Untethered Soft Robots Research Articles

Power Autonomy and Agility Control of an Untethered Insect-Scale Soft Robot.

It is still challenging to achieve agility and trajectory control for untethered soft robots on an insect scale given their low mechanical impedance and compact structures. In this study, fast translational movements and swift turning motions are demonstrated on a power autonomous soft robot with a piezoelectric-thin-film-actuated body and electrostatic turning footpads. A high relative running speed of 2.5 body length per second compared with existing untethered robots is realized on a 24-mm-long untethered prototype integrated with power source, control, and wireless communication modules. An arc-shaped leg structure is adopted to self-regulate the frication forces on different footpads during turning by an inclination-induced redistribution of the payload gravity on legs and footpads. The trajectory maneuverability is demonstrated by navigating a 380 mg robot prototype with an 1810 mg payload to pass through a 58-cm-long S-shaped path with wireless control in 43.4 s. Due to the flexibility of the all-polymer body structure, the robustness of the untethered robot to large strain is demonstrated when compressed by 91 times the weight of the robot. A maximum travel distance of 58.6 m is achieved for the robot equipped with a 40 mA·h lithium battery, corresponding to the cost of transport of 261. This work provides a feasible solution to achieve high agility and advance the practicability of untethered soft robots on an insect scale.

An Ultra High Gain Converter for Driving HASEL Actuator Used in Soft Mobile Robots

Soft robots have the potential to fundamentally change interactions between robots and the surrounding environment, and between robots and animals, and robots and humans in ways that today's hard robots are incapable of doing. However, to realize this potential, soft robot actuators require extremely high voltage supplies of more than 4 kV. The electronics that can satisfy this need currently are either too large and bulky or unable to achieve the high power efficiency required for mobile systems. To meet this challenge, this paper conceptualizes, analyzes, designs, and validates a hardware prototype of an ultra-high gain (UHG) converter that can support extremely large conversion ratios up to ∼1000× to provide up to 5 kV output voltage from an input voltage of ∼5-10 V. This converter is demonstrated to be able to drive HASEL (Hydraulically Amplified Self-Healing Electrostatic) actuators, a promising candidate to realize future soft mobile robotic fishes, from an input voltage range of a 1-cell battery pack. The circuit topology employs a unique hybrid combination of a high-gain switched magnetic element (HGSME) and a diode and capacitor-based voltage multiplier rectifier (DCVMR) to enable compact magnetic elements, efficient soft-charging in all flying capacitors, and adjustable output voltage capability with simple duty-cycle modulation. Achieving an efficiency of 78.2% at 15 W output power, while providing 3.85 kV output from 8.5 V input, the proposed UGH converter proves to be a promising candidate for future untethered soft robots.

Gecko-and-inchworm-inspired untethered soft robot for climbing on walls and ceilings

Gecko-and-inchworm-inspired untethered soft robot for climbing on walls and ceilings

Untethered unidirectionally crawling gels driven by asymmetry in contact forces

Reversible thermoresponsive hydrogels, which swell and shrink (deswell) in the temperature range of 30° to 60°C, provide an attractive material class for operating untethered soft robots in human physiological and ambient conditions. Crawling has been demonstrated previously with thermoresponsive hydrogels but required patterned or constrained gels or substrates to break symmetry for unidirectional motion. Here, we demonstrate a locomotion mechanism for unidirectionally crawling gels driven by spontaneous asymmetries in contact forces during swelling and deswelling of segmented active thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) and passive polyacrylamide (pAAM) bilayers with suspended linkers. Actuation studies demonstrate the consistent unidirectional movement of these gel crawlers across multiple thermal cycles on flat, unpatterned substrates. We explain the mechanism using finite element simulations and by varying experimental parameters such as the linker stiffness and the number of bilayer segments. We elucidate design criteria and validate experiments using image analysis and finite element models. We anticipate that this mechanism could potentially be applied to other shape-changing locomotors.

Multi-Modal Locomotion of Caenorhabditis elegans by Magnetic Reconfiguration of 3D Microtopography.

Miniaturized untethered soft robots are recently exploited to imitate multi-modal curvilinear locomotion of living creatures that perceive change of surrounding environments. Herein, the use of Caenorhabditis elegans(C. elegans) is proposed as a microscale model capable of curvilinear locomotion with mechanosensing, controlled by magnetically reconfigured 3D microtopography. Static entropic microbarriers prevent C. elegans from randomly swimming with the omega turns and provide linear translational locomotion with velocity of ≈0.14 BL s-1 . This velocity varies from ≈0.09 (for circumventing movement) to ≈0.46 (for climbing) BL s-1 , depending on magnetic bending and twisting actuation coupled with assembly of microbarriers. Furthermore, different types of neuronal mutants prevent C. elegans from implementing certain locomotion modes, indicating the potential for investigating the correlation between neurons and mechanosensing functions. This strategy promotes a platform for the contactless manipulation of miniaturized biobots and initiates interdisciplinary research for investigating sensory neurons and human diseases.

A magnetic soft robot with rolling and grasping capabilities

Soft robots offer a great application potential in the field of military, medical, and industry. In this study, a rolling gait was adopted to the design of the soft robot so as to improve the locomotion velocity of the soft robot actuated by the magnetic field. The soft robot was fabricated as a square frame structure (6 × 6 × 0.6 mm) to achieve grasping function. The experimental results indicated that the designed soft robot could roll on a slope with a maximum inclination around 30° under the given conditions. The velocity of the soft robot during the rolling process would decrease with the increase of the inclination. Meanwhile, the soft robot could carry objects 3.75 times its own weight when it rolled on a level pavement. Further analysis and experiments suggested that the rolling locomotion of the soft robot relied on the magnetic torque and the friction force. The locomotion velocity of the soft robot was related to the perimeter of the contour composed of the soft robot and the object being grasped. Moreover, a modular assembly strategy was proposed to improve the locomotion and grasping performance of the soft robot. The findings of this work will help to promote the application of untethered soft robots.

Design and Closed‐Loop Motion Planning of an Untethered Swimming Soft Robot Using 2D Discrete Elastic Rods Simulations

Despite tremendous progress in the development of untethered soft robots in recent years, existing systems lack the mobility, model‐based control, and motion planning capabilities of their piecewise rigid counterparts. As in conventional robotic systems, the development of versatile locomotion of soft robots is aided by the integration of hardware design and control with modeling tools that account for their unique mechanics and environmental interactions. Here, a framework for physics‐based modeling, motion planning, and control of a fully untethered swimming soft robot is introduced. This framework enables offline co‐design in the simulation of robot parameters and gaits to produce effective open‐loop behaviors and enables closed‐loop planning over motion primitives for feedback control of a frog‐inspired soft robot testbed. This pipeline uses a discrete elastic rods (DERs) physics engine that discretizes the soft robot as many stretchable and bendable rods. On hardware, an untethered aquatic soft robot that performs frog‐like rowing behaviors is engineered. Hardware validation verifies that the simulation has sufficient accuracy to find the best candidates for sets of parameters offline. The simulator is then used to generate a trajectory library of the robot's motion in simulation that is used in real‐time closed‐loop path following experiments on hardware.

High‐Gain Microfluidic Amplifiers: The Bridge between Microfluidic Controllers and Fluidic Soft Actuators

Soft fluidic robots are typically controlled using manifolds containing large and rigid electromechanical valves. These bulky controllers limit scalability and hinder motion, in particular for untethered soft robots. There has been recent interest in using fluidic controllers analogous to electrical logic gates and microcontrollers to replace rigid valve systems. However, these microfluidic networks typically operate with small volumes, low flow rates, and low pressures relative to what is needed to power fluidic soft actuators. This article presents the design, fabrication, and analysis of a soft, fluidic amplifier as the “missing link” between microfluidic analogies of microcontrollers and the high fluidic power loads representative of soft actuators. The article demonstrates amplification gains of pressure signals up to a factor of four. The amplifier is a step toward fully autonomous soft robots by allowing designers to develop control strategies from soft materials with minimal additional rigid components or tethering.

Electrically actuated soft actuator integrated with an electrochemical reactor

The soft actuators integrated with the reversible proton exchange membrane fuel cell (RPEMFC) structure exhibit the potential to serve in portable and untethered soft robots with advantages of electric actuation, and compact and lightweight structure. But their performances in actuation strain, load capacity, controllability, and robustness are expected to be further improved. Here, a soft electrochemical actuator (SECA) that is integrated with an electrochemical reactor (ECR) designed based on the RPEMFC structure is reported. The ECR works as a pneumatic source that provides regulated pneumatic pressure through the water electrolysis reaction and hydrogen–oxygen fuel cell reaction. With the designed water-containing structure, the ECR has an improved performance of robustness to pose variations. The SECA can reach controllable deformation by the proposed actuation and deformation control methods which require a low-voltage (<3 V) and simple electrical system. With the new structural design, the SECA achieves excellent performances of large actuation deformation (150% actuation strain) and a high load-to-weight ratio (25:1) with a lightweight, compact, and easy-to-fabricate structure.

Bioinspired untethered soft robot with pumpless phase change soft actuators by bidirectional thermoelectrics

Soft pneumatic actuators play an important role in the technological advancement of soft robotics, but they face intrinsic limitations due to the presence of complex and bulky air compressors and tubes, that make it difficult to design completely soft robotic systems. In this study, we develop a first demonstration of the soft thermo-pneumatic actuating module that operates based on the thermally controlled gas-liquid phase transition with the soft thermoelectric device. The bi-directional thermal managing ability (heating and cooling) of the thermoelectric device is capable of controlling the inflation and deflation of the pneumatic chamber with the aid of the thermally conductive silicone rubber that enhances the heat transfer. For the first time to the authors’ best knowledge, we demonstrate the active cooling of the thermoelectric device that accelerates the deflation rate by the active cooling functionality that significantly reduces the whole operating time. Additionally, to validate its practical usage in soft robotics, the soft actuating module shows different motions, such as bending and rectilinear stretching, based on the assembly design that is employed to develop the soft gripper and the entirely untethered soft earthworm robot without a complicated and bulky compressor system.

On-demand anchoring of wireless soft miniature robots on soft surfaces

Untethered soft miniature robots capable of accessing hard-to-reach regions can enable new, disruptive, and minimally invasive medical procedures. However, once the control input is removed, these robots easily move from their target location because of the dynamic motion of body tissues or fluids, thereby restricting their use in many long-term medical applications. To overcome this, we propose a wireless spring-preloaded barbed needle release mechanism, which can provide up to 1.6 N of force to drive a barbed needle into soft tissues to allow robust on-demand anchoring on three-dimensional (3D) surfaces. The mechanism is wirelessly triggered using radio-frequency remote heating and can be easily integrated into existing untethered soft robotic platforms without sacrificing their mobility. Design guidelines aimed at maximizing anchoring over the range of the most biological tissues (kPa range) and extending the operating depth of the device inside the body (up to 75%) are also presented. Enabled by these advances, we achieve robust anchoring on a variety of ex vivo tissues and demonstrate the usage of such a device when integrated with existing soft robotic platforms and medical imaging. Moreover, by simply changing the needle, we demonstrate additional functionalities such as controlled detachment and subsurface drug delivery into 3D cancer spheroids. Given these capabilities, our proposed mechanism could enable the development of a new class of biomedical-related functionalities, such as local drug delivery, disease monitoring, and hyperthermia for future untethered soft medical robots.

A Method for 3D Printing and Rapid Prototyping of Fieldable Untethered Soft Robots.

Because they are made of elastically deformable and compliant materials, soft robots can passively change shape and conform to their environment, providing potential advantages over traditional robotics approaches. However, existing manufacturing workflows are often labor intensive and limited in their ability to create highly integrated three-dimensional (3D) heterogeneous material systems. In this study, we address this with a streamlined workflow to produce field-deployable soft robots based on 3D printing with digital light processing (DLP) of silicone-like soft materials. DLP-based 3D printing is used to create soft actuators (2.2 g) capable of exerting up to 0.5 Newtons of force that are integrated into a bioinspired untethered soft robot. The robot walks underwater at speeds comparable with its biological analog, the brittle star. Using a model-free planning algorithm and feedback, the robot follows remote commands to move to desired positions. Moreover, we show that the robot is able to perform untethered locomotion outside of a laboratory and in a natural aquatic environment. Our results represent progress in soft robot manufacturing autonomy for a 3D printed untethered soft robot.

Light responsive plasmonic silicone elastomer/hydrogel soft actuator.

Soft robots offer unique advantages in their ability to interact with fragile organisms. Light responsive soft actuators are promising for the development of the untethered soft robots. However, this requires a mechanism for converting the externally applied light into mechanical actuation. We designed, fabricated, and tested a light responsive soft actuator consist of silicone elastomer with plasmonic metal nanoparticle embedment and temperature sensitive hydrogel. We found that the selection of the carrier solvent for the metal nanoparticle embedment in silicone elastomer to be crucial. The fabricated soft actuator showed a relatively fast response time (< 5 min) under light illumination.

A fluidic relaxation oscillator for reprogrammable sequential actuation in soft robots

A fluidic relaxation oscillator for reprogrammable sequential actuation in soft robots

Low-Voltage Activation Based on Electrohydrodynamics in Positioning Systems for Untethered Robots

In recent years, untethered soft robots, free of the lines that restrict their mobility, have been studied extensively. Our research team has been focusing on the electrohydrodynamic phenomena (EHD) as a driving mechanism for untethered robots. EHD is a phenomenon in which a flow is generated by applying a high voltage to a dielectric liquid. We propose a method to drive a robot in an untethered manner using EHD by vertically stacking two types of liquids: conductive and dielectric. This method is simpler, more energy-efficient, and quieter than conventional systems. Although a lower voltage would prevent the enlargement of the system by limiting the electronic components, the generation of EHD requires a high voltage. Therefore, in this study, to realize the low voltage drive of untethered robots dominated by the electrostatic actuator, we tackled the reduction of the driving voltage by investigating the phenomenon. As a result, we achieved low voltage driving at 15 V and successfully drove with off-the-shelf batteries (18 V). We also investigated the output current flowing through the system to reduce power consumption. Therefore, in addition to improving the energy efficiency of the system, we confirmed that the difference of the generated current depended on the thickness of the dielectric liquid and the concentration of the conductive liquid.

Bioinspired light-fueled water-walking soft robots based on liquid crystal network actuators with polymerizable miniaturized gold nanorods

Walking on water is a unique locomotion mode found in aquatic living creatures. However, it remains challenging to create light-driven aquatic soft robots that can walk freely on water surface. Herein, water strider-inspired water-walking soft robots that can be remotely controlled by light are demonstrated through combining superhydrophobic floating surface with hierarchical nanostructures and light-driven soft-actuation legs that are fabricated through in situ embedding of judiciously designed polymerizable miniaturized gold nanorods (MiniGNR nanomonomer) in liquid crystal network (LCN)-based soft actuators. The MiniGNRs-LCN soft actuators act as driving paddles to enable propulsive motion on the water surface, and the superhydrophobic surface facilitates the aquatic soft robots with weight-bearing and drag-reducing ability. Upon cyclic exposure to near-infrared light, the LCN-based actuation leg quickly exhibits a photoinduced shape-bending deformation, enabling the one-legged soft robot to walk on water surface unidirectionally through horizontal momentum transfer of photoinduced actuation force to the water. Taking advantage of tailorable photoresponsiveness of MiniGNR nanomonomer, a tri-legged water-walking soft robot that can execute visible/infrared wavelength-dependent multi-directional motions is further demonstrated for cargo transportation. This research may provide a new toolbox for the development of untethered soft robots with sophisticated spatiotemporal actuation toward aquatic complex environments.

4D-printed untethered self-propelling soft robot with tactile perception: Rolling, racing, and exploring

<h2>Summary</h2> Fabricating an intelligent and tactile-sensitive untethered soft robot is a major challenge. In this study, we used 4D printing to manufacture a liquid crystal elastomer-based robot that can realize untethered self-propelled rolling when heated to above 160°C after morphing into a tubule. The curvature of the tubule renders a sustainable directional rolling movement to the robot. A 10-cm-long robot can attain a maximum velocity of more than 48 cm/min on the horizontal plane and climb a slope of approximately 20°. Longer tubules roll faster with improved stability, which ensures the robot's adaptability to complex terrains. Given the flexibility of the material and structure, the tactile sensing ability of the robot is similar to that of the antennae in insects. In addition, the robot can carry a load that is 40 times its own weight, which increases the potential applicability of the proposed robot in the transportation industry.

Bioinspired, Shape-Morphing Scale Battery for Untethered Soft Robots.

Geometrically multifunctional structures inspired by nature can address the challenges in the development of soft robotics. A bioinspired structure based on origami and kirigami can significantly enhance the stretchability and reliability of soft robots. This study proposes a novel structure with individual, overlapping units, similar to snake scales that can be used to construct shape-morphing batteries for untethered soft robots. The structure is created by folding well-defined, two-dimensional patterns with cutouts. The folding lines mimic the hinge structure of snakeskin, enabling stable deformations without mechanical damage to rigid cells. The structure realizes multi-axial deformability and a zero Poisson's ratio without off-axis distortion to the loading axis. Moreover, to maximize areal density, the optimal cell shape is designed as a hexagon. The structure is applied to a stretchable Li-ion battery, constructed to form an arrangement of electrically interconnected, hexagonal pouch cells. In situ electrochemical characterization and numerical simulation confirm that the shape-morphing scale battery maintains its performance under dynamic deformation with a 90% stretching ratio and 10-mm-radius bending curve, guaranteeing a long-lasting charging/discharging cycle life during cyclic bending and stretching (exceeding 36,000 cycles). Finally, the shape-morphing energy storage device is applied to movable robots, mimicking crawling and slithering, to demonstrate excellent conformability and deformability.

Untethered Soft Robots for Future Planetary Explorations?

In recent years, robotic vehicles have been deployed for planetary exploration purposes, surveying extraterrestrial terrains in search for possible signs of life and also to learn more about these unfamiliar worlds. While deploying robotic technologies for such space missions represents a significant milestone for humanity, these untethered robots may have limited dexterity and robustness due to them having rigid bodies. Therefore, there is potential to send untethered soft robots that are far more dexterous, robust, and functional than their rigid counterparts for planetary explorations in the future. Herein, some of the key advantages of deploying untethered soft robots for future planetary explorations are discussed. Also, some brief insights into how the environmental conditions on extraterrestrial terrains may affect the actuation, fabrication, and locomotion of soft robots are provided. While the emerging soft robotics technology is still relatively young, it is envisioned that these untethered soft machines have great potential to help create a paradigm shift for future planetary exploration.

Customizing a self-healing soft pump for robot

Recent advances in soft materials enable robots to possess safer human-machine interaction ways and adaptive motions, yet there remain substantial challenges to develop universal driving power sources that can achieve performance trade-offs between actuation, speed, portability, and reliability in untethered applications. Here, we introduce a class of fully soft electronic pumps that utilize electrical energy to pump liquid through electrons and ions migration mechanism. Soft pumps combine good portability with excellent actuation performances. We develop special functional liquids that merge unique properties of electrically actuation and self-healing function, providing a direction for self-healing fluid power systems. Appearances and pumpabilities of soft pumps could be customized to meet personalized needs of diverse robots. Combined with a homemade miniature high-voltage power converter, two different soft pumps are implanted into robotic fish and vehicle to achieve their untethered motions, illustrating broad potential of soft pumps as universal power sources in untethered soft robotics.