Fast-Swimming Soft Robotic Fish Actuated by Bionic Muscle

Soft underwater swimming robots actuated by smart materials have unique advantages in exploring the ocean, such as low noise, high flexibility, and friendly environment interaction ability. However, most of them typically exhibit limited swimming speed and flexibility due to the inherent characteristics of soft actuation materials. The actuation method and structural design of soft robots are key elements to improve their motion performance. Inspired by the muscle actuation and swimming mechanism of natural fish, a fast-swimming soft robotic fish actuated by a bionic muscle actuator made of dielectric elastomer is presented. The results show that by controlling the two independent actuating units of a biomimetic actuator, the robotic fish can not only achieve continuous C-shaped body motion similar to natural fish but also have a large bending angle (maximum unidirectional angle is about 40°) and thrust force (peak thrust is about 14 mN). In addition, the coupling relationship between the swimming speed and actuating parameters of the robotic fish is established through experiments and theoretical analysis. By optimizing the control strategy, the robotic fish can demonstrate a fast swimming speed of 76 mm/s (0.76 body length/s), which is much faster than most of the reported soft robotic fish driven by nonbiological soft materials that swim in body and/or caudal fin propulsion mode. What's more, by applying programmed voltage excitation to the actuating units of the bionic muscle, the robotic fish can be steered along specific trajectories, such as continuous turning motions and an S-shaped routine. This study is beneficial for promoting the design and development of high-performance soft underwater robots, and the adopted biomimetic mechanisms, as well as actuating methods, can be extended to other various flexible devices and soft robots.

Multimodal locomotion ultra-thin soft robots for exploration of narrow spaces

From power plants on land to bridges over the sea, safety-critical built environments require periodic inspections for detecting issues to avoid functional discontinuities of these installations. However, navigation paths in these environments are usually challenging as they often contain difficult-to-access spaces (near-millimetre and submillimetre-high gaps) and multiple domains (solid, liquid and even aerial). In this paper, we address these challenges by developing a class of Thin Soft Robots (TS-Robot: thickness, 1.7 mm) that can access narrow spaces and perform cross-domain multimodal locomotion. We adopted a dual-actuation sandwich structure with a tuneable Poisson's ratio tensioning mechanism for developing the TS-Robots driven by dielectric elastomers, providing them with two types of gaits (linear and undulating), remarkable output force ( ~ 41 times their weight) and speed (1.16 times Body Length/s and 13.06 times Body Thickness/s). Here, we demonstrated that TS-Robots can crawl, climb, swim and collaborate for transitioning between domains in environments with narrow entries.

Electroactive Polymer-Based Soft Actuator with Integrated Functions of Multi-Degree-of-Freedom Motion and Perception

Soft actuators have received extensive attention in the fields of soft robotics, biomedicine, and intelligence systems owing to their advantages of pliancy, silence, and essential safety. However, most existing soft actuators have only single actuation elements and lack sensing. Therefore, it is difficult for them to perform complex motions with multiple degrees of freedom (multi-DOFs) and high precision. This article reports a miniature columnar dielectric elastomer actuator (DEA) with multi-DOF actuation and sensing, which was fabricated with an electroactive polymer acrylic film (Very High Bond [VHB] acrylic film by 3M Company) and carbon black grease electrodes. The arrangement of the simulation electrodes on the VHB was optimized to realize multi-DOF actuation, and the sensing electrodes were configured on the outer part of the DEA to realize real-time sensing. The results showed that the soft actuator can achieve all-round actuation through the selective power of the stimulation electrodes with a controllable voltage. The maximum bending angle and axial strain of the actuator reached 50° and 13%, respectively. Moreover, the deformation modes, direction, and quantity could be precisely measured using the integrative sensing function. In addition, to demonstrate the advantages of the proposed actuator, a manipulator with multiple actuators was designed and controlled to realize different actions of screwing and grasping with sensing. This research is useful not only for the design of multifunctional soft actuators but also for the development of soft robots with flexible, complex, and precisely controllable motions.

Multistable shape programming of variable-stiffness electromagnetic devices

Programmable shape morphing enables soft machines to safely and effectively interact with the environment. Stimuli-responsive materials can transform 2D sheets into 3D geometries. However, most solutions cannot hold their shape at zero power, are limited to predetermined configurations, or lack sufficient mechanical stiffness to manipulate common objects. We demonstrate here segmented soft electromagnetic actuators integrated with shape memory polymer (SMP) films, capable of deforming and latching into a broad range of configurations. The device consists of liquid metal (LM) coils in an elastomer shell, laminated between two SMP films. The coils are linked by narrow joints, on which stretchable heaters are patterned. Heating the SMP greatly reduces its stiffness. Driving current through an LM coil in the presence of a magnetic field then leads to large bending or twisting. Cooling the SMP locks in the shape, leading to load-bearing capacity. Complex shapes are obtained from an initially flat device.

Bistable and Multistable Actuators for Soft Robots: Structures, Materials, and Functionalities

Snap-through bistability is often observed in nature (e.g., fast snapping to closure of Venus flytrap) and the life (e.g., bottle caps and hair clippers). Recently, harnessing bistability and multistability in different structures and soft materials has attracted growing interest for high-performance soft actuators and soft robots. They have demonstrated broad and unique applications in high-speed locomotion on land and under water, adaptive sensing and fast grasping, shape reconfiguration, electronics-free controls with a single input, and logic computation. Here, an overview of integrating bistable and multistable structures with soft actuating materials for diverse soft actuators and soft/flexible robots is given. The mechanics-guided structural design principles for five categories of basic bistable elements from 1D to 3D (i.e., constrained beams, curved plates, dome shells, compliant mechanisms of linkages with flexible hinges and deformable origami, and balloon structures) are first presented, alongside brief discussions of typical soft actuating materials (i.e., fluidic elastomers and stimuli-responsive materials such as electro-, photo-, thermo-, magnetic-, and hydro-responsive polymers). Following that, integrating these soft materials with each category of bistable elements for soft bistable and multistable actuators and their diverse robotic applications are discussed. To conclude, perspectives on the challenges and opportunities in this emerging field are considered.

One Soft Step: Bio-Inspired Artificial Muscle Mechanisms for Space Applications

Soft robots, devices with deformable bodies and powered by soft actuators, may fill a hitherto unexplored niche in outer space. All space-bound payloads are heavily limited in terms of mass and volume, due to the cost of launch and the size of spacecraft. Being constructed from stretchable materials allows many possibilities for compacting soft robots for launch and later deploying into a much larger volume, through folding, rolling, and inflation. This morphability can also be beneficial for adapting to operation in different environments, providing versatility, and robustness. To be truly soft, a robot must be powered by soft actuators. Dielectric elastomer transducers (DETs) offer many advantages as artificial muscles. They are lightweight, have a high work density, and are capable of artificial proprioception. Taking inspiration from nature, in particular the starfish podia, we present here bio-inspired inflatable DET actuators powering low-mass robots capable of performing complex motion that can be compacted to a fraction of their operating size.

Muscle-fiber array inspired, multiple-mode, pneumatic artificial muscles through planar design and one-step rolling fabrication

Advances in development of artificial muscles have enabled creation of soft robots with biological dexterity and self-adaption in unstructured environments; however, production of scalable artificial muscles with multiple-mode actuations remains elusive. Inspired by muscle-fiber arrays in muscular hydrostats, we present a class of versatile artificial muscles called MAIPAMs (muscle-fiber array inspired pneumatic artificial muscles), capable of multiple-mode actuations (such as parallel elongation-bending-spiraling actuations, 10 parallel bending actuations and cascaded elongation-bending-spiraling actuations). Our MAIPAMs consist of active 3D elastomer-balloon arrays reinforced by a passive elastomer membrane, achieved through a planar design and one-step rolling fabrication approach. We introduce prototypical designs for the MAIPAMs and demonstrate their muscle-mimic structures and versatility, as well as their scalable ability to integrate flexible but non-stretchable layers for contraction and twisting actuation modes and compliant electrodes for self-sensing. We further demonstrate that this class of artificial muscles shows potential for versatile robotic applications, such as carrying a camera for recording videos, gripping or manipulating objects, and climbing a pipe-line.

Bioinspired Bistable Dielectric Elastomer Actuators: Programmable Shapes and Application as Binary Valves

Nature has plenty of imitable examples of bistable thin structures that can actuate in response to mechanical and environmental stimuli, such as touch, light, and moisture. Scientists and engineers have used these as models to develop real-world systems with enhanced shape stability, energy efficiency, and power output. The bistable leaf of the Venus Flytrap (VFT) has a uniquely simple structure that enables exquisite actuation to trap the prey instantly. In this study, we present a strategy, inspired and derived from the VFT, which incorporates dielectric elastomer (DE) layers in a bistable actuator capable of reversible snapping through electrical stimulation. The trilayered laminated actuator is composed of two prestrained layers and a strain-limiting middle layer. The balance between elastic energy and bending energy of the laminates results in bistable shapes. We explore a broad design space of the bistable architecture through analysis and experiments to validate the fabrication parameters. The rapid snap-through between the two stable configurations is activated by a voltage pulse applied on the DE layers that change the laminate's strain field. Whereas a high electric field is used as the actuation trigger, the self-stabilization characteristic of the bistable structure obviates the need for continuous voltage supply. Finally, we recommended a new method of flow control by modulating porosity on curved surfaces through operating bistable dielectric elastomer actuators as binary valves.

Modelling dielectric elastomer circuit networks for soft biomimetics

In order to obtain entirely soft bio-inspired robots, fully soft electronic circuits are needed. Dielectric elastomers (DEs) are electroactive polymers that have demonstrated multifunctionality. The same material can achieve different tasks like actuation, sensing, or energy harvesting. It has been shown that basic logic and memory functions can be realized with similar soft structures by combining multiple DE actuators and DE switches. Thus it would be possible to build, with the same materials and processes, a soft structure that mimics a biological being with all these capabilities. This contribution is focused on the modelling of the aforementioned soft electro-mechanical circuit networks. It is here reported the building process of a comprehensive SIMULINK model including the electro-mechanical behaviour of DE logic units and their interconnections. Conventional models deal with a single aspect of DEs, generating complex finite-element simulations. This contribution, based on a former model for an inverter-based DEO, shows how to integrate these various mathematical models and, with the help of direct measurements, create a software representation of DE circuit networks. This work is intended to demonstrate the validity of a recently introduced model and apply it to more complex circuit networks that have a higher number of components. Since, at the present state, the building processes are by hand, adding components generates more variability due to sample-to-sample variation and human error. Despite this, the model still shows a qualitatively good prediction of the devices' behaviour. Furthermore, the introduction of new materials and automatic processes will help to reduce this variability, allowing the model to reach even better performance.

Hysteresis modeling and compensation of a rotary series elastic actuator with nonlinear stiffness

Series elastic actuators (SEAs) have widely been adapted in robots where safe human-robot interaction is required for accurate and robust force control. Recent research on the SEAs has shown that the SEA with a user-defined variable stiffness possesses several advantages over the constant stiffness SEA, such as large force range and bandwidth while keeping low output impedance and high force fidelity. However, a limitation of this type of SEA is that an obvious hysteresis effect exists and the associated torque curves are nonlinear and vary with amplitudes. Conventional mathematical hysteresis models are usually developed with some kind of black-box modeling, and the model parameters are adjusted through parameter identification methods. It is challenging to tune the model parameters to match the experimental data well among inputs with different amplitudes, let alone the inverse model of the hysteresis, which is necessary to compensate the hysteresis effect in control. In this paper, a rotary SEA (rSEA) with nonlinear stiffness is proposed. A concept called "virtual deformation" is introduced to mathematically transform the nonlinear curve into a polyline hysteresis model. This eases torque estimation with respect to the deformation of the rSEA. A hysteresis compensation torque controller is implemented for precise torque control. A prototype of the rSEA was fabricated, and the experimental results verified modeling accuracy of the proposed model. Our results showed that, with the new model, the computation cost was greatly reduced while keeping the modeling accuracy almost the same compared with the nonlinear backlash model.

Viscoelasticity Modeling of Dielectric Elastomers by Kelvin Voigt-Generalized Maxwell Model

Dielectric elastomers (DEs) are polymer materials consisting of a network of polymer chains connected by covalent cross-links. This type of structural feature allows DEs to generate large displacement outputs owing to the nonlinear electromechanical coupling and time-dependent viscoelastic behavior. The major challenge is to properly actuate the nonlinear soft materials in applications of robotic manipulations. To characterize the complex time-dependent viscoelasticity of the DEs, a nonlinear rheological model is proposed to describe the time-dependent viscoelastic behaviors of DEs by combining the advantages of the Kelvin-Voigt model and the generalized Maxwell model. We adopt a Monte Carlo statistical simulation method as an auxiliary method, to the best knowledge of the author which has never reportedly been used in this field, to improve the quantitative prediction ability of the generalized model. The proposed model can simultaneously describe the DE deformation processes under step voltage and alternating voltage excitation. Comparisons between the numerical simulation results and experimental data demonstrate the effectiveness of the proposed generalized rheological model with a maximum prediction error of 3.762% and root-mean-square prediction error of 9.03%. The results presented herein can provide theoretical guidance for the design of viscoelastic DE actuators and serve as a basis for manipulation control to suppress the viscoelastic creep and increase the speed response of the dielectric elastomer actuators (DEA).

Control of a Rehabilitation Robotic Device Driven by Antagonistic Soft Actuators

Stroke is becoming a widely concerned social problem, and robot-assisted devices have made considerable contributions in the training and treatment of rehabilitation. Due to the compliance and continuous deformation capacity, rehabilitation devices driven by soft actuators are attached to widespread attention. Considering the large output force of pneumatic artificial muscle (PAM) and the biological musculoskeletal structure, an antagonistic PAM-driven rehabilitation robotic device is developed. To fulfill the need for control of the proposed device, a knowledge-guided data-driven modeling approach is used and an adaptive feedforward–feedback control approach is presented to ensure the motion accuracy under large deformation motion with high frequency. Finally, several simulations and experiments are carried out to evaluate the performance of the developed system, and the results show that the developed system with the proposed controller can achieve expected control performance under various operations.

Materials, Actuators, and Sensors for Soft Bioinspired Robots

Biological systems can perform complex tasks with high compliance levels. This makes them a great source of inspiration for soft robotics. Indeed, the union of these fields has brought about bioinspired soft robotics, with hundreds of publications on novel research each year. This review aims to survey fundamental advances in bioinspired soft actuators and sensors with a focus on the progress between 2017 and 2020, providing a primer for the materials used in their design.

A Simple Dynamic Characterization Method for Thin Stacked Dielectric Elastomer Actuators by Suspending a Weight in Air and Electrical Excitation

This paper proposes a simple but effective method for characterizing dielectric elastomer actuators (DEAs), especially for thin stacked DEAs, which are promising for haptic devices but which measure the dynamic elastic modulus with great difficulty. The difficulty of the measurement of such a thin stacked DEA arises from the friction and local deformation of the surface between the DEA and a contact, as shown in this paper. In the proposed method, a DEA is vertically suspended and a weight is attached to it. The proposed method requires no contact with the surface of a DEA and uses only a weighting mass. Experimental results demonstrated the proposed method can estimate almost essential constants, such as the dynamic elastic modulus (Young’s modulus and damping time constant), the electrical constants (permittivity and resistivity), and the coefficient of electromechanical coupling, through the forced vibration induced by voltage actuation.

DEAP Actuator Composed of a Soft Pneumatic Spring Bias with Pressure Signal Sensing

Dielectric electroactive actuators are novel and significant smart actuators. The crucial aspect of construction of these devices is the bias mechanism. The current literature presents three main types of biases used in the construction of the DEAP actuators. In these solutions, the bias is caused by the action of a spring, a force of a permanent magnet or an applied mass. The purpose of this article is to present a novel type of DEAP bias mechanism using soft pneumatic spring. In contrast to the solutions presented so far, the soft pneumatic spring has been equipped with a sensor that measures the variable pressure of its inner chamber. We performed the modeling process of a soft pneumatic spring with the finite element method to predict its mechanical behavior. Furthermore, a prototype of the soft spring was molded and used to construct a dielectric electroactive polymer actuator. The principle of operation has been confirmed by the experiments with measurement of static and dynamics characteristics. The presented device can be used to control systems with an additional pressure-sensing feedback.

Dynamic modeling of dielectric elastomer actuator with conical shape

With desirable physical performances of impressive actuation strain, high energy density, high degree of electromechanical coupling and high mechanical compliance, dielectric elastomer actuators (DEAs) are widely employed to actuate the soft robots. However, there are many challenges to establish the dynamic models for DEAs, such as their inherent nonlinearity, complex electromechanical coupling, and time-dependent viscoelastic behavior. Moreover, most previous studies concentrated on the planar DEAs, but the studies on DEAs with some other functional shapes are insufficient. In this paper, by investigating a conical DEA with the material of polydimethylsiloxane and considering the influence of inertia, we propose a dynamic model based on the principles of nonequilibrium thermodynamics. This dynamic model can describe the complex motion characteristics of the conical DEA. Based on the experimental data, the differential evolution algorithm is employed to identify the undetermined parameters of the developed dynamic model. The result of the model validation demonstrates the effectiveness of the model.

Development of an annelid-like peristaltic crawling soft robot using dielectric elastomer actuators

The annelid, which consists of several identical segments, exploits its soft structures to move effectively in complex natural environments. Elongation and shortening of different segments produce a reverse peristaltic wave while retractable setae generate a variable friction, enabling bidirectional crawling locomotion. Although several designs have applied soft technologies towards the construction of annelid-like robots, these robots do not exhibit the homonymous segmentation, reverse peristaltic wave and variable friction. This paper reports the development of an annelid-like soft robot based on an improved dielectric elastomer (DE) minimum energy structure actuator to have these annelidan features. Each biomimetic segment of the robot is supported by a polyethylene terephthalate (PET) frame adhered to the DE actuator. The DE actuator induces segment elongation or shortening, which causes silica gel pads attached to the PET frame to contact or separate from the ground, producing a variable friction. The designed robot, whose identical segments conform to the homonymous segmentation, achieves forward or backward movement via the cooperative efforts of all the biomimetic segments. This cooperative movement, which produces the reverse peristaltic wave, strongly resembles that of natural annelidan locomotion. In addition, the kinematic analysis of the robot is investigated. Experimental results confirm that the designed robot is capable of bidirectional and rapid locomotion. The robot can achieve a maximum velocity of 11.5 mm s^-1 and a maximum velocity/mass ratio of 86.25 mm (min^-1 g^-1). Compared to other existing annelid-like soft robots, this designed robot exhibits a superior average velocity, velocity/length ratio, body length/cycle, and velocity/mass ratio, and its performance affords the best approximation to that of the natural annelid.

Linear-Quadratic Regulator for Control of Multi-Wall Carbon Nanotube/Polydimethylsiloxane Based Conical Dielectric Elastomer Actuators

Conventional rigid actuators, such as DC servo motors, face challenges in utilizing them in artificial muscles and soft robotics. Dielectric elastomer actuators (DEAs) overcome all these limitations, as they exhibit complex and fast motions, quietness, lightness, and softness. Recently, there has been much focus on studies of the DEAs material’s non-linearity, the non-linear electromechanical coupling, and viscoelastic behavior of VHB and silicone-based conical DEAs having compliant electrodes that are based on graphite powder and carbon grease. However, the mitigation of overshoot that arises from fast response conical DEAs made with solid electrodes has not received much research focus. In this paper, we fabricated a conical configuration of multi-walled carbon nanotube/polydimethylsiloxane (MWCNT/PDMS) based DEAs with a rise time of 10 ms, and 50% peak overshoot. We developed a full feedback state-based linear-quadratic regulator (LQR) having Luenberger observer to mitigate the DEAs overshoot in both the voltage ON and OFF instances. The cone DEA’s model was identified and a stable and well-fitting transfer function with a fit of 94% was obtained. Optimal parameters Q = 70,000, R = 0.1, and Q = 7000, R = 0.01 resulted in the DEA response having a rise time value of 20 ms with zero overshoot, in both simulations and experiments. The LQR approach can be useful for the control of fast response DEAs and this would expand the potential use of the DEAs as artificial muscles in soft robotics.

A Unidirectional Soft Dielectric Elastomer Actuator Enabled by Built-In Honeycomb Metastructures

Dielectric elastomer actuators (DEAs) are able to undergo large deformation in response to external electric stimuli and have been widely used to drive soft robotic systems, due to their advantageous attributes comparable to biological muscles. However, due to their isotropic material properties, it has been challenging to generate programmable actuation, e.g., along a predefined direction. In this paper, we provide an innovative solution to this problem by harnessing honeycomb metastructures to program the mechanical behavior of dielectric elastomers. The honeycomb metastructures not only provide mechanical prestretches for DEAs but, more importantly, transfer the areal expansion of DEAs into directional deformation, by virtue of the inherent anisotropy. To achieve uniaxial actuation and maximize its magnitude, we develop a finite element analysis model and study how the prestretch ratios and the honeycomb structuring tailor the voltage-induced deformation. We also provide an easy-to-implement and scalable fabrication solution by directly printing honeycomb lattices made of thermoplastic polyurethane on dielectric membranes with natural bonding. The preliminary experiments demonstrate that our designed DEA is able to undergo unidirectional motion, with the nominal strain reaching up to 15.8%. Our work represents an initial step to program deformation of DEAs with metastructures.

Dielectric Elastomer Actuator for Soft Robotics Applications and Challenges

This paper reviews state-of-the-art dielectric elastomer actuators (DEAs) and their future perspectives as soft actuators which have recently been considered as a key power generation component for soft robots. This paper begins with the introduction of the working principle of the dielectric elastomer actuators. Because the operation of DEA includes the physics of both mechanical viscoelastic properties and dielectric characteristics, we describe theoretical modeling methods for the DEA before introducing applications. In addition, the design of artificial muscles based on DEA is also introduced. This paper reviews four popular subjects for the application of DEA: soft robot hand, locomotion robots, wearable devices, and tunable optical components. Other potential applications and challenging issues are described in the conclusion.

Iterative Learning Control for Motion Trajectory Tracking of a Circular Soft Crawling Robot

Soft robots have recently received much attention with their infinite degrees of freedoms and continuously deformable structures, which allow them to adapt well to the unstructured environment. A new type of soft actuator, namely, dielectric elastomer actuator (DEA) which has several excellent properties such as large deformation and high energy density is investigated in this study. Furthermore, a DEA-based soft robot is designed and developed. Due to the difficulty of accurate modeling caused by nonlinear electromechanical coupling and viscoelasticity, the iterative learning control (ILC) method is employed for the motion trajectory tracking with an uncertain model of the DEA. A D² type ILC algorithm is proposed for the task. Furthermore, a knowledge-based model framework with kinematic analysis is explored to prove the convergence of the proposed ILC. Finally, both simulations and experiments are conducted to demonstrate the effectiveness of the ILC, which results show that excellent tracking performance can be achieved by the soft crawling robot.

Cell Nanomechanics Based on Dielectric Elastomer Actuator Device

As a frontier of biology, mechanobiology plays an important role in tissue and biomedical engineering. It is a common sense that mechanical cues under extracellular microenvironment affect a lot in regulating the behaviors of cells such as proliferation and gene expression, etc. In such an interdisciplinary field, engineering methods like the pneumatic and motor-driven devices have been employed for years. Nevertheless, such techniques usually rely on complex structures, which cost much but not so easy to control. Dielectric elastomer actuators (DEAs) are well known as a kind of soft actuation technology, and their research prospect in biomechanical field is gradually concerned due to their properties just like large deformation (> 100%) and fast response (< 1 ms). In addition, DEAs are usually optically transparent and can be fabricated into small volume, which make them easy to cooperate with regular microscope to realize real-time dynamic imaging of cells. This paper first reviews the basic components, principle, and evaluation of DEAs and then overview some corresponding applications of DEAs for cellular mechanobiology research. We also provide a comparison between DEA-based bioreactors and current custom-built devices and share some opinions about their potential applications in the future according to widely reported results via other methods.

Highly fluorescent triazolopyridine-thiophene D-A-D oligomers for efficient pH sensing both in solution and in the solid state

Conjugated fluorophores have been extensively used for fluorescence sensing of various substances in the field of life processes and environmental science, due to their noninvasiveness, sensitivity, simplicity and rapidity. Most existing conjugated fluorophores exhibit excellent light-emitting performance in dilute solutions, but their properties substantially decrease or even completely vanish due to severe aggregation quenching in the solid state. Herein, we synthesize a series of triazolopyridine-thiophene donor-acceptor-donor (D-A-D) type conjugated molecules with high absolute fluorescence quantum yields (ΦF) ranging from 80% to 89% in solution. These molecules also show unusual light-emitting properties in the solid state with ΦF of up to 26%. We find that owing to the protonation-deprotonation process of the pyridine ring, these compounds display obvious changes in both fluorescence wavelength and intensity upon addition of acids, and these changes can be readily recovered by the successive introduction of bases. By harnessing this phenomenon, we further show that these fluorophores can be employed for efficient and reversible fluorescence sensing of hydrogen ions in a broad pH range (0.0-7.0). With the fabrication of pH testing papers and ink-printed complex patterns including butterflies and letters on substrates, we demonstrate the application of such sensors to fluorescence indication or solid state pH detection for real samples such as volatile acidic/basic gas and water-quality analysis.

Control of a muscle-like soft actuator via a bioinspired approach

Soft actuators have played an indispensable role in generating compliant motions of soft robots. Among the various soft actuators explored for soft robotic applications, dielectric elastomer actuators (DEAs) have caught the eye with their intriguing attributes similar to biological muscles. However, the control challenge of DEAs due to their strong nonlinear behaviors has hindered the development of DEA-based soft robots. To overcome the control challenge, this paper proposes a bioinspired control approach of DEAs. A three-dimensional muscle-like DEA, capable of large forces and giant deformation, is fabricated and adopted as the control platform. To facilitate the controller design, the dynamic model of the DEA is developed through experimental analysis, which takes electromechanical coupling, viscoelastic effects and dynamics uncertainties into consideration. Motivated by the proprioception of the biological muscles, the self-sensing capability of the actuator is explored and exhibits good accuracy. Thus the self-sensing of the actuator is utilized to provide the sensory feedback in the control loop without the need of additional external sensors. Inspired from the role of the cerebellum in motor learning, a cerebellum model articulation nonlinear controller is proposed to compensate the dynamics uncertainties and to provide motion correction. Finally, the effectiveness of the proposed control approach is verified by both the simulation and the experiments.

Modeling the Viscoelastic Hysteresis of Dielectric Elastomer Actuators with a Modified Rate-Dependent Prandtl⁻Ishlinskii Model

Dielectric elastomer actuators (DEAs) are known as a type of electric-driven artificial muscle that have shown promising potential in the field of soft robotics. However, the inherent viscoelastic nonlinearity makes the modeling and control of DEAs challenging. In this paper, we propose a new phenomenological modeling approach with the Prandtl–Ishlinskii (P–I) model to characterize the viscoelastic hysteresis nonlinearity of DEAs. Differently from the commonly used physics-based models, the developed phenomenological model, called the modified rate-dependent P–I model (MRPIM), produces behavior similar to that of physics-based models but without necessarily considering physical insight into the modeling problem. In this way, the developed MRPIM can characterize the asymmetric and rate-dependent viscoelastic hysteresis with a relative simple mathematical format using only the experimental data. To validate the development, experimental tests were conducted with seven different frequencies; four were selected to identify the model parameters and the rest of the data were used to further verify the model. Comparisons between the model prediction and experimental data demonstrate that the MRPIM can precisely describe the viscoelastic hysteresis effect of DEAs with a maximum prediction error of 9.03% and root-mean-square prediction error of 4.50%.

3D Printed Electrically-Driven Soft Actuators

Soft robotics is an emerging field enabled by advances in the development of soft materials with properties commensurate to their biological counterparts, for the purpose of reproducing locomotion and other distinctive capabilities of active biological organisms. The development of soft actuators is fundamental to the advancement of soft robots and bio-inspired machines. Among the different material systems incorporated in the fabrication of soft devices, ionic hydrogel-elastomer hybrids have recently attracted vast attention due to their favorable characteristics, including their analogy with human skin. Here, we demonstrate that this hybrid material system can be 3D printed as a soft dielectric elastomer actuator (DEA) with a unimorph configuration that is capable of generating high bending motion in response to an applied electrical stimulus. We characterized the device actuation performance via applied (i) ramp-up electrical input, (ii) cyclic electrical loading, and (iii) payload masses. A maximum vertical tip displacement of 9.78 ± 2.52 mm at 5.44 kV was achieved from the tested 3D printed DEAs. Furthermore, the nonlinear actuation behavior of the unimorph DEA was successfully modeled using analytical energetic formulation and a finite element method (FEM).

A reusable fluorescent sensor from electrosynthesized water-soluble oligo(1-pyrenesulfonic acid) for effective detection of Fe3+

Electrosynthesized oligo(1-pyrenesulfonic acid) displays high selectivity, low detection limit and outstanding reversibility in the detection of Fe³⁺ in aqueous solution.