Active Mechanical Metamaterial Research Articles

3D printing of active mechanical metamaterials: A critical review

The emergence of mechanical metamaterials from 4D printing has paved the way for developing advanced hierarchical structures with superior multifunctionalities. In particular, 4D-printed mechanical metamaterials exhibit extraordinary mechanical performance by integrating multiphysics stimuli with advanced structures when actuated by external factors, thereby altering their shapes, properties, and functionalities. This critical review offers readers a comprehensive overview of the rapidly growing 4D printing technology for developing novel mechanical metamaterials. It provides essential information about the multifunctionalities of 4D-printed mechanical metamaterials, including energy absorption and shape-morphing behavior in response to physical, chemical, or mechanical stimuli. These capabilities are key to developing smart and intelligent structures for multifunctional applications such as biomedical, photonics, acoustics, energy storage, and thermal insulation. The primary focus of this review is to describe the structural and functional applications of mechanical metamaterials developed through 4D printing. This technology leverages the shape-shifting functions of smart materials in applications such as micro-grippers, soft robots, biomedical devices, and self-deployable structures. Additionally, the review addresses current progress and challenges in the field of 4D-printed mechanical metamaterials. In conclusion, recent developments in 4D-printed mechanical metamaterials could establish a new paradigm for applications in engineering and science.

Phase-transforming mechanical metamaterials with dynamically controllable shape-locking performance.

Active mechanical metamaterials with customizable structures and deformations, active reversible deformation, dynamically controllable shape-locking performance and stretchability are highly suitable for applications in soft robotics and flexible electronics, yet it is challenging to integrate them due to their mutual conflicts. Here, we introduce a class of phase-transforming mechanical metamaterials (PMMs) that integrate the above properties. Periodically arranging basic actuating units according to the designed pattern configuration and positional relationship, PMMs can customize complex and diverse structures and deformations. Liquid-vapor phase transformation provides active reversible large deformation while a silicone matrix offers stretchability. The contained carbonyl iron powder endows PMMs with dynamically controllable shape-locking performance, thereby achieving magnetically assisted shape locking and energy storing in different working modes. We build a theoretical model and finite element simulation to guide the design process of PMMs, so as to develop a variety of PMMs with different functions suitable for different applications, such as a programmed PMM, reconfigurable antenna, soft lens, soft mechanical memory, biomimetic hand, biomimetic flytrap and self-contained soft gripper. PMMs are applicable to achieve various 2D deformations and 2D-to-3D deformations, and integrate multiple properties, including customizable structures and deformations, active reversible deformation, rapid reversible shape locking, adjustable energy storing and stretchability, which could open a new application avenue in soft robotics and flexible electronics.

Extreme on-demand contactless modulation of elastic properties in magnetostrictive lattices

2D lattices are widely popular in micro-architected metamaterial design as they are easy to manufacture and provide lightweight multifunctional properties. The mechanical properties of such lattice structures are predominantly an intrinsic geometric function of the microstructural topology, which are generally referred to as passive metamaterials since there is no possibility to alter the properties after manufacturing if the application requirement changes. A few studies have been conducted recently to show that the active modulation of elastic properties is possible in piezoelectric hybrid lattice structures, wherein the major drawback is that complicated electrical circuits are required to be physically attached to the micro-beams. This paper proposes a novel hybrid lattice structure by incorporating magnetostrictive patches that allow contactless active modulation of Young’s modulus and Poisson’s ratio as per real-time demands. We have presented closed-form expressions of the elastic properties based on a bottom-up approach considering both axial and bending deformations at the unit cell level. The generic expressions can be used for different configurations (both unimorph or bimorph) and unit cell topologies under variable vertical or horizontal magnetic field intensity. The study reveals that extreme on-demand contactless modulation including sign reversal of Young’s modulus and Poisson’s ratio (such as auxetic behavior in a structurally non-auxetic configuration, or vice-versa) is achievable by controlling the magnetic field remotely. Orders of difference in the magnitude of Young’s modulus can be realized actively in the metamaterial, which necessarily means that the same material can behave both like a soft polymer or a stiff metal depending on the functional demands. The new class of active mechanical metamaterials proposed in this article will bring about a wide variety of design and application paradigms in the field of functional materials and structures.

Mechanical coupling effects of 2D lattices uncovered by decoupled micropolar elasticity tensor and symmetry operation

Mechanical couplings such as axial–shear and axial–bending have great potential in the design of active mechanical metamaterials with directional control of input and output loads in sensors and actuators. However, the current ad hoc design of mechanical coupling without theoretical support of elasticity cannot provide design guidelines for mechanical coupling with lattice geometries. Moreover, the correlation between mechanical coupling effects and geometric symmetry is not yet clearly understood. In this work, we systematically search for all possible mechanical couplings in 2D lattice structures by determining the non-zero diagonal terms in the decomposed micropolar elasticity tensor. We also correlate the mechanical couplings with the point-group symmetry of 2D lattices by applying the symmetry operation to the decomposed micropolar elasticity tensor. The decoupled micropolar constitutive equation uncovers eight coupling effects for 2D lattice structures. The symmetry operation of the decoupled micropolar elasticity tensor reveals the correlation of the mechanical coupling with the point groups. Our findings can strengthen the design of mechanical metamaterials with potential applications in areas including sensors, actuators, soft robots, and active metamaterials for elastic/acoustic wave guidance and thermal management.

Active mechanical metamaterial with embedded piezoelectric actuation

Metamaterials are artificially structured materials and exhibit properties that are uncommon or non-existent in nature. Mechanical metamaterials show exotic mechanical properties, such as negative stiffness, vanishing shear modulus, or negative Poisson’s ratio. These properties stem from the geometry and arrangement of the metamaterial unit elements and, therefore, cannot be altered after fabrication. Active mechanical metamaterials aim to overcome this limitation by embedding actuation into the metamaterial unit elements to alter the material properties or mechanical state. This could pave the way for a variety of applications in industries, such as aerospace, robotics, and high-tech engineering. This work proposes and studies an active mechanical metamaterial concept that can actively control the force and deformation distribution within its lattice. Individually controllable actuation units are designed based on piezostack actuators and compliant mechanisms and interconnected into an active metamaterial lattice. Both the actuation units and the metamaterial lattice are modeled, built, and experimentally studied. In experiments, the actuation units attained 240 and 1510 µm extensions, respectively, in quasi-static and resonant operation at 81 Hz, and 0.3 N blocked force at frequencies up to 100 Hz. Quasi-static experiments on the active metamaterial lattice prototype demonstrated morphing into four different configurations: Tilt left, tilt right, convex, and concave profiles. This demonstrated the feasibility of altering the force and deformation distribution within the mechanical metamaterial lattice. Much more research is expected to follow in this field since the actively tuneable mechanical state and properties can enable qualitatively new engineering solutions.

Recent Progress in Active Mechanical Metamaterials and Construction Principles.

Active mechanical metamaterials (AMMs) (or smart mechanical metamaterials) that combine the configurations of mechanical metamaterials and the active control of stimuli‐responsive materials have been widely investigated in recent decades. The elaborate artificial microstructures of mechanical metamaterials and the stimulus response characteristics of smart materials both contribute to AMMs, making them achieve excellent properties beyond the conventional metamaterials. The micro and macro structures of the AMMs are designed based on structural construction principles such as, phase transition, strain mismatch, and mechanical instability. Considering the controllability and efficiency of the stimuli‐responsive materials, physical fields such as, the temperature, chemicals, light, electric current, magnetic field, and pressure have been adopted as the external stimuli in practice. In this paper, the frontier works and the latest progress in AMMs from the aspects of the mechanics and materials are reviewed. The functions and engineering applications of the AMMs are also discussed. Finally, existing issues and future perspectives in this field are briefly described. This review is expected to provide the basis and inspiration for the follow‐up research on AMMs.

Optimal electromechanical bandgaps in piezo-embedded mechanical metamaterials

Elastic mechanical metamaterials are the exemplar of periodic structures. These are artificially designed structures having idiosyncratic physical properties like negative mass and negative Young’s modulus in specific frequency ranges. These extreme physical properties are due to the spatial periodicity of mechanical unit cells, which exhibit local resonance. That is why scientists are researching the dynamics of these structures for decades. This unusual dynamic behavior is frequency contingent, which modulates wave propagation through these structures. Locally resonant units in the designed metamaterial facilitate bandgap formation virtually at any frequency for wavelengths much higher than the lattice length of a unit. Here, we analyze the band structure of piezo-embedded negative mass metamaterial using the generalized Bloch theorem. For a finite number of the metamaterial units coupled equation of motion of the system is deduced, considering purely resistive and shunted inductor energy harvesting circuits. Successively, the voltage and power produced by piezoelectric material along with transmissibility of the system are computed using the backward substitution method. The addition of the piezoelectric material at the resonating unit increases the complexity of the solution. The results elucidate, the insertion of the piezoelectric material in the resonating unit provides better tunability in the band structure for simultaneous energy harvesting and vibration attenuation. Non-dimensional analysis of the system gives physical parameters that govern the formation of mechanical and electromechanical bandgaps. Optimized numerical values of these system parameters are also found for maximum first attenuation bandwidth. Thus, broader bandgap generation enhances vibration attenuation, and energy harvesting can be simultaneously available, making these structures multifunctional. This exploration can be considered as a step towards the active elastic mechanical metamaterials design.

Observation of non-Hermitian topology and its bulk–edge correspondence in an active mechanical metamaterial

Topological edge modes are excitations that are localized at the materials' edges and yet are characterized by a topological invariant defined in the bulk. Such bulk-edge correspondence has enabled the creation of robust electronic, electromagnetic, and mechanical transport properties across a wide range of systems, from cold atoms to metamaterials, active matter, and geophysical flows. Recently, the advent of non-Hermitian topological systems-wherein energy is not conserved-has sparked considerable theoretical advances. In particular, novel topological phases that can only exist in non-Hermitian systems have been introduced. However, whether such phases can be experimentally observed, and what their properties are, have remained open questions. Here, we identify and observe a form of bulk-edge correspondence for a particular non-Hermitian topological phase. We find that a change in the bulk non-Hermitian topological invariant leads to a change of topological edge-mode localization together with peculiar purely non-Hermitian properties. Using a quantum-to-classical analogy, we create a mechanical metamaterial with nonreciprocal interactions, in which we observe experimentally the predicted bulk-edge correspondence, demonstrating its robustness. Our results open avenues for the field of non-Hermitian topology and for manipulating waves in unprecedented fashions.

Recent advances in additive manufacturing of active mechanical metamaterials

Active mechanical metamaterials are emerging materials receiving tremendous attention in the past decades. Additive manufacturing (AM, or 3D printing) techniques empower the rational design and fabrication of complex, multiscale architectures that enable unprecedented mechanical properties and functionality of metamaterials. Moreover, the use of smart or stimuli-responsive materials for AM of active mechanical metamaterials offers new capabilities to program the mechanical, acoustic, or other functional properties. Herein, we present an overview of recent advances in AM of active mechanical metamaterials. The primary AM techniques used for mechanical metamaterial fabrication are discussed first. Several mechanical metamaterial structures and designs enabled by AM are summarized. Active mechanical metamaterial designs utilizing different stimuli, such as solvents, heat, and magnetic field, are introduced. Additionally, some functional applications of active material systems to create transforming mechanical metamaterials are included. Finally, the outlook and challenges for future research in this field are provided.

External Mean Flow on Sound Radiation of Active Mechanical Metamaterials

Sound pressure level (SPL) is one of the significant parameters of the acoustic–structure coupling for the elastic wave metamaterial. This paper presents the use of the external mean flow and the active feedback control system. Based on the locally resonant mechanism, the dynamic effective mass density is also discussed and the negative density appears at a lower frequency region. From the results, it can be seen that both the SPL and the effective mass density can be controlled by the active feedback system. As the frequency region of the negative density increases obviously, it emerges two negative regions with the changing of the displacement feedback control action and leads to the changing of the SPL. Besides the Mach number of the external mean flow, other influences on the SPL are also discussed, such as the structural damping, lattice constants, excitation position, elevation angle, and azimuth angle. Finally, these interesting effects on the elastic wave metamaterial are presented and explored.

Foundations for Soft, Smart Matter by Active Mechanical Metamaterials.

Emerging interest to synthesize active, engineered matter suggests a future where smart material systems and structures operate autonomously around people, serving diverse roles in engineering, medical, and scientific applications. Similar to biological organisms, a realization of active, engineered matter necessitates functionality culminating from a combination of sensory and control mechanisms in a versatile material frame. Recently, metamaterial platforms with integrated sensing and control have been exploited, so that outstanding non‐natural material behaviors are empowered by synergistic microstructures and controlled by smart materials and systems. This emerging body of science around active mechanical metamaterials offers a first glimpse at future foundations for autonomous engineered systems referred to here as soft, smart matter. Using natural inspirations, synergy across disciplines, and exploiting multiple length scales as well as multiple physics, researchers are devising compelling exemplars of actively controlled metamaterials, inspiring concepts for autonomous engineered matter. While scientific breakthroughs multiply in these fields, future technical challenges remain to be overcome to fulfill the vision of soft, smart matter. This Review surveys the intrinsically multidisciplinary body of science targeted to realize soft, smart matter via innovations in active mechanical metamaterials and proposes ongoing research targets that may deliver the promise of autonomous, engineered matter to full fruition.

Reversible signal transmission in an active mechanical metamaterial.

Mechanical metamaterials are designed to enable unique functionalities, but are typically limited by an initial energy state and require an independent energy input to function repeatedly. Our study introduces a theoretical active mechanical metamaterial that incorporates a biological reaction mechanism to overcome this key limitation of passive metamaterials. Our material allows for reversible mechanical signal transmission, where energy is reintroduced by the biologically motivated reaction mechanism. By analysing a coarse-grained continuous analogue of the discrete model, we find that signals can be propagated through the material by a travelling wave. Analysis of the continuum model provides the region of the parameter space that allows signal transmission, and reveals similarities with the well-known FitzHugh-Nagumo system. We also find explicit formulae that approximate the effect of the time scale of the reaction mechanism on the signal transmission speed, which is essential for controlling the material.

Atomimetic Mechanical Structures with Nonlinear Topological Domain Evolution Kinetics.

A mechanical metamaterial, a simple, periodic mechanical structure, is reported, which reproduces the nonlinear dynamic behavior of materials undergoing phase transitions and domain switching at the structural level. Tunable multistability is exploited to produce switching and transition phenomena whose kinetics are governed by the same Allen-Cahn law commonly used to describe material-level, structural-transition processes. The reported purely elastic mechanical system displays several key features commonly found in atomic- or mesoscale physics of solids. The rotating-mass network shows qualitatively analogous features as, e.g., ferroic ceramics or phase-transforming solids, and the discrete governing equation is shown to approach the phase field equation commonly used to simulate the above processes. This offers untapped opportunities for reproducing material-level, dissipative and diffusive kinetic phenomena at the structural level, which, in turn, invites experimental realization and paves the road for new active, intelligent, or phase-transforming mechanical metamaterials bringing small-scale processes to the macroscopically observable scale.