response; Shim et al. 24 systematically investigated the role of circular hole arrangement on the post-buckling behavior of the periodic porous structures. Remarkably, these reversible pattern transformations have been demonstrated to be instrumental to design tunable color displays, 25,26 and phononic [27][28][29] and photonic 30 switches. The design of the periodic elastomeric porous structures is based on various distributions of voids in a single phase matrix material. ...
Article
Full-text available
We investigate the instability-induced pattern transformations in 3D-printed soft composites consisting of stiff inclusions and voids periodically distributed in soft matrix. These soft auxetic composites are prone to elastic instabilities giving rise to negative Poisson’s ratio (NPR) behavior. Upon reaching the instability point, the composite microstructure rearranges into new morphology attaining NPR regime. Remarkably, identical composites can morph into distinct patterns depending on the loading direction. These fully determined instability-induced distinct patterns are characterized by significantly different NPR behaviors, thus, giving rise to enhanced tunability of the composite properties. Finally, we illustrate a potential application of these reversible pattern transformations as tunable acoustic-elastic metamaterials capable of selectively filtering low frequency ranges controlled by deformation.
... Some groups demonstrated multi-switchable patterns with a multi-shape memory effect of Nafion [30,31]. Li et al. [32] showed a switchable SMP membrane with color diffraction to a transparent film while harnessing the mechanical instability and shape memory effect. They all showed the novel design, demonstrating large deformations of cellular structures with SMP. ...
Experiment Findings
Most thermally triggered reconfigurable mesostructures with shape memory polymers (SMPs) require direct mechanical training at high temperatures. In this work, we suggest a method to generate reconfiguration of mesostructures using preload and reverse stiffness combined with shape memory effects, producing programmable deformations at high temperatures. We analyze the transformation mechanism of the reconfigurable structures, providing a design guideline on the thermomechanical deformation for preloading and geometric conditions of multi-materials. Applying preload to a mechanical assembly, we demonstrate a temperature-triggered shape-change of mesostructures using a reverse stiffness effect at a temperature above its glass transition, followed by recovery with a shape memory effect. Using an analytical model verified by experiments and finite element (FE)-based simulations, we demonstrate the unconventional thermal transformation with recovery for three patterns: circle-triangle, circle-square, and circle-hexagon. This work shows that the strain energy conversion by reverse stiffness of two materials, which are designed with prestressing and triggered by temperature, can open a new field of the design of reconfigurable metamaterials.
... Shape memory polymers (SMPs) [1][2][3][4] can remember one or more temporary deformations and recover to their permanent shapes under certain external stimuli. Compared to one-way SMPs, two-way SMPs have attracted increasing attentions in recent years, mainly because the latter can reversibly morph so that they can be possibly used in soft robots [5][6][7][8][9][10][11], actuators [12][13][14], artificial muscles [15][16][17][18][19], shape changing substrates [20][21][22][23][24], intelligent fibers [25], 4D printing [26,27], etc. According to the criterion whether external stress is necessary for operation, the two-way shape memory effect (SME) is classified into quasi two-way shape memory effect [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] and true two-way shape memory effect . ...
Article
In the present work, a new strategy for preparing authentic two-way shape memory polymer was proposed by using a conventional crosslinked polyurethane (PU) containing crystalline poly(ε-caprolactone) (PCL) as the proof-ofconcept material. Lauroyl peroxide (LPO) was added as a chemical crosslinker for inducing secondary crosslinking during the programming. Having been stretched and heat treated without additional ingredients and chemicals, the trained PU showed the desired two-way shape memory effect. The crosslinking network created by LPO was successfully converted into internal stress supplier, which represents the core progress of this research. As the temperature changed, the reversible melting/re-crystallization of the crystalline phases elaborately cooperated with the compressed crosslinking network, leading to the implementation of two-way shape memory effect. Through the optimization of the LPO quality, an average reversible strain of up to ~21% in the direction of stretching was measured. In principle, all semi-crystalline polymers can be imparted with two-way shape memory effect following the above-proposed method. Given the great convenience of material selection, preparation, programming and application, the current research may have opened a new way for the production and usage of the smart materials in practice.
... AIenabled functionalities) [273]. The former includes the algorithms of artificial neural networks, computer vision, fuzzy logic, genetic algorithms, logic programming, natural language processing, nonmonotonic reasoning, problem solving and planning, robotics, learning and planning, and different types of hybrid systems that are combined with two or more of the branches [238,240,[274][275][276][277]. The latter includes the functionalities of reasoning, programming, artificial life, belief revision, data mining, evolutionary computation, knowledge representation, natural language understanding, theorem proving, constraint satisfaction, reactive machines, limited memory, theory of mind, self-awareness, etc. [238,240]. ...
Article
Full-text available
Mechanical metamaterials have opened an exciting venue for control and manipulation of architected structures in recent years. Research in the area of mechanical metamaterials has covered many of their fabrication, mechanism characterisation and application aspects. More recently, however, a paradigm shift has emerged to an exciting research direction towards designing, optimising and characterising mechanical metamaterials using artificial intelligence (AI) techniques. This new line of research aims at addressing the difficulties in mechanical metamaterials (i.e. design, analysis, fabrication and industrial application). This review article discusses the advent and development of mechanical metamaterials, and the future trends of applying AI to obtain smart mechanical metamaterials with programmable mechanical response. We explain why architected materials and structures have prominent advantages, what are the main challenges in the mechanical metamaterial research domain, and how to surpass the limit of mechanical metamaterials via the AI techniques. We finally envision the potential research avenues and emerging trends for using the AI-enabled mechanical metamaterials for future innovations.
... While most of the properties of these metamaterials are fixed, compliance, resulting from the use of soft or relatively thin materials, can be used as a paradigm to design reconfigurable metamaterials with tunable functionality. Applications range from materials with adaptive auxetic behavior, [7][8][9][10] tunable stiffness, [11] or ad-hoc optical, [12,13] phononic, [14,15] and acoustic [16] properties, to tunable surface properties such as the drag coefficient, [17] wettability, [18] and chemistry. [19] A particularly interesting avenue has been to harness mechanical instabilities in the design of reconfigurable metamaterials, which changes the continuous nature of the and evolutionary approaches that are inspired by mechanical allostery. ...
Article
Full-text available
Metamaterials are man‐made materials which get their properties from their structure rather than their chemical composition. Their mesostructure is specifically designed to create functionalities not found in nature. However, despite the broad variety of metamaterials developed in recent years, a straightforward procedure to design these complex materials with tailored properties has not yet been established. Here, the inverse design problem is tackled by introducing a general optimization tool to explore the range of material properties that can be achieved. Specifically, a stochastic optimization algorithm is applied and its applicability to disjoint problems is demonstrated, with a focus on tuning the buckling properties of mechanical metamaterials, including experimental verification of the predictions. Besides this problem, this algorithm can be applied to a large variety of systems that, because of their complexity, would be challenging otherwise. Potential applications range from the design of optomechanical resonators, acoustic band gap materials, to dielectric metasurfaces. In this study, a stochastic topology optimization algorithm is extended to include an Ising‐inspired energy potential to inversely design mechanical metamaterials with targeted buckling properties. Both objective functions targeting the lowest and higher modes are considered. As such, this optimization strategy is capable of tackling disjoint optimization problems. Furthermore, the numerical results are validated by experiments on fabricated optimized geometries.
... Zhang and others applied heat and changed the FDM printed pattern using residual thermal stress [202]. Li and others applied the mechanical deformation of 3D printed (vat photopolymerization) PDMS membrane with a hot press [203]. Liu and coworkers demonstrated cylindrical auxetic structures fabricated by polyjet process with a zigzag pattern having isotropic Poisson's ratio over a large range of strains [204,205]. ...
Article
Full-text available
Additive manufacturing is rapidly evolving and opening new possibilities for many industries. This article gives an overview of the current status of additive manufacturing with polymers and polymer composites. Various types of reinforcements in polymers and architectured cellular material printing including the auxetic metamaterials and the triply periodic minimal surface structures are discussed. Finally, applications, current challenges, and future directions are highlighted here.
... Under compression, elastic buckling of the interhole ligaments causes the circular holes to transform into mutually orthogonal ellipses, resulting in a lateral contraction of the sheet. These soft, holey sheets have found application in phononics [19,20] and photonics [21,22], as wave guides [23] and in colour displays [24]. Industrial application is limited, however, by the material properties of the bulk, in particular; the low strength, low melting point and low resistance to chemical corrosion of the base elastomer. ...
Article
Full-text available
Auxetics are materials that contract laterally when compressed, rather than expand, in contrast to common experience. Here we show that common metals and plastics can be rendered auxetic through the introduction of a regular array of holes. Under compression, these hard holey materials bypass localized failure modes, such as shear banding, and instead deform via a global pattern transformation previously reported in elastomeric structures. Despite significant variations in internal structure, the pattern transformation responsible for auxetic behaviour in both metals and plastics is governed by the buckling of the slender struts that comprise the microarchitecture. Furthermore, in contrast to elastomeric structures, holey sheets made from hard materials exhibit significant negative post-buckling stiffness. This suggests that, beyond the geometrical nonlinearities associated with topological modifications, material nonlinearities which arise during plastic deformation offer further potential for altering the material properties of the constituent.
... Thus, the revealed twinning microstructures can be used for achieving fully reversible switchable properties and functionalities associated with the dramatic microstructure transformations. Our findings can help advance further development of reconfigurable material system platforms for tunable optical 34 and acoustic 35 devices, soft robotics 36 , stretchable electronics 37 , and biomedical devices 38 . In addition, we have identified shared characteristic instability feature in soft composites for which long-wave instability mode is predicted. ...
Article
Full-text available
Nature frequently employs the buckling phenomenon to facilitate the formation of complicated patterns across length-scales. Current knowledge, however, is limited to a small set of buckling-induced microstructure transformations in soft composites; and the pattern formation phenomenon remains largely unknown for a vast pool of material morphologies. Here, we investigate the unexplored rich domain of soft heterogeneous composites. We experimentally observe the formation of instability-driven domains in stratified composites with a non-dilute stiff phase. We illustrate that the discovered domain patterns are energetically favorable over wrinkling. Moreover, we introduce a closed-form analytical expression allowing us to predict the evolution of the patterns in the post-buckling regime. Finally, we show that various patterns can be pre-designed via altering material compositions. These findings can help advance our understanding of the mechanisms governing pattern formations in soft biological tissues, and potentially enable the platform for mechanical metamaterials.
... SMP-based micro-optical devices operated through compressing, cooling, and heating process have also been reported in other works, such as quasistructural color display and diffractive optical images. The basic structural units used are generally gratings and micropores [186][187][188]. However, colors produced by these devices are with poor monochromaticity and low purity and that limited their further application. ...
Article
Full-text available
With their ultrathin characteristics as well as the powerful and flexible capabilities of wavefront modulation, optical metasurfaces have brought a new understanding of the interaction between light and matter and provided a powerful way to constrain and manage light. However, the unmodifiable structures and the immutable materials used in the construction lead to the unsatisfactory applications in most functional devices. The emergence of tunable optical metasurfaces breaks the aforementioned limitations and enables us to achieve dynamic control of the optical response. The work in recent years has focused on achieving tunability of optical metasurfaces through material property transition and structural reconfiguration. In this review, some tunable optical metasurfaces in recent years are introduced and summarized, as well as the advantages and limitations of various materials and mechanisms used for this purpose. The corresponding applications in functional devices based on tunability are also discussed. The review is terminated with a short section on the possible future developments and perspectives for future applications.
... However, all of these studies focus on large specimens, typically with unit cells of centimetre scale. There are some studies in the literature on mico-scale metamaterials such as: pattern transformations in porous materials during the processing (Singamaneni et al., 2009b,a;Singamaneni and Tsukruk, 2010); study of recoverable pattern transformations in cellular membranes made of shape memory polymers (Li et al., 2012); analysis of different patterns induced by swelling in cellular hydrogel membranes (Wu et al., 2014); and microfabrication of sub-millimetre metamaterials (Dong et al., 2018). These examples have in comon that they do not study the kinematics nor size effects of cellular metamaterials in detail. ...
Preprint
Full-text available
Cellular elastomeric metamaterials are interesting for various applications, e.g. soft robotics, as they may exhibit multiple microstructural pattern transformations, each with its characteristic mechanical behavior. Numerical literature studies revealed that pattern formation is restricted in (thick) boundary layers causing significant mechanical size effects. This paper aims to experimentally validate these findings on miniaturized specimens, relevant for real applications, and to investigate the effect of increased geometrical and material imperfections resulting from specimen miniaturization. To this end, miniaturized cellular metamaterial specimens are manufactured with different scale ratios, subjected to in-situ micro-compression tests combined with digital image correlation yielding full-field kinematics, and compared to complementary numerical simulations. The specimens' global behavior agrees well with the numerical predictions, in terms of pre-buckling stiffness, buckling strain and post-buckling stress. Their local behavior, i.e. pattern transformation and boundary layer formation, is also consistent between experiments and simulations. Comparison of these results with idealized numerical studies from literature reveals the influence of the boundary conditions in real cellular metamaterial applications, e.g. lateral confinement, on the mechanical response in terms of size effects and boundary layer formation.
... In the 19th century, a concentrated effort was made to characterize critical loads at the onset of mechanical instabilities [7][8][9][10][11], because engineers had to design stable and safe buildings, structures, and machines. However, in recent years, it has become a trend to exploit these instabilities in order to make socalled mechanical metamaterials in a wide range of applications including flexible electronics [12][13][14], flexible photovoltaics [15][16][17][18], tunable surface properties (drag, adhesion, hydrophobicity/hydrophilicity) [19][20][21], tunable photonic and phononic band gaps [22][23][24][25], mechanical cloaks [26,27], self-assembled/self-folded robots and structures [28][29][30], shape-changing materials [31][32][33], and mechanical topological metamaterials [34][35][36][37][38]. ...
Preprint
Full-text available
Steady progress in the miniaturization of structures and devices has reached a scale where thermal fluctuations become relevant and it is thus important to understand how such fluctuations affect their mechanical stability. Here, we investigate the buckling of thermalized sheets and we demonstrate that thermal fluctuations increase the critical buckling load due to the enhanced scale-dependent bending rigidity for sheets that are much larger than a characteristic thermal length scale. The presented results are universal and apply to a wide range of microscopic sheets. These results are especially relevant for atomically thin 2D materials, where thermal fluctuations can significantly increase the critical buckling load because the thermal length scale is on the order of nanometers at room temperature.
... However, all of these studies focus on large specimens, typically with unit cells of centimetre scale. There are some studies in the literature on mico-scale metamaterials such as: pattern transformations in porous materials during the processing (Singamaneni et al., 2009b,a;Singamaneni and Tsukruk, 2010); study of recoverable pattern transformations in cellular membranes made of shape memory polymers (Li et al., 2012); analysis of different patterns induced by swelling in cellular hydrogel membranes (Wu et al., 2014); and microfabrication of sub-millimetre metamaterials (Dong et al., 2018). These examples have in comon that they do not study the kinematics nor size effects of cellular metamaterials in detail. ...
Article
Full-text available
Cellular elastomeric metamaterials are interesting for various applications, e.g. soft robotics, as they may exhibit multiple microstructural pattern transformations, each with its characteristic mechanical behaviour. Numerical literature studies revealed that pattern formation is restricted in (thick) boundary layers causing significant mechanical size effects. This paper aims to experimentally validate these findings on miniaturized specimens, relevant for real applications, and to investigate the effect of increased geometrical and material imperfections resulting from specimen miniaturization. To this end, miniaturized cellular metamaterial specimens are manufactured with different scale ratios, subjected to in-situ micro-compression tests combined with digital image correlation yielding full-field kinematics, and compared to complementary numerical simulations. The specimens' global behaviour agrees well with the numerical predictions, in terms of pre-buckling stiffness, buckling strain and post-buckling stress. Their local behaviour, i.e. pattern transformation and boundary layer formation, is also consistent between experiments and simulations. Comparison of these results with idealized numerical studies from literature reveals the influence of the boundary conditions in real cellular metamaterial applications, e.g. lateral confinement, on the mechanical response in terms of size effects and boundary layer formation.
... Recently, however, the instability phenomenon has been embraced for designing new materials with unusual properties, switchable microstructures, and functions. Elastic instabilities give rise to sudden changes in microstructures [17] that can be leveraged for designing materials with negative Poisson's ratio behavior [18][19][20][21][22], shapemorphing abilities [23], tunable stiffness [24], controllable surface properties (adhesion and wettability) [25], tunable color [26], and phononic [27][28][29][30] and photonic [31] switches. Moreover, buckling-induced microstructure transformations [17] are frequently observed in nature [32] and have been employed to enable new actuation mechanisms [33,34] for soft robotics. ...
Article
Elastic instabilities can trigger dramatic microstructure transformations giving rise to unusual behavior in soft matter. Motivated by this phenomenon, we study instability-induced pattern formations in soft magnetoactive elastomer (MAE) composites deforming in the presence of a magnetic field. We show that identical MAE composites with periodically distributed particles can switch to a variety of new patterns with different periodicity upon developments of instabilities. The newly formed patterns and postbuckling behavior of the MAEs are dictated by the magnitude of the applied magnetic field. We identify the particular levels of magnetic fields that give rise to strictly doubled or multiplied periodicity upon the onset of instabilities in the periodic particulate soft MAE. Thus, the predicted phenomenon can be potentially used for designing new reconfigurable soft materials with tunable material microstructures remotely controlled by a magnetic field.
Article
We present a new type of optomechanical soft metamaterials, which is different from conventional mechanical metamaterials, in that they are simple isotropic and homogenous materials without resorting to any complex nano/microstructures. This metamaterial is unique in the sense that its responses to uniaxial forcing can be tailored by programmed laser inputs to manifest different nonlinear constitutive behaviors, such as monotonic, S-shape, plateau, and non-monotonic snapping performance. To demonstrate the novel metamaterial, a thin sheet of soft material impinged by two counterpropagating lasers along its thickness direction and stretched by an in-plane tensile mechanical force is considered. A theoretical model is formulated to characterize the resulting optomechanical behavior of the thin sheet by combining the nonlinear elasticity theory of soft materials and the optical radiation stress theory. The optical radiation stresses predicted by the proposed model are validated by simulations based on the method of finite elements. Programmed optomechanical behaviors are subsequently explored using the validated model under different initial sheet thicknesses and different optical inputs, and the first- and second-order tangential stiffness of the metamaterial are used to plot the phase diagram of its nonlinear constitutive behaviors. The proposed optomechanical soft metamaterial shows great potential in biological medicine, microfluidic manipulation, and other fields.
Article
A bio-inspired honeycomb pattern that exhibits shape memory behavior is successfully fabricated via the breath figure process. By the surface modification along with a chemical crosslinking process to enhance the recoverability, this honeycomb-like structure with shape memory behavior is realized. The surface wettability is dependent on the surface topography controlled by the deformation of temporary shape (elliptical circle) and recovery of the permanent shape (round circle) at the microscopic scales without the need of micromolding or expensive lithography strategy. This approach opens a facile route to an efficient, inexpensive, and versatile method to prepare films with switchable wettability.
Chapter
Mechanical instabilities in periodic porous elastic structures may lead to the formation of homogeneous patterns, opening avenues to a wide range of applications that are related to the geometry of the system. Here, we focus on a square lattice of elastic beams and show that under equibiaxial compression a pattern transformation can be induced by buckling. Interestingly, this pattern transformation can be effectively used to tune the propagation of elastic waves through the system, enhancing the tunability of its dynamic response.
Article
Structural colors derived from nature have attracted extensive interest due to their unique optical performances and various applications in the field of biomimetic optics. Many organisms have evolved different strategies to respond to external stimuli, so as to obtain attention or hide themselves by changing their surface color. Inspired by this, various biomimetic micro/nanostructures with periodic size have been developed and realized tunable structural colors. Functional materials utilizing the synergy of thermal polymer and photonic microstructure to realize the temperature-responsive structural colors have broad application prospects in the fields of sensor, display, anti-counterfeiting and biomedical treatments, etc. In this review, the research progress on photonic crystals or other mechanisms for generating structural colors with various thermo-sensitive polymers is systematically summarized. The artificial systems with diverse types of temperature-stimulated reactions are presented. Finally, emerging research directions and current challenges in this field are also discussed.
Article
Mechanical metamaterials exhibit properties and functionalities that cannot be realized in conventional materials. Originally, the field focused on achieving unusual (zero or negative) values for familiar mechanical parameters, such as density, Poisson's ratio or compressibility, but more recently, new classes of metamaterials — including shape-morphing, topological and nonlinear metamaterials — have emerged. These materials exhibit exotic functionalities, such as pattern and shape transformations in response to mechanical forces, unidirectional guiding of motion and waves, and reprogrammable stiffness or dissipation. In this Review, we identify the design principles leading to these properties and discuss, in particular, linear and mechanism-based metamaterials (such as origami-based and kirigami-based metamaterials), metamaterials harnessing instabilities and frustration, and topological metamaterials. We conclude by outlining future challenges for the design, creation and conceptualization of advanced mechanical metamaterials.
Article
Smart windows that regulate solar irradiation (including ultraviolet UV, visible and infrared) have emerged as an exciting research area for energy conservation, privacy protection and organism healthcare. However, scalable, low cost and robust smart windows with broadband light modulation are still challengeable for practical applications. Here, an integrated smart membrane with capability of managing the light wavelength from UV, visible (Vis) to near infrared (NIR) band via facile spray coating is reported. The smart membrane consists of the upper ITO rich layer, bottom TiO2/SiO2 NPs layer and a bulk elastomeric PDMS as matrix. Due to the NIR reflection and UV absorption, the smart membrane shows high Vis transmittance and moderate UV and NIR transmittance under relaxed state. Due to Mie scattering from vacuum cavity, under stretching (50%), UV, Vis and NIR transmittance could be dramatically decreased from 40% to 5% (380 nm), from 70% to 5% and from 70% to 10%, respectively, indicating highly efficient and sensitive management. In particular, the application in greenhouse exhibits excellent properties in plants protection, IR cooling and privacy preserving. We believe that this work will pave a facile and low-cost approach for a novel class of integrated smart windows via broadband light modulation.
Article
Mechano-optical materials are of great importance in smart window, security, display and camouflages. However, fabricating ultrasensitive optical devices with wide range still remains great challenging. Inspired from the fast tail amputation of gecko utilizing weak non-ossificated septum, herein, we report an ultrasensitive mechano-optical membrane based on weak dye layer between silica nanoparticles and soft polydimethylsiloxane matrix. Owing to the light scattering from dye-induced internal cavitation, the transmittance dramatically decreases by ~44% at initial small strain level (15%) and a high sensitivity (S > 2) is kept within an abroad strain range (0~35%). Moreover, a high total transmittance tuning range of ~75% is exhibited. Simulation and experimental monitoring demonstrate the effective weakening of interface energy. The potential applications in smart window, sensor, anti-counterfeiting also exhibit the multifunctionality of our smart membranes. This facile and low-cost approach enables the large-scale production and applications of mechano-optical membranes and this interface design provides a general way to develop ultrasensitive mechano-responsive materials.
Article
Shape memory polymers (SMPs) are a class of smart materials that change shape when stimulated by environmental stimuli. Different from the shape memory effect at the macro level, the introduction of micro-patterning technology into SMPs strengthens the exploration of the shape memory effect at the micro/nano level. The emergence of shape memory micro/nano patterns provides a new direction for the future development of smart polymers, and their applications in the fields of biomedicine/textile/micro-optics/adhesives show huge potential. In this review, the authors introduce the types of shape memory micro/nano patterns, summarize the preparation methods, then explore the imminent and potential applications in various fields. In the end, their shortcomings and future development direction are also proposed.
Article
Highly intricate surface architectures derived from patterned polymer microstructures have received increasing concern in recent years. Directional photo-manipulation (DPM) of azopolymers is one of the effective strategies to tune the patterned polymer microstructures through directional mass migration (DMM) upon polarized light illumination. In this feature article, we emphasize the latest advances of DPM on azopatterns created by self-assembly. The mechanism of DMM, the photo-manipulation performance, and functions of manipulated patterns are introduced in sequence. As presented, DPM can manipulate the as-prepared microstructures feasibly by taking the advantages of non-contacting and nondestructive characters. Moreover, the challenges and opportunities of DPM strategy are discussed in conclusion.
Article
When soft cellular structures are compressed axially beyond critical limits, elastic instabilities induce buckling behavior. Although the nonlinear response of periodic materials with different shape voids has been widely investigated, the effect of the friction on the structural response has not yet been explored. In this paper, we develop a simple theoretical model for the buckling of holey column with holes. Meanwhile, we also numerically and experimentally explore the effect of friction on the buckling behavior of the cellular structures. We find out that friction could prevent conventional, global Euler buckling for holey column, which tends to choose the pattern switching mode, and our study also provides future perspectives for mechanics of buckling or optimal design for the cellular structures.
Article
Programmable materials hold great potential for many applications such as deployable structures, soft robotics, and wave control; however, the presence of instability and disorder might hinder their utilization. Through a combination of analytical, numerical, and experimental analyses, we harness the interplay between instabilities, geometric frustration, and mechanical deformations to control the propagation of sound waves within self-assembled soft materials. We consider levitated magnetic disks confined by a magnetic boundary in-plane. The assemblies can be either ordered or disordered depending on the intrinsic disk symmetry. By applying an external load to the assembly, we observe the nucleation and propagation of different topological defects within the lattices. In the presence of instabilities, the defect propagation gives rise to time-independent localized transition waves. Surprisingly, in the presence of frustration, the applied load briefly introduces deformation-induced order to the material. By further deforming the lattices, new patterns emerge across all disk symmetries. We utilize these patterns to tune sound propagation through the material. Our findings could open new possibilities for designing exotic materials with potential applications ranging from sound control to soft robotics.
Article
Nano-layered films of PVAc/PU systems were fabricated by forced assembly coextrusion method. The bulk shape memory properties of PVAc/PU systems were utilized to program nanoscale patterns such as diffraction grating which exhibit iridescence after patterning. A hot embossing process has been utilized to imprint diffraction grating patterns as nano-scale information onto the surface of the thin multilayer films. Three levels of hierarchy i.e. layer thickness, spacing and heights of patterns, governs the functionality of the patterned multilayer film. The time and temperature dependent viscoelastic shape memory behavior determines the opto-mechanical tunability of the film. Mechanical switching of the patterns also leads to optical switching of the films which corresponds to their efficiency of information retrieval. The recovery of patterns as well as the diffractive property depends on the layer thickness (l) of films and heights of patterns (h 0 ). The results illustrate that the higher ratio of h 0 /l better is the recovery of the grating patterns and the corresponding diffractive properties. This scaling effect enables versatile applications in information security by tuning the layer structure of the multilayer shape memory films.
Article
Two-way shape memory polymers (2W-SMPs) are a novel class of shape memory polymers with the reversible and programmable shape-changing behavior which have gained considerable attention compared to one-way shape memory polymers (1W-SMP). Although several promising polymers have been developed to exhibit two-way shape memory effect and there are a number of reviews discussing the general concepts of shape memory polymers, the focus has been attributed to 1W-SMPs. A review focusing on only 2W-SMPs structure, mechanism, and application is still lacking. Herein, a comprehensive review focusing on 2W-SMPs structures, mechanisms, and applications is presented. This review will also emphasize on future insight of 2W-SMPs as an independent theme. The molecular strategies of 2W-SMPs, liquid crystalline elastomers, semi-crystalline networks, composites and interpenetrating networks are thoroughly discussed. Molecular mechanisms attributed to net-points, switching segments, and entropic energy changes are also explained. Considerable efforts have been devoted to explore the underlying mechanisms and strategies of 2W-SMPs in response to different stimulations, enabling their applications in emerging fields including biomedical engineering, aerospace and electronics.
Article
The present work is focused on developing external stress-free two-way triple shape memory polymers (SMPs). Accordingly, a series of innovative approaches are proposed for the material design and preparation. Polyurethane prepolymers carrying crystalline polytetrahydrofuran (PTMEG) and poly(ε-caprolactone) (PCL) as the switching phases are respectively synthesized in advance and then crosslinked to produce the target material. The stepwise method is believed to be conducive to manipulation of the relative contribution of PCL and PTMEG. Moreover, the chain extender, 2-amino-5-(2-hydroxyethyl)-6-methylpyrimidin-4-ol (UPy), is incorporated to establish hydrogen bonds among the macromolecules. By straightforward stretching treatment at different temperatures, the hydrogen bond networks are successfully converted into internal stress provider, which overcomes the challenge of stress relaxation of the melted low melting temperature polymer (i.e. PTMEG) and increases the efficiency of stress transfer. Meanwhile, the contraction force of the switching phases is tuned to match the internal tensile stress. As a result, the internal stress provider can closely collaborate with melting/recrystallization of the crystalline domains, leading to the repeated multiple shape memory effects. The crosslinked polyurethane is thus able to reversibly morph among three shapes and displays its potentials as soft robot and actuator. The strategy reported here has the advantages of easy accessible raw materials, simple reaction and facile programing/deprograming/reprograming, so that it possesses wide applicability.
Article
2D soft periodic structures can produce reversible compaction and relaxation behaviors with complex and diverse compacting patterns during loading and unloading. The (trans)formation and control of the recoverable and adjustable compacting pattern are essential in their wide range of application prospects. In addition to the traditional schemes, this paper proposes and discusses the possibility of using combined loading to control the compacting pattern of finite-size 2D soft periodic structures. Based on the detailed analysis and design of the loading schemes, the buckling and post-buckling behaviors of four kinds of 2D soft periodic structures with finite size under biaxial loading and combined normal and shear loading are investigated by a validated ABAQUS finite element analysis. The buckling model shapes of each structure under each loading combination are discussed and classified in detail to determine the Representative Compacting Patterns (RCPs) and the Critical Points of model transformation Controlled by Loading (CPCL) on the loading plane. Of course, some new modes have been found. Moreover, the CPCL between RCPs without conversion relationship is found to have a corresponding relationship with the variation of the difference between the first eigenvalue and the second eigenvalue of the structure on the loading plane. By using the rotation angle of the hole walls as an index, the compacting behavior and characteristics of each pattern in the process of post-buckling are analyzed quantitatively. The research ideas and results of this paper will provide a new design space for various application fields of finite-size 2D soft periodic structures.
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Shape-memory polymers (SMPs) have attracted significant attention from both industrial and academic researchers due to their useful and fascinating functionality. This review thoroughly examines progress in shape-memory polymers, including the very recent past, achieved by numerous groups around the world and our own research group. Considering all of the shape- memory polymers reviewed, we identify a classification scheme wherein nearly all SMPs may be associated with one of four classes in accordance with their shape fixing and recovering mechanisms and as dictated by macromolecular details. We discuss how the described shape- memory polymers show great potential for diverse applications, including in the medical arena, sensors, and actuators.
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We report capillary force induced instability from drying water swollen poly(2-hydroxyethyl methacrylate) (PHEMA) based hydrogel membranes with micron-sized holes in a square array. When the PHEMA membrane was exposed to deionized-water, the size of the holes became smaller but retained the shape, so-called breathing mode instability. However, during the drying process, the square pore array buckled into a diamond plate pattern. The deformed pattern could be recovered upon re-exposure to water. The instability mechanism was confirmed by comparing the observations from optical and scanning electron microscopy (SEM) images with theoretical prediction. When thermoresponsive poly(N-isopropylacrylamide) was introduced to the PHEMA gel, the poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide) (PHEMA-co-PNIPAAm) membrane underwent pattern transformation only if dried below the lower critical solution temperature of PNIPAAm. Along the pattern transformation, we observed a dramatic change of the optical property of the film, from colourful reflection to transparent window.
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We study structural symmetries of two-dimensional (2D) photonic crystals with anisotropic unit cells, including square- and rectangular-lattices with orientationally modulated elliptic motifs, and a compound structure consisting of circles with sixfold rotational symmetry and elliptical lines with twofold symmetry, which are created through elastic deformation of a single elastomeric membrane with circular pores. We then investigate the photonic bandgap (PBG) properties of the corresponding 2D Si posts and their tolerance to the structural deviation. We find that in the compound structure the overall PBGs are dominated by the sublattice with a higher symmetry, while the total symmetry is determined by the one with a lower symmetry.
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Many cephalopods exhibit remarkable dermal iridescence, a component of their complex, dynamic camouflage and communication. In the species Euprymna scolopes, the light-organ iridescence is static and is due to reflectin protein-based platelets assembled into lamellar thin-film reflectors called iridosomes, contained within iridescent cells called iridocytes. Squid in the family Loliginidae appear to be unique in which the dermis possesses a dynamic iridescent component with reflective, coloured structures that are assembled and disassembled under the control of the muscarinic cholinergic system and the associated neurotransmitter acetylcholine (ACh). Here we present the sequences and characterization of three new members of the reflectin family associated with the dynamically changeable iridescence in Loligo and not found in static Euprymna iridophores. In addition, we show that application of genistein, a protein tyrosine kinase inhibitor, suppresses ACh- and calcium-induced iridescence in Loligo. We further demonstrate that two of these novel reflectins are extensively phosphorylated in concert with the activation of iridescence by exogenous ACh. This phosphorylation and the correlated iridescence can be blocked with genistein. Our results suggest that tyrosine phosphorylation of reflectin proteins is involved in the regulation of dynamic iridescence in Loligo.
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Shape-memory polymers represent a promising class of materials that can move from one shape to another in response to a stimulus such as heat. Thus far, these systems are dual-shape materials. Here, we report a triple-shape polymer able to change from a first shape (A) to a second shape (B) and from there to a third shape (C). Shapes B and C are recalled by subsequent temperature increases. Whereas shapes A and B are fixed by physical cross-links, shape C is defined by covalent cross-links established during network formation. The triple-shape effect is a general concept that requires the application of a two-step programming process to suitable polymers and can be realized for various polymer networks whose molecular structure allows formation of at least two separated domains providing pronounced physical cross-links. These domains can act as the switches, which are used in the two-step programming process for temporarily fixing shapes A and B. It is demonstrated that different combinations of shapes A and B for a polymer network in a given shape C can be obtained by adjusting specific parameters of the programming process. Dual-shape materials have already found various applications. However, as later discussed and illustrated by two examples, the ability to induce two shape changes that are not limited to be unidirectional rather than one could potentially offer unique opportunities, such as in medical devices or fasteners. • active polymer • polymer network • shape-memory polymer • stimuli-sensitive polymer • two-step programming process
Article
The results of an experimental investigation into pattern switching phenomena in three-dimensional cellular structures under compression are reported. It is found that the switch is mediated above a critical strain by an elastic instability which is coupled throughout the structure. Surprisingly, the phenomena are found to be essentially two-dimensional in nature with a uniform pattern switch in one of the directions orthogonal to the applied uniform strain. Selection of the direction of the pattern switch is realized using biaxial loading and the results interpreted in terms of multiple bifurcations. The relevance of the results to the construction of ordered 3D cellular structures and their use in phononic and photonic devices is discussed.
Article
As recently demonstrated, after programming, thermo-responsive shape memorypolymers can exhibit the multi-shape memory effect (SME) upon heating. In addition, it is confirmed that the temperature corresponding to the maximum recovery stress in constrained recovery is roughly the temperature at which pre-deformation is conducted, a phenomenon known as the temperature memory effect (TME). In this paper, we propose a framework to investigate the underlying mechanisms behind both effects and provide the conditions for the TME. According to this framework, we can achieve fully controllable shape recovery following a very complicated sequence in a continuous manner.
Article
The photonic band structure and optical transmittance of two-dimensional periodic elastomeric photonic crystals are studied computationally to understand the effects of large strains on optical properties of the structures. The large compressive deformation patterns of the two-dimensional periodic structure studied by Mullin and coworkers [Mullin, T., Deschanel, S., Bertoldi, K., Boyce, M.C., 2007. Pattern transformation triggered by deformation. Physical Review Letters 99(8), 084301] are first reproduced using hyperelastic material models for the elastomer SU-8. Finite element analysis is then used to solve Maxwell's equations to obtain light transmittance through both the undeformed and deformed structures; simultaneously the wave equation resulting from the appropriate two-dimensional form of Maxwell's equations is solved as an eigenvalue problem to obtain the band structure. The deformation-induced shift in transmission spectrum valleys for different bands is calculated, and the changes in the width of these reflectance peaks are also obtained. The band structure calculation shows that there are no complete photonic band gaps as expected for the low dielectric contrast system. However, the effect of the observed reversible, symmetry-breaking deformation pattern is to uncouple many of the photonic bands in all three high symmetry directions, i.e. Γ–X, X–M, and Γ–M. New non-degenerate deformation-induced optical modes appear in both the real space transmittance spectra and the band structure with lower reflectance values. Analyses of the deformation pattern, the optical mode shapes, and the photonic band structure reveal that localized regions of large rotation are responsible for the significant changes in optical transmittance. The results have practical importance for the design of strain-tunable optomechanical materials for sensing and actuation.
Article
A critical parameter for a shape memory polymer (SMP) lies in its shape memory transition temperature. For an amorphous SMP polymer, it is highly desirable to develop methods to tailor its Tg, which corresponds to its shape memory transition temperature. Starting with an amine cured aromatic epoxy system, epoxy polymers were synthesized by either reducing the crosslink density or introducing flexible aliphatic epoxy chains. The thermal and thermomechanical properties of these epoxy polymers were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). All the crosslinked epoxy polymers with Tg's above room temperature were found to possess shape memory properties. Overall, our approach represents a facile method to precisely tune the Tg of epoxy SMP polymers ranging from room temperature to 89°C.
Article
Recently, novel and uniform deformation-induced pattern transformations have been found in periodic elastomeric cellular solids upon reaching a critical value of applied load [Mullin, T., Deschanel, S., Bertoldi, K., Boyce, M.C., 2007. Pattern transformation triggered by deformation. Phys. Rev. Lett. 99, 084301; Boyce, M.C., Prange, S.M., Bertoldi, K., Deschanel, S., Mullin, T., 2008. Mechanics of periodic elastomeric structures. In: Boukamel, Laiarinandrasana, Meo, Verron (Eds.), Constitutive Models for Rubber, vol. V. Taylor & Francis Group, London, pp. 3–7]. Here, the mechanics of the deformation behavior of several periodically patterned two-dimensional elastomeric sheets are investigated experimentally and through numerical simulation. Square and oblique lattices of circular voids and rectangular lattices of elliptical voids are studied. The numerical results clearly show the mechanism of the pattern switch for each microstructure to be a form of local elastic instability, giving reversible and repeatable transformation events as confirmed by experiments. Post-deformation transformation is observed to accentuate the new pattern and is found to be elastic and to occur at nearly constant stress, resulting in a superelastic behavior. The deformation-induced transformations have been physically realized on structures constructed at the millimeter length scale. This behavior should also persist at the micro and nano length scales, providing opportunities for transformative photonic and phononic crystals which can switch in a controlled manner and also exploiting the phenomenon to imprint complex patterns.
Article
Wrinkling patterns in shape memory polymer (SMP) sputter deposited with a thin layer (10 s nm in thickness) of gold atop are systematically investigated under various conditions. Depending on the surface condition, heating temperature and pre-straining, different patterns of micro/nano-scaled wrinkles are produced. Although elastic buckling of the gold layer is the mechanism behind all types of wrinkles, the shape memory effect (SME) and thermal expansion mismatch (TEM) are the driving force for different patterns after heating to different temperatures, i.e. the evolution of wrinkle pattern is due to the SME after heating to low temperatures and the TEM after heating to high temperatures. The flexibility and convenience in using SMP to achieve different wrinkling patterns is demonstrated.
Article
Utilizing the significant shape recovery ability (in the order of 100% strain) in shape memory polymers (SMPs), we propose two simple approaches, namely laser heating and indentation, to produce micro-sized protrusion arrays. In the former, after local laser heating a pre-compressed SMP, protrusive bumps can be produced. In the latter, through an indentation-polishing-heating process, various shaped protrusive bumps can be produced. It is to demonstrate that indentation is a more convenient and powerful approach than laser heating, since well controlled, different shaped protrusion arrays can be realized.
Article
A flexible and high-throughput approach to produce colloidal particles with well-defined anisotropic shape and surface chemistry through the use of interference lithography with chemically amplified photoresists has been proposed. In interference lithography, the size and shape of the patterns can be controlled and tuned by varying the beam parameters, including the phase, polarization, and intensity. Monodisperse particles were synthesized at a rate of ten millions of particles per single exposure, which can be easily scaled up by increasing the exposure area. The approach allows particles ranging from disklike to rodlike with an aspect ratio up to 10 and diameters from 300 nm to a few micrometers to be varied conveniently by changing exposure intensity and spin-coating speed. The approach also allowed to functionalize the photoresist surface uniformly or selectively through chemical coupling, grafting, or selective deposition of a thin film of AU, to introduce amino groups.
Article
Cooling-induced crystallization of cross-linked poly(cyclooctene) films under a tensile load results in significant elongation and subsequent heating to melt the network reverses this elongation (contracting), yielding a net two-way shape memory (2W-SM) effect. The influence of cross-linking density on the thermal transitions, mechanical properties, and the related 2W-SM effect was studied by varying the concentration of cross-linking agent dicumyl peroxide (DCP) and using differential scanning calorimetry (DSC), gel fraction measurements, dynamic mechanical analysis (DMA), and customized 2W-SM analysis. The latter showed that there is crystallization-induced elongation on cooling and melting-induced shrinkage on heating (2W-SM), with lower cross-link density leading to higher elongation at the same applied stress. For a given cross-link density, however, increasing the tensile stress applied during cooling resulted in greater stress-induced crystallization. We further observed that the onset temperatures for elongation on cooling (T c) and contraction on heating (T m) shifted to higher temperatures with decreasing cross-link density. Similarly, the degree of molecular orientation achieved upon deformation was found to increase with decreasing cross-link density. The impact of stress on the 2W-SM effect was examined using wide-angle X-ray diffraction (WAXD), revealing a transition from bimodal to unimodal orientation. As the crystalline structure evolves from bimodal (low stress) to unimodal (high stress), the crystallization occurs along a single preferred orientation thus inducing greater elongation along the stretching direction. We anticipate that the observed 2W-SM property in a semicrystalline network will enable applications heretofore possible only with costly shape memory alloys and liquid crystalline elastomers.
Article
The transformation of periodic microporous structures fabricated by interference lithography followed by their freezing below glass transition is described. Periodic porous microstructures subjected to internal compressive stresses can undergo sudden structural transformation at a critical strain. The pattern transformation of collapsed pores is caused by the stresses originated during the polymerization of acrylic acid (rubbery component) inside of cylindrical pores and the subsequent solvent evaporation in the organized microporous structure. By confining the polymerization of acrylic acid to localized porous areas complex microscopic periodic structures can be obtained. The control over the mechanical instabilities in periodic porous solids at a sub-micron scale demonstrated here suggests the potential mechanical tunability of photonic, transport, adhesive, and phononic properties of such periodic porous solids.
Article
Surface wrinkles are created on a metallic film supported on a shape memory polymer substrate. The wrinkle wavelength approaches that of visible lights, resulting in diffraction colors. The spatial and geometric distribution of the surface wrinkles can be controlled in an arbitrary fashion, allowing the capture of a three dimensional arbitrary image on a macroscopically flat surface.
Article
Pattern transformation in periodic microporous elastoplastic solid coatings is caused by a buckling of the struts and a rotation of the nodes under compressive stresses. The results of a nonlinear numerical investigation confirm the critical role of the bifurcation of the periodic solid under compressive stresses. In striking contrast to the earlier observations of elastic instabilities in porous elastomeric solids, the elastic-plastic nature of the cross-linked periodic microstructure studied here provides the ability to lock in the transformed pattern with complete relaxation of the internal stresses. The study unveils a novel deformation mode in porous periodic solids in the form of organized buckling instability of weak strut elements.
Article
Shape memory polymers are materials that can memorize temporary shapes and revert to their permanent shape upon exposure to an external stimulus such as heat, light, moisture or magnetic field. Such properties have enabled a variety of applications including deployable space structures, biomedical devices, adaptive optical devices, smart dry adhesives and fasteners. The ultimate potential for a shape memory polymer, however, is limited by the number of temporary shapes it can memorize in each shape memory cycle and the ability to tune the shape memory transition temperature(s) for the targeted applications. Currently known shape memory polymers are capable of memorizing one or two temporary shapes, corresponding to dual- and triple-shape memory effects (also counting the permanent shape), respectively. At the molecular level, the maximum number of temporary shapes a shape memory polymer can memorize correlates directly to the number of discrete reversible phase transitions (shape memory transitions) in the polymer. Intuitively, one might deduce that multi-shape memory effects are achievable simply by introducing additional reversible phase transitions. The task of synthesizing a polymer with more than two distinctive and strongly bonded reversible phases, however, is extremely challenging. Tuning shape memory effects, on the other hand, is often achieved through tailoring the shape memory transition temperatures, which requires alteration in the material composition. Here I show that the perfluorosulphonic acid ionomer (PFSA), which has only one broad reversible phase transition, exhibits dual-, triple-, and at least quadruple-shape memory effects, all highly tunable without any change to the material composition.
Article
Negative Poisson's ratio behavior has been uncovered in cellular solids that comprise a solid matrix with a square array of circular voids. The simplicity of the fabrication implies robust behavior which is relevant over a range of scales. The behavior results from an elastic instability which induces a pattern transformation and excellent quantitative agreement is found between calculation and experiment.
Article
We fabricated diamond-like microstructures from epoxy-functionalized cyclohexyl polyhedral oligomeric silsesquioxanes (POSS) through four-beam interference lithography. The 3D structure was maintained when calcined at a temperature up to 1100 degrees C, and crack-free samples over a large area ( approximately 5 mm in diameter) were obtained when the POSS films were heated at 500 degrees C under an Ar environment or treated with a low intensity oxygen plasma. In the latter, the volume fraction of the 3D porous structures could be fine-tuned by plasma etching time and power. Both Fourier transform infrared (FT-IR) spectroscopy and energy-dispersive X-ray (EDX) spectroscopy analysis suggested that the presence of carbon materials in the films enhanced the crack resistance of 3D POSS structures treated under Ar or oxygen plasma. Since POSS and its derivatives could be easily removed by HF solution at room temperature, we demonstrated high fidelity replication of the 3D porous structures to biocompatible poly(glycidyl methacrylate) (PGMA) and elastomeric poly(dimethylsiloxane) (PDMS). Importantly, the whole fabrication process (template fabrication, infiltration, and removal) was carried out at room temperature. Finally, we illustrated the application of 3D PDMS film as a reversible and repeatable color-changing, flexible photonic crystal.
Article
Shape-memory polymers are a class of smart materials that have recently been used in intelligent biomedical devices and industrial applications for their ability to change shape under a predetermined stimulus. In this study, photopolymerized thermoset shape-memory networks with tailored thermomechanics are evaluated to link polymer structure to recovery behavior. Methyl methacrylate (MMA) and poly(ethylene glycol) dimethacrylate (PEGDMA) are copolymerized to create networks with independently adjusted glass transition temperatures (T(g)) and rubbery modulus values ranging from 56 to 92 °C and 9.3 to 23.0 MPa, respectively. Free-strain recovery under isothermal and transient temperature conditions is highly influenced by the T(g) of the networks, while the rubbery moduli of the networks has a negligible effect on this response. The magnitude of stress generation of fixed-strain recovery correlates with network rubbery moduli, while fixed-strain recovery under isothermal conditions shows a complex evolution for varying T(g). The results are intended to help aid in future shape-memory device design and the MMA-co-PEGDMA network is presented as a possible high strength shape-memory biomaterial.
Article
We report a fully reversible and robust shape-memory effect in a two-dimensional nanoscale periodic structure composed of three steps, the elastic instability governing the transformation, the plasticity that locks in the transformed pattern as a result of an increase in glass transition temperature (T(g)), and the subsequent elastic recovery due to the vapor-induced decrease in T(g). Solvent swelling of a cross-linked epoxy/air cylinder structure induces an elastic instability that causes a reversible change in the shape of the void regions from circular to oval. The pattern symmetry changes from symmorphic p6mm to nonsymmorphic p2gg brought via the introduction of new glide symmetry elements and leads to a significant change in the phononic band structure, specifically in the opening of a new narrow-band gap due to anticrossing of bands, quite distinct from gaps originating from typical Bragg scattering. We also demonstrate that numerical simulations correctly capture the three steps of the shape-memory cycle observed experimentally.
Article
The introduction of biodegradable implant materials as well as minimally invasive surgical procedures in medicine has substantially improved health care within the past few decades. This report describes a group of degradable thermoplastic polymers that are able to change their shape after an increase in temperature. Their shape-memory capability enables bulky implants to be placed in the body through small incisions or to perform complex mechanical deformations automatically. A smart degradable suture was created to illustrate the potential of these shape-memory thermoplastics in biomedical applications.
Article
Materials are said to show a shape-memory effect if they can be deformed and fixed into a temporary shape, and recover their original, permanent shape only on exposure to an external stimulus. Shape-memory polymers have received increasing attention because of their scientific and technological significance. In principle, a thermally induced shape-memory effect can be activated by an increase in temperature (also obtained by heating on exposure to an electrical current or light illumination). Several papers have described light-induced changes in the shape of polymers and gels, such as contraction, bending or volume changes. Here we report that polymers containing cinnamic groups can be deformed and fixed into pre-determined shapes--such as (but not exclusively) elongated films and tubes, arches or spirals--by ultraviolet light illumination. These new shapes are stable for long time periods, even when heated to 50 degrees C, and they can recover their original shape at ambient temperatures when exposed to ultraviolet light of a different wavelength. The ability of polymers to form different pre-determined temporary shapes and subsequently recover their original shape at ambient temperatures by remote light activation could lead to a variety of potential medical and other applications.
Article
A deployable, shape memory polymer adapter is investigated for reducing the hemodynamic stress caused by dialysis needle flow impingement within an arteriovenous graft. Computational fluid dynamics simulations of dialysis sessions with and without the adapter demonstrate that the adapter provides a significant decrease in the wall shear stress. Preliminary in vitro flow visualization measurements are made within a graft model following delivery and actuation of a prototype shape memory polymer adapter. Both the simulations and the qualitative flow visualization measurements demonstrate that the adapter reduces the severity of the dialysis needle flow impingement on the vascular access graft.
Article
We report on a simple yet robust method to produce orientationally modulated two-dimensional patterns with sub-100 nm features over cm2 regions via a solvent-induced swelling instability of an elastomeric film with micrometer-scale perforations. The dramatic reduction of feature size ( approximately 10 times) is achieved in a single step, and the process is reversible and repeatable without the requirement of delicate surface preparation or chemistry. By suspending ferrous and other functional nanoparticles in the solvent, we have faithfully printed the emergent patterns onto flat and curved substrates. We model this elastic instability in terms of elastically interacting "dislocation dipoles" and find complete agreement between the theoretical ground-state and the observed pattern. Our understanding allows us to manipulate the structural details of the membrane to tailor the elastic distortions and generate a variety of nanostructures.
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