Auxetic Mechanical Metamaterials Research Articles

Coupling Chiral Cuboids with Wholly Auxetic Response.

Auxetic materials have been extensively studied for their design, fabrication and mechanical properties. These material systems exhibit unique mechanical characteristics such as high impact resistance, shear strength, and energy absorption capacity. Most existing auxetic materials are two-dimensional (2D) and demonstrate half-auxetic behavior, characterized by a negative Poisson's ratio when subjected to either tensile or compressive forces. Here, we present novel three-dimensional (3D) auxetic mechanical metamaterials, termed coupling chiral cuboids, capable of achieving negative Poisson's ratio under both tension and compression. We perform experiments, theoretical analysis, and numerical simulations to validate the wholly auxetic response of the proposed coupling chiral cuboids. Parametric studies are carried out to investigate the effects of structural parameters on the elastic modulus and Poisson's ratio of the coupling chiral cuboids. The results imply that the Poisson's ratio sign-switching from negative to positive can be implemented by manipulating the thickness of Z-shaped ligaments. Finally, the potential application of the coupling chiral cuboids as inner cores for impact-resistant sandwich panels is envisioned and validated. Test results demonstrate a remarkable 49.3% enhancement in energy absorption compared to conventional solid materials.

A review on the auxetic mechanical metamaterials and their applications in the field of applied engineering

Metamaterials are artificially created materials or structures with properties not found in nature. They encompass electromagnetic, acoustic, and mechanical metamaterials, which are particularly significant in applied engineering. Mechanical metamaterials exhibit unique mechanical properties such as vanishing shear modulus, negative Poisson’s ratio, negative compressibility, etc. This paper reviews the most commonly used mechanical metamaterials and discusses their applications in the field of applied engineering, specifically in vibration isolation, energy absorption, and vibration reduction. The prospects for future developments in this field are also presented.

Auxetic Biomedical Metamaterials for Orthopedic Surgery Applications: A Comprehensive Review.

Poisson's ratio in auxetic materials shifts from typically positive to negative, causing lateral expansion during axial tension. This scale-independent characteristic, originating from tailored architectures, exhibits specific physical properties, including energy adsorption, shear resistance, and fracture resistance. These metamaterials demonstrate exotic mechanical properties with potential applications in several engineering fields, but biomedical applications seem to be one of the most relevant, with an increasing number of articles published in recent years, which present opportunities ranging from cellular repair to organ reconstruction with outstanding mechanical performance, mechanical conduction, and biological activity compared with traditional biomedical metamaterials. Therefore, focusing on understanding the potential of these structures and promoting theoretical and experimental investigations into the benefits of their unique mechanical properties is necessary for achieving high-performance biomedical applications. Considering the demand for advanced biomaterial implants in surgical technology and the profound advancement of additive manufacturing technology that are particularly relevant to fabricating complex and customizable auxetic mechanical metamaterials, this review focuses on the fundamental geometric configuration and unique physical properties of negative Poisson's ratio materials, then categorizes and summarizes auxetic material applications across some surgical departments, revealing efficacy in joint surgery, spinal surgery, trauma surgery, and sports medicine contexts. Additionally, it emphasizes the substantial potential of auxetic materials as innovative biomedical solutions in orthopedics and demonstrates the significant potential for comprehensive surgical application in the future.

Glass Sponge-inspired Auxetic Mechanical Metamaterials for Energy Absorption

Glass Sponge-inspired Auxetic Mechanical Metamaterials for Energy Absorption

Auxetic Two‐Phase Chevron Mechanical Metamaterial

This investigation explores novel two‐phase chevron mechanical metamaterials that exhibit auxetic properties. Unlike traditional foam‐like cellular or porous auxetic materials, these designs are composed of chevron patterned layers embedded in anisotropic matrix. This innovation design allows for auxeticity in two orthogonal in‐plane directions (bi‐auxeticity) or in all in‐plane directions (complete auxeticity), providing not only a general strategy but also detailed guidelines for designing non‐porous auxetic mechanical metamaterials with tunable auxetic behaviors. One goal of this work is to explore the mechanical behavior, specifically effective stiffness and Poisson's ratio, of these new designs and to identify the design space for auxetic behavior using numerical and experimental methods. Systematic finite element (FE) simulations are conducted using ABAQUS and Python scripts to quantify effective stiffness and Poisson's ratio within a small strain range. To validate the numerical predictions, three representative designs are selected and fabricated via multi‐material polymer jetting. Uniaxial tension experiments are conducted on these specimens. Design spaces for non‐auxeticity, partial‐auxeticity, and complete auxeticity are identified through an integrated numerical approach. Theoretical criteria for determining the completeness of auxeticity are proposed and verified via FE simulations.

Compressive properties and energy absorption of 4D printed auxetic mechanical metamaterials

Auxetic mechanical metamaterials contract or expand laterally when subjected to compressive or tensile load in the axial direction and offer potential benefits in areas such as flexible electronics, aerospace, and soft actuators. Nevertheless, when metamaterials are fabricated using conventional methods, their configurations and properties are fixed, which prohibits adaptation to the specific geometric requirements of their application environment. This limits their further development. This paper proposes an auxetic mechanical metamaterial with a negative Poisson's ratio and energy absorption capability. Compression specimens are fabricated with LCD photo-curable printing technology. The deformation mechanism and in-plane compression characteristics are examined through experimental and numerical simulation methods. The influence of unit cell geometrical control parameters on Poisson's ratio and energy absorption are also explored. The results indicate the dominant deformation mechanism of metamaterial during in-plane compression is bending deformation. The negative Poisson's ratio behavior is more pronounced when compressing along the X direction than along the Y direction, and the specimens also exhibit energy absorption capacity. In addition, the metamaterials prepared by 4D printing technology can be transformed from one configuration to another under external stimuli stimulation and force, and exhibit different mechanical properties, realizing programmable and reconfigurable, further expanding their application range.

Auxetic mechanical metamaterials with symmetry-broken Re-entrant units

Three innovative auxetic metamaterials with symmetry-broken unit cells and possible applicability in the construction and automotive industries are presented by breaking the mirror symmetries of the conventional re-entrant unit cell. The single symmetry-broken re-entrant (SBR), double symmetry-broken re-entrant (DBR), and hybrid symmetry-broken re-entrant (HBR) structures are fabricated via 3D printing and examined by experimental compression tests and finite element analysis (FEA). The proposed metamaterials exhibit remarkable specific energy absorption (SEA) properties in comparison to the benchmark symmetric-unit honeycomb, with the SBR model showing 103.9 % higher SEA. The novel structures also show superior stability under large compressive deformation, exhibiting excellent auxeticity in comparison to the symmetric-unit model. Parametric investigations reveal that the level of asymmetricity of the unit cells plays a key role in the stability and superior mechanical properties of the proposed metamaterials under compression. One set of parametric studies discovers an HBR structure with a special combination of SBR and DBR units which outperforms the parent structures under compression. Finally, parametric investigation of transverse and inclined compression of the structures indicated superiority of the DBR metamaterial to other structures due to having a deformation mechanism with organized self-contact regions. The DBR metamaterial owns 314 % and 86 % higher SEA rather than the benchmark honeycomb under transverse and inclined compression, respectively.

Auxetic mechanical metamaterials and their futuristic developments: A state-of-art review

Auxetic mechanical metamaterials and their futuristic developments: A state-of-art review

A bioinspired 3D-printable flexure joint with cellular mechanical metamaterial architecture for soft robotic hands.

Compliant flexure joints have been widely used for cable-driven soft robotic hands and grippers due to their safe interaction with humans and objects. This paper presents a soft and compliant revolute flexure joint based on the auxetic cellular mechanical metamaterials with a heterogeneous structure. The heterogeneous architecture of the proposed metamaterial flexure joint (MFJ), which is inspired by the human finger joints, provides mechanically tunable multi-stiffness bending motion and large range of bending angle in comparison to conventional flexure joints. The multi-level variation of the joint stiffness over the range of bending motion can be tuned through the geometrical parameters of the cellular mechanical metamaterial unit cells. The proposed flexure joints are 3D printed with single flexible material in monolithic fashion using a standard benchtop 3D printer. The application of the MFJ is demonstrated in robotic in-hand manipulation and grasping thin and deformable objects such as wires and cables. The results show the capability and advantages of the proposed MFJ in soft robotic grippers and highly functional bionic hands.

A 4D-printed programmable soft network with fractal design and adjustable hydrophobic performance

<h2>Summary</h2> The concept of creating novel structures by drawing inspiration from the natural world has broad prospects in bio-integrated electronics. Auxetic mechanical metamaterials have wide applications in flexible sensors, implantable medical devices, and soft robots. Here, by combining 4D-printed fractal-inspired metamaterials with microlevel biomimetic electrodes, we propose a shape memory sensor with functional and structural reconfiguration, enhanced sensitivity, and high flexibility. First, we propose the design conception and mechanical model of the 4D-printed metamaterials. Then, the metamaterial skeleton is integrated with shape memory biomimetic electrodes, which prove to possess adjustable hydrophobic and electrical performance. The derived health-monitoring sensor proves to possess reconfiguration ability in electrical and mechanical properties according to the external stimulus and high sensitivity to signal feedback with high flexibility and compliance to the human body. This work demonstrates potential applications of 4D-printed reconfigurable sensors in the fields of wearable electronics and human-computer interaction.

Energy dissipation in multistable auxetic mechanical metamaterials

Multistable mechanical metamaterials are periodic structures whose unit cells show bistable configurations. The snap-through behavior of the unit cell enhances the energy dissipation of the metamaterials. The materials are reusable and can dissipate energy multiple times since the deformation is within the elastic region. This paper presents two types of two-dimensional auxetic metamaterials: the shuriken-shaped motif with four axes of symmetry and the triangle motif with six axes of symmetry. The cylindrical and three-dimensional (3D) configurations are designed with the geometric topology method. We carry out experiments with 3D printed samples, finite element analysis (FEA), and theoretical investigation to reveal the various mechanisms of energy dissipation. The results demonstrate that the planar metamaterials exhibit different energy dissipation along various loading directions. Each type of metamaterial exhibits discrepant force-displacement characteristics. The triangle motif performs better than the shuriken-shaped motif in terms of energy dissipation along some loading directions. The proposed auxetic metamaterials perform better than the conventional multistable structures with zero Poisson’s ratio. Energy dissipation and auxetic behavior of the typically cylindrical and 3D metamaterials are verified numerically and experimentally.

A pixel design method for mechanical metamaterials based on topology optimization

Mechanical metamaterials attract a lot of interest because of the peculiar mechanical properties. Especially in adjustment of Poisson’s ratio of structures, the auxetic mechanical metamaterials have shown their outstanding advantages. Some topology optimization methods are also used to optimize the auxetic structures. In this article, we propose a pixel design method for mechanical metamaterials arrangement based on topology optimization. Metamaterials optimized with two different objectives: the maximum stiffness (MS) and the maximum stiffness with minimum Poisson’s ratio (MSMP) are obtained by this method. The lattice of the metamaterial is in uniform size and its stiffness can be adjusted. The topology optimization is used to achieve the optimal stiffness distribution of the design area. Then, the stiffness distribution is pixelated according to the lattice size. The required metamaterials can be achieved by filling the lattice structure into this pixel image. The experimental results are consistent with the simulation.

Mechanical behaviors of a novel auxetic honeycomb characterized by re-entrant combined-wall hierarchical substructures

The current focus of the metamaterials is to further improve their performance by the unit cell innovation, while for the auxetic metamaterials, the compromise between the mechanical properties and auxetic effect still needs more efforts. Given this issue, here we developed a novel auxetic honeycomb, named re-entrant combined-wall (RCW) honeycomb, by introducing four hierarchical substructures to the RE cell. Analytical expressions were derived and used to study the in-plane elastic constants of the RCW honeycomb, which were well confirmed by the established finite element model. Further, we investigated its crushing behaviors under large deformation by the explicit numerical method, and the quasi-static crushing experiments were also carried out by the 3D-printed specimens. Results show that the properties of the proposed RCW honeycomb have a high degree of orthogonality and tunability. Compared with the traditional RE honeycomb, the Young’s modulus of the RCW honeycomb in the y direction increases by more than 120%, and the Poisson’s ratio decreases by about 43%. Besides, behaviors of the cell wall contact induced by the adding substructure can lead to an interesting stress enhancement phenomenon under large deformation, which significantly increases its crushing strength, up to 140%, compared with the RE honeycomb. Therefore, the results in this work effectively demonstrate the improved mechanical properties and auxetic performance of the proposed RCW honeycomb. Besides, the adopted design strategy of hierarchical substructure also exhibits great potential for developing novel and excellent auxetic mechanical metamaterials.

Multiscale Strain Field Characterization in Flexible Planar Auxetic Metamaterials with Rotating Squares

Auxetic mechanical metamaterials show significant potential to impact many engineering fields and have been a topic of considerable research interest in recent years. Existing literature on the topic often aims to achieve larger negative Poisson's ratios or tailorable responses by carefully designed and distributed unit cells. Herein, it is aimed to investigate the relationships between global and local strain fields in rectangular center‐symmetric perforated planar structures, thus highlighting the role of local morphology on the macroscopic material response. Additively manufactured samples with hyperelastic constitutive behavior are characterized under tension. The structures are designed and developed with several perforation aspect ratios, leading to various degrees of auxeticity. Global and local strain fields are characterized using a multiscale digital image correlation measurement approach. The local rotation and in‐plane strain fields generated within the solid portions of the unit cells are correlated with the global strain fields and macroscopic Poisson's ratios for a range of cell geometries. The interplay between cell rotation and strain at the meso (unit cell) scale is shown to be the dominant factor in the strain‐dependent evolution of the Poisson's ratio in the structures.

Additively Manufactured Hierarchical Auxetic Mechanical Metamaterials.

Due to the ability to create structures with complex geometry at micro- and nanoscales, modern additive technologies make it possible to produce artificial materials (metamaterials) with properties different from those of conventional materials found in nature. One of the classes with special properties is auxetic materials—materials with a negative Poisson’s ratio. In the review, we collect research results on the properties of auxetics, based on analytical, experimental and numerical methods. Special attention of this review is paid to the consideration of the results obtained in studies of hierarchical auxetic materials. The wide interest in the hierarchical subclass of auxetics is explained by the additional advantages of structures, such as more flexible adjustment of the desired mechanical characteristics (the porosity, stiffness, specific energy absorption, degree of material release, etc.). Possibilities of biomedical applications of hierarchical auxetic materials, such as coronary stents, filtration and drug delivery systems, implants and many others, where the ability for high-precision tuning is required, are underlined.

Bio-inspired auxetic mechanical metamaterials evolved from rotating squares unit

This work investigates the mechanical response of bio-inspired titanium mechanical metamaterials with negative Poisson's ratio evolved from rotating squares unit. The systems were designed and optimized using finite element analysis, with peculiar focus on the shape profile at the interconnection regions of the rotating units. The proposed solution consists of a combined auxetic rotating/chiral architecture with enhanced mechanical and topological properties which exhibits a 3% of global elastic strain of the structure and a Poisson's ratio equal to −0.94. Numerical results are in good agreement with those obtained from experimental tests on a 3D printed Onyx prototype. We also examined the effect on the structural response of the metamaterial subject to off-axis mechanical conditions revealing the strong correlation to the geometrically-related anti-tetrachiral honeycombs. The work confirms the great potential of biologically inspired auxetic metamaterials, which can be designed to obtain tailored mechanical properties while improving the elastic strains capabilities of the system.

Review: Auxetic Polymer-Based Mechanical Metamaterials for Biomedical Applications.

Over the last three decades but more particularly during the last 5 years, auxetic mechanical metamaterials constructed from precisely architected polymer-based materials have attracted considerable attention due to their fascinating mechanical properties. These materials present a negative Poisson's ratio and therefore unusual mechanical behavior, which has resulted in enhanced static modulus, energy adsorption, and shear resistance, as compared with the bulk properties of polymers. Novel advanced polymer processing and fabrication techniques, and in particular additive manufacturing, allow one to design complex and customizable polymer architectures that are particularly relevant to fabricate auxetic mechanical metamaterials. Although these metamaterials exhibit exotic mechanical properties with potential applications in several engineering fields, biomedical applications seem to be one of the most relevant with a growing number of articles published over recent years. As a result, special focus is needed to understand the potential of these structures and foster theoretical and experimental investigations on the potential benefits of the unusual mechanical properties of these materials on the way to high performance biomedical applications. The present Review provides up to date information on the recent progress of polymer-based auxetic mechanical metamaterials mainly fabricated using additive manufacturing methods with a special focus toward biomedical applications including tissue engineering as well as medical devices including stents and sensors.

The Synergy of Biomimetic Design Strategies for Tissue Constructs

AbstractThe regeneration and repair of complex structures, interfaces, and mechanical properties present in natural tissues remains a challenge. To move beyond simplified tissue engineered constructs, nature is a source of inspiration for complex, hierarchical scaffold designs. Recent advances in additive manufacturing allow for increasingly complex fabrication of architectures that better mimic the multiscale structure‐function relationship found in natural tissues. In this review, scaffold‐based and scaffold‐free approaches and the synergistic use of fabrication technologies (two things make a third) to produce more biomimetic implants are described. Recent advanced scaffold designs such as auxetic mechanical metamaterials and induced fibrillar alignment are highlighted. Next, the pre‐programmed assembly of spheroids, tissue strands, and other modular building blocks without the need for permanent exogenous scaffold support are discussed. Furthermore, the application of hybridized manufacturing processes to fabricate hierarchical functional constructs is outlined for the osteochondral unit, vascular grafts, and peripheral nerves.

Tailoring auxetic mechanical metamaterials to achieve patterned wire strain sensors with controllable high sensitivity

A novel design concept of auxetic mechanical metamaterials is proposed to achieve controllable high sensitivity as well as good stretchability and durability of wire strain sensors. Semi-auxetic and contractile architectures are derived from an auxetic architecture and collected as three master units. To achieve an adjustable structural Poisson’s ratio (ν), patterned wire strain sensors with six units are constructed. Sensitivity of wire strain sensors is tuned by tailoring the ratio and location of three master units. High sensitivity, good stretchability, and durability are related to continuous stretching and structural synergetic effects. Based on the results of finite element modeling and sensing performance, relationship between structural ν, effective contributions, and sensitivity is presented. To evaluate the effectiveness of proposed scheme, wire strain sensors with different shapes and a 2D auxetic network sensor are fabricated. The analysis reveals that wire strain sensors with six auxetic units show high sensitivity (GF = 21.8 at ∼ 80–130% strain), high stretchability (130%), excellent durability (up to 1000 r), and fast response time. Morphological evolution is tracked to explain the excellent durability. Because of controllable sensitivity, our sensors can be used for weight and displacement monitoring of objects with a small mass.

Micro-Scale Auxetic Hierarchical Mechanical Metamaterials for Shape Morphing.

Shape morphing and the possibility of having control over mechanical properties via designed deformations have attracted a lot of attention in the materials community and led to a variety of applications with an emphasis on the space industry. However, current materials normally do not allow to have a full control over the deformation pattern and often fail to replicate such behavior at low scales which is essential in flexible electronics. Thus, in this paper, novel 2Dand 3D microscopic hierarchical mechanical metamaterials using mutually-competing substructures within the system that are capable of exhibiting a broad range of the highly unusual auxetic behavior are proposed. Using experiments (3D microprinted polymers) supported by computer simulations, it is shown that such ability can be controlled through geometric design parameters. Finally it is demonstrated that the considered structure can form a composite capable of shape morphing allowing it to deform to a predefined shape.