Variable Stiffness Materials Research Articles

Tendon‐Driven Stiffness‐Tunable Soft Actuator via Thermoelectric‐based Bidirectional Temperature Control

AbstractSoft robots have excellent spatial adaptability and high flexibility, but they are limited by the low stiffness of their constituent materials when faced with high‐load tasks. In recent years, there have been many works on the development of stiffness‐tunable soft actuators by introducing variable stiffness materials into soft actuators, but the existing solutions usually suffer from the problems of slow response, complex structure, and the need of many auxiliary devices to support the completion of the stiffness tuning cycle. This paper proposes a tendon‐driven stiffness‐tunable soft actuator that addresses these issues. Benefiting from the bidirectional temperature control of thermoelectric modules and the excellent in‐plane thermal conductivity of graphene, the actuator is capable of achieving the heating and cooling process by transferring the heat flow through the graphene structure into and out of the shape‐memory polymer (SMP) layer of the tendon‐driven actuator. This enables stiffness tuning via a single device, reducing the dependence on complex external cooling systems. The use of tendon‐driven actuators further eliminates the complex bellow structure of conventional pneumatic actuators and dramatically reduces the size and manufacturing difficulty of individual actuators. Finally, the high load capacity and shape adaptability of the actuator are demonstrated by a gripper equipped with three actuators, which successfully grips objects of various shapes and weights, ranging from less than 10 g to up to 1.6 kg.

Ultra-Tough, yet Rigid and Healable Supramolecular Polymers with Variable Stiffness for Multimodal Actuators.

Variable stiffness materials have shown considerable application in soft robotics. However, previously reported materials often struggle to reconcile high stiffness, stretchability, toughness, and self-healing ability, because of the inherently conflicting requisite of these properties in molecular design. Herein, we propose a novel strategy that involves incorporating acid-base ionic pairs capable of from strong crosslinking sites into a dense and robust hydrogen-bonding network to construct rigid self-healing polymers with tunable stiffness and excellent toughness. To demonstrate these distinct features, the polymer was employed to serve as the strain-regulation layers within a fiber-reinforced pneumatic actuator (FPA). The exceptional synergy between the configuration versatility of FPA and the dynamic molecular behavior of the supramolecular polymers equips the actuator with simultaneous improvement in motion dexterity, multimodality, loading capacity, robustness, and durability. Additionally, the concept of integrating high dexterity at both macro- and micro-scale is prospective to inspire the design of intelligent yet robust devices across various domains.

A Reconfigurable Soft Helical Actuator with Variable Stiffness Skeleton.

Due to their exceptional adaptability, inherent compliance, and high flexibility, soft actuators have significant advantages over traditional rigid actuators in human-machine interaction and in grasping irregular or fragile objects. Most existing soft actuators are designed using preprogramming methods, which schedule complex motions into flexible structures by correctly designing deformation constraints. These constraints restrict undesired deformation, allowing the actuator to achieve the preprogrammed motion when stimulated. Therefore, these actuators can only achieve a certain type of motion, such as extension, bending, or twisting, since it is impossible to adjust the deformation constraints once they are embedded into the structures. In this study, we propose the use of variable stiffness materials, such as shape memory polymer (SMP), in the structural design of soft actuators to achieve variable stiffness constraints. A reconfigurable soft helical actuator with a variable stiffness skeleton is developed based on this concept. The skeleton, made of SMP, is encased at the bottom of a fiber-reinforced chamber. In its high-stiffness state, the SMP constrains the deformation toward the skeleton when the actuator is pressurized. This constraint is removed once the SMP skeleton is heated, endowing the actuator with the ability to switch between bending and helical motion in real-time. A theoretical model is proposed to predict the behavior of the actuator when driven by pressure, and experiments are conducted to verify the model's accuracy. In addition, the influence of different design parameters is investigated based on experimental results, providing reference guidelines for the design of the actuator.

Minimal-surface-based multiphase metamaterials with highly variable stiffness

Variable-stiffness materials have a unique ability to change their stiffness reversibly in response to external stimuli or conditions. However, achieving ultrahigh stiffness change is often constrained by the geometric organization of the microstructures in most materials that exhibit variable stiffness. Therefore, to overcome this limitation, we introduce a metamaterial design inspired by triply periodic minimal surfaces for fabricating multiphase metamaterials. The specific geometric features of minimal surface designs facilitate interlocking bi- or tri-continuous interpenetrating phases such as air, resin, and alloy within a single multiphase metamaterial. These multiphase metamaterials are constructed by injecting a low-melting-point alloy (LMPA) into a 3D-printed elastic resin mold. The thermally-induced solid-liquid phase transition of the LMPA governs the stiffness change in multiphase metamaterials, ranging from Kilopascals to Gigapascals. Further contributing to this phenomenon, the superior resilience of the elastic resin enhances the shape-memory effect of the multiphase metamaterials. Applications of these materials in origami and deployable structures have been successfully demonstrated, highlighting their reconfigurability and volume compressibility. This innovative design strategy provides the foundation for crafting other metamaterials with intricately arranged internal phases. In conclusion, the proposed multiphase metamaterials have promising potential for various engineering applications where adaptability and morphing capabilities are essential.

Kirigami layer jamming

Variable stiffness materials bridge the desirable qualities of soft and rigid robotic components, forming the basis of new machines that contextually adapt their structural properties. Recent developments have led to the creation of jamming materials: Collections of media that undergo pressure-dependent kinematic and frictional interactions to radically, rapidly, and reversibly change in stiffness. Among the types of jamming, laminar jamming harnesses an ensemble of sheets for stiffness changes. Most implementations of laminar jamming employ unaltered, continuous sheets, though the vast design space of mechanical anisotropy programmed via structured cuts invites a new jamming paradigm: kirigami laminar jamming. Through analytical, experimental, and numerical studies, we show how intentional topological augmentation of laminar jamming sheets through shell, void, and cut geometric primitives can elicit bespoke mechanical responses. In particular, we optimize jamming specimens to enhance the jammed to unjammed flexural stiffness ratio across a range of deflections, which unlocks new move-and-hold capabilities. We also demonstrate pressure-induced shape morphing and both sharp-edged and doubly-curved surface matching with kirigami patterns. Results hold broad significance for robotics, morphing structures, and the systematic design of variable stiffness materials.

Low melting point alloy modified polymer composite with sharp stiffness switch for soft actuator

Materials possessing switchable stiffness, particularly dynamically switchable stiffness, have generated a lot of attention in the soft actuator. Herein, we proposed a facial poly (vinyl alcohol) (PVA) coating method to fabricate the micron-sized Field's metal (FM) phase-changing particles with an average particle size of about 1.2 μm, and obtained a series of stiffness variable polymer composites via incorporating FM particles into the designed stiffness variable polymer matrix. Owing to the rapid solid-liquid change and ultra-high modulus of FM, the rigid/soft stiffness ratio of the composites improved to 1442-folds. Moreover, the shape fixity (Rf) and shape recovery ratio (Rr) values of the shape memory properties could reach up to ultra-high values of 99.4% and 99.6%, respectively. Subsequently, the prepared composites were selected and used a 3D printing soft actuator as the stiffness variable units. The soft actuator exhibits short heating-cooling time of 36s under 1.2A current and with 4 °C water as the coolant, and it is able to lift a 200 g weight at the rigid state. Furthermore, the stiffness of obtained actuator can achieve high value 779 mN/mm. The results indicate that the obtained soft actuator possesses excellent actuate behavior and stiffness switchable ability. This work might provide faster and more rigid variable stiffness materials for soft actuators application in the real world.

A Palm-Shape Variable-Stiffness Gripper Based on 3D-Printed Fabric Jamming

Soft grippers have excellent adaptability for a variety of objects and tasks. Jamming-based variable stiffness materials can further increase soft grippers' gripping force and capacity. Previous universal grippers enabled by granular jamming have shown great capability of handling objects with various shapes and weight. However, they require a large pushing force on the object during gripping, which is not suitable for very soft or free-hanging objects. In this paper, we create a novel palm-shape anthropomorphic variable-stiffness gripper enabled by jamming of 3D printed fabrics. This gripper is conformable and gentle to objects with different shapes, requires little pushing force, and increases gripping strength only when necessary. We present the design, fabrication and performance of this gripper and tested its conformability and gripping capacity. Our design utilizes soft pneumatic actuators to drive two wide palms to enclose objects, thanks to the excellent conformability of the structured fabrics. While the pinch force is low, the palm can significantly increase stiffness to lift heavy objects with a maximum gripping force of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$17\,$</tex-math></inline-formula> N and grip-to-pinch force ratio of 42. We also explore different variable-stiffness materials in the gripper, including sheets for layer jamming, to compare their performances. We conduct gripping tests on standard objects and daily items to show the great capacity of our gripper design.

Sizing optimization and experimental characterization of a variable stiffness shape memory polymer filled honeycomb composite

Variable stiffness structures and materials have been considered for many applications, including active vibration control and shape morphing. With regards to shape morphing, variable stiffness materials and composites have been considered for reconfigurable skin materials in aerospace vehicles. Of the many concepts that have been developed for such applications, shape memory polymers (SMPs) are one such promising materials for shape morphing. SMPs exhibit both high modulus ratios and recoverable strains but suffer from a low overall modulus and often require reinforcements, such as honeycomb. This work investigates the design space of such honeycomb reinforced SMPs as variable stiffness materials. Unit cell finite element models are developed for the material, and parametric studies are completed for varying honeycomb cell geometries. A multiobjective, constrained Pareto front optimization is completed for two honeycomb material models and in two loading directions using selected sizing design variables. Pareto fronts are established, and cell geometries are selected and fabricated to experimentally verify the optimized model predictions. The results both predict and demonstrate the advantages of using honeycomb reinforcements for SMPs. Effective in-plane moduli as high as 45 GPa are predicted while achieving a change in modulus of 450X. Compared to existing reinforcement strategies for shape memory polymers, these composites exhibit favorable combinations of both high stiffness and high changes in stiffness with a high degree of tailorability through the honeycomb cell geometry and predicted performances that meet and exceed the state of the art.

Printable multi-stage variable stiffness material enabled by low melting point particle additives

A novel phase-changing composite that gains multi-stage stiffness under the thermal stimulus has been developed to make a mechanism system to adapt to the complex environment, and complex design structures can be fabricated by 4D printing.

A Magnetically and Thermally Controlled Liquid Metal Variable Stiffness Material

Smart materials that can actively tune their stiffness are of great interest to many fields, including the construction industry, medical devices, industrial machines, and soft robotics. However, developing a material that can offer a large range of stiffness change and rapid tuning remains a challenge. Herein, a liquid metal variable stiffness material (LMVSM) that can actively and rapidly tune its stiffness by applying an external magnetic field or by changing the temperature is developed. The LMVSM is composed of three layers: a gallium–iron magnetorheological fluid (Ga–Fe MRF) layer for providing variable stiffness, a nickel–chromium wire layer for Joule heating, and a soft heat dissipation layer for accelerating heating and rapid cooling. The stiffness can be rapidly increased by 4 times upon the application of a magnetic field or 10 times by solidifying the Ga–Fe MRF. Finally, the LMVSM is attached to a pneumatically controlled soft robotic gripper to actively tune its load capacity, demonstrating its potential to be further developed into smart components that can be widely adopted by smart devices.

Versatile Adhesion-Based Gripping via an Unstructured Variable Stiffness Membrane.

Reversible and variable dry adhesion is a promising approach for versatile robotic grasping. Variable stiffness materials with a modulus that can be tuned using an external stimulus offer a unique approach to realize dynamic control of adhesion. In this study, an unstructured shape memory polymer (SMP) membrane with variable stiffness is used to pick-and-place three-dimensional objects. The variable stiffness of the SMP allows the membrane to conform to and make good contact with objects of various shapes in its soft state and then achieve high adhesive load capacity by switching to the stiff state. Release of objects is realized by switching to the soft state. The ratio between the high-adhesion and low-adhesion state is demonstrated to be >2000 on a curved substrate and ∼115 on a flat substrate. This gripper exhibits no adhesion in the unactivated state and maintains adhesion passively once actuation is complete.

Detachable Soft Actuators with Tunable Stiffness Based on Wire Jamming

The integration of variable stiffness materials and structures into soft robots is a popular trend, allowing soft robots to switch between soft and rigid states in different situations. This concept combines the advantages of rigid mechanisms and soft robots, resulting in not only excellent flexibility but also tunable stiffness for high load capacity and fast and precise operation. Here, a stiffness-tunable soft actuator based on wire/fiber jamming structure is proposed, where the fiber-reinforced soft actuator is responsible for the bending motion, and the jamming structure acts as a stiffness-tunable layer controlled by vacuum pressure. The primary design objective of this study is to fabricate a jamming structure with wide-range stiffness, universal adaptability and high dexterity. Thus, the behaviors of wire/fiber jamming structures with different layouts, materials and wire arrangements are analyzed, and a theoretical model is developed to predict the effect of geometric parameters. Experimental characterizations show that the stiffness can be significantly enhanced in the bending direction, while the stiffness is smaller in the torsion direction. Additionally, by integrating Velcro strips into the design, a quick and detachable scheme for the stiffness-tunable soft actuator is achieved. Application examples exhibit high load capacity and good shape adaptability.

Dynamic Cloth Manipulation Considering Variable Stiffness and Cloth Material Change Using Deep Predictive Model Learning

Dynamic Cloth Manipulation Considering Variable Stiffness and Cloth Material Change Using Deep Predictive Model Learning

Stiffness-tunable and shape-locking soft actuators based on 3D-printed hybrid multi-materials

Soft actuators have been receiving tremendous attention as a result of their excellent adaptability to the environment. However, due to their inherently low stiffness, soft actuators are difficult to adapt to high-load tasks. Despite previous efforts in developing stiffness-tunable actuators by utilizing variable stiffness materials, they still suffer from limitations, including relatively low load and locking capacity to grasp weights and difficulties regarding their fabrication with complex structures. This work reports a novel stiffness-tunable and shape-locking soft (Tri-S) actuator using hybrid multi-material 3D printing. The Tri-S actuator consists of polylactic acid, thermoplastic polyurethane and a flexible carbon fiber heating wire. Its stiffness can be effectively tuned by Joule heating. A soft robotic gripper equipped with three Tri-S actuators demonstrates their stiffness-tunable and shape-locking capability by grasping and holding objects of various shapes and weights. The gripper can grasp weights up to 2.2 kg with an external driving force by tuning the stiffness and hold weights up to 310 g depending on its own shape locking without an external driving power source.

Thermoset Shape Memory Polymer Variable Stiffness 4D Robotic Catheters.

Variable stiffness catheters are typically composed of an encapsulated core. The core is usually composed of a low melting point alloy (LMPA) or a thermoplastic polymer (TP). In both cases, there is a need to encapsulate the core with an elastic material. This imposes a limit to the volume of variable stiffness (VS) material and limits miniaturization. This paper proposes a new approach that relies on the use of thermosetting materials. The variable stiffness catheter (VSC) proposed in this work eliminates the necessity for an encapsulation layer and is made of a unique biocompatible thermoset polymer with an embedded heating system. This significantly reduces the final diameter, improves manufacturability, and increases safety in the event of complications. The device can be scaled to sub‐millimeter dimensions, while maintaining a high stiffness change. In addition, integration into a magnetic actuation system allows for precise actuation of one or multiple tools.

An element-free study of variable stiffness composite plates with cutouts for enhanced buckling and post-buckling performance

In this paper, an efficient element-free modeling framework for variable stiffness composite laminates with cutouts is developed. In comparison with other numerical methods or semi-analytical methods, element-free meshless methods offer many advantages in modeling variable stiffness materials and structures, e.g. complex geometries with cut-outs, providing accurate stress computations and stable nonlinear behavior modeling. In this study, an element-free Galerkin (EFG) method based modeling framework is developed to model variable stiffness composite plates containing circular holes, specifically for buckling and post-buckling analysis. Weighted orthogonal basis functions are used to further reduce the computational costs of the meshless method. Modeling accuracy for variable stiffness composite structures using node-based fiber angle definitions in this element-free framework are validated against FEM results obtained from ABAQUS. The critical buckling loads of linearly varying fiber composite laminates with different sizes of central circular holes are then computed. Based on the proposed element-free modeling scheme, nonlinear post-buckling equilibrium paths of variable stiffness composite plates are traced using an incremental loading step control method, which enables efficient post-buckling analysis. Changes in both critical buckling loads and nonlinear post-buckling performance with respect to hole size and the linear variation of fiber angles are investigated with the results presented in the form of parametric studies. The numerical results show that stress redistribution remains the major factor driving improvements in the buckling and post-buckling performance of variable stiffness composite plates with cut-outs. The research work presented in this paper for composite plates with linearly varying stiffness provides a basis for the analysis of general variable stiffness soft materials in the future.

A Layer Jamming Actuator for Tunable Stiffness and Shape-Changing Devices.

Changing the shape and the stiffness of a device in a dynamic and controlled way enables important advancements in the field of robotics and wearable robotics. Variable stiffness materials and technologies can be used to address this challenge. In particular, layer jamming actuation is a very promising technology, featured by high efficiency and low cost. In this article, a stiffness- and shape-changing device based on a novel mechanism including a multiple-chamber structure is proposed. It allows to effectively modulate the shape and stiffness of a device, by activating two jamming chambers while pressurizing/depressurizing one or more interposed inflatable chambers. Prototypes with a size of 45 × 270 mm2 and an average thickness ranging from 4.4 to 13 mm were developed and their ability to undergo a stiffness change over two orders of magnitude was demonstrated. The prototypes were also able to change their shape according to the position and inflation level of the interposed inflatable chambers, thus resulting in an overall deflection >10 mm. The possibility to wear the system as an orthotic brace was also demonstrated: this technology increased the patient comfort in static positions, yet keeping a supportive function when needed (e.g., in dynamic conditions). The device working principle highlighted in this article could also be exploited in other domains, for example, to build walking soft robots, prostheses, or grippers, as demonstrated through additional tests.

Dual-Stimuli-Responsive Polymer Composite with Ultrawide Tunable Stiffness Range Triggered by Water and Temperature

Variable stiffness materials are an important class of materials that are useful in soft robotics, reconfigurable devices, deployable vehicles, and artificial muscles. Most variable stiffness polym...

Variable stiffness morphing limb for amphibious legged robots inspired by chelonian environmental adaptations

Robotic vehicles capable of transition from aquatic to terrestrial locomotion face considerable challenges associated with propulsive efficiency and performance in each environment. Here we present a morphing amphibious robotic limb that combines the locomotor adaptations of sea turtles for swimming and tortoises for walking. The limb can transform between the streamlined morphology of a sea turtle flipper and the load-bearing geometry of a tortoise leg using a variable stiffness material coupled to a pneumatic actuator system. Herein, we describe the fabrication and characterization of the morphing limb, and quantitatively show how morphing between hydrodynamic and axial-load bearing states can enhance the locomotive performance of a single design over land and in water.

A novel design of shape-memory alloy-based soft robotic gripper with variable stiffness

Soft robotic grippers with compliance have great superiority in grabbing objects with irregular shape or fragility compared with traditional rigid grippers. The main limitations of such systems are small grasping force resulted from properties of soft actuators and lacking variable stiffness of soft robotic grippers, which prevent them from a larger wide range of applications. This article proposes a shape-memory alloy (SMA)-based soft gripper with variable stiffness composed of three robotic fingers for grasping compliantly at low stiffness and holding robustly at high stiffness. Each robotic finger mainly consisted of stiff parts and two variable stiffness joints is installed on the base with a specific angle. The paraffin as a variable stiffness material in the joint can be heated or cooled to change the stiffness of the robotic fingers. Results of experiments have shown that a single robotic finger can approximately achieve 18-fold stiffness enhancement. Each finger with two joints can actively achieve multiple postures by both changing the corresponding stiffness of joints and actuating the SMA wire. Based on these principles, the gripper can be applied to grasp objects with different shapes and a large range of weights, and the maximum grasping force of the gripper is increased to about 10 times using the variable stiffness joints. The final experiment is conducted to validate variable stiffness of the proposed soft grippers grasping an object.