Applications In Soft Robotics Research Articles

Soft Manipulator for Soft Robotic Applications: a Review

Soft Manipulator for Soft Robotic Applications: a Review

Characteristic Analysis of Heterochiral TCP Muscle as a Extensile Actuator for Soft Robotics Applications

A soft actuator is an essential component in a soft robot that enables it to perform complex movements by combining different fundamental motion modes. One type of soft actuator that has received significant attention is the twisted and coiled polymer artificial muscle (TCP actuator). Despite many recent advancements in TCP actuator research, its use as an extensile actuator is less common in the literature. This works introduces the concept of using TCP actuators as thermal-driven extensile actuators for robotics applications. The low-profile actuator can be easily fabricated to offer two unique deformation capabilities. Results from the characterization indicate that extensile actuators, made with various rod diameters and under different load conditions, display remarkable elongation deformation. Additionally, a proof-of-concept soft-earthworm robot was developed to showcase the potential application of the extensile actuator and to demonstrate the benefits of combining different types of motion modes.

Light-driven soft climbing robot based on negative pressure adsorption

Many organisms rely on a dynamic attachment mechanism to climb surfaces. Inspired by these organisms, researchers have developed soft climbing robots. Despite their rapid development in recent times, such robots have restricted application in complex environments, such as the lack of remote wireless actuation and attachment ability for climbing tilted or slippery surfaces. To address these issues, we report alight-driven dynamic adsorption/desorption soft suction cup inspired by rock-climbing fish (Beaufortia kweichowensis). Our suction cups utilize the volumetric change caused by changes in the gas–liquid phase in the inner chamber to generate suction and thrust. The repetitive changes in the gas–liquid phase, controlled by photothermal conversion, help achieve adsorption and desorption. We demonstrated that the suction cups based on negative pressure adsorption can easily help miniaturize soft robotic devices and adapt to multiple types of substrates, making our approach applicable to tilted or slippery surfaces. By integrating the soft suction cups for surface attachment by dynamic control and a shape memory alloy as a deformable body, we developed a soft climbing robot that can perform precise, facile, and programmed controlled climbing under different conditions via remote light manipulation. Four continuous and directional climbing sequences on tilted and slippery surfaces were demonstrated. The results are expected to help advance the application of light-driven soft robots on tilted or slippery surfaces

Mechanics of regulatable hydrogel adhesion with elastic heterogeneity

Hydrogel adhesion regulation remains a key challenge in applications of stretchable electronics, tissue engineering, and soft robotics. Whereas existing strategies are focused on enhancing adhesion, the underlying mechanics of on-demand mechanical regulation from adhesion promotion to adhesion reduction receives insufficient attention. We propose an effective method to manipulate the energy process zone near the crack front by invoking periodic elastic heterogeneity, whose adhesion toughness exhibits high dependence on the periodic structures and peeling direction. Without modifying surface chemistry, the adhesion of heterogeneous interfaces implemented by 3D printing can enhance peak force by 6-fold or weaken interfacial toughness to 1/30, as compared to the case of a homogeneous interface. Based on a variational principle, analytical formulations and finite element simulations are performed to quantify the heterogeneous behaviors. The analysis captures the instability phenomenon and predicts the maximum peel force. Combined experimental, theoretical, and finite element results demonstrate the effects of geometric parameters and peeling directions on the overall adhesion toughness. This data elucidates an energy storage mechanism, namely that the discrete elastic ligaments free of lateral constraints could enable the interface to accommodate large stretching and store intensified strain energy. The results also indicate the necessity and significance of peeling directions. The work provides a design guideline for on-demand enhancement and detachment in hydrogel interface applications.

Bioinspired soft robots based on organic polymer-crystal hybrid materials with response to temperature and humidity

The capability of stimulated response by mechanical deformation to induce motion or actuation is the foundation of lightweight organic, dynamic materials for designing light and soft robots. Various biomimetic soft robots are constructed to demonstrate the vast versatility of responses and flexibility in shape-shifting. We now report that the integration of organic molecular crystals and polymers brings about synergistic improvement in the performance of both materials as a hybrid materials class, with the polymers adding hygroresponsive and thermally responsive functionalities to the crystals. The resulting hybrid dynamic elements respond within milliseconds, which represents several orders of magnitude of improvement in the time response relative to some other type of common actuators. Combining molecular crystals with polymers brings crystals as largely overlooked materials much closer to specific applications in soft (micro)robotics and related fields.

Controlling C2C12 Cytotoxicity on Liquid Metal Embedded Elastomer (LMEE).

Liquid metal embedded elastomers (LMEEs) are highly stretchable composites comprising microscopic droplets of eutectic gallium-indium (EGaIn) liquid metal embedded in a soft rubber matrix. They have a unique combination of mechanical, electrical, and thermal properties that make them attractive for potential applications in flexible electronics, thermal management, wearable computing, and soft robotics. However, the use of LMEEs in direct contact with human tissue or organs requires an understanding of their biocompatibility and cell cytotoxicity. In this study, the cytotoxicity of C2C12 cells in contact with LMEE composites composed of EGaIn droplets embedded with a polydimethylsiloxane (PDMS) matrix is investigated. In particular, the influence of EGaIn volume ratio and shear mixing time during synthesis on cell proliferation and viability is examined. The special case of electrically-conductive LMEE composites in which a percolating network of EGaIn droplets is created through "mechanical sintering" is also examined. This study in C2C12 cytotoxicity represents a first step in determining whether LMEE is safe for use in implantable biomedical devices and biohybrid systems.

Stretchable Sensors for Soft Robotic Grippers in Edge-Intelligent IoT Applications.

The rapid development of electronic material and sensing technology has enabled research to be conducted on liquid metal-based soft sensors. The application of soft sensors is widespread and has many applications in soft robotics, smart prosthetics, and human-machine interfaces, where these sensors can be integrated for precise and sensitive monitoring. Soft sensors can be easily integrated for soft robotic applications, where traditional sensors are incompatible with robotic applications as these types of sensors show large deformation and very flexible. These liquid-metal-based sensors have been widely used for biomedical, agricultural and underwater applications. In this research, we have designed and fabricated a novel soft sensor that yields microfluidic channel arrays embedded with liquid metal Galinstan alloy. First of all, the article presents different fabrication steps such as 3D modeling, printing, and liquid metal injection. Different sensing performances such as stretchability, linearity, and durability results are measured and characterized. The fabricated soft sensor demonstrated excellent stability and reliability and exhibited promising sensitivity with respect to different pressures and conditions.

Extremely Large‐Stroke Hair Artificial Muscles with Fast Recovery Prepared by a Facile and Green Method

Twisted artificial muscles based on natural fibers are of special interest in diverse fields. However, current research is limited either by the material itself, or by the preparation methods requiring complex processing or specialized equipment. In this study, a facile and green method is introduced to prepare hair artificial muscles with extremely large tensile stroke and fast recovery. The maximum tensile stroke for the hair artificial muscles upon water actuation is as large as 10 000% and the large‐stroke muscles can recover fast in ethanol. In addition, these hair artificial muscles maintain their excellent performance after either 100 water–ethanol stimulation cycles or staying in open air for 5 months. Moreover, the hair artificial muscle is able to contract by 59% when lifting 10 times its own weight, pull a wheel model or climb a long distance under water, and work as a smart water‐sensitive switch. This work provides new insights into the development of simpler and more environmentally friendly methods for preparing advanced natural fiber‐based artificial muscles that have broad applications in soft robotics and biomedical engineering. An interactive preprint version of the article can be found here: https://doi.org/10.22541/au.167385630.01748766/v1.

Tardigrade-inspired extremotolerant glycerogels

We developed extremotolerant glycerogels (GGs) with well-modulated polymer structures, functions, and properties, inspired by the tun formation of tardigrades. GGs comprising extreme protected intra- and intermolecular networks are obtained through a very slow structure building process, which includes the smooth replacement of water in predesigned hydrogels with glycerol and thermal annealing while retaining the structures and functions of the original hydrogels. Four different GGs are fabricated as proofs-of-concept using different crosslinkers and polymers. Although various polyol-based wide-temperature-tolerant gels fabricated by conventional methods fail to demonstrate stabilities at low and high temperature extremes simultaneously, the GGs fabricated by our bioinspired method exhibit long-term stability (approaching one month) over an extremely wide temperature range (−50–80 °C) and thermal-shock-absorption capabilities at 150 °C. Furthermore, our versatile method enables us to program GGs with wide ranges of stiffness, strength, stretchability, and toughness values and elasticity, plasticity, hysteresis, and self-recoverability capabilities. The self-weldability, electrical patternability, and applicability characteristics of the GGs as electrolytes and supercapacitors demonstrate their complex 3D designability and facile functionalization capability aspects. The various functional GGs developed through the proposed method are applicable for the design of diverse extremotolerant, flexible, and stretchable devices for biological, electrical/electronic, and soft robotics applications.

The Influence of Industrial Environmental Factors on Soft Robot Materials.

This work aims to identify the effects that a series of environmental factors, specific to the industrial conditions, have on the materials in the structure of soft robots and, therefore, on soft robotics systems. The purpose is to understand the changes in the mechanical characteristics of silicone materials, with the aim of transferring soft robotics applications from the sphere of services in the industrial field. Distilled water, hydraulic oil, cooling oil, and UV rays are the environmental factors considered in which the specimens were immersed/exposed for 24 h according to ISO-62/2008. The analysis was carried out on two of the most widely used materials in the field, belonging to the category of silicone rubber, which were subjected to uniaxial tensile tests on the strength testing machine Titan 2 Universal. The results show that the greatest impact on the characteristics of the two materials was when exposed to UV rays, while the other media tested had relatively little impact on the mechanical and elastic properties (tensile strength, elongation at break, and tensile modulus) of these materials.

The role of chemical microstructures and compositions on the actuation performance of dielectric elastomers: A materials research perspective

AbstractDielectric elastomer actuators (DEAs) are one of the emerging areas of investigation in smart elastomer research and have attracted considerable interests due to their wide‐ranging applications, for instance in automation and soft robotic components. Commonly utilized dielectric elastomers (DEs) such as nitrile butadiene rubber (NBR) and its hydrogenated and carboxylated derivatives along with the dielectric fillers (titanium oxide, barium titanate, etc.) were considered to be one of the most established composite systems demonstrating high actuation performance. When subjected to external electrical field, the DEAs derived from the composites were, therefore, capable of reproducing high energy density and large deformation with fast response time. The recent research emphasizes that tunable material properties eventually triggered the improved actuation performance. The exploration of compliant electrodes fabricated by the conventional and state‐of‐the‐art lithographic methods was also crucial to improve actuation performance for the soft robotic applications. This review summarizes the commonly used DEs and dielectric fillers, their performances and challenges, and outlines the research progress (experimental and theoretical) in the field of actuators. Finally, a brief introduction to DEA‐based soft robotics application is also presented.

Non-equilibrium-Growing Aesthetic Ionic Skin for Fingertip-Like Strain-Undisturbed Tactile Sensation and Texture Recognition.

Humans use periodically ridged fingertips to precisely perceive the characteristics of objects via ion-based fast- and slow-adaptive mechanotransduction. However, designing artificial ionic skins with fingertip-like tactile capabilities remains challenging because of the contradiction between structural compliance and pressure sensing accuracy (e.g., anti-interference from stretch and texture recognition). Inspired by the formation and modulus-contrast hierarchical structure of fingertips, an aesthetic ionic skin grown from a non-equilibrium Liesegang patterning process is introduced. This ionic skin with periodic stiff ridges embedded in a soft hydrogel matrix enables strain-undisturbed triboelectric dynamic pressure sensing as well as vibrotactile texture recognition. By coupling with another piezoresistive ionogel, an artificial tactile sensory system is further fabricated as a soft robotic skin to mimic the simultaneous fast- and slow-adaptive multimodal sensations of fingers in grasping actions. This approach may inspire the future design of high-performance ionic tactile sensors for intelligent applications in soft robotics and prosthetics.

The world of exotic crystals: Raman spectro-microscopy for probing local structure

In this article, we review briefly the recent progress on mechanically responsive molecular crystals which defy the common perception that “crystals are rigid”. Crystals which respond to different external stimuli are emerging fast and may find applications in miniature technologies, sensors, flexible devices, and soft-robotics. Here, we present some early examples of molecular crystals from our group, along with other examples of our choice, to emphasize on the use of Raman spectromicroscopy technique for characterizing crystals, polymorphic forms and strained crystals in the context of structure- property correlation and crystal engineering.

Dielectric elastomer artificial muscle materials advancement and soft robotic applications

AbstractConventional robotic systems are built with rigid materials to deal with large forces and predetermined processes. Soft robotics, however, is an emerging field seeking to develop adaptable robots that can perform tasks in unpredictable environments and biocompatible devices that close the gap between humans and machines. Dielectric elastomers (DEs) have emerged as a soft actuation technology that imitates the properties and performance of natural muscles, making them an attractive material choice for soft robotics. However, conventional DE materials suffer from electromechanical instability (EMI), which reduces their performance and limits their applications in soft robotics. This review discusses key innovations in DE artificial muscles from a material standpoint, followed by a survey on their representative demonstrations of soft robotics. Specifically, we introduce modifications of DE materials that enable large strains, fast responses, and high energy densities by suppressing EMI. Additionally, we examine materials that allow variable stiffness and self‐healing abilities in DE actuators. Finally, we review dielectric elastomer actuator (DEA) applications in soft robotics in four categories, including automation, manipulation, locomotion, and human interaction.

Adaptive Magnetoactive Soft Composites for Modular and Reconfigurable Actuators

AbstractMagnetoactive soft materials, typically composed of magnetic particles dispersed in a soft polymer matrix, are finding many applications in soft robotics due to their reversible and remote shape transformations under magnetic fields. To achieve complex shape transformations, anisotropic, and heterogeneous magnetization profiles must be programmed in the material. However, once programmed and assembled, magnetic soft actuators cannot be easily reconfigured, repurposed, or repaired, which limits their application, their durability, and versatility in their design. Here, magnetoactive soft composites are developed from squid‐derived biopolymers and NdFeB microparticles with tunable ferromagnetic and thermomechanical properties. By leveraging reversible crosslinking nanostructures in the biopolymer matrix, a healing‐assisted assembly process is developed that allows for on‐demand reconfiguration and magnetic reprogramming of magnetoactive composites. This concept in multi‐material modular actuators is demonstrated with programmable deformation modes, self‐healing properties to recover their function after mechanical damage, and shape‐memory behavior to lock in their preferred configuration and un‐actuated catch states. These dynamic magnetic soft composites can enable the modular design and assembly of new types of magnetic actuators, not only eliminating device vulnerabilities through healing and repair but also by providing adaptive mechanisms to reconfigure their function on demand.

Aromatic Zipper Topology Dictates Water-Responsive Actuation in Phenylalanine-Based Crystals.

Water-responsive (WR) materials that reversibly deform in response to relative humidity (RH) changes are gaining increasing interest for their potential in energy harvesting and soft robotics applications. Despite progress, there are significant gaps in the understanding of how supramolecular structure underpins the reconfiguration and performance of WR materials. Here, three crystals are compared based on the amino acid phenylalanine (F) that contain water channels and F packing domains that are either layered (F), continuously connected (phenylalanyl-phenylalanine, FF), or isolated (histidyl-tyrosyl-phenylalanine, HYF). Hydration-induced reconfiguration is analyzed through changes in hydrogen-bond interactions and aromatic zipper topology. F crystals show the greatest WR deformation (WR energy density of 19.8MJm-3 ) followed by HYF (6.5MJm-3 ), while FF exhibits no observable response. The difference in water-responsiveness strongly correlates to the deformability of aromatic regions, with FF crystals being too stiff to deform, whereas HYF is too soft to efficiently transfer water tension to external loads. Thesefindings reveal aromatic topology design rules for WR crystals and provide insight into general mechanisms of high-performance WR actuation. Moreover, the best-performing crystal, F emerges as an efficient WR material for applications at scale and low cost.

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.