Applications In Soft Robotics Research Articles

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.

4D Printing of Reprogrammable Liquid Crystal Elastomers with Synergistic Photochromism and Photoactuation.

4D printing of liquid crystal elastomers (LCEs) via direct ink writing has opened up great opportunities to create stimuli-responsive actuations for applications such as soft robotics. However, most 4D-printed LCEs are limited to thermal actuation and fixed shape morphing, posing a challenge for achieving multiple programmable functionalities and reprogrammability. Here, a 4D-printable photochromic titanium-based nanocrystal (TiNC)/LCE composite ink is developed, which enables the reprogrammable photochromism and photoactuation of a single 4D-printed architecture. The printed TiNC/LCE composite exhibits reversible color-switching between white and black in response to ultraviolet (UV) irradiation and oxygen exposure. Upon near-infrared (NIR) irradiation, the UV-irradiated region can undergo photothermal actuation, allowing for robust grasping and weightlifting. By precisely controlling the structural design and the light irradiation, the single 4D-printed TiNC/LCE object can be globally or locally programmed, erased, and reprogrammed to achieve desirable photocontrollable color patterns and 3D structure constructions, such as barcode patterns and origami- and kirigami-inspired structures. This work provides a novel concept for designing and engineering adaptive structures with unique and tunable multifunctionalities, which have potential applications in biomimetic soft robotics, smart construction engineering, camouflage, multilevel information storage, etc.

Photothermally-driven oxazine hydrogel actuators at the water-air interface

The design of synthetic hydrogels capable of exhibiting smart adaptive behaviors in response to light has gained significant interest to mimic the dynamic structures and functions as we observed in nature. Here we report on the molecular design of a photoresponsive hydrogel by covalently integrating oxazine-based compounds into the crosslinked polymer network through a free radical copolymerization strategy. The resultant oxazine hydrogels are found to be able to display well-controlled temperature variation through a photothermal effect in response to external irradiation with visible light. The light-induced non-uniform temperature variation of these hydrogels at the water-air interface lead to the generation of Marangoni propulsion, which subsequently drives them move as a swimmer. In addition to unidirectional swimming, such hydrogel swimmers are able to display steering, rotating, path-following motion and cargo transporting function by control of external irradiation parameters. Our work provides a useful molecular design strategy for development of other photoactive hydrogels with applications in smart actuators and soft robotics.

Chiral design of tough spring-shaped hydrogels for smart umbrellas

Developing hydrogel artificial muscles to mimic the motion of natural muscles has long attracted scientists from the perspective of materials science for potential applications in soft robotics. However, rational design of hydrogel artificial muscles with large stroke, rapid actuation speed, and high work capacity remains a major challenge. Herein, we reported two kinds of chiral spring-shaped hydrogels that were prepared via consecutive shaping process (e.g., stretching, twisting, folding, coiling, and fixing). By switching the chirality of coil, homochiral muscle and heterochiral muscle were obtained, respectively. Homochiral muscle could rapidly expand to 560% with an average speed of 6.7 % s−1 in response to NIR irradiation, whose maximum work capacity reached 45 J kg−1. On contrary, heterochiral muscle contracted 69% within 1 min under NIR irradiation with a maximum work capacity of 33 J kg−1. Interestingly, the parasol containing homochiral muscles opened autonomously during dehydration process, while the umbrellas containing heterochiral muscle could opened rapidly when water was applied. This work provided an innovative strategy for developing tough hydrogel muscles with opposite chiralities.

Ultrahigh tough, self-healing copolymer elastomer crosslinked by reversible imine system

Improvement of toughness is still a challenge for self-healing materials. Most of self-healing materials difficultly balance the mechanical and self-healing property. Here, a molecular design strategy of copolymer is proposed based on three combinations of reversible imine bond, chain motion and thermosensitive hydrogen bond together. The approach initially forms a co-continuous microphase separation structure and then cross-linked by aromatic imine via esterification reaction and Schiff base reaction, which features efficient self-healing, ultrahigh strength and toughness. The resultant elastomer exhibits a high stress at break (≈3 MPa) and high fracture strain (≈600 %). Additionally, the elastomer can reach 95 % self-healing efficiency after healing at 100 °C for 12 h. Extra reversible imine crosslinking displays a doubly promotion for fracture stress, and unchanging strain as well as 1times increase for self-healing efficiency, compared with merely chain motion and hydrogen bonding. This elastomer can be successfully spun into stretchable, conductive, self-healing filaments with core-shell structure by wet spinning and coating methods. It shows excellent corrosion resistance against NaCl, and conductive and self-healing property after coating by PANI polymer. This new type of self-healing polymer is promising to expand the application in future tissue engineering, soft robotics, and biomedical devices.

High-Performance Supramolecular Organogel Adhesives for Antimicrobial Applications in Diverse Conditions.

Supramolecular organogel coatings that can disinfect the deposited microbial pathogens are emerging as an effective vehicle to prevent pathogen transmission. However, the development of anti-pathogen supramolecular adhesives with mechanical robustness and controlled oil inclusion is technically challenging. Here, we report supramolecular adhesives with mechanical integrity and robust interfacial adhesion over a wide range of biogenic antimicrobial oil. Bifunctional monomers are synthesized and assembled into linear polymers with semicrystalline stackings through hierarchical hydrogen bonds, where incorporated bioactive oil could regulate the semicrystalline stackings into nanosized crystalline domains through intermolecular hydrogen bonds. The abundant bonding motifs provided by the supramolecular cross-linked networks could accommodate oil molecules with high inclusion capability and provide more interfacial binding sites with high adhesion strength, and the nanosized crystalline domains could stabilize the organogel network and compensate for the interactions with oil molecules to enhance structural and mechanical stability. In addition, rapid healing, robust adhesion, and antimicrobial and antiviral properties of the resultant organogel coatings are demonstrated. This study paves the way for the development of high-performance antimicrobial supramolecular adhesives with controlled oil inclusion, showing potential applications in soft robotics, tissue engineering, and biomedical devices.

Ultrastretchable and highly conductive hydrogels based on Fe3+- lignin nanoparticles for subzero wearable strain sensor

Conductive hydrogels have attracted considerable interest for potential applications in soft robotics, electronic skin and human monitoring. However, insufficient mechanical characteristics, low adhesion and unsatisfactory electrical conductivity severely restrict future application possibilities of hydrogels. Herein, lignin nanoparticles (LNPs)-Fe3+-ammonium persulfate (APS) catalytic system was introduced to assemble Poly(2-hydroxyethyl methacrylate)/LNPs/Ca2+ (PHEMA/LNPs/Ca) hydrogels. Due to the abundant metal coordination and hydrogen bonds, the composite hydrogel displayed ultrahigh stretchable capacity (3769 %), adhesion properties (248 kPa for skin) and self-healing performance. Importantly, hydrogel sensors possess with high durability, strain sensitivity (GF = 8.75), fast response time and freeze resistance (−20 °C) that could be employed to monitor motion signals in low-temperature regime. Therefore, the LNPs-Fe3+ catalytic system has great potential in preparing hydrogel for various applications such as human-computer interaction, artificial intelligence, personal healthcare and subzero wearable devices. At the same time, incorporation of natural macromolecules into polymer hydrogels is tremendous research significance for investigating high-value utilization of lignin.

Tough and Photo-Plastic Liquid Crystal Elastomer with a 2-Fold Dynamic Linker for Artificial Muscles.

Liquid crystal elastomers (LCEs) have been optimized by combining cross-linkers and dynamic bonds to achieve a reversible actuation behavior comparable to living skeletal muscles. In this study, one unique type of segment with 2-fold dynamic properties was introduced into LCEs, which offered not only dynamic diselenide covalent bonds for thermo-/photoplasticity but also H-bond arrays for dynamic cross-linking and mechanical robustness. Besides self-healing, self-welding, and recyclability, the LCEs were reprogrammable with elevated temperature or intensive visible light irradiation. The resultant LCEs gave an actuation blocking stress of 1.96 MPa and an elastic modulus of 14.4 MPa at 80 °C. The actuation work capacity reached 135.2 kJ m-3. When incorporating the Joule electrode or photothermal materials, the LCEs could be programmed as the electricity-driven and photothermal artificial muscles and thereby promised the application both as a biomimetic artificial hand and as an energy collector from sunlight. Thus, the 2-fold dynamic LCEs offered the pathway of enabling the reversible actuation behavior comparable to living skeletal muscles and promising applications in sustainable actuators, artificial muscles, and soft robots.

Photografting of Surface-assembled Hydrogel Prepolymers to Elastomeric Substrates for Production of Stimuli-Responsive Microlens Arrays.

Hydrogels have emerged as prototypical stimuli-responsive materials with potential applications in soft robotics, microfluidics, tissue engineering, and adaptive optics. To leverage the full potential of these materials, fabrication techniques capable of simultaneous control of microstructure, device architecture, and interfacial stability, i.e., adhesion of hydrogel components to support substrates, are needed. A universal strategy for the microfabrication of hydrogel-based devices with robust substrate adhesion amenable to use in liquid environments would enable numerous applications. This manuscript reports a general approach for the facile production of covalently attached, ordered arrays of microscale hydrogels (microgels) on silicone supports. Specifically, silicone-based templates were used to: i) drive mechanical assembly of prepolymer droplets into well-defined geometries and morphologies, and ii) present appropriate conjugation moieties to fix gels in place during photoinitiated crosslinking via a "graft from" polymerization scheme. Automated processing enabled rapid microgel array production for characterization, testing, and application. Furthermore, the stimuli-responsive microlensing properties of these arrays, via contractile modulated refractive index, were demonstrated. This process is directly applicable to the fabrication of adaptive optofluidic systems and can be further applied to advanced functional systems such as soft actuators and robotics and 3D cell culture technologies.

Development of a Soft Pneumatic Actuator with In-built Flexible Sensing Element for Soft Robotic Applications

Development of a Soft Pneumatic Actuator with In-built Flexible Sensing Element for Soft Robotic Applications

Elastocapillary rolling transfer weaves soft materials to spatial structures.

Spatial structures of soft materials have attracted great attention because of emerging applications in wearable electronics, biomedical devices, and soft robotics, but there are no facile technologies available to assemble the soft materials into spatial structures. Here, we report a mechanical transfer route enabled by the rotational motion of curved substrates relative to the soft materials on liquid surface. This transfer can weave soft materials into a broad variety of spatial structures with controllable global weaving chirality and orders and could also produce local ear-like folds with programmable numbers and distributions. We further prove that multiple pieces of soft materials in different forms including wire, ribbon, and large-area film can be woven onto curved substrates with various three-dimensional geometry shapes. Application demonstrations on the woven freestanding spatial structures with on-demand weaving patterns and orders have been conducted to show the temperature-driven multimodal actuating functionalities for programmable robotic postures.

3D Printing of Interpenetrating Network Flexible Hydrogels with Enhancement of Adhesiveness.

3D printing of hydrogels has been widely explored for the rapid fabrication of complex soft structures and devices. However, using 3D printing to customize hydrogels with both adequate adhesiveness and toughness remains a fundamental challenge. Here, we demonstrate mussel-inspired (polydopamine) PDA hydrogel through the incorporation of a classical double network (2-acrylamido-2-methylpropanesulfonic acid) PAMPS/(polyacrylamide) PAAm to achieve simultaneously tailored adhesiveness, toughness, and biocompatibility and validate the 3D printability of such a hydrogel into customized architectures. The strategy of combining PDA with PAMPS/PAAm hydrogels leads to favorable adhesion on either hydrophilic or hydrophobic surfaces. The hydrogel also shows excellent flexibility, which is attributed to the reversible cross-linking of PDA and PAMPS, together with the long-chain PAAm cross-linking network. Among them, the reversible cross-linking of PDA and PAMPS is capable of dissipating mechanical energy under deformation. Meanwhile, the long-chain PAAm network contributes to maintaining a high deformation capability. We establish a theoretical framework to quantify the contribution of the interpenetrating networks to the overall toughness of the hydrogel, which also provides guidance for the rational design of materials with the desired properties. Our work manifests a new paradigm of printing adhesive, tough, and biocompatible interpenetrating network hydrogels to meet the requirements of broad potential applications in biomedical engineering, soft robotics, and intelligent and superabsorbent devices.

Nature's Blueprint in Bioinspired Materials for Robotics

AbstractSoft robotics, an emerging field that focuses on the development of robots utilizing soft, flexible, and deformable materials, is revolutionizing traditional robotics (reliant on rigid materials and motors) and broadening its range of applications and potential uses. In addition, by emulating the structure, function, and characteristics of biological systems, bioinspired materials are facilitating significant progress in a diverse array of soft robotic applications. This review offers an overview of bioinspired materials employed in soft robotics, exploring their potential applications, challenges, and future research directions. For an intuitive understanding, soft robots based on the primary abilities required and the habitats (terrestrial, aquatic, aerial) of the animals and plants they mimic are categorized. Furthermore, real‐world applications of developed soft robots in everyday human life are presented. The novel category classification and comprehensive analysis presented in this review provide insights into the development of soft robotic systems with the potential to transform various industries and enhance quality of life.

Bioinspired rotary flight of light-driven composite films

Light-driven actuators have great potential in different types of applications. However, it is still challenging to apply them in flying devices owing to their slow response, small deflection and force output and low frequency response. Herein, inspired by the structure of vine maple seeds, we report a helicopter-like rotary flying photoactuator (in response to 0.6 W/cm2 near-infrared (NIR) light) with ultrafast rotation (~7200 revolutions per minute) and rapid response (~650 ms). This photoactuator is operated based on a fundamentally different mechanism that depends on the synergistic interactions between the photothermal graphene and the hygroscopic agar/silk fibroin components, the subsequent aerodynamically favorable airscrew formation, the jet propulsion, and the aerodynamics-based flying. The soft helicopter-like photoactuator exhibits controlled flight and steering behaviors, making it promising for applications in soft robotics and other miniature devices.

Latent Representation-Based Learning Controller for Pneumatic and Hydraulic Dual Actuation of Pressure-Driven Soft Actuators.

The pneumatic and hydraulic dual actuation of pressure-driven soft actuators (PSAs) is promising because of their potential to develop novel practical soft robots and expand the range of soft robot applications. However, the physical characteristics of air and water are largely different, which makes it challenging to quickly adapt to a selected actuation method and achieve method-independent accurate control performance. Herein, we propose a novel LAtent Representation-based Feedforward Neural Network (LAR-FNN) for dual actuation. The LAR-FNN consists of an autoencoder (AE) and a feedforward neural network (FNN). The AE generates a latent representation of a PSA from a 30-s stairstep response. Subsequently, the FNN provides an individual inverse model of the target PSA and calculates feedforward control input by using the latent representation. The experimental results with PSAs demonstrate that the LAR-FNN can meet the requirements of dual actuation control (i.e., accurate control performance regardless of the actuation method with a short adaptation time) with a single neural network. The results suggest that a LAR-FNN can contribute to soft dual-actuation robot development and the field of soft robotics.

Soft Architecture: Application of Soft Robotics in the Design Responsive-Interactive Architecture

Soft Architecture: Application of Soft Robotics in the Design Responsive-Interactive Architecture

Synthesis of Self-healing and Light-, Thermal-, and Humidity-induced Deformative Polyurethane Actuator.

Intelligent actuating materials have drawn enormous attention because of their potential applications in soft robots, smart sensors, bionics, etc. Aiming to integrate light, thermal, and humidity stimuli deformations and self-healing function into a single polymer, a smart actuating polyurethane material CPPU-50 is designed and successfully synthesized through co-polymerization of azobenzene-containing Azo-C12 , polyethylene glycol 200 (PEG200), and 4,4'-diphenylmethane diisocyanate (MDI) at a ratio of 1:1:2. The obtained polyurethane CPPU-50 exhibits good photoinduced bending, thermal responsive shape memory effect, humidity triggered deflections and self-healing properties. Furthermore, an actuator combining light and thermal stimuli is created and the self-healing CPPU-50 film can withstand the object of 1800 times without tearing. This work can pave a way for further development of long-lived multi-stimuli-responsive actuating devices and intelligent materials.