Soft Active Materials Research Articles

Topology Optimization of Hard-Magnetic Soft Phononic Structures for Wide Magnetically Tunable Band Gaps

Abstract Hard-magnetic soft materials, which exhibit finite deformation under magnetic loading, have emerged as a promising class of soft active materials for the development of phononic structures with tunable elastic wave band gap characteristics. In this paper, we present a gradient-based topology optimization framework for designing the hard-magnetic soft materials-based two-phase phononic structures with wide and magnetically tunable anti-plane shear wave band gaps. The incompressible Gent hyperelastic material model, along with the ideal hard-magnetic soft material model, is used to characterize the constitutive behavior of the hard-magnetic soft phononic structure phases. To extract the dispersion curves, an in-house finite element model in conjunction with Bloch’s theorem is employed. The method of moving asymptotes is used to iteratively update the design variables and obtain the optimal distribution of the hard-magnetic soft phases within the phononic structure unit cell. Analytical sensitivity analysis is performed to evaluate the gradient of the band gap maximization function with respect to each one of the design variables. Numerical results show that the optimized phononic structures exhibit a wide band gap width in comparison to a standard hard-magnetic soft phononic structure with a central circular inclusion, demonstrating the effectiveness of the proposed numerical framework. The numerical framework presented in this study, along with the derived conclusions, can serve as a valuable guide for the design and development of futuristic tunable wave manipulators.

Minimal actuation and control of a soft hydrogel swimmer from flutter instability

Micro-organisms propel themselves in viscous environments by the periodic, nonreciprocal beating of slender appendages known as flagella. Active materials have been widely exploited to mimic this form of locomotion. However, the realization of such coordinated beating in biomimetic flagella requires complex actuation modulated in space and time. We prove through experiments on polyelectrolyte hydrogel samples that directed undulatory locomotion of a soft robotic swimmer can be achieved by untethered actuation from a uniform and static electric field. A minimal mathematical model is sufficient to reproduce, and thus explain, the observed behavior. The periodic beating of the swimming hydrogel robot emerges from flutter instability thanks to the interplay between its active and passive reconfigurations in the viscous environment. Interestingly, the flutter-driven soft robot exhibits a form of electrotaxis whereby its swimming trajectory can be controlled by simply reorienting the electric field. Our findings trace the route for the embodiment of mechanical intelligence in soft robotic systems by the exploitation of flutter instability to achieve complex functional responses to simple stimuli. While the experimental study is conducted on millimeter-scale hydrogel swimmers, the design principle we introduce requires simple geometry and is hence amenable for miniaturization via micro-fabrication techniques. We believe it may also be transferred to a wider class of soft active materials.

Extrusion-based 3D printing of soft active materials.

Active materials are capable of responding to external stimuli, as observed in both natural and synthetic systems, from sensitive plants to temperature-responsive hydrogels. Extrusion-based 3D printing of soft active materials facilitates the fabrication of intricate geometries with spatially programmed compositions and architectures at various scales, further enhancing the functionality of materials. This Feature Article summarizes recent advances in extrusion-based 3D printing of active materials in both non-living (i.e., synthetic) and living systems. It highlights emerging ink formulations and architectural designs that enable programmable properties, with a focus on complex shape morphing and controllable light-emitting patterns. The article also spotlights strategies for engineering living materials that can produce genetically encoded material responses and react to a variety of environmental stimuli. Lastly, it discusses the challenges and prospects for advancements in both synthetic and living composite materials from the perspectives of chemistry, modeling, and integration.

Controlling liquid-liquid phase behaviour with an active fluid.

Demixing binary liquids is a ubiquitous transition explained using a well-established thermodynamic formalism that requires the equality of intensive thermodynamics parameters across phase boundaries. Demixing transitions also occur when binary fluid mixtures are driven away from equilibrium, but predicting and designing such out-of-equilibrium transitions remains a challenge. Here we study the liquid-liquid phase separation of attractive DNA nanostars driven away from equilibrium using a microtubule-based active fluid. We find that activity lowers the critical temperature and narrows the range of coexistence concentrations, but only in the presence of mechanical bonds between the liquid droplets and reconfiguring active fluid. Similar behaviours are observed in numerical simulations, suggesting that the activity suppression of the critical point is a generic feature of active liquid-liquid phase separation. Our work describes a versatile platform for building soft active materials with feedback control and providing an insight into self-organization in cell biology.

Model-driven identification framework for optimal constitutive modeling from kinematics and rheological arrangement

Soft materials such as biological tissues or magnetorheological elastomers present complex mechanical behaviors that include large deformations, numerous nonlinearities, time- or even external field (magnetic)-dependent responses. The description of their constitutive modeling is challenging and often time-consuming. Numerical algorithms to automatically calibrate model parameters have provided invaluable tools to help this purpose. However, these are mostly limited to the fitting of a set of pre-defined parameters associated with the model used. In this work, we go a step further by developing a machine learning framework capable of automatically identifying not only such model parameters but also the optimal kinematics and rheological model. To this end, we present a multiphysics model-driven framework that optimally selects the most suitable model kinematics, its rheological components and their arrangement for a given set of experimental curves. Subsequently, it calibrates all the material constants belonging to such a model, independent of its complexity. We demonstrate the versatility and capabilities of this framework with examples on hyperelastic, viscohyperelastic and magneto-viscohyperelastic materials. The present work opens new routes to not only fit model parameters but to identify the constitutive ingredients and underlying mechanisms needed to describe nonlinear responses of soft active materials.

Methods for numerical simulation of soft actively contractile materials

Soft materials that can demonstrate on demand reconfigurability and changing compliance are highly sought after as actuator materials in many fields such as soft robotics and biotechnology. Whilst there are numerous proof of concept materials and devices, rigorous predictive models of deformation have not been well-established or widely adopted. In this paper, we discuss programming complex three-dimensional deformations of a soft intrinsically anisotropic material by controlling the orientation of the contractile units and/or direction of the applied electric field. Programming is achieved by patterning contractile units and/or selectively activating spatial regions. A new constitutive model is derived to describe the soft intrinsic anisotropy of soft materials. The model is developed within a continuum mechanics framework using an invariant-based formulation. Computational implementation allows us to simulate the complex three-dimensional shape response when activated by electric field. Several examples of the achievable Gauss-curved surfaces are demonstrated. Our computational analysis introduces a mechanics-based framework for design when considering soft morphing materials with intrinsic anisotropy, and is meant to inspire the development of new soft active materials.

A mechanics model of hard-magnetic soft rod with deformable cross-section under three-dimensional large deformation

Hard-magnetic soft (HMS) materials are soft active materials prepared by embedding the hard-magnetic particles (e.g. NdFeB) into soft elastomer, which can be actuated by applied magnetic fields. With the ability to retain remnant magnetism, HMS materials have many applications in soft robotics, haptic sensors and other fields. For HMS structures used in soft robotics, mechanics models have been developed for better exploiting the potential of the HMS materials. However, for rod-like structures made of soft materials, the nonlinear stress–strain relation and the areal change of the cross-section have not been considered in previous modeling work. In the present three-dimensional HMS rod model, started with the geometrically exact beam theory, the rigid cross-section assumption is replaced with a planar areal change assumption for soft rods, incompressible neo-Hookean material model is used to model the hyperelastic behavior of elastomers, which will be more accurate than linear relation under large deformation. Consistent with the deformable cross-section assumption, the magneto-elastic energy distribution is formulated and reduced to the line of centroid of the soft rod. To verify the accuracy and efficiency of the model, some simulation examples and experiments are performed. The error between the experimental results and the simulation results calculated by using 5 elements is within 10%. Our model can accurately and efficiently predict the 3D large deformation of the HMS rod, which can reduce the design and optimization cost and shorten the design cycle of HMS structure.

Liquid Crystalline Elastomer for Separate or Collective Sensing and Actuation Functions.

A porous liquid crystalline elastomer actuator filled with an ionic liquid (PLCE-IL) is shown to exhibit the functions of two classes of materials: electrically responsive, deformable materials for sensing and soft active materials for stimuli-triggered actuation. On one hand, upon the order-disorder phase transition of aligned mesogens, PLCE-IL behaves like a typical actuator capable of reversible shape change and can be used to assemble light-fuelled soft robot. On the other hand, at temperatures below the phase transition, PLCE-IL is an elastomer that can sustain and sense large deformations of various modes as well as environmental condition changes by reporting the corresponding electrical resistance variation. The two distinguished functions can also be used collectively with PLCE-IL integrated in one device. This intelligent feature is demonstrated with an artificial arm. When the arm is manually powered to fold and unfold, the PLCE-IL strip serves as a deformation sensor; while when the manual power is not available, the role of the PLCE-IL strip is switched to an actuator that enables light-driven folding and unfolding of the arm. This study shows that electrically responsive LCEs are a potential materials platform that offers possibilities for merging deformable electronic and actuation applications.

Shape memory behaviors of 3D printed liquid crystal elastomers

As soft active materials, shape-memory polymers (SMPs) and liquid crystal elastomers (LCEs) have attracted considerable research interest due to their potential applications in various areas. SMPs refer to polymeric materials that can return to their permanent shape in response to external stimuli, such as heat, light, and solvent. In this sense, LCEs can exhibit intrinsic shape-memory behaviors since LCEs can switch between two shapes with temperature change due to the order-disorder transition of liquid crystals. In this work, we fabricate both the polydomain and monodomain nematic LCEs through direct ink writing 3D printing. With increasing the temperature of the substrates, the printed LCEs change from the monodomain state to the polydomain state. For polydomain LCEs, a reversible shape change can occur upon constant loading, while the monodomain can switch the shape with temperature in the stress-free state. This two-way shape-memory behavior is caused by the nematic-isotropic phase transition. We further show that the printed LCEs exhibit a good one-way shape-memory effect due to glass transition. The shape recovery region increases with the programming temperature, which is a typical temperature memory effect. Finally, it is demonstrated that complex shape-memory performance can be designed by combining one-way and two-way shape-memory effects. Specifically, for the monodomain LCEs, with increasing temperature, the programmed shape first recovers, and a second shape change can further occur due to the nematic-isotropic transition.

Recent advances in the design, fabrication, actuation mechanisms and applications of liquid crystal elastomers

Liquid crystal elastomers (LCEs), as an intriguing class of soft active materials, exhibit excellent actuation performances and biocompatible properties, as well as a high degree of design flexibility, which have been of increasing interest in many disciplines. This review summarizes recent developments in this inspiring area, providing an overview of fabrication methods, design schemes, actuation mechanisms, and diverse applications of LCEs. Firstly, two-stage and one-pot synthesis methods, as well as emerging fabrication techniques (e.g., 3D/4D printing and top-down microfabrication techniques) are introduced. Secondly, the design and actuation mechanisms are discussed according to the different types of stimuli (e.g., heat, light, and electric/magnetic fields, among others). Thirdly, the representative applications are summarized, including soft robotics, temperature/strain sensors, biomedical devices, stretchable displays, and smart textiles. Finally, outlooks on the scientific challenges and open opportunities are provided.

Petroleum pitch derived carbon as both cathode and anode materials for advanced potassium-ion hybrid capacitors

Potassium-ion hybrid capacitors (PIHCs) are considered as ideal devices for large-scale electrochemical energy storage because of their low reduction potential, environmental-friendly and abundant potassium resources in the earth. However, the simple and eco-friendly preparation of well-matched cathode and anode materials is still with great challenges. With the same petroleum pitch (with asphaltenes removed by n-heptane) as carbon source, herein we reported low-cost and non-corrosive processes to construct two carbon materials, namely soft carbon nanosheets (SCNs) and active carbon nanosheets (ACNs), as anode and cathode materials, respectively, to assemble high performance PIHCs. The SCNs with expanded graphitic interlayer spacing of 0.396 nm and abundant surface defects present super rate capability and high pseudocapacitive proportion. In addition, the ACNs with hierarchical pores structure, which accelerates the adsorption and transport of electrolyte ions, exhibit outstanding capacitive performance. By optimizing the voltage window and mass ratio of cathode and anode, the SCNs//ACNs PIHCs deliver a high energy density of 124.0 Wh kg−1 at a power density output of 198.9 W kg−1 and an ultralong lifespan over 9000 cycles. This work opens a new avenue to increase the value-added utilization of petroleum pitch for innovative potassium-based energy storage technology.

Finite Element Model for Investigating the Thermo-Electro-Mechanical Response of Inhomogeneously Deforming Dielectric Elastomer Actuators

Among the available soft active materials, Dielectric elastomers (DEs) possess the capability of achieving the large actuation strain under the application of high electric field. The material behavior of such elastomers is affected significantly by the change in temperature. This paper reports a 3-D finite element framework based on the coupled nonlinear theory of thermo-electro-elasticity for investigating the thermal effects on the electromechanical performance of inhomogeneously deforming dielectric elastomer actuators (DEAs). The material behavior of the actuator is modeled using the neo-Hookean model of hyperelasticity with temperature dependent shear modulus. An in-house computational code is developed to implement the coupled finite element framework. Firstly, the accuracy of the developed FE code is verified by simulating the temperature effects on the actuation response and pull-in instability of the benchmark homogeneously deforming planar DE actuator. Further, the influence of temperature on the electromechanical responses of complex bi-layered bending actuator and buckling pump actuator involving inhomogeneous deformation is investigated. The numerical framework and the associated inferences can find their potential use in addressing the effect of temperature in the design of electro-active polymer based actuators.

Soft Actuators and Robotic Devices for Rehabilitation and Assistance

Soft actuators and robotic devices have been increasingly applied to the field of rehabilitation and assistance, where safe human and machine interaction is of particular importance. Compared with their widely used rigid counterparts, soft actuators and robotic devices can provide a range of significant advantages; these include safe interaction, a range of complex motions, ease of fabrication, and resilience to a variety of environments. In recent decades, significant effort has been invested in the development of soft rehabilitation and assistive devices for improving a range of medical treatments and quality of life. This review provides an overview of the current state‐of‐the‐art in soft actuators and robotic devices for rehabilitation and assistance, in particular systems that achieve actuation by pneumatic and hydraulic fluid‐power, electrical motors, chemical reactions, and soft active materials such as dielectric elastomers, shape memory alloys, magnetoactive elastomers, liquid crystal elastomers, and piezoelectric materials. Current research on soft rehabilitation and assistive devices is in its infancy, and new device designs and control strategies for improved performance and safe human–machine interaction are identified as particularly untapped areas of research. Finally, insights into future research directions are outlined.

Ionic Transportation Characteristics in All Solid-State Battery Electrode with Sulfide and Halide Solid Electrolytes

Bulk-type all solid-state lithium-ion battery electrodes are manufactured by compressing composite powder of active material and solid electrolyte powder. This compression increases contact area between the powder particles and reduces ion transportation resistance in the electrode. In many cases, mechanical characteristics of the solid electrolyte and active material are different, and complex stress distribution is generated in the compressed composite powder. In authors previous studies, ion transportation characteristics of electrode with soft active material and hard active material are studied experimentally and numerically. The electrode with soft active material has high ionic conductivity due to high stress is applied to the solid electrolyte in the electrode and high contact area between solid electrolyte powder is achieved. As showed from this study, Young’s modulus of active material is an important factor of ionic transportation in electrode of all solid-state battery. Based on this result, solid electrolyte the Young's modulus of the solid electrolyte is also an important factor of ionic transportation in electrode. Therefore, in this study, we conducted experiments and numerical simulation of ionic conductivity of the electrode with changing the Young’s modulus of the solid electrolyte to elucidate the influence of Young’s modulus of the solid electrolyte on ionic transportation characteristics of the electrode.Two solid electrolytes are employed in this study. One is sulfide solid electrolyte of Li5.5PS4.5Br1.5. This solid electrolyte is made by mechanical milling with vibration mill and heat treatment. The other is halide solid electrolyte of Li3InCl6. This halide solid electrolyte is made with wet process from LiCl and InCl3. These solid electrolytes are mixed with dummy active material of ZrO2 and nylon. The dummy active materials used to simulate only ionic transportation by a solid electrolyte and mechanical conditions. Young’s modulus of ZrO2 is about 200 GPa and is almost same as Young’s modulus of hard active materials such as LCO and NCM. Young’s modulus of nylon is about 6 GPa and is almost same as Young’s modulus of soft active materials such as graphite. From these two solid electrolytes and two dummy active materials, four dummy electrodes.Fig.1 illustrates effective Young’s modulus of Li5.5PS4.5Br1.5 powder and Li3InCl6 powder measured with changing the applied pressure from 0MPa to 100MPa. From this figure, the Young’s modulus of Li5.5PS4.5Br1.5 is from 100MPa to 700MPa and that of Li3InCl6 is about half of Li5.5PS4.5Br1.5. Therefore, it can be said that, compared to sulfide solid electrolyte, halide solid electrolyte is relatively soft solid electrolyte.Fig.2 illustrates relative ionic conductivity as a function of the volumetric fraction of dummy active material. Here, the ionic conductivity is normalized by the ionic conductivity of pure solid electrolyte (= 0% volumetric fraction of dummy active material). As shown in fig.2 (a), the relative ionic conductivity of sulfide solid electrolyte and halide solid electrolyte are almost same when the dummy active material is soft (nylon). However, in the case with hard dummy active material (ZrO2), as shown in fig.2 (b), the relative ionic conductivity with halide solid electrolyte is lower than that with sulfide solid electrolyte at high dummy active material volumetric fraction. With hard active material, mechanical pillar is generated by active material in the electrode and strain of the solid electrolyte is low. In the case with sulfide solid electrolyte, relatively high stress can be generated with the low strain, but in the case with halide solid electrolyte, stress is relatively low with the low strain. This is because, as shown in fig.1, Young’s modulus of sulfide solid electrolyte is higher than that of the halide solid electrolyte, and high stress for high ionic conductivity can be generated even with low strain in the case with sulfide solid electrolyteThis Study is supported by JKA foundation (2021M-188) Figure 1

Liquid Crystal Elastomer Metamaterials with Giant Biaxial Thermal Shrinkage for Enhancing Skin Regeneration.

Liquid crystal elastomers (LCEs) are a class of soft active materials of increasing interest, because of their excellent actuation and optical performances. While LCEs show biomimetic mechanical properties (e.g., elastic modulus and strength) that can be matched with those of soft biological tissues, their biointegrated applications have been rarely explored, in part, due to their high actuation temperatures (typically above 60°C) and low biaxial actuation performances (e.g., actuation strain typically below 10%). Here, unique mechanics-guided designs and fabrication schemes of LCE metamaterials are developed that allow access to unprecedented biaxial actuation strain (-53%) and biaxial coefficient of thermal expansion (-33125ppmK-1 ), significantly surpassing those (e.g., -20% and -5950ppmK-1 ) reported previously. A low-temperature synthesis method with use of optimized composition ratios enables LCE metamaterials to offer reasonably high actuation stresses/strains at a substantially reduced actuation temperature (46°C). Such biocompatible LCE metamaterials are integrated with medical dressing to develop a breathable, shrinkable, hemostatic patch as a means of noninvasive treatment. In vivo animal experiments of skin repair with both round and cross-shaped wounds demonstrate advantages of the hemostatic patch over conventional strategies (e.g., medical dressing and suturing) in accelerating skin regeneration, while avoiding scar and keloid generation.

Strong, Ultrastretchable Hydrogel‐Based Multilayered Soft Actuator Composites Enhancing Biologically Inspired Pumping Systems

Diverse solutions for active fluid movement are known in nature and in human‐made devices. However, commercial peristaltic pumps are mostly rigid, noncompliant, and tough to integrate into biocompatible materials. This work aims to approximate actuator‐like behavior concerning nonhemolytic pumping action and higher energy density to develop biorobotic physical models and biomedical assistive devices with life‐like motion profiles. To achieve this, dielectric elastomers (DEs) offer themselves. DE connected via very high bonding (VHB) tape's pumping performance is tested and compared to a novel configuration. Comparative analysis of the VHB‐based DE pump vis‐a‐vis the novel design solution involving composite layering of hydrogel and electroactive polymer (HEAP) with interfacial toughness of ≈1522 ± 188 J m−2 exhibits increases in pressure change of up to 68 mmHg at measured flow rates of 16.8 mL s−1 with low viscoelastic losses (% at biaxial prestretch of 3 × 3, 10% stretch rate, and 20 cycles postoperation). The HEAP‐sandwiched layer embracing hydrogel‐based ionotronics presents 2,205% ultimate strain and sustains compressive stress of 632 kPa. This pilot thus demonstrates the advantages of greater incorporation of hydrogel‐based biocompatible polymers in conjunction with soft active materials and proposes performance characterization for cardiovascular trials and related biofluid pumping applications.

Unusual stress and strain concentration behaviors at the circular hole of a large monodomain liquid crystal elastomer sheet

Liquid crystal elastomers combine the hyperelasticity of elastomers with the multi-functionality of liquid crystals and have emerged as an important class of soft active materials. Monodomain liquid crystal elastomers under loading exhibit the soft elastic behavior due to the stress induced director rotation and the resulting spontaneous strain. Here, we numerically study their stress and strain concentration behavior by considering the classical example, a large sheet with a small circular hole in the middle under uniaxial loading. The concentration behavior is found to be very different from regular elastomers. Firstly, the concentration factors are much bigger at both small and large strains. Secondly, the locations of the strain concentration may not coincide with that of the stress concentration. Detailed analysis of the director rotation and the resulting spontaneous strain around the free edge of the hole are shown to be the main causes for the unusual concentration behavior. Moreover, under a given strain, the stress level of the LCE sample, and therefore the free energy, is slightly lower than that of the neo-Hookean material, while the local free energy density on the hole edge is much bigger due to the severer concentrations. By considering material parameters obtained from various samples of polysiloxane and polyacrylate side-chain nematic elastomers, we find that their stress and strain concentration behaviors are qualitatively similar but quantitatively quite different. As a result, at a prescribed strain, the samples with a larger maximal spontaneous strain has severer stress and strain concentrations. The stress concentration factor difference scaled by a coupling constant that combines the effect of the two material parameters, r and a, increases as the semi-soft coefficient a decreases. Similar results are found for the scaled maximal spontaneous strains and the scaled strain concentration factor difference at small strains.

An anisotropic constitutive model for 3D printed hydrogel-fiber composites

Fiber-reinforced soft composites with anisotropic mechanical properties exist widely in nature, forming complex shape-morphing structures during the growth or hydration processes. In this paper, we employ the 3D printing method, termed direct ink writing (DIW), to fabricate the hydrogel-fiber composites. It is found that the fiber distribution is controlled by printing parameters, such as printing direction and nozzle size, which results in complicated anisotropic mechanical and swelling responses. We develop an anisotropic constitutive model for the printed hydrogel-fiber composites. Built upon the isotropic hydrogel model, we add a decoupled free-energy function for fibers to incorporate the three-dimensional distribution of fibers. The model parameters can be determined from the swelling and uniaxial tension tests. We further implement the model for finite element analysis. The results show that the model can describe the stress response of fiber-reinforced hydrogel composites with different degrees of fiber orientations. More importantly, the model can reproduce the observed anisotropic swelling behaviors in the soft composites. The model is employed to design the shape-morphing structures mimicking a saddle-shaped leaf and a twisted orchid petal. The outcome is verified through comparison with the experimental results. The work provides a mechanics tool for 3D printed hydrogel-fiber composites and can promote the broad applications of soft active materials.

Thermo-mechanical modeling of viscoelastic crystallizable shape memory polymers

Shape memory polymers (SMPs) are a class of soft active materials that have the ability to retain one or multiple temporary shapes and exhibit deformation as a response to stimuli. The temporary shapes involved can be very complex but SMPs can recover large amounts of strains as compared to other smart materials. They have a wide range of applications in fields as diverse as healthcare, transportation, energy generation, etc. and their mechanism can be easily altered. Hence, there is an increasing need to model their behavior. This paper discusses the behavior of a thermally activated subclass of SMPs in which the temporary shape is fixed by a crystalline phase and the return to original shape is due to the transition of this crystalline phase. We simulate its thermo-mechanical behavior using a framework based on the theory of multiple natural configurations. Viscoelastic effects have been incorporated using a rate type model along with the anisotropy of the crystalline phase. The model has been applied to boundary value problems of homogeneous deformations such as uniaxial extension of dual shape and triple shape memory polymers under controlled stress and strain conditions. The model has also been applied to boundary value problems related to inhomogeneous deformations such as circular shear, inflation, and extension of a hollow cylinder. The effects of parameters such as temperature, external stress, crystallinity, shear modulus and relaxation time on the nonlinear deformation have been studied. The results are found to be consistent with the experimental data and show how the complicated thermo-mechanical behavior and shape memory effect are influenced by these parameters.

Soft active materials and soft robots surviving extremely high hydrostatic pressure

The experiment proves that the soft robot can be driven under extremely high hydrostatic pressure (110 MPa), with actuators made of soft active materials and the electromechanical system constructed via the soft-hard hybrid method.