AbstractIn recent years, there has been a growing interest in the development of universal soft grippers that can handle objects of varying form factors (including flat objects), surface condition (including moistened or oily objects), and mechanical properties (deformable and fragile). Yet, there is no single gripper that can gently grip objects with such a wide range of properties. In this paper, we present a soft gripper that combines granular jamming (GJ) and electroadhesion (EA) to gently grasp and release a large set of diverse objects. The gripper can operate in GJ mode only, in EA mode only, or in a combination mode that simultaneously activates GJ and EA. In GJ mode, the gripper can grasp objects with different surface properties, lift objects 38 times its own weight using negative pressure, and release objects by applying positive pressure, but has difficulty in handling flat and fragile objects. In EA mode, the gripper can manipulate flat and fragile objects but encounters difficulties with different surface properties such as oily or moistened. In the combination mode, the gripper can generates grasping forces up to 35% higher than in the GJ mode for all object sizes and certain shapes such as a cylinder.IntroductionThe softness of the human hand is a critical factor that allows us to hold, lift, and manipulate a variety of objects and has inspired roboticists to incorporate softness in gripper design and materials. The compliance of soft materials enables passive adaptation of the gripper during grasping operations allowing manipulation of a wide range of objects without bringing additional control complexities.[1,2] In recent years, there has been a growing interest in the development of universal soft grippers that can work with objects of different form factors, rigidity, surface properties, and level of fragility.[3–6] A possible approach to create such highly versatile grippers is to combine different gripping technologies that complement their individual limitations.[1,3,7–10] Yet, it is still challenging to develop a single gripper that can grasp and release objects of different form factor including flat objects, surface conditions (wet, porous, oily, and powdered), and mechanical properties (fragile and deformable).In this paper, we present a soft gripper capable of manipulating different objects with varying physical properties, such as shape, surface conditions, and rigidity. The proposed gripper combines two different technologies: granular jamming to control stiffness and electroadhesion to control adhesion. Here we show that not only does this combination mutually compensate for the limitations of each individual technology, but it also makes the gripper capable of performing multi-stage grasping tasks that consist of diverse grasping and releasing operations on objects made of different material, surface, and shape. The manipulation of a book is an example of multi-stage operation that requires grasping and turning a rigid cover and flipping through single pages.Granular jamming (GJ) enables reversible stiffness change between soft and rigid configurations by means of negative pressure[5,11,12]. High compliance in the soft state allows a GJ gripper to envelope the manipulated object by pressing on it. When negative pressure is applied, the gripper becomes stiff and holds the encaged object.[2] Variable stiffness can also be achieved by integrating phase-change materials that vary mechanical properties under thermal stimulation.[7,13] However, granular jamming offers comparatively faster response time (~100ms), independence from environmental temperature, higher lifting force, easier fabrication, higher robustness, and lower cost.[2,5,14,15] The grasping force produced by granular jamming is sufficient to grasp objects of different morphologies, almost independently of the surface conditions of the object.[5,11,16] The grasping force of GJ grippers can vary from 0.09 to 1.2 kN.[12] GJ has been combined with soft pneumatic actuators to provide more dexterous grasp and lift heavier objects because of the enhanced holding forces.[10,17] However, GJ grippers cannot lift flat objects, such as a sheet of paper. Also, the grasping performance of delicate, fragile and easily deformable objects such as a thin layer of cloth, an egg, or water balloons, as well as larger objects than the active area of the granular bag can be challenging and have not been demonstrated so far.Electroadhesion (EA) instead is an adhesive technology that leverages the shear force generated by electrostatic forces.[18] Electroadhesive pads have been combined with different actuation technologies, such as dielectric elastomer actuation,[1] soft pneumatic actuation,[19] layer jamming,[20] and Fin-Ray structured actuation[21]. The enhanced shear force makes EA-based grippers capable of delicately grasping both flat and fragile objects without squeezing or breaking them.[1,22–26] While the adhesive force of EA pads can be tuned by regulating electrical input, EA effectiveness is highly dependent on the environmental and surface conditions of the object being grasped.[18] In particular electroadhesion is less effective for objects that are greasy, rough, or wet.[2] An additional challenge of soft grippers that rely on electroadhesion is the residual electrostatic charge that remains for a few seconds after removing the voltage and can result in difficult release of light objects.[27]

AbstractBiodegradable materials decompose and return to nature. This functionality can be applied to derive robotic systems that are environmentally friendly. This study presents a fully biodegradable soft actuator, which is one of the key elements in “green” soft robotics. The working of the actuator is based on an electrohydraulic principle, which is similar to that of hydraulically amplified self-healing electrostatic actuators. The actuator developed in this study consists of a dielectric film made of polylactic acid (PLA) and polybutylene adipate-co-terephthalate (PBAT), with soybean oil as the dielectric liquid and electrodes made from a mixture of gelatin, glycerol, and sodium chloride (NaCl). The synthesized biodegradable electrode material exhibits a Young’s modulus of 0.06 MPa and resistivity of 258 Ω·m when the mass fraction of NaCl relative to the amount of gelatin and glycerol is 10 wt%. The softness and conductivity of the electrode material results in actuation strain values of 3.4% (at 1 kV, corresponding to 1.2 kV/mm) and 18.6% (at 10 kV corresponding to 9.6 kV/mm) for the linear-type and circular-type actuator, respectively. These values obtained for the biodegradable electrohydraulic soft actuators are comparable to those of non-biodegradable actuators of the same type, representing the successful implementation of the concept.1. IntroductionSoft robotics has a high potential owing to the high compliance from which a wide variety of functional robots and applications can be derived.[1–8] Synthetic polymers such as silicone rubbers are the most widespread materials used in soft robotics. They are low cost,[9] easy to handle,[10] and compatible with various fabrication methods, such as casting, molding, and printing.[11] Synthetic polymers are also chemically stable, making them suitable for soft robots operated in diverse situations and environments, such as on the ground,[12] underwater,[13]in snowstorms,[14] and even in radiation environments.[15] On the contrary, their stable nature and irreversible synthetic process like thermoset[16,17] make them non-biodegradable, which may lead to environmental destruction; this can particularly occur when the robots performing tasks in natural fields are discarded as the result of malfunctions or accidents. In addition, polymeric materials used in soft robotics are mostly difficult to recycle and have a high environmental impact. Considering these perspectives, it is important to incorporate biodegradability into soft robots.Researchers have demonstrated biodegradable soft robotic elements that are focused on actuators. Their working principle includes pneumatic actuation,[18–24]piezoelectricity,[25] ion migration,[26–30] and swelling.[31,32] Pneumatic actuators are relatively easy to fabricate and can provide large outputs; however, their performance is dependent on bulky external pumps and compressors, which can lead to difficulty in constructing robots according to their types and specifications. From a system perspective, actuators based on piezoelectricity and ion migration have been driven electrically using a portable power source. However, actuation strain generated by piezoelectricity tends to be small (4%[33]) and the actuation speed achieved with ion migration is normally low (2.3%/s[34]), thus limiting the actuation performance. Similarly, actuation based on swelling has a limitation on speed (over 6 h required for achieving a fully swelled state[31]) and controllability of actuated deformation because its working principle requires material injection[31] and cannot perform multiple actuations[32].In recent years, electrohydraulic soft actuators, also known as hydraulically amplified self-healing electrostatic (HASEL) actuators, are emerging.[35]This type of actuators consists of a pair of opposing electrodes covering a portion of the surface of a flexible pouch encapsulating a dielectric liquid. When a high voltage is applied, electrostatic forces between the electrodes squeeze the pouch, causing the local position of the liquid to change, resulting in a hydraulic deformation of the entire structure as actuation. Electrohydraulic soft actuators exhibit large actuation strain (107% linear strain[36]) and force (actuation stress of ~114 kPa[36]), high power density (358 W/kg[36]), and high speed (strain rate of 900%/s[37]). Their structure is simple, allowing to tailor them in various shapes.In this paper, we present a biodegradable soft actuator based on the electrohydraulic principle. This type of actuation principle requires compliant and conductive electrodes. First, we investigated the mechanical and electrical properties of the electrode for different compositions. Then, we fabricated and characterized two types of actuators that have linear and circular shapes to study the effect of incorporating biodegradable materials into the existing actuation principle and to validate our hypothesis.