Human hand-inspired all-hydrogel gripper with a high load capacity formed by the split-brushing adhesion of diverse hydrogels

Rewritable wavelength-selective hydrogel actuators grafted with fluorophores

Recent efforts have focused on developing stimuli-responsive soft actuators that mimic the adaptive, complex, and reversible movements found in natural species. However, most hydrogel actuators are limited by their inability to combine wavelength-selectivity with reprogrammable shape changes, thereby reducing their degree of freedom in motion. To address this challenge, we present a novel strategy that integrates these capabilities by grafting fluorophores onto temperature-responsive hydrogels. By harnessing the photothermal effects of fluorophores responsive to specific light wavelengths, we achieve wavelength-selective shape morphing under light irradiation at wavelengths of 405, 520, and 638 nm. Furthermore, iterative chemical bleaching of the fluorophores allows for multiple rewritable shape configurations from a single actuator. Using this approach, we successfully demonstrate multiple shape configurations with a single hydrogel actuator that are precisely controlled with both wavelength-selectivity and rewritability. This approach significantly advances the field of soft robotics, paving the way for adaptive, reprogrammable actuators that could serve as intelligent, light-driven soft robots in the future.

Hydrogel-based 3D fabrication of multiple replicas with varying sizes and materials from a single template via iterative shrinking

3D printing technologies have been widely used for the rapid prototyping of 3D structures, but their application in a broader context has been hampered by their low printing throughput. For the same structures to be produced in a variety of sizes and materials, each must be printed separately, which increases time and cost. Replicating 3D-printed structures in a variety of sizes using a molding process with size-tunable molds could be a solution, but it has only been applied to simple structures, such as those with tapered or vertical profiles. This work demonstrates the generation of multiple replicas of varying sizes and materials from a single 3D-printed template with complex geometries by using molds made of stretchable hydrogel that shrink isotropically. We optimize hydrogel compositions to synthesize a hydrogel that is highly stretchable and shrinks isotropically in all directions. The high stretchability of this hydrogel allows for the removal of complex 3D-printed templates from hydrogel molds. The cavities of the hydrogel molds are then filled with polycaprolactone (PCL) and dried at 80 °C. As the hydrogel shrinks due to drying, the melted PCL fragments completely fill the cavities. The entire process can be repeated to produce multiple replicas in a variety of sizes and materials. Replicas that are one-tenth of the size of the original printed template can be produced. Finally, we demonstrate how our method can be used to reduce the size of interconnected geometries, which would be impossible to achieve using traditional molding processes.

Selectively Detachable Hydrogel Adhesion Enabled by Stimulus-Specific Cleavable Cross-Linkers

The development of detachable hydrogel adhesion presents an advancement in the fields of soft electronics and bioengineering as it offers additional functionalities to these applications. However, conventional methods typically rely on a single detachment trigger, so it is unclear whether unintentional detachment might occur in the specific environments of other detachment systems. This makes it difficult to directly introduce two independent detachment triggers directly. In this article, we present a strategy for selective detachable adhesion based on two types of cleavable cross-linkers, N,N'-bis(acryloyl)cystamine (BAC) and N,N'-(1,2-dihydroxyethylene)bis(acrylamide) (DHEBA), each with an independent cleavage trigger. BAC can be cleaved through the reduction of disulfide bonds using reducing agents, while DHEBA can be hydrolyzed through heating. We constructed stitching polymer networks for topological adhesion using two types of cleavable cross-linkers, allowing the networks to be selectively degraded depending on which cross-linker was used. Our findings show that the use of cleavable cross-linkers achieved selectively detachable adhesion in various hydrogels, with adhesion energy that reached up to 1223 J m-2 in polyacrylamide-alginate (PAAm-alginate) tough hydrogel. This strategy also proved versatile as it led to effective adhesion with various substrates, including aluminum, copper, glass, and polyester film (PET). Furthermore, we took advantage of the high programmability of this approach to construct hydrogel-based YES and AND logic gates, whose output changed depending on the applied input triggers. In addition, we designed a selective-release capsule model capable of dual-solution release, which emphasizes the potential of our strategy in creating programmable and responsive soft materials.

Advancement in Soft Hydrogel Grippers: Comprehensive Insights into Materials, Fabrication Strategies, Grasping Mechanism, and Applications

Soft hydrogel grippers have attracted considerable attention due to their flexible/elastic bodies, stimuli-responsive grasping and releasing capacity, and novel applications in specific task fields. To create soft hydrogel grippers with robust grasping of various types of objects, high load capability, fast grab response, and long-time service life, researchers delve deeper into hydrogel materials, fabrication strategies, and underlying actuation mechanisms. This article provides a systematic overview of hydrogel materials used in soft grippers, focusing on materials composition, chemical functional groups, and characteristics and the strategies for integrating these responsive hydrogel materials into soft grippers, including one-step polymerization, additive manufacturing, and structural modification are reviewed in detail. Moreover, ongoing research about actuating mechanisms (e.g., thermal/electrical/magnetic/chemical) and grasping applications of soft hydrogel grippers is summarized. Some remaining challenges and future perspectives in soft hydrogel grippers are also provided. This work highlights the recent advances of soft hydrogel grippers, which provides useful insights into the development of the new generation of functional soft hydrogel grippers.

The Role of 3D Printing Technologies in Soft Grippers

Soft grippers are essential for precise and gentle handling of delicate, fragile, and easy-to-break objects, such as glassware, electronic components, food items, and biological samples, without causing any damage or deformation. This is especially important in industries such as healthcare, manufacturing, agriculture, food handling, and biomedical, where accuracy, safety, and preservation of the objects being handled are critical. This article reviews the use of 3D printing technologies in soft grippers, including those made of functional materials, nonfunctional materials, and those with sensors. 3D printing processes that can be used to fabricate each class of soft grippers are discussed. Available 3D printing technologies that are often used in soft grippers are primarily extrusion-based printing (fused deposition modeling and direct ink writing), jet-based printing (polymer jet), and immersion printing (stereolithography and digital light processing). The materials selected for fabricating soft grippers include thermoplastic polymers, UV-curable polymers, polymer gels, soft conductive composites, and hydrogels. It is conclude that 3D printing technologies revolutionize the way soft grippers are being fabricated, expanding their application domains and reducing the difficulties in customization, fabrication, and production.

Double layered asymmetrical hydrogels enhanced by thermosensitive microgels for high-performance mechanosensors and actuators

Thermosensitive hydrogels have found extensive applications in soft devices, but they often suffer from limited functionalities, low response rate and small response amplitude. In this work, double layered asymmetrical hydrogels composed of a thermosensitive layer and a non-thermosensitive layer are developed to simultaneously achieve high-performance mechanosensing and actuating properties in a single hydrogel. In thermosensitive layer, thermosensitive microgels are introduced to construct hierarchical structure, which accounts for the enhanced thermosensitive behaviors by cooperative responsiveness. In non-thermosensitive layer, poly(acrylamide-co-acrylic acid) (P(AM-co-AA)) hydrogel is constructed. KCl is introduced as conductive component. Mechanosensors for monitoring various mechanical stimuli in daily life have been fabricated utilizing such hydrogels and high gauge factors (GF) have been achieved, 0.38 for resistive strain sensors, 9.40 kPa-1 for piezoresistive pressure sensors and 3.92 kPa-1 for capacitive pressure sensors. Because of the asymmetrical structure, such hydrogels also exhibit outstanding actuating properties with a fast response rate of 863°/min and a bending amplitude about 360°. Interestingly, grasping-releasing of target objects utilizing an octopus-shaped hydrogel actuator and temperature alerting based on hydrogel actuator are also demonstrated. Overall, the double layered asymmetrical ionic hydrogels have provided a new clue to construct hydrogel devices with multiple functionalities and enhanced response properties.

Mechanical Robust GO/PVA Hydrogel for Strong and Recyclable Adhesion in Air, Underwater, and Underoil Environments

Adhesive hydrogels are considered to be promising interfacial adhesive materials for various applications; however, their adhesive strength is significantly reduced when immersed in liquid environments (water and oil) due to obstruction of the liquid layer or swelling in liquid, and they could not always be reused when the failure of the adhesive performance occurred. Herein, a graphite oxide/poly(vinyl alcohol) (GO/PVA) hydrogel with strong adhesion in air and under liquid environments was developed by rationally regulating the interactions of water and dimethyl sulfoxide (DMSO) in the binary liquid system. The strong interaction between water and DMSO allowed the water layer of the GO/PVA hydrogel on the hydrogel surface to act as a shield to repel oil in air, under water, and even when immersed in oil, and it also endowed the obtained hydrogel with antiswelling property when immersed in water and oil. Importantly, the GO/PVA hydrogel could serve as an advanced adhesive to firmly bond different substrates in air, under water, and under oil, and interestingly, its dry and wet adhesive performance was repeatable and recyclable. This work is expected to be an important addition to the field of adhesive soft materials.

Variable stiffness soft robotic gripper: design, development, and prospects

The advent of variable stiffness soft robotic grippers furnishes a conduit for exploration and manipulation within uncharted, non-structured environments. The paper provides a comprehensive review of the necessary technologies for the configuration design of soft robotic grippers with variable stiffness, serving as a reference for innovative gripper design. The design of variable stiffness soft robotic grippers typically encompasses the design of soft robotic grippers and variable stiffness modules. To adapt to unfamiliar environments and grasp unknown objects, a categorization and discussion have been undertaken based on the contact and motion manifestations between the gripper and the things across various dimensions: points contact, lines contact, surfaces contact, and full-bodies contact, elucidating the advantages and characteristics of each gripping type. Furthermore, when designing soft robotic grippers, we must consider the effectiveness of object grasping methods but also the applicability of the actuation in the target environment. The actuation is the propelling force behind the gripping motion, holding utmost significance in shaping the structure of the gripper. Given the challenge of matching the actuation of robotic grippers with the target scenario, we reviewed the actuation of soft robotic grippers. We analyzed the strengths and limitations of various soft actuation, providing insights into the actuation design for soft robotic grippers. As a crucial technique for variable stiffness soft robotic grippers, variable stiffness technology can effectively address issues such as poor load-bearing capacity and instability caused by the softness of materials. Through a retrospective analysis of variable stiffness theory, we comprehensively introduce the development of variable stiffness theory in soft robotic grippers and showcase the application of variable stiffness grasping technology through specific case studies. Finally, we discuss the future prospects of variable stiffness grasping robots from several perspectives of applications and technologies.