Switchable Adhesion of Hydrogels to Plant and Animal Tissues

The ability to "switch on" adhesion between a thin hydrogel and a biological tissue can be useful in biomedical applications such as surgery. One way to accomplish this is with an electric field, a phenomenon termed electroadhesion (EA). Here, it is shown that cationic gels can be adhered by EA to tissues across all of biology. This includes tissues from animals, including humans and other mammals; birds; fish; reptiles (e.g., lizards); amphibians (e.g., frogs), and invertebrates (e.g., shrimp, worms). Gels can also be adhered to soft tissues from plants, including fruit (e.g., plums) and vegetables (e.g; carrot). In all cases, EA is induced by a low electric field (DC, 10 V) applied for a short time (20 s). After the field is removed, the adhesion persists. The adhesion can also be reversed by applying the field with opposite polarity. In mammals, EA is strong for many tissues (e.g., arteries, muscles, and cornea), but not others (e.g., adipose, brain). Tissues with anisotropic structure show anisotropic adhesion strength by EA. The higher the concentration of anionic polymers in a tissue, the stronger its adhesion to cationic gels. This underscores that EA is mediated by the electrophoresis of chain segments across the gel-tissue interface.

Influence of Polymer Nature on Electroadhesion

Electroadhesive systems are promising for creating delicate robotic manipulators operating both in the natural environment and in space conditions. Using thermosetting epoxy resin, polyurethane and polyester resin as examples, the influence of the polymers' natures, potential differences and current strengths on electroadhesive interactions in polymer-polymer systems was studied. The investigations were carried out by recording the force of normal separation of substrates from electroadhesives using contact and contactless methods at various electrical parameters of the systems and their components. A correlation was established between the relative permittivity and the electroadhesive force. The relaxation nature of the electroadhesion phenomenon after removing the electrical voltage was revealed. The influence of the potential difference and current strength on the effect of electroadhesion for polymer substrates of various natures was established. The obtained dependencies describe the main regularities of electroadhesive interactions necessary for creating promising electroadhesive materials.

Superior electroadhesion force with permittivity-engineered bilayer films using electrostatic simulation and machine learning approaches

Electroadhesive forces are crucial in various applications, including grasping devices, electro-sticky boards, electrostatic levitation, and climbing robots. However, the design of electroadhesive devices relies on speculative or empirical error approaches. Therefore, we present a theoretical model comprising predictive coplanar electrodes and protective layers for analyzing the electrostatic fields between an object and electroadhesive device. The model considers the role of protective layer and the air gap between the electrode surface and the object. To exert a higher electroadhesive force, the higher permeability of the protective layer is required. However, a high permeability of the protective layer is hard to withstand high applied voltage. To overcome this, two materials with different permeabilities were employed as protective layers-a low-permeability inner layer and a high-permeability outer layer-to maintain a high voltage and generate a large electroadhesive force. Because a low-permeability inner layer material was selected, a more permeable outer layer material was considered. A theoretical analysis revealed complex relationships between various design parameters. The impact of key design parameters and working environments on the electroadhesion behavior was further investigated. This study reveals the fundamental principles of electroadhesion and proposes prospective methods to enhance the design of electroadhesive devices for various engineering applications.

A review of bioinspired dry adhesives: from achieving strong adhesion to realizing switchable adhesion

In the early twenty-first century, extensive research has been conducted on geckos' ability to climb vertical walls with the advancement of microscopy technology. Unprecedented studies and developments have focused on the adhesion mechanism, structural design, preparation methods, and applications of bioinspired dry adhesives. Notably, strong adhesion that adheres to both the principles of contact splitting and stress uniform distribution has been discovered and proposed. The increasing popularity of flexible electronic skins, soft crawling robots, and smart assembly systems has made switchable adhesion properties essential for smart adhesives. These adhesives are designed to be programmable and switchable in response to external stimuli such as magnetic fields, thermal changes, electrical signals, light exposure as well as mechanical processes. This paper provides a comprehensive review of the development history of bioinspired dry adhesives from achieving strong adhesion to realizing switchable adhesion.

Sensor-Enhanced Smart Gripper Development for Automated Meat Processing

Grasping and object manipulation have been considered key domains of Cyber-Physical Systems (CPS) since the beginning of automation, as they are the most common interactions between systems, or a system and its environment. As the demand for automation is spreading to increasingly complex fields of industry, smart tools with sensors and internal decision-making become necessities. CPS, such as robots and smart autonomous machinery, have been introduced in the meat industry in recent decades; however, the natural diversity of animals, potential anatomical disorders and soft, slippery animal tissues require the use of a wide range of sensors, software and intelligent tools. This paper presents the development of a smart robotic gripper for deployment in the meat industry. A comprehensive review of the available robotic grippers employed in the sector is presented along with the relevant recent research projects. Based on the identified needs, a new mechatronic design and early development process of the smart gripper is described. The integrated force sensing method based on strain measurement and magnetic encoders is described, including the adjacent laboratory and on-site tests. Furthermore, a combined slip detection system is presented, which relies on an optical flow-based image processing algorithm using the video feed of a built-in endoscopic camera. Basic user tests and application assessments are presented.

Snail-inspired water-enhanced soft sliding suction for climbing robots

Snails can stably slide across a surface with only a single high-payload sucker, offering an efficient adhesive locomotion mechanism for next-generation climbing robots. The critical factor for snails' sliding suction behaviour is mucus secretion, which reduces friction and enhances suction. Inspired by this, we proposed an artificial sliding suction mechanism. The sliding suction utilizes water as an artificial mucus, which is widely available and evaporates with no residue. The sliding suction allows a lightweight robot (96 g) to slide vertically and upside down, achieving high speeds (rotation of 53°/s and translation of 19 mm/s) and high payload (1 kg as tested and 5.03 kg in theory), and does not require energy during adhesion. Here, we show that the sliding suction is a low-cost, energy-efficient, high-payload and clean adhesive locomotion strategy, which has high potential for use in climbing robots, outdoor inspection robots and robotic transportation.

An Amphibious Fully-Soft Centimeter-Scale Miniature Crawling Robot Powered by Electrohydraulic Fluid Kinetic Energy

Miniature locomotion robots with the ability to navigate confined environments show great promise for a wide range of tasks, including search and rescue operations. Soft miniature locomotion robots, as a burgeoning field, have attracted significant research interest due to their exceptional terrain adaptability and safety features. Here, a fully-soft centimeter-scale miniature crawling robot directly powered by fluid kinetic energy generated by an electrohydraulic actuator is introduced. Through optimization of the operating voltage and design parameters, the average crawling velocity of the robot is dramatically enhanced, reaching 16 mm s-1. The optimized robot weighs 6.3 g and measures 5 cm in length, 5 cm in width, and 6 mm in height. By combining two robots in parallel, the robot can achieve a turning rate of ≈3° s-1. Additionally, by reconfiguring the distribution of electrodes in the electrohydraulic actuator, the robot can achieve 2 degrees-of-freedom translational motion, improving its maneuverability in narrow spaces. Finally, the use of a soft water-proof skin is demonstrated for underwater locomotion and actuation. In comparison with other soft miniature crawling robots, this robot with full softness can achieve relatively high crawling velocity as well as increased robustness and recovery.

Reversibly Sticking Metals and Graphite to Hydrogels and Tissues

We have discovered that hard, electrical conductors (e.g., metals or graphite) can be adhered to soft, aqueous materials (e.g., hydrogels, fruit, or animal tissue) without the use of an adhesive. The adhesion is induced by a low DC electric field. As an example, when 5 V DC is applied to graphite slabs spanning a tall cylindrical gel of acrylamide (AAm), a strong adhesion develops between the anode (+) and the gel in about 3 min. This adhesion endures after the field is removed, and we term it as hard-soft electroadhesion or EA[HS]. Depending on the material, adhesion occurs at the anode (+), cathode (-), or both electrodes. In many cases, EA[HS] can be reversed by reapplying the field with reversed polarity. Adhesion via EA[HS] to AAm gels follows the electrochemical series: e.g., it occurs with copper, lead, and tin but not nickel, iron, or zinc. We show that EA[HS] arises via electrochemical reactions that generate chemical bonds between the electrode and the polymers in the gel. EA[HS] can create new hybrid materials, thus enabling applications in robotics, energy storage, and biomedical implants. Interestingly, EA[HS] can even be achieved underwater, where typical adhesives cannot be used.

A Low-Voltage, High-Force Capacity Electroadhesive Clutch Based on Ionoelastomer Heterojunctions

Electroadhesive devices with dielectric films can electrically program changes in stiffness and adhesion, but require hundreds of volts and are subject to failure by dielectric breakdown. Recent work on ionoelastomer heterojunctions has enabled reversible electroadhesion with low voltages, but these materials exhibit limited force capacities and high detachment forces. It is a grand challenge to engineer electroadhesives with large force capacities and programmable detachment at low voltages (<10 V). In this work, tough ionoelastomer/metal mesh composites with low surface energies are synthesized and surface roughness is controlled to realize sub-ten-volt clutches that are small, strong, and easily detachable. Models based on fracture and contact mechanics explain how clutch compliance and surface texture affect force capacity and contact area, which is validated over different geometries and voltages. These ionoelastomer clutches outperform the best existing electroadhesive clutches by fivefold in force capacity per unit area (102 N cm-2 ), with a 40-fold reduction in operating voltage (± 7.5 V). Finally, the ability of the ionoelastomer clutches to resist bending moments in a finger wearable and as a reversible adhesive in an adjustable phone mount is demonstrated.

Strong Reliable Electrostatic Actuation Based on Self-Clearing Using a Thin Conductive Layer

Electrostatic adhesion, as a promising actuation technique for soft robotics, severely suffers from the failure caused by the unpredictable electrical breakdown. This study proposes a novel self-clearing mechanism for electrostatic actuators, particularly for electrostatic adhesion. By simply employing an enough thin conductive layer (e.g., <7 μm for copper), this method can spontaneously clear the conductor around the breakdown sites effectively once breakdowns onset and survive the actuator shortly after the electrical damage. Compared with previous self-clearing methods, which typically rely on new specific materials, this mechanism is easy to operate and compatible with various materials and fabrication processes. In our tests, it can improve the maximum available voltage by 260% and the maximum electrostatic adhesive force by 276%. In addition, the robustness and repeatability of the self-clearing mechanism are validated by surviving consecutive breakdowns and self-clearing of 173 times during 65 min. This method is also demonstrated to be capable of recovering the electrostatic pad from severe physical damages such as punctures, penetrations, and cuttings successfully and enabling stable and reliable operation of the electrostatic clutch, or gripping, for example, even after the short-circuit takes place for hundreds of times. Therefore, the proposed self-clearing method sheds new light on high performance and more extensive practical applications of electrostatic actuators in the future.

Measuring electro-adhesion pressure before and after contact

Electro-adhesion (EA) is a low-power, tunable, fast and reversible electrically-controlled adhesion method, effective on both conducting and insulating objects. Typically, only the electro-adhesive detachment force, i.e., the force required to separate an object from the EA patch, is measured. Here, we report a method that enables comparing the pre-contact EA attachment forces with post-contact EA detachment forces. We observe that pre-contact pressures are 1 to 100 times lower than post-contact detachment pressures, indicating the large role played by surface forces, charge injection, and polarization inertia. We characterize the time-dependence of pre- and post-contact EA forces as a function of the applied voltage waveform, observing that using an AC drive allowing for much faster release than DC operation. We measure both EA forces on conductive and insulating objects, using over 100 different EA patches covering a wide range of electrode dimensions. At 400 V, the EA release pressures for conductive objects range from 1 to 100 kPa, and are 1 to 10 times higher than pre-contact adhesion force. For dielectric objects, release pressures are 1 to 100 higher than pre-contact adhesion pressures. The methodology presented in this paper can enable standardized EA characterization while varying numerous parameters.

Predicting the Electrical, Mechanical, and Geometric Contributions to Soft Electroadhesives through Fracture Mechanics

Electroadhesion is the modulation of adhesive forces through electrostatic interactions and has potential applications in a number of next-generation technologies. Recent efforts have focused on using electroadhesion in soft robotics, haptics, and biointerfaces that often involve compliant materials and nonplanar geometries. Current models for electroadhesion provide limited insight on other contributions that are known to influence adhesion performance, such as geometry and material properties. This study presents a fracture mechanics framework for understanding electroadhesion that incorporates geometric and electrostatic contributions for soft electroadhesives. We demonstrate the validity of this model with two material systems that exhibit disparate electroadhesive mechanisms, indicating that this formalism is applicable to a variety of electroadhesives. The results show the importance of material compliance and geometric confinement in enhancing electroadhesive performance and providing structure-property relationships for designing electroadhesive devices.

Universal Way to "Glue" Capsules and Gels into 3D Structures by Electroadhesion

We demonstrate the use of electroadhesion (EA), i.e., adhesion induced by an electric field, to connect a variety of soft materials into 3D structures. EA requires a cationic and an anionic material, but these can be of diverse origin, including covalently cross-linked hydrogels made by polymerizing charged monomers or physical gels/capsules formed by the ionic cross-linking of biopolymers (e.g., alginate and chitosan). Between each cationic/anionic pair, EA is induced rapidly (in ∼10 s) by low voltages (∼10 V DC)─and the adhesion is permanent after the field is turned off. The adhesion is strong enough to allow millimeter-scale capsules/gels to be assembled in 3D into robust structures such as capsule-capsule chains, capsule arrays on a base gel, and a 3D cube of capsules. EA-based assembly of spherical building blocks can be done more precisely, rapidly, and easily than by any alternative techniques. Moreover, the adhesion can be reversed (by switching the polarity of the field)─hence any errors during assembly can be undone and fixed. EA can also be used for selective sorting of charged soft matter─for example, a 'finger robot' can selectively 'pick up' capsules of the opposite charge by EA and subsequently 'drop off' these structures by reversing the polarity. Overall, our work shows how electric fields can be used to connect soft matter without the need for an adhesive or glue.

A Spiny Climbing Robot with Dual-Rail Mechanism

Easy detachment is as important as reliable an attachment to climbing robots in achieving stable climbing on vertical surfaces. To deal with the difficulty of detachment occurring in wheeled and track-type climbing robots using bio-inspired spines, a novel climbing robot utilizing spiny track and dual-rail mechanism is proposed in this paper. The spiny track consists of dozens of spiny feet, and the movement of each spiny foot is guided by the specially designed dual-rail mechanism to achieve reliable attachment and easy detachment. First, the design of the climbing robot and the dual-rail mechanism are presented. Then, the dual-rail model is constructed to analyze the attaching and detaching movements of the spiny feet, and a mechanical model is established to analyze the force distribution on the spiny track. Finally, a robot prototype is developed, and the analysis results are verified by the experiment results. Experiments on the prototype demonstrated that it could climb on various rough vertical surfaces at a speed of 36 mm/s, including sandpaper, brick surfaces, concrete walls with pebbles, and coarse stucco walls.

Influence of the Dynamic Effects and Grasping Location on the Performance of an Adaptive Vacuum Gripper

A rigid in-plane matrix of suction cups is widely used in robotic end-effectors to grasp objects with flat surfaces. However, this grasping strategy fails with objects having different geometry e.g., spherical and cylindrical. Articulated rigid grippers equipped with suction cups are an underinvestigated solution to extend the ability of vacuum grippers to grasp heavy objects with various shapes. This paper extends previous work by the authors in the development of a novel underactuated vacuum gripper named Polypus by analyzing the impact of dynamic effects and grasping location on the vacuum force required during a manipulation cycle. An articulated gripper with suction cups, such as Polypus, can grasp objects by adhering to two adjacent faces, resulting in a decrease of the required suction action. Moreover, in the case of irregular objects, many possible grasping locations exist. The model explained in this work contributes to the choice of the most convenient grasping location that ensures the minimum vacuum force required to manipulate the object. Results obtained from an extensive set of simulations are included to support the validity of the proposed analytical approach.

Materials with Electroprogrammable Stiffness

Stiffness is a mechanical property of vital importance to any material system and is typically considered a static quantity. Recent work, however, has shown that novel materials with programmable stiffness can enhance the performance and simplify the design of engineered systems, such as morphing wings, robotic grippers, and wearable exoskeletons. For many of these applications, the ability to program stiffness with electrical activation is advantageous because of the natural compatibility with electrical sensing, control, and power networks ubiquitous in autonomous machines and robots. The numerous applications for materials with electrically driven stiffness modulation has driven a rapid increase in the number of publications in this field. Here, a comprehensive review of the available materials that realize electroprogrammable stiffness is provided, showing that all current approaches can be categorized as using electrostatics or electrically activated phase changes, and summarizing the advantages, limitations, and applications of these materials. Finally, a perspective identifies state-of-the-art trends and an outlook of future opportunities for the development and use of materials with electroprogrammable stiffness.

3D printable and fringe electric field adhesion enabled variable stiffness artificial muscles for semi-active vibration attenuation

Soft robots are able to generate large and compliant deformation in an unconstructed environment, but their operation capability is limited by low stiffness. Thus, developing the function of variable stiffness while preserving its compliance is a challenging issue. This study proposes a new variable stiffness artificial muscle, as a complementary component for soft robots, using the principle of fringe electric field adhesion. Taking inspiration from the mechanism of multi-layer structures in biological muscles, the artificial muscle is composed of patterned conductive layers and interlayers and is 3D printable by direct ink writing (DIW). To further demonstrate the application, a vibration absorber by stacking this artificial muscle is proposed, whose natural frequency is tunable by the varying stiffness. The advantages of the fringe electric field-enabled variable stiffness (FEVS) artificial muscles include lightweight and irrelevance of the stiffness to the thickness of the interlayer, which can be beneficial to soft robots to achieve variable stiffness and semi-active vibration attenuation without extra weighting load.

Limiting Mechanisms and Scaling of Electrostatically Controlled Adhesion of Soft Nanocomposite Surfaces for Robotic Gripping

Surfaces with switchable adhesive properties are employed by robots to quickly grip and release objects and thereby to perform dexterous manipulation and locomotion tasks. Robotic grippers with switchable adhesion have been developed using structured polymers and electrostatic mechanisms. However, manipulating delicate items can be challenging as this requires strong, switchable gripping forces that do not damage the target object. Soft nanocomposite electroadhesives (SNEs) were recently introduced as an option for handling such objects. The technology integrates an electrostatic adhesion mechanism into a mechanically compliant surface formed from dielectric-coated carbon nanotubes (CNTs) to ensure soft contact with target objects. In this study we explore the scaling of the electrostatic adhesion of SNEs, toward their potential application in macroscale grasping and manipulation. We measure electroadhesive pressures on millimeter-scale areas of up to ∼20 kPa with an on/off adhesion ratio of ∼700. Based on the measured forces and simple modeling, we conclude that the maximum achievable SNE adhesion forces are determined by dielectric breakdown in the insulating coating and surrounding air. Consequently, the SNE surface behaves as a parallel capacitor plate placed at an effective distance of 2.9 μm from the target object, despite being in contact with the target and therefore having the contacting CNTs separated from the surface by ∼2 nm dielectric coating. This mechanistic understanding of soft nanocomposite electroadhesives outlines the capabilities of the technology and informs their design for advanced manufacturing applications.

Modeling and Optimization of Electrostatic Film Actuators Based on the Method of Moments

Electrostatic film actuators represent a promising new approach to drive a soft robot, but they lack a comprehensive model to link the design parameters and actuation performance, making actuator design and parameter optimization challenging. To solve this problem, we build a mathematical model based on the method of moments by assuming that each electrode consists of a large number of line charges. This model can directly deduce fluctuation in thrust and adhesive forces during actuator movement, as well as the distribution of electric potential and field strength, for analysis and optimization. It consumes shorter computing time and fewer computing resources, but with comparable accuracy, in comparison with previous indirect means. It is validated by results from both previous studies and on-site experiments. Based on this model, we generate numerous values of actuator output force for different structural parameters. By analyzing the tendency, we summarize a parameter optimization workflow and write an open-sourced program as an example to facilitate the parameter selection for actuator design starting from scratch.

Spinning artificial spiderwebs

Sensing, adhesion, and self-cleaning capabilities are demonstrated in artificial spiderwebs through electrostatic actuation and a dirt-shirking coating.

Low-Voltage Reversible Electroadhesion of Ionoelastomer Junctions

Electroadhesion provides a simple route to rapidly and reversibly control adhesion using applied electric potentials, offering promise for a variety of applications including haptics and robotics. Current electroadhesives, however, suffer from key limitations associated with the use of high operating voltages (>kV) and corresponding failure due to dielectric breakdown. Here, a new type of electroadhesion based on heterojunctions between iono-elastomer of opposite polarity is demonstrated, which can be operated at potentials as low as ≈1 V. The large electric field developed across the molecular-scale ionic double layer (IDL) when the junction is placed under reverse bias allows for strong adhesion at low voltages. In contrast, under forward bias, the electric field across the IDL is destroyed, substantially lowering the adhesion in a reversible fashion. These ionoelastomer electroadhesives are highly efficient with respect to the force capacity per electrostatic capacitive energy and are robust to defects or damage that typically lead to catastrophic failure of conventional dielectric electroadhesives. The findings provide new fundamental insight into low-voltage electroadhesion and broaden its possible applications.