Swimming Robot Research Articles

Ultrafast, Programmable, and Electronics‐Free Soft Robots Enabled by Snapping Metacaps

Soft robots offer a myriad of potential because of their intrinsically compliant bodies, enabling safe interactions with humans and adaptability to unpredictable environments. However, most of them have limited actuation speeds, require complex control systems, and lack sensing capabilities. To address these challenges, herein, a class of metacaps is geometrically designed by introducing an array of ribs to a spherical cap with programmable bistabilities and snapping behaviors, enabling several unprecedented soft robotic functionalities. Specifically, a centimeter‐sized, sensor‐less metacap gripper is demonstrated that can grasp objects in 3.75 ms upon physical contact or pneumatic actuation with tunable behaviors that have little dependence on the rate of input. The grippers can be readily integrated into a robotic platform for practical applications. The metacap can further enable propelling of a swimming robot, exhibiting amplified swimming speed as well as untethered, electronics‐free swimming with tunable speeds using an oscillating valve. The metacap designs provide new strategies to enable the next‐generation soft robots to achieve high transient output energy and autonomous and electronics‐free maneuvering.

Development of a Stiffness‐Adjustable Articulated Paddle and its Application to a Swimming Robot

Stiffness of a swimming appendage is the key mediator between thrust generated and its beating frequency. Due to the advantageous role of flexible propulsors, they are widely adopted in previous swimming robots. As an optimal propulsor, stiffness is highly dependent on its beating frequency, and stiffness modulation is crucial when a robot is swimming with multiple beating frequencies. Herein, a novel swimming paddle that can switch two different stiffness states by sliding a laminate inside and its application to a swimming robot is studied. This paddle has 8 articulated joints and 20 passive flaps to achieve drag asymmetry with minimum control effort. A semiempirical model to estimate the stiffness change in good accuracy is also studied. The thrust modulation caused by stiffness change is comprehensively studied by varying frequency and range of motion. In addition, a nontethered swimming robot propelled by a bilateral pair of paddles is developed to investigate when and how the stiffness adjustment is useful. There is a threshold frequency dividing two regimes where one stiffness excels the other stiffness with respect to cost of transport. Finally, it is shown that the paddle thickness is closely related to the necessity of stiffness change mechanism.

Interfacial Electrostatic Self-Assembly of Amyloid Fibrils into Multifunctional Protein Films.

Amyloid fibrils have generated steadily increasing traction in the development of natural and artificial materials. However, it remains a challenge to construct bulk amyloid films directly from amyloid fibrils due to their intrinsic brittleness. Here, a facile and general methodology to fabricate macroscopic and tunable amyloid films via fast electrostatic self-assembly of amyloid fibrils at the air-water interface is introduced. Benefiting from the excellent templating properties of amyloid fibrils for nanoparticles (such as conductive carbon nanotubes or magnetic Fe3 O4 nanoparticles), multifunctional amyloid films with tunable properties are constructed. As proof-of-concept demonstrations, a magnetically oriented soft robotic swimmer with well-confined movement trajectory is prepared. In addition, a smart magnetic sensor with high sensitivity to external magnetic fields is fabricated via the combination of the conductive and magnetic amyloid films. This strategy provides a convenient, efficient, and controllable approach for the preparation of amyloid-based multifunctional films and related smart devices.

Magnetic-field-induced propulsion of jellyfish-inspired soft robotic swimmers.

The multifaceted appearance of soft robots in the form of swimmers, catheters, surgical devices, and drug-carrier vehicles in biomedical and microfluidic applications is ubiquitous today. Jellyfish-inspired soft robotic swimmers (jellyfishbots) have been fabricated and experimentally characterized by several researchers that reported their swimming kinematics and multimodal locomotion. However, the underlying physical mechanisms that govern magnetic-field-induced propulsion are not yet fully understood. Here, we use a robust and efficient computational framework to study the jellyfishbot swimming kinematics and the induced flow field dynamics through numerical simulation. We consider a two-dimensional model jellyfishbot that has flexible lappets, which are symmetric about the jellyfishbot center. These lappets exhibit flexural deformation when subjected to external magnetic fields to displace the surrounding fluid, thereby generating the thrust required for propulsion. We perform a parametric sweep to explore the jellyfishbot kinematic performance for different system parameters-structural, fluidic, and magnetic. In jellyfishbots, the soft magnetic composite elastomeric lappets exhibit temporal and spatial asymmetries when subjected to unsteady external magnetic fields. The average speed is observed to be dependent on both these asymmetries, quantified by the glide magnitude and the net area swept by the lappet tips per swimming cycle, respectively. We observe that a judicious choice of the applied magnetic field and remnant magnetization profile in the jellyfishbot lappets enhances both these asymmetries. Furthermore, the dependence of the jellyfishbot swimming speed upon the net area swept (spatial asymmetry) is twice as high as the dependence of speed on the glide ratio (temporal asymmetry). Finally, functional relationships between the swimming speed and different kinematic parameters and nondimensional numbers are developed. Our results provide guidelines for the design of improved jellyfish-inspired magnetic soft robotic swimmers.

Bio-inspired magnetic-driven folded diaphragm for biomimetic robot

Functional soft materials, exhibiting multiple types of deformation, have shown their potential/abilities to achieve complicated biomimetic behaviors (soft robots). Inspired by the locomotion of earthworm, which is conducted through the contraction and stretching between body segments, this study proposes a type of one-piece-mold folded diaphragm, consisting of the structure of body segments with radial magnetization property, to achieve large 3D and bi-directional deformation with inside-volume change capability subjected to the low homogeneous magnetically driving field (40 mT). Moreover, the appearance based on the proposed magnetic-driven folded diaphragm is able to be easily customized to desired ones and then implanted into different untethered soft robotic systems as soft drivers. To verify the above points, we design the diaphragm pump providing unique properties of lightweight, powerful output and rapid response, and the soft robot including the bio-earthworm crawling robot and swimming robot inspired by squid to exhibit the flexible and rapid locomotion excited by single homogeneous magnetic fields.

Ultrafast Miniature Robotic Swimmers with Upstream Motility.

With the development of materials science and micro-nano fabrication techniques, miniature soft robots at millimeter or submillimeter size can be manufactured and actuated remotely. The small-scaled robots have the unique capability to access hard-to-reach regions in the human body in a noninvasive manner. To date, it is still challenging for miniature robots to accurately move in the diverse and dynamic environments in the human body (e.g., in blood flow). To effectively locomote in the vascular system, miniature swimmers with upstream swimming capability are required. Herein, we design and fabricate a miniature robotic swimmer capable of performing ultrafast swimming in a fluidic environment. The maximum velocity of the swimmer in water is 30 cm/s, which is 60 body lengths. Moreover, in a tubular environment, the swimmer can still obtain a swimming velocity of 17 cm/s. The swimmer can also perform upstream swimming in a tubular environment with a velocity of 5 cm/s when the flow speed is 10 cm/s. The ultrasound-guided navigation of the swimmer in a phantom mimicking a blood vessel is also realized. This work gives insight into the design of agile undulatory milliswimmers for future biomedical applications.

A Torsion-Bending Antagonistic Bistable Actuator Enables Untethered Crawling and Swimming of Miniature Robots.

Miniature robots show great potential in exploring narrow and confined spaces to perform various tasks, but many applications are limited by the dependence of these robots on electrical or pneumatic tethers to power supplies outboard. Developing an onboard actuator that is small in size and powerful enough to carry all the components onboard is a major challenge to eliminate the need for a tether. Bistability can trigger a dramatic energy release during switching between the 2 stable states, thus providing a promising way to overcome the intrinsic limitation of insufficient power of small actuators. In this work, the antagonistic action between torsional deflection and bending deflection in a lamina emergent torsional joint is utilized to achieve bistability, yielding a buckling-free bistable design. The unique configuration of this bistable design enables integrating of a single bending electroactive artificial muscle in the structure to form a compact, self-switching bistable actuator. A low-voltage ionic polymer-metal composites artificial muscle is employed, yielding a bistable actuator capable of generating an instantaneous angular velocity exceeding 300 °/s by a 3.75-V voltage. Two untethered robotic demonstrations using the bistable actuator are presented, including a crawling robot (gross weight of 2.7 g, including actuator, battery, and on-board circuit) that can generate a maximum instantaneous velocity of 40 mm/s and a swimming robot equipped with a pair of origami-inspired paddles that swims breaststroke. The low-voltage bistable actuator shows potential for achieving autonomous motion of various fully untethered miniature robots.

A robotic fish capable of fast underwater swimming and water leaping with high Froude number

Natural underwater species outperform man-made underwater vehicles in many aspects. In this work, as our first effort to eventually mimic flying fish, a robotic fish capable of swimming fast and leaping out of water was developed, named KUFish. The thrust of KUFish was produced by a tail-beating mechanism driven by a DC motor in combination with reduction gears, four-bar linkage, and pulley-string mechanisms. The passive dynamic stability was implemented by the symmetric mass distribution, positive buoyancy, and lower center of gravity. A series of swimming experiments indicated that the KUFish could swim 0.68 m and leap out of the water with a speed of 1.35 m/s (6.1 BL/s) at an instant of 0.68 s after release. In addition to experimentation, a two-dimensional dynamic model was developed to predict the swimming behavior of the robot. The proposed dynamic model could reasonably capture the measured swimming performance of the robot before water leaping. The water leaping capability of the KUFish can be well supported by the computed Froude number of (1.08 or 0.92) in terms of the body faring length or robot length, respectively. The results from the current work can be useful for developing a flying-fish-like swimming robot in the future.

Plane-parallel motion of a trimaran capsubot controlled with an internal flywheel

Swimming robots driven by internal mass motion are demanded for applications in aggressive media. A novel type of such robot is proposed – a trimaran controlled by a single internal flywheel. The robot consists of a platform, three blades rigidly connected to it, and a flywheel located at the platform. A control torque is applied to the flywheel. The control is the sum of periodic excitation and feedback terms. It provides oscillations of the flywheel. Oscillations of the flywheel cause oscillations of the platform with blades. Oscillating blades submerged in fluid provide the thrust force. The mathematical model of the system is constructed basing on the quasi-steady description of hydrodynamic loads. Parameters of the control law are adjusted to increase the propulsion speed. It is shown that the lateral force acting on each blade plays crucial role in the propulsion. In a wide range of parameters, the system possesses two types of attracting periodic modes of propulsion: high-speed bow-first motion and low-speed stern-first motion. Both these modes are irreversible with respect to the main direction of propulsion. The self-start of the robot to a high-speed mode depends on geometrical parameters and on the control law. The laboratory prototype of the robot is constructed.

Bioinspired Superhydrophobic Swimming Robots with Embedded Microfluidic Networks and Photothermal Switch for Controllable Marangoni Propulsion

AbstractChemical Marangoni propulsion enables dynamic and untethered motion by generating surface tension gradient through chemical release, thereby having great potential for the development of insect‐scale self‐propelled robots. However, as the release and diffusion of chemical “fuels” are commonly uncontrollable, the Marangoni propulsion is unstable, thereby restricting robotic applications. Herein, the laser fabrication of superhydrophobic swimming robots to develop controllable Marangoni propulsion based on a photothermal composite of graphene and polydimethylsiloxane is reported. By combining the microfluidic channels with photothermal air chambers, a light‐triggered switch that can control the release of chemical “fuels” is proposed. Furthermore, a superhydrophobic surface is fabricated on the swimming robot by laser treatment, which reduced water resistance and promoted propulsion. On‐demand actuation and swimming route planning are realized by programming the alcohol/air segments in the releasing channels, on‐demand actuation and swimming route planning have been realized. As a proof‐of‐concept, a Marangoni swimming robot equipped with a miniature digital camera is used in an actual environment. Therefore, this study is expected to advance the practical applications of the chemical Marangoni effect in swimming robots.

Controlling Maneuverability of a Bio-Inspired Swimming Robot Through Morphological Transformation: Morphology Driven Control of a Swimming Robot

Biology provides many examples of how body adaption can be used to achieve a change in functionality. The feather star, an underwater crinoid that uses feather arms to locomote and feed, is one such system; it releases its arms to distract prey and vary its maneuverability to help escape predators. Using this crinoid as inspiration, we develop a robotic system that can alter its interaction with the environment by changing its morphology. We propose a robot that can actuate layers of flexible feathers and detach them at will. We first optimize the geometric and control parameters for a flexible feather using a hydrodynamic simulation followed by physical experiments. Second, we provide a theoretical framework for understanding how body change affects controllability. Third, we present a novel design of a soft swimming robot ( <xref ref-type="fig" rid="fig1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Figure 1</xref> ) with the ability of changing its morphology. Using this optimized feather and theoretical framework, we demonstrate, on a robotic setup, how the detachment of feathers can be used to change the motion path while maintaining the same low-level controller.

Butterfly stroke propels swimming robot to record speed

Butterfly stroke propels swimming robot to record speed

Rolled Dielectric Elastomer Antagonistic Actuators for Biomimetic Underwater Robots.

In this study, an antagonistic actuator using dielectric elastomer actuators (DEAs) is developed to investigate the use of rolled DEAs in underwater robots. The actuator consists of a backbone, an elastic hinge, and two rolled DEAs placed in an antagonistic fashion, allowing for the generation of bidirectional movements of the actuator tip. To prove this concept, an analytical model of the actuator is built. The experimental samples are fabricated based on the specification determined by the model. In the fabricated actuator, each rolled DEA has a diameter of 6 mm and a length of 21 mm. The whole device weighs 1.7 g. In the tested voltage range of 0-1200 V, the actuator exhibits a voltage-controllable angle and torque of up to 2.2° and 11.3 mN∙mm, respectively. The actuator is then implemented into a swimming robot, which shows forward speed of 0.9 mm/s at the applied voltage of 1000 V and the driving frequency of 10 Hz. The results demonstrate the feasibility of using rolled DEAs in underwater robots.

Soft Underwater Swimming Robots Based on Artificial Muscle

AbstractWith the increasing requirements of underwater missions and the rapid development of soft robotics technologies, soft underwater swimming robots have become a hot topic of research due to their low noise, high flexibility, and high environmental adaptability. In the past 10 years, research into soft underwater robots based on artificial muscle actuation has made considerable progress. Herein, a comprehensive survey on recent advances in soft underwater swimming robots based on different actuation methods is reviewed systematically, including pressure actuation, intelligent material actuation, and biomaterial actuation. First, for each type of actuation, the actuating principle, structural design, and swimming performance of soft underwater robots are introduced in detail. Then, according to the different swimming modes of underwater organisms, the aforementioned soft underwater robots are classified and compared. The results show that for different swimming tasks and application scenarios, the robot needs to realize the optimal design by reasonably selecting the actuating method and the swimming mode. This review summarizes the advanced information and critical technology in designing high‐performance soft underwater robots in the aspect of structural design, actuating methods, and swimming modes and offers an insightful outlook for the future development of soft robots.

Dynamic modeling and experiment of hind leg swimming of beaver-like underwater robot

Abstract. When the beaver-like underwater robot is swimming, its hind legs provide the main propulsion force for the body, which is the source of power for the robot's movement. Hind leg swimming dynamics is the basis for analyzing the generation and change of propulsion force during the robot swimming process, which directly determines how the robot swimming trajectory is planned. However, there are few researches on the swimming dynamics of the hind leg of a beaver-like underwater robot. This paper proposes a rigid–liquid fusion dynamics modeling method, which simplifies the swimming dynamics of hind legs of beaver-like robot to hydrodynamics of webbed feet and rigid body dynamics of thighs and calves. The hydrodynamics of the bendable webbed foot is established based on the integral hydrodynamics method, and the rigid body dynamic model of the thigh and calf is constructed using the Newton–Euler method. Through the force transmission, the overall swimming dynamic model of the hind leg is established, and the propulsion and lift force of the hind leg to body are obtained. The ANSYS Fluent simulation of the movement of robot's hind leg and underwater single-leg swimming experiments verify the correctness and effectiveness of the dynamics model. Comparing the theory, simulation, and experimental results of the propulsion and lift force of the robot's hind legs under bionic swimming, increased amplitude swimming, and reduced amplitude swimming, it further verifies the correctness of the proposed rigid–liquid fusion dynamic modeling method, and proves the superiority of the robot's bionic swimming trajectory. This study can provide new ideas for the leg dynamic modeling of underwater swimming robots with bendable webbed feet, and lay a theoretical foundation for exploring the swimming mechanical process of underwater robots.

Active and Robust Twisting Morphing Wings With Geometric Constraints for Flying or Swimming Robots

Typically, a twisting morphing wing of flying or swimming robots has one spanwise shaft and many ribs, the ribs can swing in their individual planes perpendicular to the shaft or the spanwise direction of the wing, showing different swing angles and different speed ratios with respect to the rotation of the shaft, thus the wing can form various degrees of spanwise twisting as the shaft rotates. While feasible solutions and mechanical implementations via gear transmission with tight geometric constraints are largely unexplored. This article considers the tight geometric constraints for such robotic twisting wing, and provides and particularly expands the analytical feasible solutions for the constraints, as well as provides mechanical implementations for active twisting of the wing that is driven by only one motor installed at the wing base, which implementations are compact with low inertia, low control complexity, and also high robustness that tolerates the deformations (e.g., bending due to load) of the shaft. The results serve as a design guidance and can be used for morphing wings of flying or swimming robots, particularly for accurately active spanwise twisting and large load transmissions to the movable parts.

Development of a High-Speed Swimming Robot With the Capability of Fish-Like Leaping

Achieving fast and agile swimming still remains extremely challenging for a self-propelled robotic fish due to the constraint of actuator’s propulsion capability. In this article, we report an untethered bioinspired robotic fish, which combines a high-frequency oscillation and a compliant passive mechanism to realize fast swimming, high pitch maneuvers, and even the leaping motion. For pursuing the explosive propulsion of the robotic fish, we propose an actuation system with a powerful output and a compact structure. A dynamic model is established and indicates that the compliant joint is able to modulate the power transmitted to a caudal fin to affect its velocity in the return stroke for generating more peak thrust. The design is validated with extensive experimental results. Namely, the robotic fish can surprisingly reach up to a speed of 3.8 body lengths per second (BL/s). Compared to the case with a rigid joint, dramatic improvements, involving a speed of 1.2 BL/s and a swimming distance of 141.2 m (70.6%), have been obtained, which reveal that besides the high-frequency oscillation, the compliant passive mechanism is also of great significance to perform high-speed swimming. Additionally, the robotic fish demonstrates its high pitch maneuvers by performing an agile front flip motion with a radius of 0.4 BL and an average angular velocity of 439 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> /s. Most importantly, with a simple control strategy, our robotic fish can remarkably leap out of water completely. Results from this study provide significant insights into the innovative designs of next-generation robotic fishes, which require high speed and maneuverability.

Mixing in arrays of villi-like actuators

This paper is concerned with computational modeling of fluid mixing by arrays of villi-like actuators. There are numerous applications of such actuators motivated by the motility and mixing induced by natural villi in the small intestine, such as microbial fuel cells and swimming robots—understanding how mixing occurs from viscous-dominated to inertia-dominated flows is paramount. Here, we analyze mixing in two-dimensional arrays of actuators, where neighboring actuators perform in-phase or anti-phase oscillations. We show that in both these cases, the temporal behavior becomes progressively more complex as inertia, or the Reynolds number, is increased. This behavior is classified into three regimes or stages with distinct behaviors and flow structures. We show that mixing can be substantially enhanced in the direction parallel to the wall the actuators are mounted on. We show this mixing is effectively constrained to a peripheral region or layer above the actuator tips. This layer is thicker in the anti-phase case than the in-phase case; however, in both cases this layer thickness saturates at high Reynolds number. Particle tracking results are used to define a mixing number, which shows the anti-phase pattern to be the most effective at mixing both along and across this peripheral layer, and this is linked to the flow structures generated in each stage. Our results provide a map for a range of behaviors that can be achieved through coordinated active motions of villi-like structures that we hope will be useful for the design of future robotics and fluidic-control systems.

Self-Healing Approach toward Catalytic Soft Robots

Soft robotics is a rapidly evolving research field that focuses on developing robots with bioinspired actuation/sensing mechanisms and highly flexible soft materials, some of which are similar to those found in living organisms. The hydrogel has the characteristics of excellent biocompatibility, softness, and elasticity, which makes it an ideal candidate material for the preparation of soft robots. Here we utilized a self-healing approach to develop a catalytically driven soft robot, which was constructed by dynamic imine bonds between modular hydrogels. One of the modules was a hydrogel formed by dynamic aldimine cross-linking of chitosan and glutaraldehyde, and the other module was a hydrogel embedded with catalase. The soft hydrogel robot moved because of catalytic reactions between the robot and environment [hydrogen peroxide (H2O2) fuel], giving rise to a fluidic release that supports propulsion, as inspired by the jet-propulsive mechanism in swimming dragonfly larvae. The speed of the soft robot can be mediated by adjusting the concentration of H2O2 and enable/disable movement based on the folding and unfolding of enzymes. In addition, the hydrogel formed by replacing glutaraldehyde with dialdehyde-functionalized PEG2000 had excellent elastic properties, and the soft robot based on PEG2000 had a higher movement speed than that based on glutaraldehyde under the same H2O2 concentration. Moreover, the addition of iron oxide nanoparticles can realize the magnetic guidance of the soft robot and the combination of different modules can realize different motion modes. The highly configurable self-healing catalytic soft robot holds great potential for a variety of interesting applications, including swimming robots, robot-assisted water treatment, and drug release.

Robotic swimming in curved space via geometric phase

Locomotion by shape changes or gas expulsion is assumed to require environmental interaction, due to conservation of momentum. However, as first noted in [J. Wisdom, Science 299, 1865-1869 (2003)] and later in [E. Guéron, Sci. Am. 301, 38-45 (2009)] and [J. Avron, O. Kenneth, New J. Phys, 8, 68 (2006)], the noncommutativity of translations permits translation without momentum exchange in either gravitationally curved spacetime or the curved surfaces encountered by locomotors in real-world environments. To realize this idea which remained unvalidated in experiments for almost 20 y, we show that a precision robophysical apparatus consisting of motors driven on curved tracks (and thereby confined to a spherical surface without a solid substrate) can self-propel without environmental momentum exchange. It produces shape changes comparable to the environment's inverse curvatures and generates movement of [Formula: see text]cm per gait. While this simple geometric effect predominates over short time, eventually the dissipative (frictional) and conservative forces, ubiquitous in real systems, couple to it to generate an emergent dynamics in which the swimming motion produces a force that is counter-balanced against residual gravitational forces. In this way, the robot both swims forward without momentum and becomes fixed in place with a finite momentum that can be released by ceasing the swimming motion. We envision that our work will be of use in a broad variety of contexts, such as active matter in curved space and robots navigating real-world environments with curved surfaces.