Bistable Joints Enable the Morphing of Hydrogel Sheets with Multistable Configurations

Advanced Liquid Metal-Based Hydrogels for Flexible Electronics

With the rapid development of flexible electronics in wearable devices, healthcare devices, and the Internet of Things (IoT), liquid metals (LMs)-based hydrogels have emerged as cutting-edge functional materials due to their high electrical conductivity, tunable mechanical properties, excellent biocompatibility, and unique self-healing properties. Through various physical or chemical methods, LMs can be integrated to form multifunctional LMs-based hydrogels, thus broadening the potential application fields. In this Review, the recent advancement in LMs-based hydrogels for flexible electronics is comprehensively and systematically reviewed from three aspects of synthesis methods, properties, and applications. For the first time, the existing innovative synthesis methods of LMs-based hydrogels are classified and summarized, including patterned LMs on/inside hydrogel substrates, LMs as conductive fillers in polymeric hydrogels, LMs as initiators in hydrogels, and LMs as cross-linkers with organic/inorganic materials. The synthesis mechanism is also stated in detail to highlight the multiple roles of LMs in adjusting the hydrogel properties. The versatile applications of LMs-based hydrogels in flexible electronics, including flexible sensors, wireless communications, electromagnetic interference (EMI) shielding, soft robot actuators, energy storage and conversion, etc., are separately described. Finally, the current challenges and future prospects of LMs-based hydrogels are proposed.

Muscle-like hydrogels with fast isochoric responses and their applications as soft robots: a minireview

Hydrogels with abundant water and responsiveness to external stimuli have emerged as promising candidates for artificial muscles and garnered significant interest for applications as soft actuators and robots. However, most hydrogels possess amorphous structures and exhibit slow, isotropic responses to external stimuli. These features are far inferior to real muscles, which have ordered structures and endow living organisms with programmable deformations and motions through fast, anisotropic responses in complex environments. In recent years, this issue has been addressed by a conceptual new strategy to develop muscle-like hydrogels with highly oriented nanosheets. These hydrogels exhibit fast, isochoric responses based on temperature-mediated electrostatic repulsion between charged nanosheets rather than water diffusion, which significantly advances the development of soft actuators and robots. This minireview summarizes the recent progress in muscle-like hydrogels and their applications as soft actuators and robots. We first introduce the synthesis of muscle-like hydrogels with monodomain structures and the unique mechanism for rapid and isochoric deformations. Then, the developments of hydrogels with complex ordered structures and hydrogel-based soft robots are discussed. The morphing mechanisms and motion kinematics of the hydrogel actuators and robots are highlighted. Finally, concluding remarks are given to discuss future opportunities and challenges in this field.

Polymorphing Hydrogels Regulated by Photo-reactive DNA Cross-links

Polymorphing hydrogels can morph into another structure on demand with reprogrammable features. This concept extends the degree of morphing beyond that of traditional shape-morphing hydrogels, which predetermine their morphing capabilities at the fabrication stage. However, current polymorphing hydrogels face limitations due to the need for complex, non-sustained responsiveness or additional chemical steps for reconfigurable morphing. Here, photo-reactive DNA-cross-linked polymorphing hydrogels are presented that enable polymorphing under patterned light. The photo-reactive DNA cross-links act as a regulator in changing the shapes of the hydrogels by adjusting the lengths of the cross-links depending on the wavelength of light, which allows precise and dynamic morphing in a programmable way through a one-pot reaction. The high programmability involving spatiotemporal controllability and reprogrammable features offers advanced solutions for multifunctional soft machines and applications requiring complex processes.

Marine Amoebae-Inspired Salting Hydrogels to Reconfigure Anisotropy for Reprogrammable Shape Morphing

Reprogrammable shape morphing is ubiquitous in living beings and highly crucial for them to move in normal situations, even to survive under dangerous conditions. There is increasing interest in using asymmetric hydrogel structures to understand and mimic living beings' shape morphing upon an external trigger in a controlled way. However, these asymmetric or heterogeneous configurations cannot be further modified once the polymer hydrogels are prepared. Therefore, it is a great challenge to achieve reprogrammable shape morphing using the existing hydrogels. Inspired by marine amoebae, which transform into several different morphologies according to the various external salt concentrations, a new strategy is developed for salting hydrogels to reconfigure their anisotropy toward reprogrammable shape morphing. Polyampholyte hydrogels with equal stoichiometric COO- and N+(CH3)3 groups were first swollen in HCl/NaCl solution. After being then transferred into water, they first swollen again by water uptake driven by the osmotic pressure, and then were spontaneously deswollen due to increase in internal pH and dialysis of ions leading to deprotonation of COOH to COO- and regeneration of COO-/N+(CH3)3 electrostatic attraction. This work provides a novel strategy to reconfigure anisotropy of hydrogel soft actuators and to open up an avenue for reprogrammable shape morphing.

A Proprioceptive Janus Fiber with Controllable Multistage Segments for Bionic Soft Robots

Smart fibers capable of integrating the multifunctionality of actuation and self-sensation into a single proprioceptive device have significant applications in soft robots and biomedicine. Especially, the achievement of self-sensing the movement patterns of different actuating segments in one fiber is still a great challenge. Herein, in this study, a fiber with the controllable Janus architecture is successfully proposed via an artful centrifugation-driven hierarchical gradient self-assembly strategy, which couples two functional components of piezoresistive carbon nanotubes and magnetic NdFeB nanoparticles into the upper and lower layers of this flexible fibrous framework with the porous sponge structure partially, respectively. As predicted, the final product exhibits the as-anticipated bionic proprioceptive behaviors of programmable actuating deformation and highly selective response to bending, stretching, and pressure with high washable stability and mechanical performances. More importantly, assisted by the different three-dimensional printing molds, the superlong Janus fibers with various controllable lengths of the reversed but sequential multistage segments can be fabricated, offering the hybrid magnetic actuation and proprioceptive sensation existing at arbitrary nodes. Therefore, several kinds of soft organism-inspired Janus fiber-derived soft robots with the arbitrarily controlled segmental characters were assembled as the proof-of-concept, which can not only realize a snake or inchworm-inspired successive contracting-stretching deformation and a sperm-inspired self-rotating crawling motion but also self-sense the signals of each segment themselves in real time and then be used to navigate an object to target position in a liquid-filled confined tube. It is believed that this work promotes the further development of proprioceptive soft robots.

Magnetic-field induced shape memory hydrogels for deformable actuators

Magnetic hydrogel actuators exhibit promising applications in the fields of soft robotics, bioactuators, and flexible sensors owing to their inherent advantages such as remote control capability, untethered deformation and motion control, as well as easily manipulable behavior. However, it is still a challenge for magnetic hydrogels to achieve adjustable stiffness and shape fixation under magnetic field actuation deformation. Herein, a simple and effective approach is proposed for the design of magnetic shape memory hydrogels to accomplish this objective. The magnetic shape memory hydrogels, consisting of methacrylamide, methacrylic acid, polyvinyl alcohol and Fe3O4 magnetic particles, which crosslinked by hydrogen bonds, are facilely prepared via one-pot polymerization. The dynamic nature of noncovalent bonds offers the magnetic hydrogels with excellent mechanical properties, precisely controlled stiffness, and effective shape fixation. The presence of Fe3O4 particles renders the hydrogels soft when subjected to an alternating current field, facilitating their deformation under the influence of an actuation magnetic field. After the elimination of the alternating current magnetic field, the hydrogels stiffen and attain a fixed actuated shape in the absence of any external magnetic field. Moreover, this remarkable magnetic shape memory hydrogel is effectively employed as an underwater soft gripper for lifting heavy objects. This work provides a novel strategy for fabricating magnetic hydrogels with non-contact reversible actuation deformation, tunable stiffness and shape locking.

Hydrogels with Differentiated Hydrogen-Bonding Networks for Bioinspired Stress Response

Stress response, an intricate and autonomously coordinated reaction in living organisms, holds a reversible, multi-path, and multi-state nature. However, existing stimuli-responsive materials often exhibit single-step and monotonous reactions due to the limited integration of structural components. Inspired by the cooperative interplay of extensor and flexor cells within Mimosa's pulvini, we present a hydrogel with differentiated hydrogen-bonding (H-bonding) networks designed to enable the biological stress response. Weak H-bonding domains resemble flexor cells, confined within a hydrophobic network stabilized by strong H-bonding clusters (acting like extensor cells). Under external force, strong H-bonding clusters are disrupted, facilitating water diffusion from the bottom layer and enabling transient expansion pressure gradient along the thickness direction. Subsequently, water diffuses upward, gradually equalizing the pressure, while weak H-bonding domains undergo cooperative elastic deformation. Consequently, the hydrogel autonomously undergoes a sequence of reversible and pluralistic motion responses, similar to Mimosa's touch-triggered stress response. Intriguingly, it exhibits stress-dependent color shifts under polarized light, highlighting its potential for applications in time-sensitive "double-lock" information encryption systems. This work achieves the coordinated stress response inspired by natural tissues using a simple hydrogel, paving the way for substantial advancements in the development of intelligent soft robots.

Biomimetic growth in polymer gels

By modeling gels growing in confined environments, we uncover a biomimetic feedback mechanism between the evolving gel and confining walls that enables significant control over the properties of the grown gel. Our new model describes the monomer adsorption, polymerization and cross-linking involved in forming new networks and the resultant morphology and mechanical behavior of the grown gel. Confined between two hard walls, a thin, flat "parent" gel undergoes buckling; removal of the walls returns the gel to the flat structure. Polymerization and cross-linking in the confined parent generates the next stage of growth, forming a random copolymer network (RCN). When the walls are removed, the RCN remains in the buckled state, simultaneously "locking in" these patterns and increasing the Young's modulus by two orders of magnitude. Confinement of thicker gels between harder or softer 3D walls leads to controllable mechanical heterogeneities, where the Young's modulus between specific domains can differ by three orders of magnitude. These systems effectively replicate the feedback between mechanics and morphology in biological growth, where mechanical forces guide the structure formation throughout stages of growth. The findings provide new guidelines for shaping "growing materials" and introducing new approaches to matching form and function in synthetic systems.

Environment-Interactive Programmable Deformation of Electronically Innervated Synergistic Fluorescence-Color/Shape Changeable Hydrogel Actuators

Utilization of life-like hydrogels to replicate synergistic shape/color changeable behaviors of living organisms has been long envisaged to produce robust functional integrated soft actuators/robots. However, it remains challenging to construct such hydrogel systems with integrated functionality of remote, localized and environment-interactive control over synergistic discoloration/actuation. Herein, inspired by the evolution-optimized bioelectricity stimulus and multilayer structure of natural reptile skins, electronically innervated fluorescence-color switchable hydrogel actuating systems with bio-inspired multilayer structure comprising of responsive fluorescent hydrogel sheet and conductive Graphene/PDMS film with electrothermal effect is presented. Such rational structure enables remote control over synergistic fluorescence-color and shape changes of the systems via the cascading "electrical trigger-Joule heat generation-hydrogel shrinkage" mechanism. Consequently, local/sequential control of discoloration/actuation are achieved due to the highly controllable electrical stimulus in terms of amplitude and circuit design. Furthermore, by joint use with acoustic sensors, soft chameleon robots with unprecedented environment-interactive adaptation are demonstrated, which can intelligently sense environment signals to adjust their color/shape-changeable behaviors. This work opens previously unidentified avenues for functional integrated soft actuators/robots and will inspire life-like intelligent systems for versatile uses.

Edible Origami Actuators Using Gelatin-Based Bioplastics

The potential of ingestible medical devices can be greatly enhanced through the use of smart structures made from stimuli-responsive materials. While hydration is a convenient stimulus for inducing shape changes in biomaterials, finding robust materials that can achieve rapid actuation, facile manufacturability, and biocompatibility suitable for ingestible medical devices poses practical challenges. Hydration is a convenient stimulus to induce shape changes in smart biomaterials; however, there are many practical challenges to identifying materials that can achieve rapid actuation and facile manufacturability while satisfying constraints associated with biocompatibility requirements and mechanical properties that are suitable for ingestible medical devices. Herein, we illustrate the formulation and processability of a moisture-responsive genipin-crosslinked gelatin bioplastic system, which can be processed into complex three-dimensional shapes. Mechanical characterization of bioplastic samples showed Young's Modulus values as high as 1845 MPa and toughness values up to 52 MJ/m3, using only food-safe ingredients. Custom molds and UV-laser processing enabled the fabrication of centimeter-scale structures with over 150 independent actuating joints. These self-actuating structures soften and unfold in response to surrounding moisture, eliminating the need for additional stimuli or actuating elements.