Applications In Soft Robotics Research Articles

Three-Dimensional Printing Strategies for Enhanced Hydrogel Applications.

This study explores the dynamic field of 3D-printed hydrogels, emphasizing advancements and challenges in customization, fabrication, and functionalization for applications in biomedical engineering, soft robotics, and tissue engineering. It delves into the significance of tailored biomedical scaffolds for tissue regeneration, the enhancement in bioinks for realistic tissue replication, and the development of bioinspired actuators. Additionally, this paper addresses fabrication issues in soft robotics, aiming to mimic biological structures through high-resolution, multimaterial printing. In tissue engineering, it highlights efforts to create environments conducive to cell migration and functional tissue development. This research also extends to drug delivery systems, focusing on controlled release and biocompatibility, and examines the integration of hydrogels with electronic components for bioelectronic applications. The interdisciplinary nature of these efforts highlights a commitment to overcoming material limitations and optimizing fabrication techniques to realize the full potential of 3D-printed hydrogels in improving health and well-being.

Nanocellulose-mediated bilayer hydrogel actuators with thermo-responsive, shape memory and self-sensing performances

Inspired by creatures, abundant stimulus-responsive hydrogel actuators with diverse functionalities have been manufactured for applications in soft robotics. However, constructing a shape memory and self-sensing bilayer hydrogel actuator with high mechanical strength and strong interfacial bonding still remains a challenge. Herein, a novel bilayer hydrogel with a stimulus-responsive TEMPO-oxidized cellulose nanofibers/poly(N-isopropylacrylamide) (TOCN/PNIPAM) layer and a non-responsive TOCN/polyacrylamide (TOCN/PAM) layer is proposed as a thermosensitive actuator. TOCNs as a nano-reinforced phase provide a high mechanical strength and endow the hydrogel actuator with a strong interfacial bonding. Due to the incorporation of TOCNs, the TOCN/PNIPAM hydrogel exhibits a high compressive strength (~89.2 kPa), elongation at break (~170.7 %) and tensile strength (~24.0 kPa). The prepared PNIPAM/TOCN/PAM hydrogel actuator performs the roles of an encapsulation, jack, temperature-controlled fluid valve and temperature-control manipulator. The incorporation of Fe3+ further endows the bilayer hydrogel actuator with a synergistic performance of shape memory and temperature-driven, which can be used as a temperature-responsive switch to detect ambient temperature. The PNIPAM/TOCN/PAM-Fe3+ conductive hydrogel can be assembled into a flexible sensor and generate sensing signals when driven by temperature changes to achieve real-time feedback. This research may lead to new insights into the design and manufacturing of intelligent flexible soft robots.

Dielectric elastomers with large actuation strain and high output force

AbstractDielectric elastomer actuator (DEA) is considered as an ideal artificial muscle with soft and light in nature, widespread application in soft robotics and sensors. Dielectric elastomer (DE), the core component of DEA, with high output force and large actuation strain under relatively low driving electric field is critical for high‐performance DEA. However, the widely used VHB series (3M) DE suffered from very large pre‐strain and high driving voltage to get their high actuation performance, limiting their scope of application. In this work, the DEs with good mechanical and dielectric properties were prepared via UV curing method consisting of acrylic acid (AA) with strong polar groups as functional monomer, n‐butyl acrylate (BA) as soft monomer and urethane acrylate as cross‐linker (c‐p(BA‐AA)). The c‐p(BA‐AA) DE films obtained the simultaneous increasing of dielectric constant and breakdown strength with the increasing content of AA, which guaranteed the good actuation performance. The c‐p(BA‐AA) film exhibited excellent actuating performance (44.11% actuation strain at 45 kV/mm) without pre‐strain. Besides, a simple linear actuator based on the c‐p(BA‐AA) film could easily output weight that is tens of times of its own active weight (654.1 mN/g at 30 kV/mm).

How to Easily Make Self-Sensing Pneumatic Inverse Artificial Muscles.

Wearable mechatronics for powered orthoses, exoskeletons and prostheses require improved soft actuation systems acting as 'artificial muscles' that are capable of large strains, high stresses, fast response and self-sensing and that show electrically safe operation, low specific weight and large compliance. Among the diversity of soft actuation technologies under investigation, pneumatic devices have been the focus, during the last couple of decades, of renewed interest as an intrinsically soft artificial muscle technology, due to technological advances stimulated by applications in soft robotics. As of today, quite a few solutions are available to endow a pneumatic soft device with linear actuation and self-sensing ability, while also easily achieving these features with off-the-shelf materials and low-cost fabrication processes. Here, we describe a simple process to make self-sensing pneumatic actuators, which may be used as 'inverse artificial muscles', as, upon pressurisation, they elongate instead of contracting. They are made of an elastomeric tube surrounded by a plastic coil, which constrains radial expansions. As a novelty relative to the state of the art, the self-sensing ability was obtained with a piezoresistive stretch sensor shaped as a conductive elastomeric body along the tube's central axis. Moreover, we detail, also by means of video clips, a step-by-step manufacturing process, which uses off-the-shelf materials and simple procedures, so as to facilitate reproducibility.

Photo- and Humidity-Responsive Liquid Crystal Copolymer Actuators Fabricated via Vapor-Assisted Alignment.

Deformable liquid crystal polymers (LCPs) driven by more than one external stimulus have received extensive attention in fields ranging from multifunctional soft robots to bionic actuators. Combining responsive liquid crystal with nonmesogenic responsive groups within polymer offers a versatile way to obtain multiresponsive LCPs. However, the incorporation of nonmesogenic responsive groups causes interruption in the assembly of mesogens and brings a challenge to the alignment of LCPs. Herein, a new method is put forward to facilitate uniform mesogen alignment by exerting water vapor in the film preparation process. Using this method, vapor-assisted alignment, the homeotropic alignment of azobenzene mesogens is achieved in a copolymer containing nonmesogenic poly(ethylene glycol) (PEG). The obtained copolymer films present photodeformation brought by azobenzene isomerization and humidity-responsive deformation resulting from the asymmetric swelling of film surfaces. The dual-responsive smart "blinds" and bionic flower actuators are fabricated to demonstrate the integration of the two different stimuli. This work is anticipated to provide a feasible alignment method for multiresponsive LCPs, showing the potential applications in soft robots, sensors, and biomimetic devices.

Theoretical analysis of inflated tube wrinkling behavior under pure bending

Inflatable tubes, owing to their lightweight and foldable characteristics, have important applications in soft robots and space expandable structures. The application of a bending load to the inflated tube might induce local instability on its compressed side, leading to the formation of wrinkles that could potentially compromise the structural strength. Therefore, it is necessary to have a comprehensive understanding of the wrinkling behavior exhibited by inflated tubes under bending load. Initially, a theoretical solution is developed for the critical wrinkling behavior of inflated tubes under pure bending, drawing upon the principle of minimum potential energy. According to the energy expression of the system, the critical wrinkling behavior of inflated tubes depends on dimensionless geometrical and internal pressure parameters. The critical wrinkling load and wrinkle pattern of the system are influenced by the thickness-radius ratio and the ratio of internal air pressure to Young's modulus. Subsequently, the theoretical solutions are validated through several finite element analysis examples and a systematic investigation is conducted into the influence of dimensionless geometrical and internal pressure parameters. Finally, the evolution of morphology in post-buckling is investigated through numerical results. The findings suggest that during post-buckling, the wrinkles may undergo a secondary bifurcation and evolve into a non-axisymmetric diamond-like pattern. These results hold significant implications for understanding the bending instability and failure mechanisms of inflated tubes.

Direct Ink Writing of Strained Carbon Nanotube-Based Sensors: Toward 4D Printable Soft Robotics.

Four-dimensional (4D) printing has attracted significant attention, because it enables structures to be reconfigured based on an external stimulus, realizing complex architectures that are useful for different applications. Nevertheless, most previously reported 4D-printed components have focused on actuators, which are just one part of a full soft robotic system. In this study, toward achieving fully 4D-printed systems, the design and direct ink writing of sensors with a straining mechanism that mimics the 4D effect are explored. Solution-processable carbon nanotubes (CNTs) were used as the sensing medium, and the effect of a heat-shrinkable shape-memory polymer-based substrate (i.e., potential 4D effect) on the electronic and structural properties of CNTs was assessed, followed by their application in various sensing devices. Herein, we reveal that substrate shrinking affords a more porous yet more conductive film owing to the compressive strain experienced by CNTs, leading to an increase in the carrier concentration. Furthermore, it improves the sensitivity of the devices without the need for chemical functionalization. Interestingly, the results show that, by engineering the potential 4D effect, the selectivity of the sensor can be tuned. Finally, the sensors were integrated into a fully 4D-printed flower structure, exhibiting their potential for different soft robotic applications.

A Molecular Rheology Dynamics Study on 3D Printing of Liquid Crystal Elastomers.

This work presents a rheological study of a biocompatible and biodegradable liquid crystal elastomer (LCE) ink for three dimensional (3D) printing. These materials have shown that their structural variations have an effect on morphology, mechanical properties, alignment, and their impact on cell response. Within the last decade LCEs are extensively studied as potential printing materials for soft robotics applications, due to the actuation properties that are produced when liquid crystal (LC) moieties are induced through external stimuli. This report utilizes experiments and coarse-grained molecular dynamics to study the macroscopic rheology of LCEs in nonlinear shear flow. Results from the shear flow simulations are in line with the outcomes of these experimental investigations. This work believes the insights from these results can be used to design and print new material with desirable properties necessary for targeted applications.

Nuclear power plant pipeline detection robot based on a new radiation-proof material

Soft robots are gradually gaining recognition in recent years for their ability to work in extreme environments. However, radiation shielding is a major issue when designing soft robots for use in nuclear environments. In this paper we present an effective soft robot design and develop new radiation shielding materials that enable the robot to perform a variety of tasks in the complex pipelines of a nuclear power plant while being sufficiently radiation shielded. To achieve this, we used a mixture of silicone and lead powder with excellent radiation shielding capabilities and designed and built a soft robot prototype consisting of multiple pneumatic actuators and a resin frame. It is capable of moving through complex pipelines and performing inspection tasks, while also providing radiation shielding for the internal components of the robot. Finite element simulations and prototype experiments were then carried out to evaluate the robot's performance in various scenarios. Simulation results show that the soft robot is able to perform inspection of nuclear power plant pipelines efficiently and effectively. Furthermore, the radiation shielding provided by the radiation-proof material is effective in protecting the internal components of the robot from radiation exposure. In conclusion, we have successfully developed a soft robot prototype that is capable of performing pipeline inspections in extreme environments such as nuclear power plants, while providing adequate radiation shielding. The proposed approach can be extended to other soft robotics applications in the nuclear environment and has the potential to improve the safety and efficiency of nuclear power plant operations.

Resistive Self‐Sensing Controllable Fabric‐Based Actuator: A Novel Approach to Creating Anisotropy

AbstractDesigning advanced soft robots with soft sensing capabilities for real‐world applications remains challenging due to the intricate integration of actuation and sensor capabilities, which require diverse materials and complex procedures. This paper introduces a fabric‐based robotic technology featuring an “all textile‐based self‐sensing pneumatic actuator” and a low‐cost resistive strain sensor created through simple sewing techniques. The novel approach eliminates the need for additional strain‐limiting woven fabric, simplifying the manufacturing process. It also enables the development of bioinspired motions such as bending, twisting, and snake‐like movements. The electromechanical behaviors of the sensor and bending actuator are tested for their performance under positive air pressure. Through mathematical modeling, the actuator's sensing capacity is estimated accurately, providing precise feedback for pressure and position control. Different closed‐loop controller types, including On–Off and Proportional Integral Derivative (PID) control, are evaluated for their effectiveness. Furthermore, the practical application of the sensing actuator is demonstrated by integrating it into a wearable glove, showcasing its enhanced sensing capabilities for finger‐like soft wearable robotic applications. This research tackles the challenges associated with designing advanced soft robots with integrated sensing capabilities, offering a promising fabric‐based solution that can drive significant advancements in real‐world applications.

From Nature-Sourced Polysaccharide Particles to Advanced Functional Materials.

Polysaccharides constitute over 90% of the carbohydrate mass in nature, which makes them a promising feedstock for manufacturing sustainable materials. Polysaccharide particles (PSPs) are used as effective scavengers, carriers of chemical and biological cargos, and building blocks for the fabrication of macroscopic materials. The biocompatibility and degradability of PSPs are advantageous for their uses as biomaterials with more environmental friendliness. This review highlights the progresses in PSP applications as advanced functional materials, by describing PSP extraction, preparation, and surface functionalization with a variety of functional groups, polymers, nanoparticles, and biologically active species. This review also outlines the fabrication of PSP-derived macroscopic materials, as well as their applications in soft robotics, sensing, scavenging, water harvesting, drug delivery, and bioengineering. The paper is concluded with an outlook providing perspectives in the development and applications of PSP-derived materials.

Fabrication and Characterization of Electrically Conductive 3D Printable TPU/MWCNT Filaments for Strain Sensing in Large Deformation Conditions

AbstractThis study investigates the development of thermoplastic polyurethane (TPU) filaments incorporating multi‐walled carbon nanotubes (MWCNT) to enhance strain‐sensing capabilities. Various MWCNT reinforcement ratios are used to produce customized feedstock for fused filament fabrication (FFF) 3D printing. Mechanical properties and the piezoresistive response of samples printed with these multifunctional filaments are concurrently evaluated. Surface morphology and microstructural observations reveal that higher MWCNT weight percentages increase filament surface roughness and rigidity. The microstructural modifications directly influence the tensile strength and strain energy of the printed samples. The study identifies an apparent percolation threshold within the 10–12 wt.% MWCNT range, indicating the formation of a conductive network. At this threshold, higher gauge factors are achieved at lower strains. A newly introduced Electro‐Mechanical Sensitivity Ratio (ESR) parameter enables the classification of composite behaviors into two distinct zones, offering the ability to tailor self‐sensing structures with on‐demand properties. Finally, flexible structures with proven application in soft robotics and shape morphing are fabricated and tested at different loading conditions to demonstrate the potential applicability of the custom filaments produced. The results highlight a pronounced piezoresistive response and superior load‐bearing performance in the examined structures.

Engineering pH-Responsive, Self-Healing Vesicle-Type Artificial Tissues with Higher-Order Cooperative Functionalities.

Multicellular organisms demonstrate a hierarchical organization where multiple cells collectively form tissues, thereby enabling higher-order cooperative functionalities beyond the capabilities of individual cells. Drawing inspiration from this biological organization, assemblies of multiple protocells are developed to create novel functional materials with emergent higher-order cooperative functionalities. This paper presents new artificial tissues derived from multiple vesicles, which serve as protocellular models. These tissues are formed and manipulated through non-covalent interactions triggered by a salt bridge. Exhibiting pH-sensitive reversible formation and destruction under neutral conditions, these artificial vesicle tissues demonstrate three distinct higher-order cooperative functionalities: transportation of large cargoes, photo-induced contractions, and enhanced survivability against external threats. The rapid assembly and disassembly of these artificial tissues in response to pH variations enable controlled mechanical task performance. Additionally, the self-healing property of these artificial tissues indicates robustness against external mechanical damage. The research suggests that these vesicles can detect specific pH environments and spontaneously assemble into artificial tissues with advanced functionalities. This leads to the possibility of developing intelligent materials with high environmental specificity, particularly for applications in soft robotics.

A solid-shell model of hard-magnetic soft materials

Hard-magnetic soft materials (HMSMs) consisting of an elastomer matrix filled with high remnant magnetic particles can exhibit flexible programmability and rapid shape changing under non-contact activation, showing promising potential applications in soft robotics, biomedical devices and flexible electronics. Precise predictions of large deformations of hard-magnetic soft materials would be a key for relevant applications. Here, we develop a magneto-mechanical theoretical framework to model large deformations of magneto-active elastomers by applying solid-shell formulations based on the variational principle. Within the 3D geometric description, the complete Green–Lagrange strain tensors are considered and thus 3D nonlinear material laws can be employed straightforwardly, without presupposing Kirchhoff–Love hypothesis and plane stress constraints. In addition, the solid-shell model avoids introducing rotational degrees of freedom in computation and thus does not require complex finite-rotation update, which also works well for thick shells at large strains. We adopt the enhanced assumed strain (EAS) and assumed natural strain (ANS) methods to overcome locking effects. Several representative examples with various deformed configurations are explored, showing precise predictions of morphological transformations of HMSMs based on our model, in good agreement with experiments. Our model provides a versatile tool to harness large deformations of magneto-elastic structures, and could be used for rational designs of active flexible devices and robots.

Flexible Grippers with Programmable Three‐Dimensional Magnetization and Motions

Programmable magnetic soft grippers are highly desirable for diverse applications in drug delivery, object manipulation, and soft robotics. However, current magnetic programming grippers need to be driven by multiple physical fields, which are complicated to operate and single in motion. Here, a method for patterning hard magnetic microparticles in an elastomer matrix is reported. Based on digital light processing (DLP), this method uses controlled reorientation of magnetic particles and selective exposure to ultraviolet (UV) light to encode magnetic particles in polydimethylsiloxane (PDMS) materials with arbitrary three‐dimensional (3D) orientation. The combination of vertical and horizontal magnetic fields can produce different forces, causing different deformation modes of the grippers. Flexible grippers can be fabricated from a single precursor in one process and produce various deformation and motion forms when a single magnetic field is applied. Moreover, the gripper has the advantages of simple manipulation, fast response, and flexible movement, which is of great significance in the application of biological devices and soft robots.

Synchronous behaviors of three coupled liquid crystal elastomer-based spring oscillators under linear temperature fields.

Self-oscillating coupled systems possess the ability to actively absorb external environmental energy to sustain their motion. This quality endows them with autonomy and sustainability, making them have application value in the fields of synchronization and clustering, thereby furthering research and exploration in these domains. Building upon the foundation of thermal responsive liquid crystal elastomer-based (LCE-based) spring oscillators, a synchronous system comprising three LCE-based spring oscillators interconnected by springs is established. In this paper, the synchronization phenomenon is described, and the self-oscillation mechanism is revealed. The results indicate that by varying system parameters and initial conditions, three synchronization patterns emerge, namely, full synchronous mode, partial synchronous mode, and asynchronous mode. For strongly interacting systems, full synchronous mode always prevails, while for weak interactions, the adjustment of initial velocities in magnitude and direction yields the three synchronization patterns. Additionally, this study explores the impact of several system parameters, including LCE elasticity coefficient and spring elasticity coefficient, on the amplitude, frequency, and synchronous mode of the system. The findings in this paper can enhance our understanding of the synchronization behavior of multiple mutually coupled LCE-based spring oscillators, with promising applications in energy harvesting, soft robotics, signal monitoring, and various other fields.

Robust and sensitive conductive nanocomposite hydrogel with bridge cross-linking-dominated hierarchical structural design.

Conductive hydrogels have a remarkable potential for applications in soft electronics and robotics, owing to their noteworthy attributes, including electrical conductivity, stretchability, biocompatibility, etc. However, the limited strength and toughness of these hydrogels have traditionally impeded their practical implementation. Inspired by the hierarchical architecture of high-performance biological composites found in nature, we successfully fabricate a robust and sensitive conductive nanocomposite hydrogel through self-assembly-induced bridge cross-linking of MgB2 nanosheets and polyvinyl alcohol hydrogels. By combining the hierarchical lamellar microstructure with robust molecular B─O─C covalent bonds, the resulting conductive hydrogel exhibits an exceptional strength and toughness. Moreover, the hydrogel demonstrates exceptional sensitivity (response/relaxation time, 20 milliseconds; detection lower limit, ~1 Pascal) under external deformation. Such characteristics enable the conductive hydrogel to exhibit superior performance in soft sensing applications. This study introduces a high-performance conductive hydrogel and opens up exciting possibilities for the development of soft electronics.

Eco-Friendly Flame-Retardant Phase-Change Composite Films Based on Polyphosphazene/Phosphorene Hybrid Foam and Paraffin Wax for Light/Heat-Dual-Actuated Shape Memory.

Multiactuated shape memory materials are a class of promising intelligent materials that have received great interest in the fields of self-healing, anticounterfeiting, biomedical, soft robotic, and smart thermal management applications. To obtain a light/heat-dual-actuated shape memory material for thermal management applications in fire safety, we have designed a type of halogen-free flame-retardant phase-change composite film based on polyaryloxyphosphazene (PDAP)/phosphorene (PR) hybrid foam as a support material and paraffin wax (PW) as a phase-change material (PCM). PDAP was synthesized as a flexible foam matrix through the ring-opening polymerization of hexachlorocyclotriphosphazene, followed by a substitution reaction of aryloxy groups. The porosity of the PDAP foam is improved by introducing PR nanosheets, facilitating a high latent heat capacity of the PDAP-PR/PW composite films for thermal management applications. The PDAP-PR/PW composite films can implement rapid shape recovery within 65 s in the heating process, which is much shorter than that of the corresponding film without PR nanosheets (185 s). Furthermore, the PDAP-PR/PW composite films also exhibit light-actuated shape memory behavior thanks to their good solar-to-thermal energy absorption and conversion contributed by PR nanosheets as a highly effective photothermal material. More importantly, the presence of PR nanosheets imparts an excellent flame-retardant property to the PDAP-PR/PW composite films. The PDAP-PR/PW composite film can be self-extinguished within 2 s after the flame. Through an innovative integration of flexible polyphosphazene foam, PR nanosheets, and solid-liquid PCM to obtain a sensitive actuating response to light and heat, this study offers a new approach for developing multiactuated and eco-friendly flame-retardant shape memory materials to meet the requirement of applications with a requirement of fire safety in soft actuators, thermal therapy, control devices, and so on.

Mechanically Tough Micellar Cubic Liquid‐Crystalline Polymer Electrolytes for Electromechanical Actuators

AbstractThis study presents a novel micellar cubic ionic liquid‐crystalline polymer electrolyte, featuring an alignment‐free spherical structure with unimpeded 3D ionic pathways, aimed at enhancing the performance of an ionic electroactive polymer actuator. The development involved creating a mechanically tough and high ion‐conductive cubic polymer film through the self‐assembly of a wedge‐shaped vinyl imidazolium salt and an imidazolium ionic liquid, followed by in situ photopolymerization. The 300 µm‐thick‐trilayer films, consisting of the cubic polymer electrolyte sandwiched between poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) electrodes, exhibit remarkable capabilities. These include bearing substantial loads of 4 g with a high blocking force under a DC voltage of 2 V, achieving a high bending strain of 0.63% under a low input voltage (±2 V, 0.1 Hz), and boasting a maximum response frequency of 70 Hz. These properties position the material for potential applications in soft robots and tactile sensing devices.

Ultra-low temperature-responsive liquid crystal elastomers with tunable drive temperature range

Liquid crystal elastomers (LCEs) have recently gained significant attention in the field of smart materials due to their ability to respond to external stimuli, offering substantial potential for practical applications in soft robotics and bioengineering where actuation and sensing are crucial. However, conventional LCEs typically have high driving thresholds, significantly limiting their practical utility. Thus, reducing the driving temperature of LCEs remains a critical challenge in current research. In contrast to complex chemical molecular modifications aimed at lowering the driving threshold, our work employs a straightforward one-step cross-linking method to synthesize bisacrylate LCEs with varying compositions. These LCEs exhibit remarkably low driving temperatures and improved actuation performance. The monofunctional and difunctional acrylate liquid crystal mixtures employed in our experiments possess different ranges of anisotropy and can be effectively incorporated into the liquid crystal elastomers. Following polymerization, these LCEs demonstrate actuation capabilities at temperatures as low as −18 °C while maintaining excellent performance even at elevated temperatures of up to 60 °C. Moreover, in a series of 50 repeat experiments, the LCEs exhibited exceptional repeatability with a minimal error rate of 2.0 %. Additionally, the introduction of carbon nanotubes into the LCEs resulted in achieving a significant angular deflection of up to 100° within a narrow range of low light intensities, specifically between 26.19 mW cm−2 and 32.38 mW cm−2, highlighting the continued effectiveness of these materials even with this modification.