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Natural muscle provides excellent motilities for animals. As the basic unit of the muscle system, the skeletal muscle fibers function as a soft linear actuator. Inspired by the muscle fibers, researchers have developed various soft active devices with linear actuation. This paper reviews several soft linear actuators, such as the dielectric elastomer, thermal responsive hydrogels, pneumatic artificial muscle, and conducting polymers. The actuation mechanisms and performances of these soft linear actuators are summarized. Based on the dielectric elastomer, we propose a design of a hybrid system with linear actuation, driven by both the electric motor and dielectric elastomer cone. The electromechanical behaviors of the dielectric elastomer cone have been investigated in both experiment and finite element analysis. This work may guide the further design of soft actuators and robots.

In this paper, we develop the novel approach to design and fabricate dielectric elastomer (DE) actuator integrating with 3D printed flexible structures, demonstrating large deformation and multi-functionalities. This technique facilitates the combination of complex soft structures fused on the DE membrane without any extra adhesive. Based on the DE actuator, a soft gripper has been fabricated and characterized in terms of bending angle and blocked force. Finite element method (FEM) simulations precisely predict the electromechanical behaviors, guiding the design of the soft actuator. With optimal design, the finger-like soft actuator can generate large actuation with the bending angle about of 128 °. This study can guide the fabrication and modeling of highly designable and customized DE actuators, which are promising in the applications of soft robotics, flexible electronics, and medical devices.

Endowing the animals with agile actions, natural muscle functions as a soft actuator, which is powered by chemical reactions of metabolism and controlled by electrical nerve-signals. Inspired by natural muscle, various types of soft chemical machines (SCM) have been recently developed, possessing advantages of light weight, fast actuation, compact integration, high power and low noise. In this paper, we propose a composite structure as a soft chemical engine (SCE), which is driven by the chemical decomposition of hydrogen peroxide ( H 2 O2). The SCE directly converts chemical energy into mechanical work, achieving excellent performance of large actuation ( ∼ 33% volume expansion) and fast response ( ∼ 1 s). We advance the SCE with excellent performance to a soft chemical machine (SCM) with well-controlled functions, as a mechatronic control system. The soft robot possesses robust control with untethered operation and stable actuating cycles. We develop a model for the locomotion mechanisms, revealing that optimized mechatronic controls can significantly enhance the performance of SCM.

Soft active materials have been widely studied due to their unique attributes of high compliance, various functions and multi physics coupling effects. As a typical soft active material, dielectric elastomer (DE) is promising in various applications, possessing the advantages of large voltage-induced actuation, fast response, lightweight, low cost and high biocompatibility. Utilizing the good performance of large actuation and high compliance, dielectric elastomer is an idea material to drive bio-inspired robot as the artificial muscle. This paper has designed a Manta Ray inspired robotic fish. Utilizing the large actuation of DE membrane, the robot achieves the coupling propulsion of flapping and undulatory motions. The electronics of power and control have been co-fabricated in the body of the robot. The robot achieves good mobility and high environmental adaptability based on its fully soft and transparent structure. The experimental results and design principle of this paper may guide the future design and application of “soft-rigid” co-existed robots and bio-inspired robots.

The deep sea remains the largest unknown territory on Earth because it is so difficult to explore1–4. Owing to the extremely high pressure in the deep sea, rigid vessels5–7 and pressure-compensation systems8–10 are typically required to protect mechatronic systems. However, deep-sea creatures that lack bulky or heavy pressure-tolerant systems can thrive at extreme depths11–17. Here, inspired by the structure of a deep-sea snailfish15, we develop an untethered soft robot for deep-sea exploration, with onboard power, control and actuation protected from pressure by integrating electronics in a silicone matrix. This self-powered robot eliminates the requirement for any rigid vessel. To reduce shear stress at the interfaces between electronic components, we decentralize the electronics by increasing the distance between components or separating them from the printed circuit board. Careful design of the dielectric elastomer material used for the robot’s flapping fins allowed the robot to be actuated successfully in a field test in the Mariana Trench down to a depth of 10,900 metres and to swim freely in the South China Sea at a depth of 3,224 metres. We validate the pressure resilience of the electronic components and soft actuators through systematic experiments and theoretical analyses. Our work highlights the potential of designing soft, lightweight devices for use in extreme conditions. A free-swimming soft robot inspired by deep-sea creatures, with artificial muscle, power and control electronics spread across a polymer matrix, successfully adapts to high pressure and operates in the deep ocean.

Dielectric elastomers (DEs) is one of the promising artificial muscle for soft robots and flexible devices. As one of the key issues for practical applications, the control of DE actuators remains challenging due to the large actuation, electromechanical coupling, and viscoelastic dissipation. Feedforward control and proportional integral derivative (PID) feedback control are recently studied for the control of DE actuators. The control performance is still limited due to the complex dynamic behavior of DE actuators with both nonlinearities and modeling uncertainties. This paper proposes a model-based feedback control for DE actuator, considering nonlinearity of large deformation, electromechanical coupling, and the modeling uncertainties. A nonlinear motion model is proposed and verified by parameter identification experiments. Based on the nonlinear model, we demonstrate a robust control strategy including nonlinear model compensation and robust feedback to decrease the tracking error. The experimental results verify that the control strategy possesses excellent validity to the DE actuator with improved performance compared to the previous strategy of feedforward and PID feedback control. The system design and control strategy of this paper may guide the future design and application of DE actuators, soft robots, and flexible devices.

Compared to robots and devices made of rigid components, soft robots and flexible devices driven by soft active materials possess various advantages including high adaptability under extreme environment and compatibility with a human. Dielectric elastomer (DE) membrane, which is commonly used in building soft actuators, can achieve large actuation by the combined loadings of voltage-induced Maxwell stress and fluidic pressures (pneumatic and hydraulic pressure). This paper proposes a pneumatic–hydraulic coupled electromechanical actuator (PHCEA), which exhibits strong coupling effect of electromechanical actuation (the Maxwell stress on DE membrane), pneumatic and hydraulic pressures. Considering the moving boundary and state transition, a computational model has been developed to investigate the coupling behaviors of the PHCEA. The numerical result by this model is in accordance with the experimental measurements. The combination of experimental data and the theoretical result indicates that the state transition and moving boundary separate the potential region of electrical breakdown and mechanical damage. This model can be utilized as a practical method to characterize the performance and guide the design of soft devices. The experimental setup and computational method of the PHCEA bring new insights into the fabrication and characterization of soft robots, adaptive optics, and flexible bio-medical devices. The PHCEA possesses wide applications in underwater robots, soft muscles, and microfluidics systems. It can serve as the gas bladder of soft swimming robots, the soft actuator of hydraulic–pneumatic coupling systems, and the gas–liquid valve of flexible microfluidics systems.

Scar structures of natural animals can reinforce the wounds both mechanically and biologically to maintain the functions of the injured muscle and skin. Inspired by the scar structure, we present a dielectric elastomer (DE) with silver nanowire electrodes possessing the scar-like ability. This DE membrane can tolerate the failures by both electric breakdown and mechanical rupture. The DE actuator (DEA) can maintain their performances of force and displacement output after multiple failures. Scanning electronic microscope (SEM) images show that the scar-like structures accumulate around the electromechanical failure locations on the DE membrane as the stiffened and insulated regions, which prevent further short current and membrane rupture. J-integrals and stress distribution around the failure location have been calculated by finite element analysis to verify the mechanical reinforcements of the scar-like structures over crack propagation.

Soft chemo-mechanical (SCM) machines convert chemical energy into mechanical energy. They have the advantages of fast actuation and compliant deformation and possess wide applications in soft robots and flexible smart devices. Using experiments and theoretical modeling, this paper investigates the chemo-mechanical energy flows and the energy conversion cycle in the pressure and volume (P–V) plane of an SCM machine consisting of a soft membrane and cylindrical chambers. The machine generates 16.1 mJ of mechanical energy per cycle. We study heat dissipation as the key issue affecting the conversion efficiency. The states of the conversion cycle calculated from mechanical modeling fit well with those measured experimentally. The design principles and the theoretical methods can be used to guide the fabrication and characterization of SCM actuators and robots.