3D Printing of Stretchable, Compressible and Conductive Porous Polyurethane for Soft Robotics

The development of hydrogel adhesives with high mechanical resilience and toughness remains a challenging task. Hydrogels must exhibit high mechanical resilience to withstand the inevitable movement of the human body while simultaneously demonstrating strong wet tissue adhesion and appropriate toughness to hold and seal damaged tissues; However, tissue adhesion, toughness, and mechanical resilience are typically negatively correlated. Therefore, this paper proposes a highly resilient double-network (DN) hydrogel wound-sealing patch that exhibits a well-balanced combination of tissue adhesion, toughness, and mechanical resilience. The DN structure is formed by introducing covalently and non-covalently crosslinkable dopamine-modified crosslinkers and physically interactable linear poly(vinyl imidazole) (PVI). The resulting hydrogel adhesive exhibits high toughness and mechanical resilience due to the presence of a DN involving reversible physical intermolecular interactions such as hydrogen bonds, hydrophobic associations, cation-π interactions, π-π interactions, and chain entanglements. Moreover, the hydrogel adhesive achieves strong wet tissue adhesion through the polar hydroxyl groups of dopamine and the amine group of PVI. These mechanical attributes allow the proposed adhesive to effectively seal damaged tissues and promote wound healing by maintaining a moist environment.

Nanopapers fabricated from cellulose nanofibers (CNFs) are flexible for bending while they are rather stiff against stretching, which is a common feature shared by conventional paper-based materials in contrast with typical elastomers. Cellulose nanopapers have therefore been expected to be adopted in flexible device applications, but their lack of stretching flexibility can be a bottleneck for specific situations. The high stretching flexibility of nanopapers can effectively be realized by the implementation of Kirigami structures, but there has never been discussion on the mechanical resilience where stretching is not a single event. In this study, we experimentally revealed the mechanical resilience of nanopapers implemented with Kirigami structures for stretching flexibility by iterative tensile tests with large strains. Although the residual strains are found to increase with larger maximum strains and a larger number of stretching cycles, the high mechanical resilience was also confirmed, as expected for moderate maximum strains. Furthermore, we also showed that the round edges of cut patterns instead of bare sharp ones significantly improve the mechanical resilience for harsh stretching conditions. Thus, the design principle of relaxing the stress focusing is not only important in circumventing fractures but also in realizing mechanical resilience.

Superhydrophobic and superelastic thermoplastic polyurethane/multiwalled carbon nanotubes porous monolith for durable oil/water separation

In recent years, dielectric elastomer actuators (DEA) have been investigated as artificial muscle for soft robots, thanks to their light weight, high energy density, and silent operation. Moreover, the low stiffness of the dielectric elastomer (DE) material allows DEA to exhibit large actuation strain. On the other hand, the intrinsic softness of DEA limits their blocking and holding force. Therefore, incorporating variable stiffness structures into DEA is necessary to leverage both large actuation strain, and large holding force from such actuators. This work describes the modeling, fabrication, and characterization of a variable-stiffness DEA (VSDEA) based on interlaminar electrostatic chucking. The VSDEA consists of a multitude of stacked multilayer unimorph DEA units, where each unit consists of a passive layer and one or more active DE layers whose expansion under applied voltage induces bending of the DEA unit. Adhesion between the DEA units is mediated by electrostatic attraction caused by opposite charges accumulating on the interfacial surfaces between each unit. The bending stiffness of the VSDEA is controlled by increasing or decreasing the charge on the interfacial surfaces; large deformation can be achieved when the unit interfaces are allowed to freely slip, and a large increase in stiffness is realized when electrostatic chucking is applied.

ABSTRACTSilicon rubbers are widely used in a variety of products ranging from cooking utensils and electronics to medical devices and implants. Recently, they have sparked an interest among soft robotics researchers as they can be easily formed into various shapes and actuated in a relatively fast and easy way. In this article, we examine the nonlinear elastic response of a silicon rubber, Ecoflex, under both compressible and incompressible constraints. An experimental test on a uniaxial tension indicates a slight compressibility, and the compressibility increases with stretching. Five different constitutive material models are considered to describe the nonlinear elastic responses of Ecoflex under both compressible and incompressible conditions. In addition, finite element (FE) analysis is presented to analyze multiaxial response of structures or devices made of Ecoflex under complex boundary conditions. This study highlights the variations in the multiaxial response of structures at large deformations from different constitutive models under different compressible and incompressible constraints. For a high precision control in soft robotics applications, there is a need to understand the multiaxial response of silicon rubbers, especially under large deformations. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47025.

Dielectric elastomer actuators (DEAs) are difficult to apply to flexible grippers due to their small deformation range and low output force. Hence, a DEA with a large bending deformation range and output force was designed, and a corresponding flexible gripper was developed to realize the function of grasping objects of different shapes. The relationship between the pre-stretch ratio and DEA deformation degree was tested by experiments. Based on the performance test results of the dielectric elastomer (DE), the bending deformation process of DEAs with different shapes was simulated by Finite Element Method (FEM) simulation. DEAs with different shapes were prepared through laser cutting and the relationship between the voltage and the bending angle, and the output force of the DEAs was measured. The result shows that under uniaxial stretching, the deformation of the DEA in the stretching direction gradually increases and decreases in the unstretched direction with the increase in the pre-stretch ratio. Under biaxial stretching, DEA deformation increases with the increase in the pre-stretch ratio. The shape of the DEA has a certain influence on the bending deformation range under the same conditions, and the elliptical DEA has a larger bending deformation range and higher output force compared with the rectangular DEA and the trapezium DEA. The elliptical DEA can produce a bending deformation of 40° and an output force of 37.2 mN at a voltage of 24 kV. The three-finger flexible gripper composed of an elliptical DEA can realize the grasping of a paper cup.

Highly oriented, layered, and mechanically resilient films of polydopamine (PDA) have been synthesized from the air/water interface. The films show a unique layered structure, as shown by scanning and transmission electron studies (SEM/TEM) and X-ray diffraction analysis (XRD), which resemble that of 2D layered materials. The films exhibit a composition typical of PDA-based materials, as evidenced by X-ray photoelectron spectroscopy (XPS); moreover, the samples present the distinctive resonance modes of PDA-based nanomaterials in Raman and infrared spectroscopy (FTIR) experiments. The presence of highly ordinated 3–4 protomolecule stacking, taking place at the air/water interface, with a unique eumelanin-like supramolecular arrangement is presented. Moreover, the films show superior mechanical resilience with E = 13 ± 4 GPa and H = 0.21 ± 0.03 GPa, as revealed by nanoindentation experiments, making them highly resilient and easily transferable. Finally, the ordering induced by the interface opens many possibilities for further studies, including those regarding the supramolecular structure on PDA due to their similarity to 2D layered materials.

High water content electrically driven artificial muscles with large and stable deformation for soft robots

Shape sensing of soft robots has been a challenge due to the large deformation of the soft robots and their low stiffness. In this study, a simple yet accurate soft sensor for soft robotic application with small force ranges was proposed, modeled, prototyped, and experimentally validated. The proposed soft sensor is based on a gelatin-graphite composite that exhibited piezoresistive properties. The sensing element was molded to a cylindrical shape and was embedded in a soft flexural structure as a common type of soft flexural robots. Afterward, a mechano-electrical model for predicting the changes in the resistance of the sensing element was proposed and its predictions were validated through an experimental study. To demonstrate usability for force sensing, the sensor was calibrated with a nonlinear model and exhibited a force measurement range of 0.035-0.82N with an average absolute error of 3.7% and a resolution of 4%. Also, the mechano-electrical model was fairly accurate in predicting the piezoresistivity phenomenon of the sensing element under large bending deformations.

ABSTRACTTwisted and coiled artificial muscles (TAMs) have been extensively studied in the field of soft robots due to their exceptional properties, including high energy density, large load‐to‐weight ratio, large deformation, low driving voltage, and low hysteresis. The advancements of TAMs hold the potential for enhancing the performance and broadening their functional capabilities of soft robots, thus demonstrating substantial practical value. This review outlines the recent progress in TAMs and their diverse applications in soft robots. First, the commonly used materials to fabricate TAMs, including inorganic fibers, composite fibers, organic fibers, and natural fibers, are discussed along with their characteristics. Then, the actuation strategies are summarized across four aspects: thermal method, solvent method, electrochemical method and other non‐contact methods. Moreover, the configurations of TAMs are classified into single, parallel and braided structures. In addition, various soft robots driven by TAMs are introduced according to their functions, including manipulation, locomotion, smart textile, and sensor. Finally, the research hotspots and development trends of TAMs are evaluated. It is expected that this review article can serve as a valuable reference and source of inspiration for researchers in the field of soft actuators and robots.

This paper reports on Simscape modeling and experimental validation of compliant mechanisms and soft robotic systems. During the past decade, there has been a lot of advancements in terms of the development of novel soft robotic systems, actuators, and sensors. However, efficient modeling of these systems is still a challenge for researchers. This is due to the existence of nonlinearities such as large deformation in the structure of soft robotic systems and compliant mechanisms. While analytical models can be used for conventional rigid robots, the application of these models for soft robots is limited. Simscape is an efficient tool for modeling soft robotic systems and compliant mechanisms as it can take into account large deformation. To validate the developed Simscape models we used a 6 DOF electromagnetic position sensor for each of the example systems such as a compliant bistable mechanism, compliant linear actuator, and a soft Delta robot.

The inherent compliance and resilience allow soft robots to deal with unstructured environments. Among the various soft actuators explored for soft robotic applications, dielectric elastomer actuators (DEAs) stand out due to their intriguing advantages of large deformation, high energy density, fast response, and low cost. However, the viscoelasticity and the electromechanical coupling of these actuators increase the difficulty of the control design, especially for large deformation and/or high-speed operations. In this paper, the control issue of the DEAs is investigated. Inspired by the similarities between DEAs and biological muscles, an antagonistic actuation mechanism is proposed for DEAs control. Owing to the actions of the agonist can be compensated by the antagonist, the viscoelastic effects of the DEAs in antagonistic pairs can be compensated by each other. Moreover, a cerebellum-inspired adaptive learning controller for achieving accurate movements of the DEAs is designed and analyzed. To study the proposed actuation mechanism and controller, a simple 1-DOF manipulator based on two DEAs in an antagonistic pair is developed as a platform. Finally, several experiments have been conducted to test the developed system, and the results show that the proposed approach can achieve good performance even in large deformation and/or high-speed operations as well as strong robustness in disturbance rejection.

Wound healing in movable parts, including the joints and neck, remains a critical challenge due to frequent motions and poor flexibility of dressings, which may lead to mismatching of mechanical properties and poor fitting between dressings and wounds; thus, increasing the risk of bacterial infection. This study proposes a sprayable zwitterionic antibacterial hydrogel with outstanding flexibility and desirable adhesion. This hydrogel precursor is fabricated by combining zwitterionic sulfobetaine methacrylate (SBMA) with poly(sulfobetaine methacrylate-co-dopamine methacrylamide)-modified silver nanoparticles (PSBDA@AgNPs) through robust electrostatic interactions. About 150 s of exposure to UV light, the SBMA monomer polymerizes to form PSB chains entangled with PSBDA@AgNPs, transformed into a stable and adhesion PSB-PSB@Ag hydrogel at the wound site. The resulting hydrogel has adhesive strength (15-38kPa), large tensile strain (>400%), suitable shape adaptation, and excellent mechanical resilience. Moreover, the hydrogel displays pH-responsive behavior; the acidic microenvironment at the infected wound sites prompts the hydrogel to rapidly release AgNPs and kill bacteria. Further, the healing effect of the hydrogel is demonstrated on the rat neck skin wound, showing improved wound closing rate due to reduced inflammation and enhanced angiogenesis. Overall, the sprayable zwitterionic antibacterial hydrogel has significant potential to promote joint skin wound healing.

Dielectric elastomers (DEs) are capable of large deformation under the actuation of applied voltage and sprayed charge. Actuation of DE under voltage control is prone to electromechanical instabilities, while the DE under charge control always survives from instabilities with sacrificing a large deformation. In this article, a novel actuation mode of steady electric field is proposed. By tuning applied voltage and sprayed charge during viscoelastic creep, an invariable electric field is generated. Such an actuation method can both avoid the occurrence of electromechanical instabilities and guarantee a large deformation in DE actuation.

As is known, electromechanical deformation of voltage-controlled dielectric elastomers (DEs) is significant but is susceptible to pull-in and snap-through instabilities, while the large stable deformation can be achieved by spraying charge on DE surfaces, i.e., charge-controlled DEs. In this article, we investigate the effect of constrained fibers on electromechanical actuation of charge-controlled DEs by involving two special deformation modes: uniaxial tension and pure shear state. The coupled effects between the geometrical sizes and the mechanical tensile force of the charge-controlled DEs are also explored. The research results indicate that, different from voltage-controlled DEs, the electromechanical stretch of charge-controlled DEs with constrained fibers does not always show a beneficial merit with respect to that of the charge-controlled DEs without constrained fibers.