Large Reversible Deformation Research Articles

3D printing of reprogrammable liquid crystal elastomers with exchangeable boronic ester bonds

Liquid crystal elastomers (LCEs) are a kind of soft actuating materials with large reversible deformation ability, which can work as the “motor” to exhibit complex deformations and drive the locomotion of soft robots. The deformation of LCEs depends on the three-dimensional (3D) shape of whole structure and alignment patterns of mesogens. Various methods have been employed to fabricate the LCE structure with desired shapes and mesogen alignments. However, conventional 3D printed LCEs require continuous thermal energy input to maintain their actuated shapes. The LCEs cannot be reprocessed and reprogrammed once cured. Herein, we introduce dynamic boronic ester bonds into the ink, with which the printed LCE structures are capable of being reprogrammed from polydomain into monodomain state and vice versa. We further explore the effects of printing parameters and content of dynamic covalent bonds on the actuation performance and reprogramming ability. The actuated shape could be well predicted with finite element method. The dynamic printable LCEs developed here offer new strategy and large design space for LCE structures.

When the Poisson Ratio of Polymer Networks and Gels Is Larger Than 0.5?

We use coarse-grained molecular dynamics simulations to study deformation of networks and gels of linear and brush strands in both linear and nonlinear deformation regimes under constant pressure conditions. The simulations show that the Poisson ratio of networks and gels could exceed 0.5 in the nonlinear deformation regime. This behavior is due to the ability of the network and gel strands to sustain large reversible deformation, which, in combination with the finite strand extensibility results in strand alignment and monomer density, increases with increasing strand elongation. We developed a nonlinear network and gel deformation model which defines conditions for the Poisson ratio to exceed 0.5. The model predictions are in good agreement with the simulation results.

Powder-blown laser-based directed energy deposition of (14M) Ni-Mn-Ga magnetic shape memory alloy

Ni-Mn-Ga-based magnetic shape memory (MSM) alloys are renowned for their large magnetic-field-induced strains, making them attractive for compact magnetically controlled actuators requiring high response frequencies and large reversible deformations. In this study, Ni-Mn-Ga alloy samples were deposited using powder-blown laser-based directed energy deposition (DED-LB), employing gas-atomised powder pre-alloyed with excess Mn. The samples were deposited following a systematic experimental design, varying the applied laser power and employing two melting strategies: unidirectional and bidirectional. The results demonstrate the feasibility of achieving high relative densities (>97.5 %) in multi-layered Ni-Mn-Ga samples through DED-LB. All as-deposited samples exhibited a consistent 0.7–0.9 at% Mn loss in comparison to the powder feedstock, thus showing that the Mn overdosing requirements for DED-LB differ from those in laser powder bed fusion. Both as-deposited and heat-treated samples exhibited seven-layered modulated (14 M) martensite structures at ambient temperature. In contrast, the heat-treated samples also exhibited fully reversible martensite transformation at around 66 ºC and a Curie temperature at around 94 ºC. Distinct microstructural characteristics were observed based on the melting strategy, with bidirectional melting promoting the formation of large columnar grains with a strong <100> texture approximately along the build direction. This study highlights the potential of DED-LB processes in manufacturing relatively large (mm to cm scale) functional Ni-Mn-Ga actuators and contributes to the ongoing efforts in laser additive manufacturing of functional materials.

Liquid Crystal Orientation and Shape Optimization for the Active Response of Liquid Crystal Elastomers.

Liquid crystal elastomers (LCEs) are responsive materials that can undergo large reversible deformations upon exposure to external stimuli, such as electrical and thermal fields. Controlling the alignment of their liquid crystals mesogens to achieve desired shape changes unlocks a new design paradigm that is unavailable when using traditional materials. While experimental measurements can provide valuable insights into their behavior, computational analysis is essential to exploit their full potential. Accurate simulation is not, however, the end goal; rather, it is the means to achieve their optimal design. Such design optimization problems are best solved with algorithms that require gradients, i.e., sensitivities, of the cost and constraint functions with respect to the design parameters, to efficiently traverse the design space. In this work, a nonlinear LCE model and adjoint sensitivity analysis are implemented in a scalable and flexible finite element-based open source framework and integrated into a gradient-based design optimization tool. To display the versatility of the computational framework, LCE design problems that optimize both the material, i.e., liquid crystal orientation, and structural shape to reach a target actuated shapes or maximize energy absorption are solved. Multiple parameterizations, customized to address fabrication limitations, are investigated in both 2D and 3D. The case studies are followed by a discussion on the simulation and design optimization hurdles, as well as potential avenues for improving the robustness of similar computational frameworks for applications of interest.

Experimental Study of Nitinol Springs: Apparatus and Results

BackgroundThe behavior of shape memory alloys that admit large reversible deformations in response to thermal excitation has been extensively studied in recent years. Yet, the number of works dealing with springs made from these alloys is rather limited in spite of their attractiveness in various applications.ObjectiveTo bridge this gap we designed and constructed an experimental system for characterizing the behavior of the springs. It enables precise control of the three state variables: temperature, elongation, and force.MethodsControl of the sample temperature is achieved by immersing it in a water-filled thermal bath, where the water temperature is adjusted using a thermoelectric Peltier device. A tension-compression motorized unit sets the spring elongation and a force gauge is used for measuring the force exerted on the spring. The data is continuously monitored and acquired with a self-coded LabVIEW program. An important aspect is the calibration procedure developed for identifying the spring load-free state and ensuring the repetitiveness of the measurements.ResultsExperiments in which the elongation or the force were measured as a function of the temperature demonstrate the role of the phase transformations. Isothermal experiments enabled to characterize the variations of the force versus the elongation at different temperatures.ConclusionsThe proposed system facilitates the execution of highly accurate experiments through which the complex history-dependent behavior of shape memory springs can be revealed and studied.

Programming Motion into Materials Using Electricity-Driven Liquid Crystal Elastomer Actuators.

As thermally driven smart materials capable of large reversible deformations, liquid crystal elastomers (LCEs) have great potential for applications in bionic soft robots, artificial muscles, controllable actuators, and flexible sensors due to their ability to program controllable motion into materials. In this article, we introduce conductive LCE actuators using a liquid metal electrothermal layer and a polyethylene terephthalate substrate. Our LCE actuators can be stimulated at low currents from 2 to 4 A and produce a maximum work density of 9.4 . We illustrate the potential applications of this system by designing a palm-activated artificial muscle gripper, which can be used to grasp soft objects ranging from 5 to 55 mm in size, and even ring-shaped workpieces with precise external or internal support. Furthermore, inspired by the movement of fruit fly larvae, we designed a new soft robot capable of bioinspired crawling and turning by inducing anisotropic friction with an asymmetric design. Finally, we illustrate advanced motional control by designing an autonomously rotating wheel based on the asymmetric contraction of its spokes. To assist in the production of autonomously moving robots, we provide a thorough characterization of its motion dynamics.

Analyzing the bending deformation of van der Waals-layered materials by a semi-discrete layer model

Van der Waals (vdW)-layered materials, such as graphite, exhibit unique mechanical properties owing to their structural and mechanical anisotropies. This study reports the development of a mechanical model that reproduces the characteristics of the nonlinear and reversible bending deformation of vdW-layered materials, while taking into account the microscopic mechanism of the discrete interlayer slips. The vdW-layered material was modeled as a stack of interacting discrete deformable layers (semi-discrete layer model), and the interlayer interaction was modeled using a cohesive zone model that reproduced the localized interlayer slip. Using the finite-element method, out-of-plane bending deformation analyses were performed on the cantilevers of the highly oriented pyrolytic graphite (HOPG) and MoTe2, and the validity of the model was verified by comparing it with the experimental results. The model accurately reproduced the loading and unloading behaviors in the experiments for the submicron HOPG cantilevers or the large nonlinear and reversible deformation with a hysteresis loop. Furthermore, the model reproduced well the characteristics of the bending experiments for the micro-MoTe2 cantilevers, or the intermittent decrease in stiffness during the loading process and deformation restoration during the unloading process. These results demonstrated that the designed semi-discrete layer model can be universally applied to reproduce the bending deformation characteristics of a variety of vdW-layered materials and can be employed to effectively elucidate the underlying deformation mechanisms.

Load-induced local phase transformation and modulus of shape memory alloys under spherical indentation by finite element method

Shape memory alloys are a unique class of materials that are capable of large reversible deformations under external stimuli such as stress or temperature. The present study examines the phase transformations and mechanical responses of NiTi and NiTiHf shape memory alloys under the loading of a spherical indenter by using a finite element model. It is found that the indentation unloading curves exhibit distinct changes in slopes due to the reversible phase transformations in the SMAs. The normalized contact stiffness (F/S2) of the SMAs varies with the indentation load (depth) as opposed to being constant for conventional single-phase materials. The load-induced phase transformation that occurred under the spherical indenter was simulated numerically. It is observed that the phase transformation phenomenon in the SMA induced by an indentation load is distinctly different from that induced by a uniaxial load. A pointed indenter produces a localized deformation, resulting in a stress (load) gradient in the specimen. As a result, the transformation of phases in SMAs induced by an indenter can only be partially completed. The overall modulus of the SMAs varies continuously with the indentation load (depth) as the average volumetric fraction of the martensite phase varies. For NiTi (Ea &gt; Em), the modulus decreases with the depth, while for NiTiHf (Ea &lt; Em), the modulus increases with the depth. The predicted young modules during indentation modeling agree well with experimental results. Finally, the phase transformation of the SMAs under the indenter is not affected by the post-yield behavior of the materials.

Monodomain liquid crystal elastomer bionic muscle fibers with excellent mechanical and actuation properties

Monodomain liquid crystal elastomers (m-LCEs) exhibit large reversible deformations when subjected to light and heat stimuli. Herein, we developed a new method for the large-scale continuous preparation of m-LCE fibers. These m-LCE fibers exhibit a reversible contraction ratio of 55.6%, breaking strength of 162MPa (withstanding a load of 1 million times its weight), and maximum output power density of 1250J/kg, surpassing those of previously reported m-LCEs. These excellent mechanical properties are mainly attributed to the formation of a homogeneous molecular network. Furthermore, the fabrication of m-LCEs with permanent plasticity using m-LCEs with impermanent instability without external intervention was realized by the synergistic effects of the self-restraint of mesogens and the prolonged relaxation process of LCEs. The designed LCE fibers, which are similar to biological muscle fibers and can be easily integrated, exhibit broad application prospects in artificial muscles, soft robots, and micromechanical systems.

Bio-Inspired Soft-Rigid Hybrid Smart Artificial Muscle Based on Liquid Crystal Elastomer and Helical Metal Wire.

Artificial muscles are of significant value in robotic applications. Rigid artificial muscles possess a strong load-bearing capacity, while their deformation is small; soft artificial muscles can be shifted to a large degree; however, their load-bearing capacity is weak. Furthermore, artificial muscles are generally controlled in an open loop due to a lack of deformation-related feedback. Human arms include muscles, bones, and nerves, which ingeniously coordinate the actuation, load-bearing, and sensory systems. Inspired by this, a soft-rigid hybrid smart artificial muscle (SRH-SAM) based on liquid crystal elastomer (LCE) and helical metal wire is proposed. The thermotropic responsiveness of the LCE is adopted for large reversible deformation, and the helical metal wire is used to fulfill high bearing capacity and electric heating function requirements. During actuation, the helical metal wire's resistance changes with the LCE's electrothermal deformation, thereby achieving deformation-sensing characteristics. Based on the proposed SRH-SAM, a reconfigurable blazed grating plane and the effective switch between attachment and detachment in bionic dry adhesion are accomplished. The SRH-SAM opens a new avenue for designing smart artificial muscles and can promote the development of artificial muscle-based devices.

Tailoring Rate and Temperature-Dependent Fracture of Polyether Networks with Organoaluminum Catalysts

Polyethers are ubiquitous in engineering and biomedical applications because their oxygen-rich backbone allows them to interact with a variety of polar small molecules such as ions, gases, and pharmaceuticals. These materials are also able to sustain large reversible deformations when cross-linked at the molecular scale, leading to an interesting combination of functional and mechanical properties. We synthesized two families of polyether networks by organoaluminum-catalyzed ring-opening copolymerization of ethyl glycidyl ether (EGE) monomer and 1,4-butanediol diglycidyl ether (BDGE) cross-linker and explored the relationship between network architecture and fracture properties. The key result is that living copolymerizations, as enabled by a chelate of triethylaluminum with dimethylaminoethanol, afford access to a critical cross-link density, νx ≈ 3 × 1025 chains/m3, and loss tangent, tan(δ) ≈ 0.09, at which fracture is dominated by chain scission rather than friction. Such control over the fracture resistance of polyether networks unveils the potential of living copolymerizations to design the functional and mechanical properties of soft materials.

Flows in Straws: Viscous Flows in One-Dimensional Metamaterial

The authors study the interaction between a viscous flow and a one-dimensional mechanical metamaterial composed of bistable unit cells, much like a drinking straw. Their twofold model shows excellent agreement with experimental results. While prior studies have focused on tailoring unit-cell structure to obtain a desired multistable behavior, here the viscous fluid allows tuning of rate-dependent properties and nonlocal interaction between unit cells. The ability to manipulate both structural and nonlocal rate-related properties points to the development of smart materials with tunable kinetic relations, large reversible deformations, and tailored dynamic properties.

A programable biomimetic actuator with large and reversible deformation based on commercial poly (ethylene-co-vinyl acetate)

Due to the reversible and remote actuation under applied voltage, shape-memory polymer (SMP) based on semi-crystalline polymer are explored for developing soft actuators, where the commercial material, poly (ethylene-co-vinyl acetate) (EVA), has great potential for the large-scale application to construct an actuator, due to its cost-effective and crystal-adjustable merits. However, the small deformation of SMP based on commercial poly (ethylene-co-vinyl acetate) (EVA) (usually less than 10%) is always a problem which limits their practical application. Herein, combining the pre-stretched and photo-crosslinked EVA, a biomimetic actuator with large reversible deformation (∼20%) is designed and fabricated by compounding with polyimide (PI) with screen-printed Ag conductive network. Using chemical crosslinking points as the new skeleton domain can allow the original crystal of the skeleton domain to involve in the reversible actuation to obtain large deformation. Meanwhile, due to the chemical crosslinking points formed by photo-crosslinking in EVA, the resultant composites can be actuated at temperatures above the melting point, which is critical to reducing the constraints of actuating conditions. In addition, the perfect conductivity prepared by the Ag conductive network endows this composite with a low voltage (≤25 V/m) actuating capacity. Based on the good programming capability of composites, various modes containing zero-set reversible actuation are designed. Particularly, a quadruped robot can stand up quickly under an applied voltage of 5 V and return to a zero-set state after voltage removal, which proves the application potential of the composite in flexible robotics.

Functionally Graded Soft Dielectric Elastomer Phononic Crystals: Finite Deformation, Electro-Elastic Longitudinal Waves, and Band Gaps Tunability via Electro-Mechanical Loading

Active tuning of elastic wave band gaps received significant attention in the recent past. To this end, soft dielectric elastomers (DEs) have indicated great promise owing to their capability to undergo large reversible deformations and deformation-dependent constitutive properties. In this paper, we investigate the longitudinal wave band gaps an infinite periodic soft functionally graded dielectric elastomer subjected to electro-mechanical biasing fields. Material properties of the functionally graded DE are described by the power law. The nonlinear hyperelastic compressible neo–Hookean model is employed for characterizing the constitutive behavior of the soft functionally graded dielectric elastomer. The finite element method with Bloch–Floquet theory is employed for finding the band-structure of the infinitely periodic soft functionally graded DE structure. A parametric study brings out the effect of biasing fields and power law exponent on the band gap characteristics. The biasing mechanical prestress, the electric displacement field show significant influence on position and the width of band gaps. The inferences reported here can find their potential use in the design of soft DE wave devices with tunable band structures.

A Literature Review on Beam Column Joints with Different Loading Condition and Methods of Strengthening

Abstract: The beam–column joint is measured as the most important zone in a reinforced concrete moment resisting frame. It is subjected to large forces during earthquake and its behaviour has a major influence on the response of the entire structure. As a result, a great attention has to be paid for good detailing of such joint. The absence of transverse reinforcement in the joint, insufficient development length for the beam reinforcement and the inadequately spliced reinforcement for the column just above the joint can be considered as the most important causes for the failure of the beam–column joint under any unexpected transverse loading on the building. The recent earthquakes revealed the importance of the design of reinforced concrete (RC) structures with ductile behaviour. Ductility can be described as the ability of reinforced concrete cross sections, elements and structures to absorb the large energy released during earthquakes without losing their strength under large amplitude and reversible deformations.

A NURBS-based inverse analysis of swelling induced morphing of thin stimuli-responsive polymer gels

Gels are a mixture of cross-linked polymers and solvents, and have been widely studied in recent years for a diverse range of biomedical applications. Because gels can undergo large, reversible shape changes due to swelling, their complex physical response must be modeled by coupling large reversible deformation and mass transport. An ongoing challenge in this field is the ability to capture swelling or residual swelling-induced of such stimuli-responsive gels from initially flat two-dimensional (2D) to three-dimensional (3D) curved shapes. Specifically, because such shape changes typically involve large deformations, shape changes, and the exploitation of elastic instabilities, it remains an open question as to what external stimulus should be prescribed to generate a specific target shape. Therefore, we propose a novel formulation that tackles, using both nonlinear kinematics and material models, the coupling between elasticity and solvent transport using Kirchhoff–Love shell theory discretized using isogeometric analysis (IGA). Second, we propose an inverse methodology that chemomechanically couples large deformation and mass transport to identify the external stimuli prescribed to generate a specific target shape. Our numerical examples demonstrate the capability of identifying the required external stimuli, with the implication that the reconstructed target shapes are accurate, including cases where the shape changes due to swelling involve elastic instabilities or softening. Overall, our study can be used to effectively predict and control the large morphological changes of an important class of stimuli-responsive materials.

PH-Responsive Hydrogel Bilayer With Reversible, Bidirectional Bending Behavior

Smart hydrogels are promising materials for shape-shifting structures regarding their large reversible deformation in response to external stimuli in the absence of mechanical loading. Actuators composed of responsive hydrogels have gained significant attention due to their low power consumption, bio-compatibility, fast response, and accessibility. Among these structures, bidirectional hydrogel-based actuators are more fascinating, especially when they have similar reversible bending in both directions. This paper introduces a new design concept of a hydrogel bilayer made of a poly (HEMA-co-DMAEMA) layer and a poly (HEMA-co-AA) hydrogel layer that swells at low and high pH, respectively. This structure is capable of bending in diverse directions while the pH of the aqueous bath alters. The main characteristic of this structure is having reversible bidirectional bending, which has similar behaviors in both directions, unlike previous hydrogel-elastomer bilayers. Then, we develop an analytical method to solve the swelling-induced bidirectional bending of a pH-sensitive hydrogel bilayer. On the other hand, the finite bending of bilayer structure is studied by the finite element method in several cases to demonstrate the validity and accuracy of the proposed analytical solution. Lastly, the impacts of material composition and geometrical factors are investigated to be used for bilayer actuator design and application.

Dynamics of pneumatically coupled visco-hyperelastic dielectric elastomer actuators: Theoretical modeling and experimental investigation

Dielectric elastomers represent a class of electroactive polymers that are capable of undergoing large reversible deformations when driven electrically. Pneumatically connected DE (PCDE) actuator is one of the prevalently used configurations, wherein an electrically driven DE membrane is connected to another passive DE membrane via an intervening air column. This paper outlines the development of a theoretical model, well-supported by experimental observations, for analyzing the dynamic electromechanical response of PCDE actuators. The underlying energetics of the model is developed in the setting of the Hamilton’s principle taking into account the effects of membrane pre-stretch, initial inflation pressure, membrane viscoelasticity, and time-dependent electric loading. An idea of pseudo air-spring, responsible for the transfer of deformation from the active to passive sides of the actuator, is introduced and the analytical expressions are developed for the coupling function. Both pressurized and depressurized states are considered in the analysis. The developed nonlinear dynamic model is then used for building insights into several influential parameters, such as initial inflation pressure, time-dependent (DC and AC) electric actuation, height of the confined air column, volume of the intervening chamber, etc. The proposed model predicts the transfer efficiency of about 80% between the active and the passive membranes. The predictive capability of the theoretical model is established through comparisons with the experimental observations. The proposed investigation can find its potential use in the design and analysis of interconnected DE actuators subjected to a dynamic electromechanical actuation.

Topology optimization of soft compressible phononic laminates for widening the mechanically tunable band gaps

Active tuning of frequency bandgaps has received significant attention in the recent past. To this end, soft compressible phononic composites have indicated great promise owing to their capability to undergo large reversible deformations and deformation dependent constitutive properties. In this investigation, we present an optimization framework for arriving at the optimum topology of a two-phase soft compressible phononic composite for maximizing the frequency gap between the first four consecutive frequency bands of longitudinal wave propagation. The laminate is subjected simultaneously to an equal biaxial pre-stretch in the plane perpendicular to the direction of wave propagation and a prestress in the direction of wave propagation. The nonlinear hyperelastic neo-Hookean material model is employed for characterizing the constitutive behavior of the soft compressible laminate phases. The finite element method with Bloch–Floquet theory is employed for finding the band-structure of the infinitely periodic soft laminate. A gradient-based method of moving asymptotes is used to solve the underlying topology optimization problem. In particular, we present the optimum unit cell designs for two separate problems involving maximization of the absolute width of the band gap and that of the relative width of the band gap when compared to that in the un-deformed laminate. Test cases are presented to highlight the effects of the levels of biaxial prestretch and the longitudinal prestress. Our results indicate that a compressional longitudinal prestress and a lateral prestretch have a favorable impact on widening of the frequency band-gaps. The numerical framework and the inferences reported here can find their potential use in the optimal design of soft compressible composites used in acoustic applications.

Programmable self-folding of trilayer and bilayer-hinge structures by time-dependent swelling of tough hydrogels

Inspired by nature, active materials are used to developed complex 3D configurations considering their specific characteristics. One of the shape-shifting methods in smart structures is utilizing programmable materials in self-folding structures. Hydrogels due to their biocompatibility, controllable functionalities, large reversible deformations, and their sensitivity to environmental stimuli are vital candidates to be used in self-folding structures. To avoid the mechanical weakness of conventional hydrogels, in this paper polyampholyte tough hydrogel is inspected considering a transient electro-chemo-mechanical constitutive model combining a visco-hyperelastic model with Nernst-Planck and Poisson’s equations. After calibrating the material parameters and verifying the accuracy of the model and its implementation, we present two approaches in order to generate self-folding hydrogel-based structures: polymer structure with bilayer hinges and trilayer structure composed of a hydrogel film sandwiched between two elastomer layers. Next, diverse factors are examined in the self-folding of smart structures which conforms with experimental test data, including hydrogel swelling, structure thickness and stiffness, bilayer configuration and composition, the width of the bilayer as well as opening width in trilayer and layers thickness. Finally, several transient self-folding of 2D flat patterns which turn into 3D complex configurations are scrutinized such as the closure of box, pyramids, and flower-shaper gripper.