Soft Pneumatic Actuators Research Articles

Design of squeezing-tube-driven pump for soft pneumatic robotics based on spiral spring winding

Aiming at the demand for high-speed, easy-controllability, and integration of pneumatic soft robots and elastomer actuators, this study presents a squeezing-tube-driven pump (STDP) for soft pneumatic robotics based on spiral spring winding. This concept contains a customized spiral spring and a pneumatic tube with high-elasticity. The spiral spring is driven by an electric motor and coerced into winding deformation. Furthermore, the pneumatic tube is extruded by the spring and then the air in the tube is fast compressed to drive soft pneumatic grippers. The mechanical model and simulation are utilized to explain the operating principle of STDP. The air pressure and rotation angle of the spring under various rotation speeds are in a close linear correlation verified by the experimental results, which provides feasibility for easy controlling and rapid actuation. Finally, fast-gripping tests with an integrated gripper–pump system and a pneumatic muscle actuation test are presented to show the advantages of the proposed pump, respectively.

Silicone Elastomeric-Based Materials of Soft Pneumatic Actuator for Lower-Limb Rehabilitation: Finite Element Modelling and Prototype Experimental Validation

This study describes the basic design, material selection, fabrication, and evaluation of soft pneumatic actuators (SPA) for lower-limb rehabilitation compression therapy. SPAs can be a promising technology in proactive pressure delivery, with a wide range of dosages for treating venous-related diseases. However, the most effective design and material selection of SPAs for dynamic pressure delivery have not been fully explored. Therefore, a SPA chamber with two elastomeric layers was developed for this study, with single-side inflation. The 3D deformation profiles of the SPA chamber using three different elastomeric rubbers were analyzed using the finite element method (FEM). The best SPA-compliant behavior was displayed by food-grade silicone A10 Shore with a maximum deformation value of 25.34 mm. Next, the SPA chamber was fabricated using A10 Shore silicone and experimentally validated. During the simulation in FEM, the air pressure was applied on the inner wall of the chamber (i.e., the affected area). This is to ensure the applied pressure was evenly distributed in the inner wall while the outer wall of the chamber remained undeformed for all compression levels. During the inflation process, pressure will be applied to the SPA chamber, causing exerted pressure on the skin which is then measured for comparison. The simulation and experimental results show an excellent agreement of pressure transmission on the skin for the pressure range of 0–120 mmHg, as depicted in the Bland–Altman plots. The findings exhibited promising results in the development of the SPA chamber using low-cost and biocompatible food-grade silicone.

Kirigami-Inspired 3D Printable Soft Pneumatic Actuators with Multiple Deformation Modes for Soft Robotic Applications.

Soft robots have received much attention due to their impressive capabilities including high flexibility and inherent safety features for humans or unstructured environments compared with hard-bodied robots. Soft actuators are the crucial components of soft robotic systems. Soft robots require dexterous soft actuators to provide the desired deformation for different soft robotic applications. Most of the existing soft actuators have only one or two deformation modes. In this article, a new soft pneumatic actuator (SPA) is proposed taking inspiration from Kirigami. Kirigami-inspired cuts are applied to the actuator design, which enables the SPA to be equipped with multiple deformation modes. The proposed Kirigami-inspired soft pneumatic actuator (KiriSPA) is capable of producing bending motion, stretching motion, contraction motion, combined motion of bending and stretching, and combined motion of bending and contraction. The KiriSPA can be directly manufactured using 3D printers based on the fused deposition modeling technology. Finite element method is used to analyze and predict the deformation modes of the KiriSPA. We also investigated the step response, creep, hysteresis, actuation speed, stroke, workspace, stiffness, power density, and blocked force of the KiriSPA. Moreover, we demonstrated that KiriSPAs can be combined to expand the capabilities of various soft robotic systems including the soft robotic gripper for delicate object manipulation, the soft planar robotic manipulator for picking objects in the confined environment, the quadrupedal soft crawling robot, and the soft robot with the flipping locomotion.

A Novel and Practicable Approach for Determining the Beam Parameters of Soft Pneumatic Multi-Chamber Bending Actuators

The design of many pneumatic soft actuators is based on multiple chambers in parallel alignment. The Cosserat beam theory is an established technique for modeling this kind of actuator, where existing approaches mainly differ in the parameters being required for simulation. The modeling approach presented in this study particularly aims at finding the beam parameters necessary for a simulation even with limited experimental methods. Importantly, it provides a straightforward relationship between the bending stiffness, the extensional stiffness and the axial stretch of the actuator. If the actuator to be modeled has an elementary design, axial measurements are sufficient to identify the parameters to perform three-dimensional simulations, which is of interest to adopters with limited testing equipment. The experimentally parameterized model of such an actuator of elementary design shows high accuracy. Both without load and with a weight of 1N applied to the tip, the mean error of the tip position in vertical orientation is less than 3.4% for a constant extensional stiffness and less than 2.7% for a pressure-dependent extensional stiffness. Further reduction of the error could be achieved by more refined identification techniques that decompose the complex interrelationship of pressurization, forces and material stiffness.

Lightweight Bioinspired Exoskeleton for Wrist Rehabilitation Powered by Twisted and Coiled Artificial Muscles

Stroke, cerebral palsy, and spinal cord injuries represent the most common leading causes of upper limb impairment. In recent years, rehabilitation robotics has progressed toward developing wearable technologies to promote the portability of assistive devices and to enable home rehabilitation of the upper extremities. However, current wearable technologies mainly rely on electric motors and rigid links or soft pneumatic actuators and are usually bulky and cumbersome. To overcome the limitations of existing technologies, in this paper, a first prototype of a lightweight, ungrounded, soft exoskeleton for wrist rehabilitation powered by soft and flexible carbon fibers-based twisted and coiled artificial muscles (TCAMs) is proposed. The device, which weighs only 0.135 kg, emulates the arrangement and working mechanism of skeletal muscles in the upper extremities and is able to perform wrist flexion/extension and ulnar/radial deviation. The range of motion and the force provided by the exoskeleton is designed through simple kinematic and dynamic theoretical models, while a thermal model is used to design a thermal insulation system for TCAMs during actuation. The device’s ability to perform passive and active-resisted wrist rehabilitation exercises and EMG-based actuation is also demonstrated.

Ultrastretchable alternating current electroluminescent panels for arbitrary luminous patterns

Stretchable alternating current electroluminescent devices represent an emerging optoelectronic technology with skin-like deformability. Despite robust construction and reliable performances, the conventional sandwiched device is only compatible with simple luminous features instead of complicated patterns. An unconventional design of intrinsically stretchable ACEL panels is reported here, featuring capacitively coupled bottom interdigitated liquid metal electrodes and top hydrogel transparent electrodes. Arbitrary luminous feature resembling the top electrode pattern is readily accessible without tailored designs. A facile procedure is established to construct functional devices exhibiting ultrahigh stretchability to 400% strain and durable performance under repetitive deformations. The successful integration in a soft pneumatic actuator further demonstrates the promising potential of stretchable ACEL panels for graphic information display in soft devices and systems.

Position Control of a Cost-Effective Bellow Pneumatic Actuator Using an LQR Approach

Today, we are witnessing an increasing trend in the number of soft pneumatic actuator solutions in industrial environments, especially due to their human-safe interaction capabilities. An interesting solution in this frame is a vacuum pneumatic muscle actuator (PMA) with a bellow structure, which is characterized by a high contraction ratio and the ability to generate high forces considering its relatively small dimensions. Moreover, such a solution is generally very cost-effective since can be developed by using easily accessible, off-the-shelf components combined with additive manufacturing procedures. The presented research analyzes the precision positioning performances of a newly developed cost-effective bellow PMA in a closed-loop setting, by utilizing a Proportional-Integral-Derivative (PID) controller and a Linear Quadratic Regulator (LQR). In a first instance, the system identification was performed and a numerical model of the PMA was developed. It was experimentally shown that the actuator is characterized by nonlinear dynamical behavior. Based on the numerical model, a PID controller was developed as a benchmark. In the next phase, an LQR that involves a nonlinear pregain term was built. The point-to-point positioning experimental results showed that both controllers allow fast responses without overshoot within the whole working range. On the other hand, it was discovered that the LQR with the corresponding nonlinear pregain term allows an error of a few tens of micrometers to be achieved across the entire working range of the muscle. Additionally, two different experimental pneumatic solutions for indirect and direct vacuum control were analyzed with the aim of investigating the PMA response time and comparing their energy consumption. This research contributes to the future development of the pneumatically driven mechatronics systems used for precise position control.

Origami-Inspired Soft Pneumatic Actuators: Generalization and Design Optimization

Soft actuators are essential to soft robots and can also be used with rigid-bodied robots. This paper is focused on methods for improving the applicability of origami-inspired soft pneumatic actuators (OSPA). Our method for rapidly fabricating OSPA is shown to be capable of making a range of actuator sizes out of different materials. The largest OSPA has a force-to-weight ratio of 124, and can lift a 44 kg mass using a −85 kPa supply pressure. Experiments with a smaller OSPA demonstrate that it can perform 150,000 contraction/extension cycles while carrying a 2 kg mass with minimal degradation due to its materials and design. Compared to other OSPAs for which fatigue tests were reported, our accordion pattern OSPA has the best values of work-to-mass ratio, max. force, and fatigue life. A computationally efficient FEA-based constrained optimization method for maximizing an OSPA’s work output is then proposed. A 55% improvement in the work output was predicted, while validation experiments with OSPA prototypes showed a 53% improvement. While these improvement percentages are very similar, the values of the predicted stroke and work output are about 16% larger than the experimental values. The optimization requires only ~5 h to run on a common laptop.

Hand and face somatotopy shown using MRI-safe vibrotactile stimulation with a novel soft pneumatic actuator (SPA)-skin interface

The exact somatotopy of the human facial representation in the primary somatosensory cortex (S1) remains debated. One reason that progress has been hampered is due to the methodological challenge of how to apply automated vibrotactile stimuli to face areas in a manner that is: (1) reliable despite differences in the curvatures of face locations; and (2) MR-compatible and free of MR-interference artefacts when applied in the MR head-coil. Here we overcome this challenge by using soft pneumatic actuator (SPA) technology. SPAs are made of a soft silicon material and can be in- or deflated by means of airflow, have a small diameter, and are flexible in structure, enabling good skin contact even on curved body surfaces (as on the face). To validate our approach, we first mapped the well-characterised S1 finger layout using this novel device and confirmed that tactile stimulation of the fingers elicited characteristic somatotopic finger activations in S1. We then used the device to automatically and systematically deliver somatosensory stimulation to different face locations. We found that the forehead representation was least distant from the representation of the hand. Within the face representation, we found that the lip representation is most distant from the forehead representation, with the chin represented in between. Together, our results demonstrate that this novel MR compatible device produces robust and clear somatotopic representational patterns using vibrotactile stimulation through SPA-technology.

High‐Speed and Low‐Energy Actuation for Pneumatic Soft Robots with Internal Exhaust Air Recirculation

Multichamber soft pneumatic actuators (m‐SPAs) are widely used in soft robotic systems to achieve versatile grasping and locomotion. However, existing m‐SPAs have slow actuation speed and are either limited by a finite air supply or require energy‐consuming hardware to continuously supply compressed air. Herein, these shortcomings by introducing an internal exhaust air recirculation (IEAR) mechanism for high‐speed and low‐energy actuation of m‐SPAs are addressed. This mechanism recirculates the exhaust compressed air and recovers the energy by harnessing the rhythmic actuation of multiple chambers. A theoretical model to guide the analysis of the IEAR mechanism, which agrees well with the experimental results, is developed. Comparative experimental results of several sets of m‐SPAs show that the IEAR mechanism significantly improves the actuation speed by more than 82.4% and reduces the energy consumption per cycle by more than 47.7% under typical conditions. The promising applications of the IEAR mechanism in various pneumatic soft machines and robots such as a robotic fin, fabric‐based finger, and quadruped robot are further demonstrated. An interactive preprint version of the article can be found at: https://doi.org/10.22541/au.166428178.80668101/v1.

Continuous deformation analysis and contact force estimation for pneumatic bending actuators interacting with environment

Soft bending actuators show high adaptability for applications such as rehabilitation or grasping. Although constant curvature assumption has been extensively used for free motion modeling, these actuators do not bend circularly when interacting with the environment. In such situation, conventional bending sensors cannot provide useful information on their shape. In this paper, the Finite Rigid Elements approach is utilized to model the behavior of a soft pneumatic bending actuator in free motion and contact. With this method, the variable curvature configuration under different external loads can be modeled. Then, the contact force between the actuator and an object located in a specific position is estimated utilizing the Gradient Descent optimization method. Experimental results verified the combinatorial proposed approach for both force estimation and structure deformation.

Soft Pneumatic Actuators with Controllable Stiffness by Bio‐Inspired Lattice Chambers and Fused Deposition Modeling 3D Printing

This article shows how changing 3D printing parameters and using bio‐inspired lattice chambers can engineer soft pneumatic actuators (SPAs) with different behaviors in terms of controlling tip deflection and tip force using the same input air pressure. Fused deposition modeling (FDM) is employed to 3D print soft pneumatic actuators using varioShore thermoplastic polyurethane (TPU) materials with a foaming agent. The effects of material flow and nozzle temperature parameters on the material properties and stiffness are investigated. Auxetic, columns, face‐centered cubic, honeycomb, isotruss, oct vertex centroid, and square honeycomb lattices are designed to study actuators’ behaviors under the same input pressure. Finite‐element simulations based on the nonlinear hyper‐elastic constitutive model are carried out to precisely predict the behavior, deformation, and tip force of the actuators. A closed‐loop pneumatic system and sensors are developed to control the actuators. Results show that lattice designs can control the bending angle and generated force of actuators. Also, the lattices increase the ultimate strength by controlling the contact area inside the chambers. They demonstrate variable stiffness behaviors and deflections under the same pressure between 100 and 500 kPa. The proposed actuators could be instrumental in designing wearable hand rehabilitative devices that assist customized finger and wrist flexion‐extension.

Earthworm‐Inspired Multi‐Material, Adaptive Strain‐Limiting, Hybrid Actuators for Soft Robots

This research presents a new class of versatile multi‐material hybrid actuators (MASH actuators), powered by pneumatics and electrostatic adhesion (EA)‐based adaptive strain‐limiting layers. Soft pneumatic actuators with strain‐limiting layers as the mainstream actuation for soft robotics have been developed for decades. However, due to their permanent strain‐limiting layers, those actuators possess fewer motion patterns. Inspired by the longitudinal muscles of the earthworm, a concept of an adaptive strain‐limiting layer with the ability to vary its length, stiffness, and position is presented. By integrating two flexible EA brakes into a soft pneumatic actuator as adaptive strain‐limiting layers, the actuator can achieve multiple motion patterns including extension, contraction, and bilateral bending, with permutations among these motions. The EA‐based strain‐limiting layer can increase the original actuator length by up to 86%, and the baseline actuator stiffness by up to 605%. To demonstrate its potential in locomotion and manipulation, an earthworm‐inspired crawling robot possessing great terrain adaptability and a versatile soft robotic gripper based on this hybrid actuator concept are developed.

Neural Network-Based Active Load-Sensing Scheme and Stiffness Adjustment for Pneumatic Soft Actuators for Minimally Invasive Surgery Support

To provide a stable surgical view in Minimally Invasive Surgery (MIS), it is necessary for a flexible endoscope applied in MIS to have adjustable stiffness to resist different external loads from surrounding organs and tissues. Pneumatic soft actuators are expected to fulfill this role, since they could feed the endoscope with an internal access channel and adjust their stiffness via an antagonistic mechanism. For that purpose, it is essential to estimate the external load. In this study, we proposed a neural network (NN)-based active load-sensing scheme and stiffness adjustment for a soft actuator for MIS support with antagonistic chambers for three degrees of freedom (DoFs) of control. To deal with the influence of the nonlinearity of the soft actuating system and uncertainty of the interaction between the soft actuator and its environment, an environment exploration strategy was studied for improving the robustness of sensing. Moreover, a NN-based inverse dynamics model for controlling the stiffness of the soft actuator with different flexible endoscopes was proposed too. The results showed that the exploration strategy with different sequence lengths improved the estimation accuracy of external loads in different conditions. The proposed method for external load exploration and inverse dynamics model could be used for in-depth studies of stiffness control of soft actuators for MIS support.

Multimaterial Embedded 3D Printing of Composite Reinforced Soft Actuators.

Soft pneumatic actuators (SPAs) have attracted enormous attention in the growing field of robotics. Among different SPAs, composite reinforced actuators (CRAs) are widely used because of their simple structure and high controllability. However, multistep molding, a time-consuming method, is still the predominant fabrication method. Here, we propose a multimaterial embedded printing method (ME3P) to fabricate CRAs. In comparison with other 3-dimensional printing methods, our method improves fabrication flexibility greatly. Via the design and fabrication of the reinforced composites' patterns and different geometries of the soft body, we demonstrate actuators with programmable responses (elongation, contraction, twisting, bending, and helical and omnidirectional bending). Finite element analysis is employed for the prediction of pneumatic responses and the inverse design of actuators based on specific actuation needs. Lastly, we use tube-crawling robots as a model system to demonstrate our ability to fabricate complex soft robots for practical applications. This work demonstrates the versatility of ME3P for the future manufacturing of CRA-based soft robots.

Fabrication of a Lateral Pinch type Hand Rehabilitation Support Device by Pneumatic Soft Actuators

Fabrication of a Lateral Pinch type Hand Rehabilitation Support Device by Pneumatic Soft Actuators

Improved Motion of a Hand Rehabilitation Support Device driven by Pneumatic Soft Actuators

Improved Motion of a Hand Rehabilitation Support Device driven by Pneumatic Soft Actuators

Granular Ionogel Particle Inks for 3D Printed Tough and Stretchable Ionotronics.

Ionogels have garnered great attention as promising soft conducting materials for the fabrication of flexible energy storage devices, soft actuators, and ionotronics. However, the leakage of the ionic liquids, weak mechanical strength, and poor manufacturability have greatly limited their reliability and applications. Here, we propose a new ionogel synthesis strategy by utilizing granular zwitterionic microparticles to stabilize ionic liquids. The ionic liquids swell the microparticles and physically crosslink microparticles via either electronic interaction or hydrogen bonding. Further introducing a photocurable acrylic monomer enables the fabrication of double-network (DN) ionogels with high stretchability (>600%) and ultrahigh toughness (fracture energy > 10 kJ/m2). The synthesized ionogels exhibit a wide working temperature of -60 to 90 °C. By tuning the crosslinking density of microparticles and physical crosslinking strength of ionogels, we synthesize DN ionogel inks and print them into three-dimensional (3D) motifs. Several ionogel-based ionotronics are 3D printed as demonstrations, including strain gauges, humidity sensors, and ionic skins made of capacitive touch sensor arrays. Via covalently linking ionogels with silicone elastomers, we integrate the ionogel sensors onto pneumatic soft actuators and demonstrate their capacities in sensing large deformation. As our last demonstration, multimaterial direct ink writing is harnessed to fabricate highly stretchable and durable alternating-current electroluminescent devices with arbitrary structures. Our printable granular ionogel ink represents a versatile platform for the future manufacturing of ionotronics.

High-performance artificially reeled silkworm silk via a multi-task and high-efficiency centrifugal reeling technique and its application in soft actuators.

Silkworm silks show increasing potential in applications of bioengineering, sensors, optics, electronics, and actuators. However, their inherent irregular morphologies, structures, and properties greatly hinder the translation of these technologies to commercial applications. Herein, we report a facile and comprehensive strategy to fabricate high-performance silk materials by spinning silkworms artificially via a multi-task and high-efficiency centrifugal reeling technique. With this strategy, centrifugally reeled silks (CRSs) with long-uniform morphologies, excellent strength (844.83 ± 319.48 MPa), high toughness (121.07 ± 35.31 MJ m-3), and outstanding Young's modulus (27.72 ± 12.61 GPa) are developed. Remarkably, the maximum strength (1.45 GPa) of CRS is 3 times that of cocoon silk and even comparable to spider silk. Moreover, the centrifugal reeling technique realizes the one-step preparation of centrifugally reeled silk yarn (CRSY) from spinning silkworm, and the CRSYs show higher strength (877.38 ± 377.23 MPa) and superior torsional recovery performances. Furthermore, these CRSY-based soft pneumatic actuators (SPAs) exhibit light weight, high-loading capability, easy programmability in strength and motions, and fast responses, and therefore outperform currently reported elastomer-based SPAs, showing promising applications in flexible sensors, artificial muscles, and soft robotics. This work also provides a new guide for producing high-performance silks from silk-secreting insects and arthropods.

3D printing programmable liquid crystal elastomer soft pneumatic actuators.

Soft pneumatic actuators (SPAs) rely on anisotropic mechanical properties to generate specific motions after inflation. To achieve mechanical anisotropy, additional stiff materials or heterogeneous structures are typically introduced in isotropic base materials. However, the inherent limitations of these strategies may lead to potential interfacial problems or inefficient material usage. Herein, we develop a new strategy for fabricating SPAs based on an aligned liquid crystal elastomer (LCE) by a modified 3D printing technology. A rotating substrate enables the one-step fabrication of tubular LCE-SPAs with designed alignments in three dimensions. The alignment can be precisely programmed through printing, resulting in intrinsic mechanical anisotropy of the LCE. With a specially designed alignment, LCE-SPAs can achieve basic motions-contraction, elongation, bending, and twisting-and accomplish diverse tasks, e.g., grabbing objects and mixing water. This study provides a new perspective for the design and fabrication of SPAs.