Applications In Soft Robotics Research Articles

Mesoscopic Supramolecular Assembly of Stereolithographically Printed Microgels

AbstractMicrogels that assemble upon issuing of a trigger open up miscellaneous applications in soft robotics, drug delivery, and life‐like materials, due to their unique physical and chemical properties. In this work, a novel hydrogel photoresist system is formulated, consisting of 2‐hydroxyethyl acrylate, ethylene glycol diacrylate, and phenyl bis(2,4,6‐trimethylbenzoyl) phosphine oxide, as well as redox‐sensitive supramolecular host‐guest association motifs. Complex and multi‐compartmented geometries with precisely located recognition motifs are printed in high resolution, resulting in multifunctional structured microgel building blocks for mesoscopic self‐assembly. An enzymatic feedback system is employed to dissipatively assemble and disassemble the redox‐active microgel building blocks in a temporally and spatially controlled pathway. The self‐assembling photoresist system is characterized in terms of the lower critical solution temperature, swelling behavior, and printability. The present work provides new perspectives for the design of tailor‐made mesoscopic microgels with customizable functionality and high applicability for various technological applications.

Morphing matter: from mechanical principles to robotic applications

The adaptability of natural organisms in altering body shapes in response to the environment has inspired the development of artificial morphing matter. These materials encode the ability to transform their geometrical configurations in response to specific stimuli and have diverse applications in soft robotics, wearable electronics, and biomedical devices. However, achieving the morphing of intricate three-dimensional shapes from a two-dimensional flat state is challenging, as it requires manipulations of surface curvature in a controlled manner. In this review, we first summarize the mechanical principles extensively explored for realizing morphing matter, both at the material and structural levels. We then highlight its applications in the soft robotics field. Moreover, we offer insights into the open challenges and opportunities that this rapidly growing field faces. This review aims to inspire researchers to uncover innovative working principles and create multifunctional morphing matter for various engineering fields.

Charge-controlled swelling gradients at 200-µm resolution in an open-porous polymeric structure for compliance modulation

Plants combine active and passive liquid-mediated mechanisms across broad spatial and temporal scales, inspiring technological developments, in particular involving variable stiffness. Swelling is of particular interest due to the abundance of plant models and applicable (bio)materials, yet existing control by environmental humidity sorption limits its applications. This work combined swellable polymeric structures with electroactive control: we considered an open-porous polymeric laminate that hosted an electrokinetic medium also co-acting as a swelling agent for the same polymer. A constant volume of liquid (an electrolytic solution) was electrokinetically pumped between the symmetrical laminate’s interior and surface layers: as the second moment of inertia increases from the centre to the surface, the pumping of liquid towards the surface decreases the laminate’s bending stiffness, and vice versa. Ion electrosorption on high-specific-surface-area carbon electrodes, deposited in three layers in the laminate by simple additive spraying, facilitated the ion current necessary for the electrokinetic pumping. Flexural rigidity of the 400 µm thick laminate varied by 7% in response to 2-V input, evidencing swelling gradients forming at half-laminate (i.e., 200-µm) resolution. Charge-driven local rearrangement of liquid allows for broader adoption of bioinspired (and biological) porous architectures, where the channels are defined collectively, not individually as in, e.g., soft lithography. Sub-mm resolution and low-voltage control of liquid offer a high level of integration at minimal assembly, positioning active swelling as a promising solution for wearable and bioinspired soft robotic applications.

Visible Light Responsive Soft Actuator Based on PVA‐DR1

AbstractThere is a growing interest in creating biomimetic actuators that can perform human‐like actions in response to external stimuli. Despite the significant advancements in soft actuators, using high‐energy UV light in photoresponsive actuators remains a great challenge. Herein, we develop a film composed of polyvinyl alcohol and photo responsive Disperse Red 1 (PVA/DR1) that actuate in response to visible light. The photoresponsive film starts to actuate in the presence of green light due to trans→cis isomerization of photoresponsive dye. The actuation response of the films was investigated by varying concentrations of DR1 dye and the intensity of green light. Based on these features, we demonstrated a four‐arm soft‐gripper that can mimic human‐hand‐like motions in response to green light. This kind of actuator may find potential applications in soft robotics and biomimetic microgrippers.

A Toolbox for Designing 3D‐Printing‐Ready Pneumatic Circuits for Controlling Soft Robots

The robotics field has witnessed a rapid increase in the popularity of soft pneumatic robots due to their adaptability to unstructured environments and safe human–robot interaction. However, many of them are still controlled by rigid solenoid valves and microcontrollers. Recent research works have introduced various electronics‐free computational devices for soft robotic applications. Yet, connecting these individual components into pneumatic circuits still remains challenging. The manual circuit design, fabrication, and piping process can bring inefficient circuit layout, redundant tubing, unreliable fittings, and potentially human mistakes. This work presents an open‐source toolbox for automatically designing pneumatic circuits for soft robotic applications. The toolbox takes the desired computation function along with some space constraints as inputs, and generates a 3D‐printable file of a pneumatic circuit that can be fabricated and used directly. This is achieved by efficiently positioning the logic gates and air channels while considering the workspace constraints, fabrication limits, material cost, and system complexity. The versatility of the design automation algorithm has been tested with two different truth tables and six different workspaces. The work also showcases utilizing the toolbox to design a computational circuit that governs the movements of a soft robotic hand with four pneumatic actuators.

Technologies and Applications of Soft Robotics: A Review

A soft robot is a type of continuum robot that is made of a soft material. Soft robots show infinite degrees of freedom that enable them to track complicated paths. The advantages of soft robotics include high safety, minimal maintenance cost, and robust adaptability to the unstructured environment. Therefore, they can be used for applications in various fields such as industrial, defense, medical, and exploration. This paper discusses about the various existing technologies used in development soft robots. The paper mainly focuses on the design, manufacturing, actuation, and sensing aspects of soft robotics. A review on these aspects of soft robotics is presented along with applications.

Superhydrophobic, flexible and high thermal stable ANFs/CNTs film for controllable light driven actuators

Superhydrophobic and light-responsive actuators (SLRAs) can be driven to move on water surface remotely and in a non-contact manner, and have shown promising applications in soft robotics, environmental protection, etc. It remains a big challenge to develop high performance SLRAs with excellent thermal stability, fast light responsivity and controllable motion. Here, we propose a layer-by-layer vacuum filtration method to prepare flexible, freestanding and superhydrophobic composite films composed of alternately deposited oleylamine modified acid carbon nanotubes (O-ACNTs) and aramid nanofibers (ANFs). The composite film has excellent thermal stability with the maximum weight loss rate temperature as high as 520 ℃. The closely packed structure and strong interlayer interaction ensure the surface stability and durability of the composite film. The composite film possesses excellent photothermal conversion performance, and can maintain the structure integrity without deformation when its surface temperature reaches 208 °C. When used as a self-propelled actuator, the SLRA film can response to the light quickly and move on the water surface, and the motion speed and direction can be controlled by adjusting the irradiation intensity and position. The outstanding thermal stability guarantees the stability and reliability of the self-propelled motions. The present study opens a new avenue to design high performance SLRAs.

Lightweight Soft Conductive Composites Embedded with Liquid Metal Fiber Networks

AbstractLiquid metal composites are promising soft conductors for applications in soft electronics, sensors, and soft robotics. Existing liquid metal composites usually have a high‐volume fraction of liquid metal, which not only increases the density but also the material cost. Future applications in soft electronics and robotics highly demand liquid metal composites with low density and high conductivity for large‐scale, low‐cost, lightweight, and more sustainable applications. In this work, lightweight and highly conductive composites embedded with liquid metal fiber networks are synthesized. This new paradigm of liquid metal composites consists of an interconnected liquid metal fiber network embedded in a compliant rubber matrix. The liquid metal fiber network serves as an ultra‐lightweight conductive pathway for electrons. Experiments indicate that this soft conductive composite also possesses nearly strain‐insensitive conductance and superior cyclic stability. Potential applications of the composite films as stretchable interconnects, electrodes, and sensors are demonstrated.

A stretchable all-nanofiber iontronic pressure sensor

Flexible pressure sensors with high stretchability, sensitivity, and stability are undoubtedly urgently required for potential applications in intelligent soft robots, human-machine interaction, health monitoring, and other fields. However, most current flexible pressure sensors are unable to endure large deformation and are prone to performance degradation or even failure during frequent operation due to their multilayered structures. Here, we propose a stretchable all-nanofiber iontronic pressure sensor that is composed of ionic nanofiber membranes used as dielectric layers and liquid metal used as electrodes. This sensor exhibits a high sensitivity of 1.08 kPa-1 over a wide range of 0-300 kPa, with a fast response-relaxation time of about 18/22 ms and excellent stability. The high sensitivity comes from the electric double layer formed at the ionic film/electrode interface, while high stretchability and stability are enabled by in-situ encapsulated all-nanofiber structures. As a proof of concept, a prototype sensor array is integrated into a soft pneumatic gripper, demonstrating its capability of pressure perception and object recognition during the grasping process. Thus, the scheme provides another excellent strategy to fabricate stretchable pressure sensors with superb performance in terms of high stretchability, sensitivity, and stability.

Degradable Elastomeric Silk Biomaterial for Flexible Bioelectronics.

The integration of degradable and biomimetic approaches in material and device development can facilitate the next generation of sustainable (bio) electronics. The use of functional degradable materials presents exciting opportunities for applications in healthcare, soft robotics, energy, and electronics. These include conformability to curved surfaces, matching of stiffness of tissue, and the ability to withstand mechanical deformations. Nature-derived materials such as silk fibroin (SF) provide excellent biocompatibility, resorbability, and tunable properties toward such goals. However, fibroin alone lacks the required mechanical properties and durability for processing in biointegrated electronics and dry conditions. To overcome these limitations, we report on an elastomeric photocurable composite of silk fibroin and poly(dimethylsiloxane) (PDMS). Photofibroin (containing methacryl functionalities) is doped with photoPDMS (methacryloxypropyl-terminated poly(dimethylsiloxane)) to form an elastomeric photofibroin (ePF) composite. The elastomeric silk is photocurable, allowing for microfabrication using UV photolithography. It is suitable for circuits, strain-sensing devices, and biointegrated systems. The ePF exhibits flexibility in both wet and dry conditions, enhanced mechanical strength and long-term durability, and optical transparency. It is stable at high temperatures, compatible with electronic materials, and cytocompatible while being enzymatically degradable. This work therefore highlights a path toward combining natural and synthetic materials to achieve versatile properties and demonstrates the potential of silk fibroin composites in (bio) electronics, encapsulation, and packaging.

Development of a phase-change material-based soft actuator for soft robotic gripper functionality: In-depth analysis of material composition, ethanol microbubble distribution and lifting capabilities

Soft actuators have numerous applications in soft robotics due to their capability of mimicking quite complex motions observed in living things with deformable organs. In the present research, a programmable, low-cost soft actuator based on phase-change material (PCM) composites is developed, using silicon rubber as the matrix, ethanol as the phase-change fluid, and carbon fabric as the conductive path which generates the required heat for actuation. The required pressure for actuation is provided by the electrically-induced phase transition of the PCM (30W electrical power supply). FESEM images illustrate the homogenous distribution of ethanol microbubbles within the active area, with an average diameter of 14.66 μm, and their absence inside the passive layer. The temperature is monitored by an internal thermal sensor placed inside the composite, and controlled to prevent overheating (T > 80 °C) in the active area. The gripper fabricated by using this composite produces considerable motion using a low-voltage driving force (∼30 V). Moreover, it undergoes a sizeable displacement (∼9 cm) and bending angle (∼50°), considering the low amount of power input (30W). Additionally, the tip of the gripper reaches the maximum velocity of ∼1 mm s−1, and the designed sample is capable of lifting objects with varying shapes and weights (from ∼10g to ∼300g).

Bio-inspired soft pneumatic actuator based on a kresling-like pattern with a rigid skeleton

IntroductionBiomimetic soft pneumatic actuators (SPA) with Kresling origami patterns have unique advantages over conventional rigid robots, owing to their adaptability and safety. ObjectivesInspired by cloning and moving behaviors observed from salps, we proposed an SPA based on a Kresling-like pattern with a rigid skeleton. The elongation and output force were tested, and the effectiveness of the applications with the SPA was evaluated. MethodsThe proposed SPA consists of rigid skeletons and a soft skin. The rigid skeletons are constructed using layers of Kresling-like patterns, while a novel extensible inserting structure is devised to replace the folds found in conventional Kresling patterns. This innovative approach ensures that the SPA exhibits axial contraction/expansion motion without any twisting movement. To mimic the bionic characteristics of swimming and ingesting progress of salps, the proposed SPA can perform an axial contraction motion without twisting and a controllable bending motion based on multi-layered Kresling-like patterns; to mimic the cloning and releasing life phenomena of salps, the number of layers of Kresling-like patterns is changeable by adding or reducing skeleton components according to the practical needs. ResultsThe experimental elongation results on the SPA with multiple layers of Kresling-like patterns show that the elongation can increase to above 162% by adding layers; the experimental output force results show that the three-layer SPA can provide 6.36 N output force at an air flow rate of 10 L/min, and the output force will continue to increase as the number of layers of Kresling-like pattern increases or the air flow rate increases. Further, we demonstrate the applications of the SPA in soft grippers, scissor grippers, claw grippers and pipe crawlers. ConclusionOur proposed SPA can avoid twisting in the radial contraction motion with high elongation and output force, and provide the practical guidance for bio-inspired soft robotic applications.

Thermoresponsive double-network hydrogel/elastomer hybrid bilayer laminates with high interfacial toughness and controllable shape deformations

Thermoresponsive hydrogel-based actuators have shown great potential in tissue engineering, biosensors and soft robots, while the preparation of temperature-driven bilayer hydrogel actuators with rapid responseness and recovery properties remains a challenge. Inspired by the layered heterostructure of human skin, we developed a Janus bilayer structure actuator consisting of thermoresponsive double network (TDN) hydrogel and polydimethylsiloxane (PDMS) elastomer with tough interface adhesion. Specifically, PNaAMPS/P(NIPAM-co-AAm) TDN hydrogel via photocopolymerization of N-isopropyl acrylamide (NIPAM) and acrylamide (AAm) and reinforced by microgel poly(2-acrylamide-2-methylpropane sulfonate sodium salt) (PNaAMPS) was in situ grafted onto benzophenone-activated PDMS surface, exhibiting strong interface adhesion (628 Jm-2 adhesion energy). The thermoresponsive shape deformations of the double-layer structure actuator were controllable by adjusting hydrogel/elastomer thickness ratio and introduction of metal ions, taking advantage of the different swelling and shrinkage characteristics of the hydrogel layer and the elastomer layer. The TDN/PDMS bilayer structure actuator, which integrates the "Janus" structure, thermal responsiveness and photothermal conversion, provides new ideas for the application and remote control of soft robots and soft actuators.

Biocompatible, 3D Printable Magnetic Soft Actuators – Ink Formulation, Rheological Characterization and Hydrogel Actuator Prototypes

Abstract3D printable inks for the preparation of magnetic hydrogel actuators are difficult to formulate because magnetic nanoparticles tend to aggregate without stabilization through other ink components. At the same time, such inks need to be shear‐thinning and recover their high viscosity state sufficiently fast to be suitable for the printing process. Here, the use of chitosan as dispersing agent for Fe2O3 nanoparticles is reported. Combined with Pluronic F127 as a rheology modifier and acrylamide as the base monomer, thermoresponsive and shear‐thinning magnetic inks containing well‐dispersed particles are obtained. The ink viscosity is tuned over two orders of magnitude by varying the chitosan and Pluronics F127 content. 3D‐printed shapes with good shape fidelity are obtained at a print bed temperature of 50 °C, where aggregation of the Pluoronics F127 micelles occurs. This leads to a fast recovery of the high viscosity state of the material, so that the printed shape can then be locked in by UV cross‐linking. This treatment yielded magneto‐responsive prototypes which are promising for soft robotics applications. Thanks to the simplicity of the ink formulation, it is easily transferable also to nonspecialist laboratories, and the concept is potentially applicable also to other types of nanoparticles.

Liquid Crystal Networks Meet Water: It's Complicated!

Soft robots are composed of compliant materials that facilitate high degrees of freedom, shape-change adaptability, and safer interaction with humans. An attractive choice of material for soft robotics is crosslinked networks of liquid crystal polymers (LCNs), as they are responsive to a wide variety of external stimuli and capable of undergoing fast, programmable, complex shape morphing, which allows for their use in a wide range of soft robotic applications. However, unlike hydrogels, another popular material in soft robotics, LCNs have limited applicability in flooded or aquatic environments. This can be attributed not only to the poor efficiency of common LCN actuation methods underwater but also to the complicated relationship between LCNs and water. In this review, the relationship between water and LCNs is elaborated and the existing body of literature is surveyed where LCNs, both hygroscopic and non-hygroscopic, are utilized in aquatic soft robotic applications. Then the challenges LCNs face in widespread adaptation to aquatic soft robotic applications are discussed and, finally, possible paths forward for their successful use in aquatic environments are envisaged.

A BTO/PVDF/PDMS Piezoelectric Tangential and Normal Force Sensor Inspired by a Wind Chime.

There is a growing demand for flexible pressure sensors in environmental monitoring and human-robot interaction robotics. A flexible and susceptible sensor can discriminate multidirectional pressure, thus effectively detecting signals of small environmental changes and providing solutions for personalized medicine. This paper proposes a multidimensional force detection sensor inspired by a wind chime structure with a three-dimensional force structure to detect and analyze normal and shear forces in real time. The force-sensing structure of the sensor consists of an upper and lower membrane on a polydimethylsiloxane substrate and four surrounding cylinders. A piezoelectric hemisphere is made of BTO/PVDF/PDMS composite material. The sensor columns in the wind chime structure surround the piezoelectric layer in the middle. When pressure is applied externally, the sensor columns are connected to the piezoelectric layer with a light touch. The piezoelectric hemisphere generates a voltage signal. Due to the particular structure of the sensor, it can accurately capture multidimensional forces and identify the direction of the external force by analyzing the position of the sensor and the output voltage amplitude. The development of such sensors shows excellent potential for self-powered wearable sensors, human-computer interaction, electronic skin, and soft robotics applications.

Spatially-selective enhancement of mechanical strength through localized cross-linking of double network hydrogels

To advance the development of flexible materials for soft robotics applications, it is crucial to enhance the elastic modulus and breaking the stress of soft materials, such as hydrogels. Double network hydrogels (DN gels) have displayed remarkable mechanical strength owing to their unique network structure composed of two types of polymer networks. However, current fabrication methods for DN gels entail cross-linking two distinct hydrogel polymers throughout the entire hydrogel matrix. In this study, we focused on employing multi-photon polymerization (MPP) with femtosecond laser pulses as a cross-linking method for hydrogels to achieve spatially selective formation of DN gel structures at the micrometer scale, along with the consequent improvement in local mechanical strength. We assessed the mechanical properties of the fabricated structures and confirmed that the mechanical strength varied within the regions where DN gel was locally formed.

Soft magnetic wood composites with chain-aligned Fe3O4 nanoparticles for magnetically driven actuators

Intelligently responsive wood-derived materials have long been sought due to their sustainability, earth abundance, and low carbon footprint, with promising applications in sensors, flexible electronics, and soft robotics. However, insensitive responsiveness and unsatisfactory mechanical properties have hindered the development of intelligent drive systems. This work reports on soft magnetic wood composite (SMWC) with a sandwich structure consisting of densified wood and polydimethylsiloxane (PDMS) mixed with Fe3O4. We treat natural wood chemically to improve flexibility, especially the lignin partial removal while keeping the wood structure intact. Benefiting from densification and magnetic-assisted techniques, multilayer SMWC with chain-arranged Fe3O4 can be prepared, which exhibits 183.5 MPa mechanical strength along the longitudinal direction, flexible bending capacity, a water contact angle of 100.9° and a saturation magnetization strength of 9.81 emu g−1 for 40 wt% Fe3O4. Importantly, the chain arrangement of Fe3O4 facilitates the sensitive control of SMWC compared to non-chain arranged structures. We have designed a star-shaped actuator that demonstrates controlled magnetic response, which can perform transporting cargo through linear and rotational movements on the water. SMWC with chain-aligned Fe3O4 exhibits good flexibility, excellent mechanical properties, and superior magnetic responsiveness, which is essential for exploring the responsiveness of wireless sensors and soft robots.

Inchworm-like Soft Robot with Multi-Responsive Bilayer Films.

As an important branch of robotics, soft robots have the advantages of strong flexibility, a simple structure, and high safety. These characteristics enable soft robots to be widely used in various fields such as biomedicine, military reconnaissance, and micro space exploration. However, contemporary soft crawling robots still face problems such as the single drive mode and complex external equipment. In this study, we propose an innovative design of an inchworm-like soft crawling robot utilizing the synergistic interaction of electricity and moisture for its hybrid dual-drive locomotion. The legs of the soft robot are mainly made of GO-CNT/PE composite film, which can convert its own volume expansion into a corresponding bending motion after being stimulated by electricity or moisture. Unlike other drive methods, it requires less power and precision from external devices. The combination of the two driving methods greatly improves the environmental adaptability of the soft robot, and we developed visible light as the driving method on the basis of the dual drive. Finally, we also verified the robot's excellent load capacity, climbing ability, and optical drive effect, which laid the foundation for the application of soft robots in the future.

Organic radical ferroelectric crystals with martensitic phase transition

Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.