Academic literature on the topic 'Flexible mechanical metamaterials'

Flexible mechanical metamaterials are compliant structures designed to achieve desired mechanical properties via large deformation or rotation of their components. While their static properties (such as Poisson’s ratio) have been studied extensively, much less work has been done on their dynamic properties, especially nonlinear dynamic properties induced by large movement of internal components. Here, we examine the nonlinear dynamic response arising from impact loading of mechanical materials that consist of 1D and 2D arrangements of rotating squares, which leads to formation of solitons. Permanent magnets are added to the squares, which causes the metamaterial to become multistable. Rotations of the squares can thereby lead to sudden rearrangements of squares into new phases. We experimentally and numerically characterize the collisions of solitons in these flexible mechanical metamaterials, which, depending on their amplitude and chirality, can induce a variety of responses, including phase transitions.

Origami has been employed to build deployable mechanical metamaterials through folding and unfolding along the crease lines. Deployable metamaterials are usually flexible, particularly along their deploying and collapsing directions, which unfortunately in many cases leads to an unstable deployed state, i.e., small perturbations may collapse the structure along the same deployment path. Here we create an origami-inspired mechanical metamaterial with on-demand deployability and selective collapsibility through energy analysis. This metamaterial has autonomous deployability from the collapsed state and can be selectively collapsed along two different paths, embodying low stiffness for one path and substantially high stiffness for another path. The created mechanical metamaterial yields load-bearing capability in the deployed direction while possessing great deployability and collapsibility. The principle in this work can be utilized to design and create versatile origami-inspired mechanical metamaterials that can find many applications.

Mechanical metamaterials exhibit unusual properties, such as negative Poisson’s ratio, which are difficult to achieve in conventional materials. Rational design of mechanical metamaterials at the microscale is becoming popular partly because of the advance in three-dimensional printing technologies. However, incorporating movable building blocks inside solids, thereby enabling us to manipulate mechanical movement at the molecular scale, has been a difficult task. Here, we report a metal-organic framework, self-assembled from a porphyrin linker and a new type of Zn-based secondary building unit, serving as a joint in a hinged cube tessellation. Detailed structural analysis and theoretical calculation show that this material is a mechanical metamaterial exhibiting auxetic behavior. This work demonstrates that the topology of the framework and flexible hinges inside the structure are intimately related to the mechanical properties of the material, providing a guideline for the rational design of mechanically responsive metal-organic frameworks.

Mechanical metamaterials enable customizing the elastic properties of physical objects by altering their fine-scale structure. A broad gamut of effective material properties can be produced even from a single fabrication material by optimizing the geometry of a periodic microstructure tiling. Past work has extensively studied the capabilities of microstructures in the small-displacement regime, where periodic homogenization of linear elasticity yields computationally efficient optimal design algorithms. However, many applications involve flexible structures undergoing large deformations for which the accuracy of linear elasticity rapidly deteriorates due to geometric nonlinearities. Design of microstructures at finite strains involves a massive increase in computation and is much less explored; no computational tool yet exists to design metamaterials emulating target hyperelastic laws over finite regions of strain space. We make an initial step in this direction, developing algorithms to accelerate homogenization and metamaterial design for nonlinear elasticity and building a complete framework for the optimal design of planar metamaterials. Our nonlinear homogenization method works by efficiently constructing an accurate interpolant of a microstructure's deformation over a finite space of macroscopic strains likely to be endured by the metamaterial. From this interpolant, the homogenized energy density, stress, and tangent elasticity tensor describing the microstructure's effective properties can be inexpensively computed at any strain. Our design tool then fits the effective material properties to a target constitutive law over a region of strain space using a parametric shape optimization approach, producing a directly manufacturable geometry. We systematically test our framework by designing a catalog of materials fitting isotropic Hooke's laws as closely as possible. We demonstrate significantly improved accuracy over traditional linear metamaterial design techniques by fabricating and testing physical prototypes.

Mechanical metamaterials made of flexible building blocks can exhibit a plethora of extreme mechanical responses, such as negative elastic constants, shape-changes, programmability, and memory. To date, dissipation has largely remained overlooked for such flexible metamaterials. As a matter of fact, extensive care has often been devoted in the constitutive materials’ choice to avoid strong dissipative effects. However, in an increasing number of scenarios, where metamaterials are loaded dynamically, dissipation cannot be ignored. In this Research Update, we show that the interplay between mechanical instabilities and viscoelasticity can be crucial and that they can be harnessed to obtain new functionalities. We first show that this interplay is key to understanding the dynamical behavior of flexible dissipative metamaterials that use buckling and snapping as functional mechanisms. We further discuss the new opportunities that spatial patterning of viscoelastic properties offer for the design of mechanical metamaterials with properties that depend on the loading rate.

ABSTRACTIt is important to achieve materials with large coefficient of thermal expansion in science and engineering applications. In this paper, we propose an experimentally-validated metamaterial approach to amplify the thermal expansion of materials based on the guiding principles of flexible hinges and displacement amplification mechanism. The thermal expansion property of the designed metamaterial is demonstrated by simulation and experiment with a temperature increase of 245 K for the two-dimensional sample. Both experimental and simulation results display amplified thermal expansion property of the metamaterial. The effective coefficient of thermal expansion of the metamaterials is demonstrated to be dependent on the size parameters of the structure, which means by appropriately tailoring these parameters, the thermal expansion of materials could be amplified with different amplification factor. This work provides an important method to control the thermal expansion coefficient of materials and could be applied in various industry domain.

Ce travail est consacré à l'étude des ondes modulées se propageant le long de métamatériaux mécaniques flexibles nonlinéaires (FlexMM). Ces structures sont des matériaux architecturés constitués d'éléments souples très déformables connectés à des éléments plus rigides. Leur capacité à subir de grandes déformations locales favorise l'apparition de phénomènes d'ondes non linéaires. En utilisant une approche par éléments discrets, nous formulons des équations discrètes non linéaires qui décrivent les déplacements longitudinaux et rotationnels de chaque cellule unitaire et leur couplage mutuel. Une analyse multi-échelles est employée afin d'obtenir une équation de Schrödinger non linéaire (NLS) effective décrivant les ondes modulées pour le degré de liberté rotationnel du FlexMM. En nous appuyant sur l'équation NLS, nous identifions divers types de phénomènes d'ondes non linéaires dans le FlexMM. En particulier, nous avons observé que des ondes planes faiblement non linéaires peuvent être modulationellement stables ou instables en fonction des paramètres du système et de l'excitation utilisée. De plus, nous avons trouvé que les FlexMMs supportent des solitons-enveloppe vectoriels où le degré de liberté rotationnel des unités peut prendre la forme de solitons dits "bright" ou "dark" et, en raison du couplage, le degré de liberté de déplacement longitudinal présente un comportement de type "kink". Enfin, nous abordons le phénomène de "catastrophe de gradient", qui prédit l'émergence de structures similaires aux solitons de Peregrine dans la limite semi-classique de l'équation NLS, dans la structure FlexMM. Grâce à nos prédictions analytiques et à l'utilisation de simulations numériques, nous pouvons déterminer les conditions requises et les valeurs des paramètres physiques pour observer ces phénomènes dans les FlexMMs
This work is dedicated to the investigation of modulated waves propagating along nonlinear flexible mechanical metamaterials (FlexMM). These structures are architected materials consisting of highly deformable soft elements connected to stiffer ones. Their capacity to undergo large local deformations promotes the occurrence of nonlinear wave phenomena. Using a lump element approach, we formulate nonlinear discrete equations that describe the longitudinal land rotational displacements of each unit cell and their mutual coupling. A multiple scales analysis is employed in order to derive an effective nonlinear Schrödinger (NLS) equation describing envelope waves for the rotational degree of freedom of FlexMM. Leveraging on the NLS equation we identify various type of nonlinear waves phenomena in FlexMM. In particular we observed that weakly nonlinear plane waves can be modulationally stable or unstable depending of the system and excitation parameters. Moreover we have found that the FlexMMs support envelope vector solitons where the units rotational degree of freedom might take the form of bright or dark soliton and due to coupling, the longitudinal displacement degree of freedom has a kink-like behavior. Finally, we address the phenomenon of "gradient catastrophe", which predicts the emergence of Peregrine soliton-like structures in the semiclassical limit of the NLS equation, in FlexMM. Through our analytical predictions and by using numerical simulations, we can determine the required conditions and the values of the physical parameters in order to observe these phenomena in FlexMMs

Metamaterials are artificially structured materials which attain their unconventional macroscopic properties from their cellular configuration rather than their constituent chemical composition. The judicious design of this cellular structure opens the possibility to program and control the optical, mechanical, acoustic, or thermal responses of metamaterials. This Ph.D. dissertation focuses on scalable design and manufacturing strategies for optical and mechanical metamaterials.

The fabrication of optical metamaterials still relies heavily on low-throughput process such as electron beam lithography, which is a serial technique. Thus, there is a growing need for the development of high-throughput, parallel processes to make the fabrication of optical metamaterials more accessible and cost-effective. The first part of this dissertation presents a scalable manufacturing method, termed “roll-to-roll laser induced superplasticity” (R2RLIS), for the production of flexible optical metamaterials, specifically metallic near-perfect absorbers. R2RLIS enables the rapid and inexpensive fabrication of ultra-smooth metallic nanostructures over large areas using conventional CO₂ engravers or inexpensive diode lasers. Using low-cost metal/epoxy nanomolds, the minimum feature size obtained by R2RLIS was <40 nm, facilitating the rapid fabrication of flexible near-perfect absorbers at visible frequencies with the capability to wrap around non-planar surfaces.

The existing approaches for designing mechanical metamaterials are mostly ad hoc, and rely heavily on intuition and trial-and-error. A rational and systematic approach to create functional and programmable mechanical metamaterials is therefore desirable to unlock the vast design space of mechanical properties. The second part of this dissertation introduces a systematic, algorithmic design strategy based on Voronoi tessellation to create architected soft machines (ASMs) and twisting mechanical metamaterials (TMMs) with programmable motion and properties. ASMs are a new class of soft machines that benefit from their 3D-architected structure to expand the range of mechanical properties and behaviors achievable by 3D printed soft robots. On tendon-based actuation, ASMs deform according to the topologically encoded buckling of their structure to produce a wide range of motions such as contraction, twisting, bending, and cyclic motion. TMMs are a new class of chiral mechanical metamaterials which exhibit compression-twist coupling, a property absent in isotropic materials. This property manifests macroscopically and is independent of the flexible material chosen to fabricate the TMM. The nature of this compression-twist coupling can be programmed by simply tuning two design parameters, giving access to distinct twisting regimes and tunable onset of auxetic (negative Poisson’s ratio) behavior. Taking a metamaterial approach toward the design of soft machines substantially increases their number of degrees of freedom in deformation, thus blurring the boundary between materials and machines.

Advances in additive manufacturing technologies enable the rapid, high-throughput generation of mechanically soft microelectromechanical devices with tailored designs for many applications spanning from optical to biomedical applications. These devices can be softly interfaced with biological tissues and mechanically fragile systems, which enables to open up a whole new range of applications. However, the scalable production of these devices faces a significant challenge due to the complexity of the microfabrication process and the intolerable thermal, chemical, and mechanical conditions of their flexible polymeric substrates. To overcome these limitations, I have developed a set of advanced additive manufacturing technologies enabling (1) mechanics-driven manufacturing of quasi-three-dimensional (quasi-3D) nanoarchitectures with arbitrary substrate materials and structures; (2) repetitive replication of quasi-3D nanoarchitectures for infrared (IR) bandpass filtering; (3) electrochemical reaction-driven delamination of thin-film electronics over wafer-scale; (4) rapid custom printing of soft poroelastic materials for biomedical applications.

First, I have developed a new mechanics-driven nanomanufacturing method enabling large-scale production of quasi-3D plasmonic nanoarchitectures that are capable of controlling light at nanoscale length. This method aims to eliminate the need for repetitive uses of conventional nanolithography techniques that are time- and cost-consuming. This approach is innovative and impactful because, unlike any of the conventional manufacturing methods, the entire process requires no chemical, thermal, and mechanical treatments, enabling a large extension of types of receiver substrate to nearly arbitrary materials and structures. Pilot deterministic assembly of quasi-3D plasmonic nanoarrays with imaging sensors yields the most important advances, leading to improvements in a broad range of imaging systems. Comprehensive experimental and computational studies were performed to understand the underlying mechanism of this new manufacturing technique and thereby provide a generalizable technical guideline to the manufacturing society. The constituent quasi-3D nanoarchitectures achieved by this manufacturing technology can broaden considerations further downscaled plasmonic metamaterials suggest directions for future research.

Second, I have developed mechanics-driven nanomanufacturing that provides the capability to repetitively replicate quasi-3D plasmonic nanoarchitectures even with the presence of an extremely brittle infrared-transparent spacer, such as SU-8, thereby manipulating IR light (e.g., selectively transmitting a portion of the IR spectrum while rejecting all other wavelengths). Comprehensive experimental and computational studies were performed to understand the underlying nanomanufacturing mechanism of quasi-3D plasmonic nanoarchitectures. The spectral features such as the shape of the transmission spectrum, peak transmission and full width at half maximum (FWHM), etc. were studied to demonstrate the bandpass filtering effect of the assembled quasi-3D plasmonic nanoarchitecture.

Third, I have developed an electrochemical reaction-driven transfer printing method enabling a one-step debonding of large-scale thin-film devices. Conventional transfer printing methods have critical limitations associated with an efficient and intact separation process for flexible 3D plasmonic nanoarchitectures or bio-integrated electronics at a large scale. The one-step electrochemical reaction-driven method provides rapid delamination of large-scale quasi-3D plasmonic nanoarchitectures or bio-integrated electronics within a few minutes without any physical contact, enabling transfer onto the target substrate without any defects and damages. This manufacturing technology enables the rapid construction of quasi-3D plasmonic nanoarchitectures and bio-integrated electronics at a large scale, providing a new generation of numerous state-of-art optical and electronic systems.

Lastly, I have developed a new printing method enabling the direct ink writing (DIW) of multidimensional functional materials in an arbitrary shape and size to rapidly prototype stretchable biosensors with tailored designs to meet the requirement of adapting the geometric nonlinearity of a specific biological site in the human body. Herein, we report a new class of a poroelastic silicone composite that is exceptionally soft and insensitive to mechanical strain without generating significant hysteresis, which yields a robust integration with living tissues, thereby enabling both a high-fidelity recording of spatiotemporal electrophysiological activity and real-time ultrasound imaging for visual feedback. Comprehensive in vitro, ex vivo, and in vivo studies provide not only to understand the structure-property-performance relationships of the biosensor but also to evaluate infarct features in a murine acute myocardial infarction model. These features show a potential clinical utility in the simultaneous intraoperative recording and imaging on the epicardial surface, which may guide a definitive surgical treatment.

Abstract Information storage is an important functionality to produce a sense-decide-respond loop in active mechanical metamaterial systems. Here, we propose a new class of mechanical metamaterials with self-powered digital information storage capability. In the so-called mechanically-responsive data storage metamaterials, data is incorporated into a set of self-recovering unit cells that form the material lattice. As the metamaterial structure is loaded, the cells in each layer generate electrical signals that are coded as binary bits to represent the stored data. We show how the proposed designs can serve as sequential access memory data storage devices, where the stored data can be accessed in a deformation sequential order under mechanical stimulations. The stiffness of the metamaterial structure can be rationally designed to create either a flexible/soft or hard data storage system. We further discuss the potential of the proposed technology to create low-cost, non-volatile, and long-term storage solutions for data storage applications.

Functional metamaterials are gradually becoming the frontier of scientific research and industrial applications. Among them, reconfigurable mechanical metamaterial with inbuilt motion capability could result in unusual physical properties such as shape tunability and programmable density and stiffness. Inspired by the transformable cuboid structure that was first investigated by Ron Resch, we proposed a tilted cuboid structure that can fold into a 3D configuration. By designing the individual building units, face angles and tessellation pattern, we are able to construct a series of reconfigurable structures with various shape, twist and permeability feature. Based on our approach, a configuration method to build multi-layer metamaterial is proposed, and it can be generalized to other tilted structures with different building units. The volumetric strains of different models are analyzed, and the result shows the metamaterial has a massive deformation ability as the maximum volume can be four times of the packaged volume. The tilted cuboid structure is highly flexible with variable stiffness and permeability, and can be used to develop metamaterials, large deformation devices and kinetic architectures.

Abstract This study puts forward a metasurface design which allows the flexible tuning of the elastic wave propagation path, enabling the interrogating wave field guiding into desired monitoring regions for damage detection. As a demonstrative case study, the metasurface plate contains a rectangular array of unit cells sitting in an aluminum plate. Each unit cell is comprised of a shape memory alloy substrate and a lead stub. The controllable bandgap of such a metamaterial system can be achieved due to the stiffness change of nitinol between its martensite phase and austenite phase under a thermal load. First, a Finite Element Model (FEM) of the unit cell is constructed to calculate the band structure of the metasurface plate, demonstrating the adjustable bandgap behavior. Then, numerical modeling of the metamaterial waveguide is performed by shifting the bandgap of a specific path of the metasurface away from the excitation frequency. The modeling results demonstrate that the martensite metasurface area forms a bandgap region where guided wave energy cannot penetrate. While, the bandgap of the austenite part shifts away from the excitation frequency, opening up a transmission path for the ultrasonic waves. By delicately selecting the austenite state unit cell path, four ‘S’, ‘J’, ‘T’, ‘U’ shaped routes with a fine resolution are tailored to show a SJTU logo, demonstrating the excellent waveguiding capability and the programmable waveguide feature of this shape memory metamaterial system. The proposed tunable waveguiding methodology possesses great application potential in future Structural Health Monitoring (SHM) and Nondestructive Evaluation (NDE) applications.

Metamaterials made from flexible structures with piezoelectric laminates connected to resonant shunt circuits can exhibit vibration attenuation properties similar to those of their purely mechanical locally resonant counterparts. Thus, in analogy to purely mechanical metamaterials, electroelastic metamaterials with piezoelectric resonators can exhibit vibration attenuation bandgaps. To enable the effective design of these locally resonant electroelastic metamaterials, the electromechanical behavior of the piezoelectric patches must be reconciled with the modal behavior of the electroelastic structure. To this end, we develop a novel argument for the formation of bandgaps in bimorph piezoelectric beams, relying on modal analysis and the assumption of infinitely many segmented shunted electrodes (unit cells) on continuous piezoelectric laminates bracketing a substrate. As a case study, the frequency limits of the locally resonant bandgap that forms from resonant shunting is derived, and a design guideline is presented to place the bandgap in a desired frequency range. This method can be easily extended to more general circuit impedances, and can be used to design shunt circuits to obtain a desired frequency response in the main structure.

Abstract Soft robots composed of elastic materials can exhibit nonlinear behaviors, such as variable stiffness and adaptable deformation, that are favorable to cooperation with humans. These characteristics enable soft robots to be used in multiple applications, ranging from minimally invasive surgery and search and rescue in emergency or hazardous environments to marine or space exploration and assistive devices for people with musculoskeletal disorders. Although soft actuators composed of smart materials have been proposed as a control strategy for soft robots, most studies have focused on traditional actuators using hydraulic or pneumatic pressure. Over the years, these have made a lot of progress, but they have not been able to overcome the limitations of the complex configuration of the system and the expansion of the cross-section of the actuator when contracted. This paper merges the actuator design methodology for smart materials with the mechanical analysis of auxetic structures to present an electrically driven soft actuator architecture that achieves reliable actuation displacements. This novel soft actuator, constructed with contractile SMA springs and flexible auxetic metamaterials (FAM), has a spontaneous recovery of the shape after a contraction, a negative Poisson’s ratio, and over 90% of consistency with the performance predictions at the design stage. Our research presents a methodology for the design of a new electrically driven soft actuator, describes the manufacture of SMA springs and FAM, and concludes with the validation of the design by experimental analysis using the 2D planar soft actuator prototype. Finally, our study revealed that the application of the extraordinary characteristics of smart materials and structures together into a single architecture can be a strategy to overcome the limitations of existing soft actuator studies.

Lateral ankle sprains cost billions of dollars in medical expenses annually and frequently result in long-term functional decline and a diminished health-related quality of life. While ankle braces have been shown to be effective in prophylaxis of subsequent ankle sprains, current braces are either too stiff and affect normal gait or too flexible and provide insufficient support during high-intensity activities. In this study, we proposed an adaptive ankle brace design that employs dynamically variable stiffness components to provide minimum support under normal gait movements and maximum rigidity under large ranges of motion. To achieve these unique properties, a honeycomb geometry was designed and three dimensionally printed with thermoplastic polyurethane to exhibit nonlinear, strain-stiffening, elastic behavior. We conducted a series of tensile load tests on different honeycomb unit cell configurations. First, the influence of unit cell designs on their mechanical strength and force-strain profiles was characterized. Second, experimentally calibrated finite element models of individual components simulated the mechanical response of the geometry, which were then used to optimize the geometrical parameters of the honeycomb shape (i.e., ring size, length of lateral elements, and thickness). The results identified promising design parameters for these honeycomb geometries that could be used to realize next-generation adaptive ankle braces.

Abstract Passive thermal radiative cooling (PTRC) has drawn massive attention in the past few years due to its advantages, including excellent cooling potential, no emission of greenhouse gases, silent operation, low maintenance, and off-grid operation. PTRC has been successfully demonstrated to reduce the electricity consumption required for cooling and ventilation of buildings. Several radiative emitters have been studied in the literature, such as pigmented paints, nanoparticle-based coatings, photonic crystals, metamaterials, and polymers. Among them, polymers have proven to be inherently strong infrared (IR) emitters, scalable, low-cost, flexible, easy to apply, and durable candidates. In addition to these features, biopolymers are eco-friendly and currently abundant in the market. Despite their significant advantages, there have been limited studies on the applications of biopolymers for radiative cooling. In this study, we report promising performances from a commercially available, affordable, and applicable biopolymer, cellulose, as a PTRC emitter. We fabricated several cellulose films with various structural characteristics and thicknesses. The emissivity and reflectivity of these emitter surfaces were measured for the desired wavelengths and direction. The obtained measurements reveal relatively high magnitudes of diffuse emissivity in the atmospheric window and high reflectivity in the solar spectrum range. Using the materials’ reflectivity and emissivity data, we theoretically calculated the net cooling power and the expected temperature drop. Each emitter demonstrated high cooling power and considerable temperature reduction based on the average recorded weather conditions in Lincoln, NE.