Applications In Soft Robotics Research Articles

Layer jamming: Modeling and experimental validation

The ability to control and tune the stiffness of soft structures is one of the most important challenges in soft robotics. This is crucial in applications that require both compliance and the ability to withstand high forces. Among the various techniques, layer jamming represents a promising solution. Despite the increasing interest, the existing analytical models are not able to describe the behavior of these structures beyond the initial deformation phase. In this work, we propose an analytical model that predicts the behavior of these structures in all deformation phases, overcoming the limitations of existing models. Our previous approach is extended by explicitly taking into account the increase in stiffness due to the overhangs of the structure outside the constraining supports. We conduct experimental tests and finite element simulations to validate the predictions of the proposed model. The experimental and finite element results are in good agreement with theoretical predictions, especially considering that no fitting parameters have been used. Additionally, we analyze the effect of the main design parameters, including the number of layers, vacuum pressure and coefficient of friction, as well as the energy dissipated by friction during a load–unload cycle. We believe that this work represents a significant step forward in understanding the complex mechanisms underlying the mechanics of layer jamming structures that could be useful in helping researchers design more advanced variable stiffness applications in soft robotics.

A reconfigurable and automatic platform for the on-demand production of stretchable conductive composites

Stretchable conductive composites (SCCs) have been widely used as interconnects and sensors in stretchable electronic devices due to their tunable electromechanical properties and intrinsically high stretchability compared to solid metals. SCCs can be readily made by mixing (or breaking bulk) conductive fillers within an elastomeric polymer, which are subsequently cured. Despite the simplicity of this, most fabrication methods follow customized protocols and lack precise automatic control. These methods also require bulky and costly equipment (e.g. stirrers, mixers, ovens, and vacuuming machines). Also, variations in the production process make it challenging to maintain the consistency of SCC’s electrical and mechanical properties produced in different batches. To solve this problem, this work develops an automatic SCC production platform (ASPP) that can be programmed to produce SCCs with high consistency in properties. The versatility of ASPP is demonstrated by fabricating SCCs with single and hybrid fillers, and porous structures. The consistency of SCCs’ electromechanical properties is examined using samples fabricated in different batches following the same protocol. We further utilize the fabricated SCCs to realize various intelligent tactile sensing and heating platforms. The capability demonstrated for the ASPP shows its potential in fabricating SCCs for applications in soft robotics and wearable devices.

Biomorphic Actuation Driven Via On‐Chip Buckling of Photoresponsive Hydrogel Films

AbstractRemotely controllable photoresponsive hydrogel actuators are promising for applications in multiple fields. However, simple deformation mechanisms, which rely on the general swelling/deswelling, limit their performance and application. Herein, we report a displacement amplification mechanism based on the buckling deformation of photoresponsive hydrogel film. The on‐chip buckled architecture of the hydrogel enables actuation between a flat 2D shape and tubular 3D buckled shape with remarkable performances, including high deformation ratio (height ratio: ≈360%), tunable cycle motion frequency (0.1—1 Hz) and high cyclic stability. Moreover, localized buckling deformation, such as tube opening and closing, can be controlled in response to photostimulation. Inspired by these biomorphic shapes and motions, an intestine‐mimetic device and demonstrate segmentation with substance crushing and peristalsis motion with substance propelling were further fabricated. This study will provide a useful design principle for hydrogel actuators and shed light on diverse applications in soft robotics, dynamic microfluidics and organs‐on‐chips.

Organohydrogel Actuators with Adjustable Stimulus Responsiveness for On-Demand Morphing

Hydrogel actuators showing shape morphing in response to external stimuli are of significant interest for their applications in soft robots, artificial muscles, etc. However, there is still a lack of hydrogel actuators with adjustable stimulus responsiveness for on-demand driving. In this study, an organohydrogel actuator was prepared by a two-step interpenetrating method, resulting in the coexistence of poly(N-isopropylacrylamide-co-4-(2-sulfoethyl)-1-(4-vinylbenzyl) pyridinium betaine) (p(NIPAM-SVBP)) hydrophilic networks and poly(lauryl methacrylate) (pLMA) hydrophobic networks with gradient distribution. In the initial state, the organohydrogel actuator can be driven globally under thermal stimulation. Owing to the unique alkali-chromic performance of SVBP, the organohydrogel actuator can be endowed with photothermal properties and actuate locally under the stimulus of NIR light. More importantly, the organohydrogel will return to the original colorless state after being treated with acid solution. Our work provides a new insight into designing and fabricating novel actuators with adjustable stimulus responsiveness for on-demand morphing.

Chiral-based mechanical metamaterial with tunable normal-strain shear coupling effect

Different designs for mechanical metamaterials with tunable normal-strain shear coupling effect have been demonstrated over the last years. Their adjustable shear deformation makes them suitable as building blocks for soft robotics applications or structures with a desired deformation behavior, such as shape morphing structures. Herein, we present a modified 2.5D and 3D chiral-based mechanical metamaterials with tunable normal-strain shear coupling effect and Poisson’s Ratios close to zero. Advancing from conventional chiral-based metamaterials by introducing additional geometric freedoms into the design of the unit cell, a broad range of shear deformations, compression moduli and porosities can be achieved. 2.5D specimens with selected geometric parameters were additively manufactured with polypropylene using Fused Filament Fabrication. Compression tests were performed to investigate the mechanical properties and shear deformation. Two different numerical models were employed using ABAQUS to study the influence of the geometric parameters onto the mechanical properties and were verified by the experiments. The numerical material models were based on three-point-bending test data. The three-dimensional design was investigated with numerical simulations based on a homogenization approach to cover a broad range of geometric parameters.

Control of Droplet Impact through Magnetic Actuation of Surface Microstructures

AbstractAn effective method for on‐demand control over the impact dynamics of droplets on a magnetoresponsive surface is reported. The surface is comprised of micrometer‐sized lamellas from a magnetoactive elastomer on a copper substrate. The surface itself is fabricated using laser micromachining. The orientation of the lamellae is switched from edge‐on (orthogonal to the surface) to face‐on (parallel to the surface) by changing the direction of a moderate (<250 mT) magnetic field. This simple actuation technique can significantly change the critical velocities of droplet rebound, deposition, and splashing. Rebound and deposition regimes can be switched up to Weber number We < 13 ± 3, while deposition and splashing can be switched in the range of 32 < We < 52. Because a permanent magnet is used, no permanent power supply is required for maintaining the particular regime of droplet impact. The presented technology is highly flexible and enables selective fabrication and actuation of microstructures on complex devices. It has great potential for applications in soft robotics, microfluidics, and advanced thermal management.

Strong, Compressible, and Ultrafast Self‐Recovery Organogel with In Situ Electrical Conductivity Improvement

AbstractCoordination complexes are widely used to tune the mechanical behaviors of polymer materials, including tensile strength, stretchability, self‐healing, and toughness. However, integrating multivalent functions into one material system via solely coordination complexes is challenging, even using combinations of metal ions and polymer ligands. Herein, a single‐step process is described using silver‐based coordination complexes as cross‐linkers to enable high compressibility (>85%). The resultant organogel displays a high compressive strength (>1 MPa) with a low energy loss coefficient (<0.1 at 50% strain). Remarkably, it demonstrates an instant self‐recovery at room temperature with a speed >1200 mm s−1, potentially being utilized for designing high‐frequency‐responsive soft materials (>100 Hz). Importantly, in situ silver nanoparticles are formed, effectively endowing the organogel with high conductivity (550 S cm−1). Given the synthetic simplification to achieve multi‐valued properties in a single material system using metal‐based coordination complexes, such organogels hold significant potential for wearable electronics, tissue‐device interfaces, and soft robot applications.

4D printing of polyamide 1212 based shape memory thermoplastic polyamide elastomers by selective laser sintering

4D printing technology, also known as additive manufacturing technology for smart devices, offers a wide range of promising applications in aerospace, automobile, biomedical and soft robotics. However, there are only a limited number of commercialized elastomer materials for 3D printing which needs further development, especially smart elastomers. In this work, polyamide 1212 (PA1212) based thermoplastic polyamide elastomer (TPAE) was synthesized by one-pot melt polycondensation of 1,12-dodecanedioic acid (DA), 1,12-dodecanedioic amine (DN) and polyetheramine in a stainless steel reactor and then the TPAE powder for selective laser sintering (SLS) was obtained after freeze-crushing and sieving. Subsequently, the effects of sintering temperature, laser power, layer thickness, and scan spacing during SLS processing on the properties of the sintered parts were fully investigated. The tensile strength, elastic modulus and elongation at break of the sintered parts could reach 13.7 MPa, 124 MPa and 200 %, respectively, when the sintering temperature, laser power, layer thickness and scan spacing were set as 156 °C, 21 W, 0.10 mm and 0.10 mm, respectively. Moreover, PA1212-based TPAEs fabricated by SLS showed excellent and stable shape memory behavior. This work not only provides a designing strategy of 4D printable materials and referential SLS-processing parameters, but also broadens the application of polyamide-based elastomer materials into 4D printing fields.

Double-crosslinked reduced graphene oxide-based hydrogel actuator system with fast electro-responsive deformation and enhanced mechanical properties

Recently, soft actuators made of electroactive materials that show fast environmental response, large deformation, and can mimic multifunctional actuation of living creatures have attracted considerable attention for a variety of applications including wearable electronics, microrobotic devices, and intelligent artificial muscles. In this study, we designed an electro-responsive hydrogel actuator system with high stretchability, good self-recovery, and fast electro-responsive actuation. We prepared a hydrogel with double network structure consisting of partially reduced graphene oxide (CRGO) with carboxylic groups, poly (acrylic acid) (PA), and Fe3+ ions. The partially reduced CRGO shows excellent dispersibility and stability in an aqueous solution. The CRGO-embedded PA/CRGO/Fe hydrogels synthesized by varying the content of CRGO, FeCl3·6H2O, and crosslinker exhibited more than 75% gel fraction and a relatively high equilibrium water content (EWC) of 80.0–95.0%. Particularly, PA/CRGO/Fe hydrogels with a dual crosslinked network structure showed high stretchability of more than 1,200% and excellent self-healing properties. The electro-responsive actuation of PA/CRGO/Fe hydrogel can be precisely controlled by varying the content of CRGO, electric field strength, and concentration of the electrolyte solution. By controlling these influencing factors, PA/CRGO/Fe hydrogels exhibited rapid bending actuation of more than 100° within 1 min under an electric field. In vitro cytotoxicity results showed a relatively high cell viability of more than 97%, indicating that the prepared PA/CRGO/Fe hydrogels had no significant cytotoxicity. Therefore, these PA/CRGO/Fe hydrogels could be a promising material for electro-responsive soft robot applications.

Synergistic fluorescent hydrogel actuators with selective spatial shape/color‐changing behaviors via interfacial supramolecular assembly

AbstractBiomimetic intelligent polymeric hydrogel actuators with cooperative fluorescence‐color switchable behaviors are expected to find great potential applications in soft robotics, visual detection/display, and camouflage applications. However, it remains challenging to realize the spatial manipulation of synergistic shape/color‐changing behaviors. Herein, we report an interfacial supramolecular assembly (ISA) approach that enables the construction of robust fluorescent polymeric hydrogel actuators with spatially anisotropic structures. On the basis of this ISA approach, diverse 2D/3D soft fluorescent hydrogel actuators, including chameleon‐ and octopi‐shaped ones with spatially anisotropic structures, were facilely assembled from two different fluorescent hydrogel building blocks sharing the same physically cross‐linked agar network. Spatially control over synergistic shape/color‐changing behaviors was then realized in one single anisotropic hydrogel actuator. The proposed ISA approach is universal and expected to open promising avenues for developing powerful bioinspired intelligent soft actuators/robotics with selective spatial shape/color‐changing behaviors.

Kirigami-Inspired 3D Printable Soft Pneumatic Actuators with Multiple Deformation Modes for Soft Robotic Applications.

Soft robots have received much attention due to their impressive capabilities including high flexibility and inherent safety features for humans or unstructured environments compared with hard-bodied robots. Soft actuators are the crucial components of soft robotic systems. Soft robots require dexterous soft actuators to provide the desired deformation for different soft robotic applications. Most of the existing soft actuators have only one or two deformation modes. In this article, a new soft pneumatic actuator (SPA) is proposed taking inspiration from Kirigami. Kirigami-inspired cuts are applied to the actuator design, which enables the SPA to be equipped with multiple deformation modes. The proposed Kirigami-inspired soft pneumatic actuator (KiriSPA) is capable of producing bending motion, stretching motion, contraction motion, combined motion of bending and stretching, and combined motion of bending and contraction. The KiriSPA can be directly manufactured using 3D printers based on the fused deposition modeling technology. Finite element method is used to analyze and predict the deformation modes of the KiriSPA. We also investigated the step response, creep, hysteresis, actuation speed, stroke, workspace, stiffness, power density, and blocked force of the KiriSPA. Moreover, we demonstrated that KiriSPAs can be combined to expand the capabilities of various soft robotic systems including the soft robotic gripper for delicate object manipulation, the soft planar robotic manipulator for picking objects in the confined environment, the quadrupedal soft crawling robot, and the soft robot with the flipping locomotion.

Research Progress of the Ion Activity Coefficient of Polyelectrolytes: A Review.

Polyelectrolyte has wide applications in biomedicine, agriculture and soft robotics. However, it is among one of the least understood physical systems because of the complex interplay of electrostatics and polymer nature. In this review, a comprehensive description is presented on experimental and theoretical studies of the activity coefficient, one of the most important thermodynamic properties of polyelectrolyte. Experimental methods to measure the activity coefficient were introduced, including direct potentiometric measurement and indirect methods such as isopiestic measurement and solubility measurement. Next, progress on the various theoretical approaches was presented, ranging from analytical, empirical and simulation methods. Finally, challenges for future development are proposed on this field.

Bio‐Macromolecular Surface Coatings for Autohesive, Transparent, Elastomeric Foils

AbstractThin materials made from elastomeric polymers such as polydimethylsiloxane (PDMS) and polyurethane (PU) can be both, compliant and resilient. Their mechanical robustness and flexibility will make them great candidates for applications in the human body where space is limited and repeated deformations occur. Nonetheless, current medical applications of elastomeric foil‐like products are mainly restricted to inflatable balloon parts of stents or intubation tubes. Here, a key limiting factor is the autohesive behavior of those foils, that is, their propensity to stick to themselves. This property impedes handling and processing and can also interfere with the designated tasks of such foils. To mitigate this undesired behavior, different bio‐macromolecular coatings are applied here and assess their influence on the autohesive behavior, flexibility, and transparency of the materials. A non‐covalent, dopamine‐assisted coating approach is compared to a covalent coating strategy employing carbodiimide chemistry and investigated both, anionic and cationic macromolecules as top layers. The results show that especially the carbodiimide‐mediated mucin coating can efficiently suppress the autohesive behavior of the foils while maintaining the flexibility and transparency of the material. Thus, such coatings can not only broaden the medical application range of foil‐based elastomeric devices but may also prove beneficial for applications in soft robotics.

Wetting-Enabled Three-Dimensional Interfacial Polymerization (WET-DIP) for Bioinspired Anti-Dehydration Hydrogels.

Anti-dehydration hydrogels have attracted considerable attention due to their promising applications in stretchable sensors, flexible electronics, and soft robots. However, anti-dehydration hydrogels prepared by conventional strategies inevitably depend on additional chemicals or suffer from cumbersome preparation processes. Here, inspired by the succulent Fenestraria aurantiaca a one-step wetting-enabled three-dimensional interfacial polymerization (WET-DIP) strategy for constructing organogel-sealed anti-dehydration hydrogels is developed. By virtue of the preferential wetting on the hydrophobic-oleophilic substrate surfaces, the organogel precursor solution can spread on the three-dimensional (3D) surface and encapsulate the hydrogel precursor solution, forming anti-dehydration hydrogel with 3D shape after in situ interfacial polymerization. The WET-DIP strategy is simple and ingenious, and accessible to discretionary 3D-shaped anti-dehydration hydrogels with a controllable thickness of the organogel outer layer. Strain sensors based on this anti-dehydration hydrogel also exhibit long-term stability in signal monitoring. This WET-DIP strategy shows great potentialities for constructing hydrogel-based devices with long-term stability.

Autonomous self-burying seed carriers for aerial seeding.

Aerial seeding can quickly cover large and physically inaccessible areas1 to improve soil quality and scavenge residual nitrogen in agriculture2, and for postfire reforestation3-5 and wildland restoration6,7. However, it suffers from low germination rates, due to the direct exposure of unburied seeds to harsh sunlight, wind and granivorous birds, as well as undesirable air humidity and temperature1,8,9. Here, inspired by Erodium seeds10-14, we design and fabricate self-drilling seed carriers, turning wood veneer into highly stiff (about 4.9 GPa when dry, and about 1.3 GPa when wet) and hygromorphic bending or coiling actuators with an extremely large bending curvature (1,854 m-1), 45 times larger than the values in the literature15-18. Our three-tailed carrier has an 80% drilling success rate on flat land after two triggering cycles, due to the beneficial resting angle (25°-30°) of its tail anchoring, whereas the natural Erodium seed's success rate is 0%. Our carriers can carry payloads of various sizes and contents including biofertilizers and plant seeds as large as those of whitebark pine, which are about 11 mm in length and about 72 mg. We compare data from experiments and numerical simulation to elucidate the curvature transformation and actuation mechanisms to guide the design and optimization of the seed carriers. Our system will improve the effectiveness of aerial seeding to relieve agricultural and environmental stresses, and has potential applications in energy harvesting, soft robotics and sustainable buildings.

Ultrarobust subzero healable materials enabled by polyphenol nano-assemblies

Bio-inspired self-healing materials hold great promise for applications in wearable electronics, artificial muscles and soft robots, etc. However, self-healing at subzero temperatures remains a great challenge because the reconstruction of interactions will experience resistance of the frozen segments. Here, we present an ultrarobust subzero healable glassy polymer by incorporating polyphenol nano-assemblies with a large number of end groups into polymerizable deep eutectic solvent elastomers. The combination of multiple dynamic bonds and rapid secondary relaxations with low activation energy barrier provides a promising method to overcome the limited self-healing ability of glassy polymers, which can rarely be achieved by conventional dynamic cross-linking. The resulted material exhibits remarkably improved adhesion force at low temperature (promotes 30 times), excellent mechanical properties (30.6 MPa) and desired subzero healing efficiencies (85.7% at −20 °C). We further demonstrated that the material also possesses reliable cryogenic strain-sensing and functional-healing ability. This work provides a viable approach to fabricate ultrarobust subzero healable glassy polymers that are applicable for winter sports wearable devices, subzero temperature-suitable robots and artificial muscles.

Modeling and tracking control of dielectric elastomer actuators based on fractional calculus

Dielectric elastomer actuators (DEAs) show broad application prospects in the area of soft robots since they offer merits of fast response, large deformation, light weight and high energy conversion efficiency. Practical soft robot applications would usually require the study on the modeling and control of the DEA. However, the DEA has a memory property, which results in a highly nonlinear characteristics, bringing difficulties to the subject. To address this issue, an effective strategy is the utilization of the fractional order model, which is a type of modeling approach that can accurately characterize the memory property of a material with a small amount of parameters. Meanwhile, the fractional order controller can better handler the memory property, and it owns a better flexibility than traditional integer order controllers. With the above considerations, this paper proposes a modeling strategy and a tracking control strategy for the DEA on the basis of the fractional calculus. In the proposed strategy, a fractional order model is established to characterize the complicated nonlinear characteristics of the DEA. Then, to facilitate the computer simulation, the Oustaloup filter is used to construct an integer order approximation model (IOAM) of the fractional order model. Since the IOAM is difficult to be employed in the system controller design due to its high order, the IOAM is further simplified into a reduced order model based on the square root balance truncation algorithm. To realize the high accuracy control of the DEA, a feedforward-feedback combined controller is devised, which is composed of a feedforward controller and a fractional order proportional integral feedback controller (FOPIFC). Among which, the feedforward controller is devised based on the analytical inverse of the reduced order model to compensate the complicated nonlinear characteristics of the DEA, and the FOPIFC is devised to handle the bad influence from the modeling error and uncertainties on the control performance. Based on the proposed strategy, control experiment was conducted, and the root-mean-square errors in experiment are all below 0.7%, indicating the superiority of the presented modeling and tracking control strategies.

Soft-body dynamics induces energy efficiency in undulatory swimming: A deep learning study.

Recently, soft robotics has gained considerable attention as it promises numerous applications thanks to unique features originating from the physical compliance of the robots. Biomimetic underwater robots are a promising application in soft robotics and are expected to achieve efficient swimming comparable to the real aquatic life in nature. However, the energy efficiency of soft robots of this type has not gained much attention and has been fully investigated previously. This paper presents a comparative study to verify the effect of soft-body dynamics on energy efficiency in underwater locomotion by comparing the swimming of soft and rigid snake robots. These robots have the same motor capacity, mass, and body dimensions while maintaining the same actuation degrees of freedom. Different gait patterns are explored using a controller based on grid search and the deep reinforcement learning controller to cover the large solution space for the actuation space. The quantitative analysis of the energy consumption of these gaits indicates that the soft snake robot consumed less energy to reach the same velocity as the rigid snake robot. When the robots swim at the same average velocity of 0.024m/s, the required power for the soft-body robot is reduced by 80.4% compared to the rigid counterpart. The present study is expected to contribute to promoting a new research direction to emphasize the energy efficiency advantage of soft-body dynamics in robot design.

Muscle Contraction-Inspired Tough Hydrogels

In many animals, tough skeletal muscle contraction occurs, producing a strong force through myofilaments attaching to and sliding on fibrous actin filaments. In contrast, the strength of typical synthetic hydrogels is facilitated mainly by polymeric chains. We propose a strategy for developing strong and tough hydrogels in which the side groups on polymeric chains strongly interact with dispersing medium. The hydrogels are fabricated with a polyacrylamide-alginate double network in a choline chloride saturated solution. The hydrogels are not only highly transparent, tough, fatigue-resistant, self-recovering, self-healing, and adhesive but also water-retentive, antifreezing, and conductive. The hydrogels are strengthened by hydrogen bonds in dispersing medium with a clathrate framework structure. This work may inspire the development of tough and conductive gels for applications of e-skins, soft robots, and intelligent devices.

Circularly Polarized Light‐driven Liquid Crystal Elastomer Actuators

AbstractPhoto‐responsive liquid crystal elastomer (LCE) actuator is of paramount significance for emerging applications in soft robotics, biomimetic devices, and adaptive optical devices. However, to achieve circularly polarized light‐driven LCE actuator, which can provide an additional degree of freedom of light in the control method, is still a challenge. Herein, a circularly polarized light‐driven LCE actuator is demonstrated. The actuator is fabricated by bonding a cholesteric liquid crystal (CLC) film, which is capable of selectively reflecting and transmitting circularly polarized light, with a photothermal agent doped LCE film in splay configuration as a bilayer structure. An electrically controlled optical system with a fast switching ability of circularly polarized light's handedness has been developed to study the circularly polarized light‐response bending property of the proposed actuator under various periodic driven voltage schemes. A crawler driven by circularly polarized light based on CLC‐LCE actuator is demonstrated experimentally with different moving modes. The application of circularly polarized light in the LCE actuator driving provides an additional controlling degree of freedom for photo‐responsive LCE actuators besides the wavelength, spatial distribution, and linear polarization of light, showing wide application prospects in soft robotics, biomimetic technologies, and adaptive optical devices.