Hydrogel Rivet with Unidirectional Shape Morphing for Flexible Mechanical Assembly.

Integrating diverse materials and functions into highly additive produce has piqued global interest due to the increasing demands of intelligent soft robotics. Nevertheless, existing assembly techniques, especially supramolecular assembly which heavily rely on precise chemical design and specific recognition, may prove inadequate when confronted with diverse external demands. Inspired by the traditional mechanical assembly, rivet connection, herein, a thermo-responsive hydrogel with unidirectional shape-morphing is fabricated and a stable mechanical assembly is constructed by emulating the rivet connection mechanism. This system employed poly(acrylamide-co-acrylic acid) [P(AAm-co-AAc)] to induce continuous swelling and hexylamine-modified polyvinyl alcohol (PVA-C6) as a molecular switch to control the swelling process. The hydrogel rivet, initially threaded through pre-fabricated hollows in two components. Subsequently, upon the disassociation of alkane chains the molecular switch would activate, inducing swelling and stable mechanical assembly via anchor structures. Moreover, to enhance the assembly strength, knots are introduced to enhance assembly strength, guiding localized stress release for programmed deformations. Additionally, the system can be remotely controlled using near-infrared light (NIR) by incorporating photo-thermal nanoparticles. This work presents a universal and efficient strategy for constructing stable mechanical assemblies without compromising overall softness, offering significant potential for the fabrication of integrated soft robots.

Hydrogen-bonded Supramolecular Network Enabled Gentle Reprogramming of Liquid Crystal Elastomer toward Evolutionary Robot.

In nature, many organisms augment chances of survival by reprogramming their structures to evolving environment, among which sea squirts being a prime example. Such reprogramming has been demonstrated in liquid crystal elastomer (LCE) actuator assembled with heat assistance. However, the required temperature being higher than the actuation temperature limits its application. Here, we reported a hydrogen-bonded supramolecular network LCE to construct soft modular and reprogrammable actuator by assembling with a gentle heat treatment. Leveraging the Michael addition reaction, we introduced hydrogen bonding to the LCE matrix with functionalized pyridine monomers. Experimental and molecular dynamics modeling proved the efficient dynamic hydrogen bond exchange at 60°C, significantly lower than the actuating temperature of the LCE. This gave rise to the reversible and robust adhesion of the same collection of LCE modules capable of being built into different bilayers and performing various morphing upon a short thermal stimulation. Therefore, we demonstrated that these comparatively weak cross-links enabled reconfiguration of the LCE actuator. With the developed hydrogen-bonding LCEs, we built proof-of-concept modular reprogrammable robot, performing crawling, sailing, and microcircuit repair tasks. This bioinspired and efficient method for evolutionary LCE robot offers a viable path for further development of intelligent actuators sustainable in complex environments.

Mimicking Color-Changing Organisms to Enable the Multicolors and Multifunctions of Smart Fluorescent Polymeric Hydrogels.

Fluorescent polymer hydrogels (FPHs) are of significant interest for diverse emerging applications such as visualized sensing, smart display, camouflaging skins, soft actuators/robots, because they can synergize the features of classic fluorescent polymers and hydrogels. With great efforts in the past decades, the major challenge in this field has been believed to be not whether a given FPH of interest can be prepared but how to fabricate robust FPHs with multicolor tunability and multifunctional synergy. Such materials will conceptually minimize the contribution of passive materials to the mass and size of the final system, holding great potential to facilitate multiple applications. To this end, one promising way is to learn from the Nature that has superb capability to forge delicate or sometimes beyond-imagination materials. Chameleons and cephalopods serve as typical examples, which are famous for not only diverse skin color adaptability under changing environmental demands, but also synergistic skin color and body gesture changes to communicate, warn, camouflage, etc. Biological studies revealed their structural color-changing capacity derives from different types of skin chromatophores and their rational multilayer arrangement in under-skin tissues. Besides, their superb ability to heterogeneously integrate soft tissues with disparate functions into topology-optimized architectures has led to various multifunctional performances. Such natural strategies, if replicated and implemented in artificial systems, would significantly benefit and advance the development of robust FPHs for various applications.In this Account, we summarizes the key advances of smart FPHs mainly achieved by our groups. We start by introducing the unique hierarchical multilayer structures of skin chromatophores in structural color-changing reptiles, followed by an in-depth discussion on how a rational integration of bioinspiration and man-made design makes it possible to largely expand the fluorescence color-changing range of smart FPHs to almost cover the whole visible spectrum. Then, to closely mimic the multifunctional behaviors of chameleons and cephalopods, we further develop efficient strategies to introduce supramolecular interactions or heterogeneously integrating smart FPHs with other soft materials with disparate functions, producing a number of multifunctional fluorescent polymeric hydrogel systems. These robust FPHs can find many frontier applications, including bioinspired synergistic color/shape switchable hydrogel actuators/robots, smart systems with on-demand fluorescent patterning capacities for displaying or information encryption, as well as robust chemosensors for important food or environmental analytes. We expect that the discussion presented in this Account would promote better understanding of the discoloration systems in nature, and advance the development of bioinspired color-changing materials.

Aggregation-Induced Emissive Carbon Dots Gels for Octopus-Inspired Shape/Color Synergistically Adjustable Actuators.

Some living organisms such as the octopus have fantastic abilities to simultaneously swim away and alter body color/morphology for disguise and self-protection, especially when there is a threat perception. However, it is still quite challenging to construct artificial soft actuators with octopus-like synergistic shape/color change and directional locomotion behaviors, but such systems could enhance the functions of soft robotics dramatically. Herein, we proposed to utilize unique hydrophobic carbon dots (CDs) with rotatable surficial groups to construct the aggregation-induced emission (AIE) active glycol CDs polymer gel, which could be further employed to be interfacially bonded to an elastomer to produce anisotropic bilayer soft actuator. When putting the actuator on a water surface, glycol spontaneously diffused out from the gel layer to allow water intake, resulting in a color change from a blue dispersion fluorescence to red AIE and a shape deformation, as well as a large surface tension gradient that can promote its autonomous locomotion. Based on these findings, artificial soft swimming robots with octopus-like synergistic shape/color change and directional swimming motion were demonstrated. This study provides an elegant strategy to develop advanced multi-functional bio-inspired intelligent soft robotics.

Cephalopod-Inspired Chemical-Gated Hydrogel Actuation Systems for Information 3D-Encoding Display.

Cephalopods evolve the acetylcholine-gated actuation control function of their skin muscles, which enables their dynamic/static multimode display capacities for achieving perfectly spatial control over the colors/patterns on every inch of skin. Reproduction of artificial analogs that exhibit similar multimodal display is essential to reach advanced information three-dimensional (3D) encoding with higher security than the classic 2D-encoding strategy, but remains underdeveloped. The core difficulty is how to replicate such chemical-gated actuation control function into artificial soft actuating systems. Herein, this work proposes to develop azobenzene-functionalized poly(acrylamide) (PAAm) hydrogel systems, whose upper critical solution temperature (UCST) type actuation responsiveness can be intelligently programmed or even gated by the addition of hydrophilic α-cyclodextrin (α-CD) molecules for reversible association with pendant azobenzene moieties via supramolecular host-guest interactions. By employing such α-CD-gated hydrogel actuator as an analogue of cephalopods' skin muscle, biomimetic mechanically modulated multicolor fluorescent display systems are designed, which demonstrate a conceptually new α-CD-gated "thermal stimulation-hydrogel actuation-fluorescence output" display mechanism. Consequently, high-security 3D-encoding information carriers with an unprecedented combination of single-input multiple-output, dynamic/static dual-mode and spatially controlled display capacities are achieved. This bioinspired strategy brings functional-integrated features for artificial display systems and opens previously unidentified avenues for information security.

Entangled Mesh Hydrogels with Macroporous Topologies via Cryogelation for Rapid Atmospheric Water Harvesting.

Sorption-based atmospheric water harvesting (SAWH) is a promising technology to alleviate freshwater scarcity. Recently, hygroscopic salt-hydrogel composites (HSHCs) have emerged as attractive candidates with their high water uptake, versatile designability, and scale-up fabrication. However, achieving high-performance SAWH applications for HSHCs has been challenging because of their sluggish kinetics, attributed to their limited mass transport properties. Herein, a universal network engineering of hydrogels using a cryogelation method is presented, significantly improving the SAWH kinetics of HSHCs. As a result of the entangled mesh confinements formed during cryogelation, a stable macroporous topology is attained and maintained within the obtained entangled-mesh hydrogels (EMHs), leading to significantly enhanced mass transport properties compared to conventional dense hydrogels (CDHs). With it, corresponding hygroscopic EMHs (HEMHs) simultaneously exhibit faster moisture sorption and solar-driven water desorption. Consequently, a rapid-cycling HEMHs-based harvester delivers a practical freshwater production of 2.85 Lwater kgsorbents -1 day-1 via continuous eight sorption/desorption cycles, outperforming other state-of-the-art hydrogel-based sorbents. Significantly, the generalizability of this strategy is validated by extending it to other hydrogels used in HSHCs. Overall, this work offers a new approach to efficiently address long-standing challenges of sluggish kinetics in current HSHCs, promoting them toward the next-generation SAWH applications.

Cephalopod-Inspired Design of Photomechanically Modulated Display Systems for On-Demand Fluorescent Patterning.

Cephalopods can display variable body color/patterns upon environmental stimulation via bioelectricity-controlled muscle contraction/expansion of skin chromatophores. However, it remains challenging to produce artificial display analogs that exhibit reversible and rapid switching between multiple expected luminescent patterns, although such systems are very appealing for many practical uses (e.g., data encryption). Inspired by the bioelectromechanical display tactic of cephalopods, in this work, a conceptually new photomechanically modulated fluorescent system that enables on-demand display of fluorescent patterns via a cascading stimulation-mechanical movement-optical output conduction mechanism is presented. Specifically, this biomimetic system comprises a customizable hollow display panel and a bottom-tethered photothermally responsive fluorescent actuator. Under NIR light, the photomechanically bending movements of the fluorescent actuator will immediately cover the hollow window of the display panel and synchronously manifest as the display of fluorescent patterns. Owing to its desirable time- and light-power-dependent actuating behaviors, diverse fluorescent patterns/information can be dynamically and reversibly displayed by facilely controlling this single remote NIR signal. This bioinspired strategy is universal and promising for fabricating on-demand fluorescent display platforms that combine a wide choice of fluorophores, remote control with high spatial/temporal precision, and especially single-input multiple-output features.