Soft robots driven by stimuli-responsive materials have unique advantages over conventional rigid robots, especially in their high adaptability for field exploration and seamless interaction with humans. The grand challenge lies in achieving self-powered soft robots with high mobility, environmental tolerance, and long endurance. We are able to advance a soft electronic fish with a fully integrated onboard system for power and remote control. Without any motor, the fish is driven solely by a soft electroactive structure made of dielectric elastomer and ionically conductive hydrogel. The electronic fish can swim at a speed of 6.4 cm/s (0.69 body length per second), which is much faster than previously reported untethered soft robotic fish driven by soft responsive materials. The fish shows consistent performance in a wide temperature range and permits stealth sailing due to its nearly transparent nature. Furthermore, the fish is robust, as it uses the surrounding water as the electric ground and can operate for 3 hours with one single charge. The design principle can be potentially extended to a variety of flexible devices and soft robots.

https://advances.sciencemag.org/content/advances/3/4/e1602045.full.pdf

Fast-moving soft electronic fish

Recent advances in ionic polymer–metal composite actuators and their modeling and applications

Electrochemical actuators directly converting electrical energy to mechanical energy are critically important for artificial intelligence. However, their energy transduction efficiency is always lower than 1.0% because electrode materials lack active units in microstructure, and their assembly systems can hardly express the intrinsic properties. Here, we report a molecular-scale active graphdiyne-based electrochemical actuator with a high electro-mechanical transduction efficiency of up to 6.03%, exceeding that of the best-known piezoelectric ceramic, shape memory alloy and electroactive polymer reported before, and its energy density (11.5 kJ m−3) is comparable to that of mammalian skeletal muscle (~8 kJ m−3). Meanwhile, the actuator remains responsive at frequencies from 0.1 to 30 Hz with excellent cycling stability over 100,000 cycles. Furthermore, we verify the alkene–alkyne complex transition effect responsible for the high performance through in situ sum frequency generation spectroscopy. This discovery sheds light on our understanding of actuation mechanisms and will accelerate development of smart actuators. Transduction efficiency in electrochemical actuators hardly exceeds 1% because the current electrode materials do not allow to efficiently exploit microstructural changes to achieve large actuation effects. Here the authors demonstrate transduction efficiencies up to 6% in actuators by using graphdiyne as electrode material.

/pdf/high-performance-graphdiyne-based-electrochemical-actuators-2re844nf8v.pdf

High-performance graphdiyne-based electrochemical actuators.

Robotic artificial muscles are a subset of artificial muscles that are capable of producing biologically inspired motions useful for robot systems, i.e., large power-to-weight ratios, inherent compliance, and large range of motions. These actuators, ranging from shape memory alloys to dielectric elastomers, are increasingly popular for biomimetic robots as they may operate without using complex linkage designs or other cumbersome mechanisms. Recent achievements in fabrication, modeling, and control methods have significantly contributed to their potential utilization in a wide range of applications. However, no survey paper has gone into depth regarding considerations pertaining to their selection, design, and usage in generating biomimetic motions. In this paper, we discuss important characteristics and considerations in the selection, design, and implementation of various prominent and unique robotic artificial muscles for biomimetic robots, and provide perspectives on next-generation muscle-powered robots.

https://ieeexplore.ieee.org/ielaam/8860/8731790/8725920-aam.pdf

Robotic Artificial Muscles: Current Progress and Future Perspectives

Underwater robot designs inspired by the behavior, physiology, and anatomy of fishes can provide enhanced maneuverability, stealth, and energy efficiency. Over the last two decades, robotics researchers have developed and reported a large variety of fish-inspired robot designs. The purpose of this review is to report different types of fish-inspired robot designs based upon their intended locomotion patterns. We present a detailed comparison of various design features like sensing, actuation, autonomy, waterproofing, and morphological structure of fish-inspired robots reported in the past decade. We believe that by studying the existing robots, future designers will be able to create new designs by adopting features from the successful robots. The review also summarizes the open research issues that need to be taken up for the further advancement of the field and also for the deployment of fish-inspired robots in practice.

https://iopscience.iop.org/article/10.1088/1748-3190/11/3/031001/pdf

Fish-inspired robots: design, sensing, actuation, and autonomy--a review of research.

This paper discusses the design, fabrication, and characterization of an ionic polymer?metal composite (IPMC) actuator-based bio-inspired active fin capable of bending and twisting motion. It is pointed out that IPMC strip actuators are used in the simple cantilever configuration to create simple bending (flapping-like) motion for propulsion in underwater autonomous systems. However, the resulting motion is a simple 1D bending and performance is rather limited. To enable more complex deformation, such as the flapping (pitch and heaving) motion of real pectoral and caudal fish fins, a new approach which involves molding or integrating IPMC actuators into a soft boot material to create an active control surface (called a ?fin?) is presented. The fin can be used to realize complex deformation depending on the orientation and placement of the actuators. In contrast to previously created IPMCs with patterned electrodes for the same purpose, the proposed design avoids (1)?the more expensive process of electroless plating platinum all throughout the surface of the actuator and (2)?the need for specially patterning the electrodes. Therefore, standard shaped IPMC actuators such as those with rectangular dimensions with varying thicknesses can be used. One unique advantage of the proposed structural design is that custom shaped fins and control surfaces can be easily created without special materials processing. The molding process is cost effective and does not require functionalizing or ?activating? the boot material similar to creating IPMCs. For a prototype fin (90?mm wide ? 60?mm long ? 1.5?mm thick), the measured maximum tip displacement was approximately 44?mm and the twist angle of the fin exceeded 10?. Lift and drag measurements in water where the prototype fin with an airfoil profile was dragged through water at a velocity of 21?cm?s?1 showed that the lift and drag forces can be affected by controlling the IPMCs embedded into the fin structure. These results suggest that such IPMC-enabled fin designs can be used for developing active propeller blades or control surfaces on underwater vehicles.

/pdf/an-ipmc-enabled-bio-inspired-bending-twisting-fin-for-3wlojkfb81.pdf

An IPMC-enabled bio-inspired bending/twisting fin for underwater applications

Emerging bioinspired underwater systems, such as autonomous ocean mapping and surveillance vehicles, that maneuver through their environment by mimicking the swimming motion of aquatic animals, can benefit from soft monolithic actuators and control surfaces capable of undergoing complex deformations. Herein, an electrically driven ionic polymer-metal composite (IPMC) artificial muscle with uniquely patterned electrodes for creating complex deformations is presented. The surface electrode pattern on the IPMC is created using a simple surface machining process. By selectively activating specific regions of the IPMC, bending, twisting, flapping, and other bioinspired locomotive behavior can be achieved. For instance, the bending and twisting response of an example 50 mm × 25 mm × 0.5 mm patterned IPMC actuator is characterized to determine its range of motion, output force and torque, as well as its effectiveness as a fish-fin-like propulsor. The experimental results show that the twisting angle exceeds 8 °; the blocking tip force and torque can be as high as 16.5 mN (at 3 V) and 0.83 N·mm (at 4 V), respectively (driven at 0.05 Hz); and an average thrust force of approximately 0.4 mN (driven by 4-V sinusoidal input at 1 Hz) can be generated. These newly developed IPMC fins can be exploited to create novel and efficient propulsors for next-generation underwater robotic vehicles. An example bioinspired robotic fish is presented which exploits the capabilities of the patterned IPMCs for propulsion and maneuvering, where an average maximum swimming speed of approximately 28 mm/s is reported.

Monolithic IPMC fins for propulsion and maneuvering in bioinspired underwater robotics

With low driving voltage ( < 5 V) and the ability to be operated in aqueous environments, ionic polymer–metal composite (IPMC) materials are quickly gaining attention for use in many applications including soft bio-inspired actuators and sensors. There are, however, drawbacks to IPMC actuators, including the ‘back relaxation’ effect. Specifically, when subjected to an excessively slow input, the IPMC actuator will slowly relax back toward its original position. There is debate over the physical mechanism of back relaxation, although one prevalent theory describes an initial current, caused by the electrostatic forces of the charging electrodes, which drives water molecules across the ion exchange membrane and deforms the IPMC. Once the electrodes are fully charged, however, the dominant element of the motion is the osmotic pressure, driving the water molecules back to equilibrium, thus causing back relaxation. A new method to mitigate back relaxation is proposed, taking advantage of controlled activation of patterned (sectored) electrodes on the IPMC. By actuating sectors in opposing directions, back relaxation can be effectively canceled out. An integrated feedforward and feedback controller is employed based on this concept, and is shown to minimize back relaxation, while reducing the input voltage required, as compared to the case of the non-sectored IPMC. Experimental results show nearly an order of magnitude reduction in the tracking error compared to the uncompensated case, and that the IPMC’s position can be maintained over a period of 60 and 1200 s with minimal evidence of back relaxation.

http://iopscience.iop.org/article/10.1088/0964-1726/21/8/085002/pdf

Mitigating IPMC back relaxation through feedforward and feedback control of patterned electrodes

Ionic polymer-metal composite (IPMC) actuators with sectored (patterned) electrodes have been fabricated for realizing bending and twisting motion. Such IPMCs can be used to create next-generation artificial fish-like propulsors that can mimic the undulatory, flapping, and complex motions of real fish fins. Herein, a thorough experimental study is performed on sectored IPMCs to characterize their performance. Specifically, results are presented to show (1) the achievable twisting response; (2) blocking force and torque; (3) power consumption and effectiveness; and (4) propulsion characteristics. The results can be utilized to guide the design of practical marine systems driven by IPMC propulsors. The design of an example underwater robotic system is also described which employs the IPMC actuators, and the performance of the robotic system is reported.Copyright © 2011 by ASME

Characterization of Sectored-Electrode IPMC-Based Propulsors for Underwater Locomotion

Ionic polymer metal composites (IPMCs), a type of electroactive polymer, are quickly gaining attention for use in many applications, including soft bio-inspired actuators and sensors. Some noteworthy advantages of IPMCs include their low driving voltage (<5 V) and the ability to be operated in aqueous environments. There are, however, drawbacks to IPMC actuators, including dynamics effects, the back-relaxation effect, and time-varying behaviors. These effects can limit the performance of IPMC-based actuators. This chapter focuses on feedback and feedforward control approaches for the precision control of IPMC actuators. In particular, a model-based feedforward controller is described to compensate for dynamic effects. Repetitive control (RC) is introduced for tracking period trajectories, and a method that exploits sectored IPMCs with an integrated feedback and feedforward controller is described to mitigate back-relaxation behavior. This chapter also discusses basic IPMC manufacturing techniques and methods for sensing displacement for sensor-based feedback control.

Maxwell Fleming

Papers

An IPMC-enabled bio-inspired bending/twisting fin for underwater applications

Monolithic IPMC fins for propulsion and maneuvering in bioinspired underwater robotics

Mitigating IPMC back relaxation through feedforward and feedback control of patterned electrodes

Characterization of Sectored-Electrode IPMC-Based Propulsors for Underwater Locomotion

Chapter 11:Precision Feedback and Feedforward Control of Ionic Polymer Metal Composite Actuators