Spatiotemporal Regulation of Hydrogel Actuators by Autocatalytic Reaction Networks

Ionic fuel-powered hydrogel actuators for soft robotics

HYPOTHESIS

Hydrogel actuators powered by chemical fuels are pivotal in autonomous soft robotics. Nevertheless, chemical waste accumulation caused by chemical fuels hampers the development of programmable and reusable hydrogel actuating systems. We propose the concept of ionic fuel-powered soft robotics which are constructed by programmable salt-responsive actuators and use waste-free ionic fuels.

EXPERIMENTS

Herein, soft hydrogel actuators were developed by orchestrating the Janus bilayer hydrogels' capacity for swelling and shrinking. Decomposable and easily removable ionic fuels were applied to power the actuators. Swelling tests were used to evaluate the deformability of the hydrogels. Tensile tests were performed to investigate the modulus of the hydrogels. The bonded interface composed of the interpenetrating polymer chains from both hydrogel layers bilayer was evidenced by the optical microscopy and scanning electron microscopy. The ionic conductivities of solutions were determined by a conductivity meter. Furthermore, a range of biomimetic soft robots with various shapes and asymmetrical structures have been designed and fabricated to execute complex functions.

FINDINGS

The programmable actuators powered by ionic fuel exhibit adjustable bending orientations, amplitudes, and durations, along with consistent cyclic actuations enabled by replenishment of the fuel without noticeable loss in performance. Many life-like programmable soft robotic systems were designed, indicating spatiotemporally controllable functions.

Selenium catalysis enables negative feedback organic oscillators

The construction of materials regulated by chemical reaction networks requires regulatory motifs that can be stacked together into systems with desired properties. Multiple autocatalytic reactions producing thiols are known. However, negative feedback loop motifs are unavailable for thiol chemistry. Here, we develop a negative feedback loop based on the selenocarbonates. In this system, thiols induce the release of aromatic selenols that catalyze the oxidation of thiols by organic peroxides. This negative feedback loop has two important features. First, catalytic oxidation of thiols follows Michaelis-Menten-like kinetics, thus increasing nonlinearity for the negative feedback. Second, the strength of the negative feedback can be tuned by varying substituents in selenocarbonates. When combined with the autocatalytic production of thiols in a flow reactor, this negative feedback loop induces sustained oscillations. The availability of this negative feedback motif enables the future construction of oscillatory, homeostatic, adaptive, and other regulatory circuits in life-inspired systems and materials.

Polymer Hydrogel Sheets with Perpendicular Cross-Linking Gradient: Non-Monotonic Actuation and Ion-Specific Effects on the Actuation Kinetics

Non-monotonous actuation, that is, different kinds of motion in response to a single stimulus, is observed in some natural materials but difficult to implement in synthetic systems. Herein, polymer hydrogel sheets made from polyacrylamide (PAAm) or poly(dimethylacrylamide) (PDMAA) with a cross-linking gradient along the sheet thickness are reported. These are obtained by thermally initiated free radical polymerization using a specially designed Teflon mold with a glass lid. The resulting PAAm hydrogels undergo non-monotonous actuation (rolling into a tube and re-opening) when exposed to aqueous media as a single external stimulus. Their actuation kinetics is tuned with anions that have specific ion effects in their interaction with the surrounding solvent and the polymer itself: structure-breaking chloride enhances the hydration of the polymer backbone, structure-making sulfate decreases it, and is thus slowing down the actuation kinetics of the PAAm hydrogels. The PDMAA gel rolls up instantaneously in aqueous NaCl and only re-opens after 24 h. PDMAA actuation in aqueous Na2 SO4 is only moderate as the gel did not swell in that solvent. Bilayer hydrogels made from PAAm and PDMAA (without gradient) show monotonic actuation, closing in NaCl solution and re-opening in Na2 SO4 .

Polydopamine-Modified MXene-Integrated Poly(N-isopropylacrylamide) to Construct Ultrafast Photoresponsive Bilayer Hydrogel Actuators with Smart Adhesion

In nature, living organisms, such as octopuses, cabrito, and frogs, have already evolved admirable adhesive abilities for better movement and predation in response to the surroundings. Inspired by biological structures, researchers have made enormous efforts in developing actuators that can respond to external stimuli, while such adhesive property is very desired, yet there is still limited research in responsive hydrogel actuators. Here, a bilayer actuator with high stretchability and robust interface bonding is presented, which has a smart adhesion and thermoreception function. The system consists of an adhesive passive layer copolymerized of amphoteric ([2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl), SBMA) and acrylic acid (AA), and an active layer hydrogel composed of poly(N-isopropylacrylamide) (PNIPAm) containing polydopamine-modified MXene (P-MXene) and calcium chloride (CaCl2). The coordination of carboxylate and Ca2+ at the interface of the two layers enhances the interfacial bonding from 14 to 30 N m-1, which facilitates withstanding large strain and preventing stratification. The resulting hydrogel actuator can bend approximately 360° in a mere 10 s, exhibiting excellent photothermal effect, a large angle bending deformation, and ultrafast photoresponsive ability. As a proof of concept, the photothermal actuators are programmed to present various shapes and grab objects. Importantly, the hydrogel actuator exhibits remarkable adhesion capabilities toward diverse substrates, with a maximum peel force of up to 280 N m-1. Relying on their own adhesion and the photoresponse properties, these flexible adhesion actuators show outstanding gripping capability, enabling them to grip and release objects of different shapes and weights. More interestingly, the hydrogel exhibits a smart adjustable adhesion capability at different temperatures, which enables it as a gripper to recognize temperature signals through real-time different feedback actions based on its own adhesion. This study presents innovative insights into biomimetic hydrogel actuators, providing new opportunities for developing intelligent soft robots with multiple functions.

Transient regulation of gel properties by chemical reaction networks

Transient regulation of gel properties by chemical reaction networks (CRNs) represents an emerging and effective strategy to program or temporally control the structures, properties, and functions of gel materials in a self-regulated manner. CRNs provide significant opportunities to construct complex or sustainable gels with excellent dynamic features, thus expanding the application scope of these materials. CRN-based methods for transiently regulating the gel properties are receiving increasing attention, and the related fields are worth further studying. This feature article focuses on the CRN-mediated transient regulation of six properties of gels, which are transient gelation, transient liquefaction of gels, transient assembly of macroscopic gels, temporary actuation of gels, transient healing ability of kinetically inert gels, and cascade reaction-based self-reporting of external stimuli. Recent advances that showcase the six properties of gels controlled by CRNs are featured, the characterization and structural elucidation of gels are detailed, and the significance, achievements, and expectations of this field are discussed. The strategy of transient regulation of gel properties via CRNs is potentially useful for building the next generation of adaptive functional materials.

Organohydrogel Actuators with Adjustable Stimulus Responsiveness for On-Demand Morphing

Hydrogel actuators showing shape morphing in response to external stimuli are of significant interest for their applications in soft robots, artificial muscles, etc. However, there is still a lack of hydrogel actuators with adjustable stimulus responsiveness for on-demand driving. In this study, an organohydrogel actuator was prepared by a two-step interpenetrating method, resulting in the coexistence of poly(N-isopropylacrylamide-co-4-(2-sulfoethyl)-1-(4-vinylbenzyl) pyridinium betaine) (p(NIPAM-SVBP)) hydrophilic networks and poly(lauryl methacrylate) (pLMA) hydrophobic networks with gradient distribution. In the initial state, the organohydrogel actuator can be driven globally under thermal stimulation. Owing to the unique alkali-chromic performance of SVBP, the organohydrogel actuator can be endowed with photothermal properties and actuate locally under the stimulus of NIR light. More importantly, the organohydrogel will return to the original colorless state after being treated with acid solution. Our work provides a new insight into designing and fabricating novel actuators with adjustable stimulus responsiveness for on-demand morphing.

Remotely Controlled Light/Electric/Magnetic Multiresponsive Hydrogel for Fast Actuations

As a kind of soft smart material, hydrogel actuators have extensive development prospects, but it is still difficult for these actuators to integrate multiresponsiveness, multiple remote actuation, high strength, fast responsiveness, and programmable complex deformation. Herein, we have explored an anisotropic bilayer hydrogel actuator with an Fe3O4/co-poly(isopropylacrylamide-4-benzoylphenyl acrylate) [Fe3O4/P(NIPAM-ABP)] active layer and an isotropic conductive adhesive (ICAs) passive layer based on the layer-by-layer method. Benefiting from the fibrosis and porosity of the Fe3O4/P(NIPAM-ABP) hydrogel, the ICAs-Fe3O4/P(NIPAM-ABP) hydrogel actuator has excellent mechanical strength (tensile strength of 3.1 ± 0.3 MPa) and response speed (temperature (45 °C): bending speed of 2400.3°/s; near-infrared (NIR) light: bending speed of 356.4°/s; electricity (2 V): bending speed of 180°/s; water (10 °C): recovery speed of 30.0°/s). In addition, the good photothermal properties and magnetic conductivity of Fe3O4 nanoparticles provide precise remotely controllable light- and magnetic-actuated properties for the hydrogel actuator. The Ag microsheets with excellent conductivity (1.4 × 104 S/cm) provide remotely controllable electrical-actuated property for the hydrogel actuator. Combined with the responsiveness of P(NIPAM-ABP), the actuator can achieve short-range actuation including temperature-, ethanol-, and salt-responses. More importantly, it can achieve remote actuation including light, electrical, and magnetic responses. Finally, the Fe3O4/P(NIPAM-ABP) fibers can provide excellent anisotropic structures for the actuator to achieve precise deformational programmability. Inspired by some phenomena in nature, several actuating devices with the above characteristics have been successfully developed. This study can provide a general method for multifunctional anisotropic hydrogel actuators and will provide a new strategy for exploring smart materials suitable for complex bioinspired systems.

A UV-induced self-reinforced hydrogel based on in situ hydrophobic aggregation of strained 1,2-dithiolane rings

A novel responsive hydrogel exhibiting self-reinforcement and self-healing capacity was developed based on the hydrophobic aggregation of strained 1,2-dithiolane rings. Oligomerization of 1,2-dithiolane within hydrophobic domains under UV irradiation not only reinforced the hydrogel but also maintained its dynamic cross-linked nature by converting the intraring dynamic S-S bond to an outer one.

Chemical signal regulated injectable coacervate hydrogels

In the quest for stimuli-responsive materials with specific, controllable functions, coacervate hydrogels have become a promising candidate, featuring sensitive responsiveness to environmental signals enabling control over sol-gel transitions. However, conventional coacervation-based materials are regulated by relatively non-specific signals, such as temperature, pH or salt concentration, which limits their possible applications. In this work, we constructed a coacervate hydrogel with a Michael addition-based chemical reaction network (CRN) as a platform, where the state of coacervate materials can be easily tuned by specific chemical signals. We designed a pyridine-based ABA triblock copolymer, whose quaternization can be regulated by an allyl acetate electrophile and an amine nucleophile, leading to gel construction and collapse in the presence of polyanions. Our coacervate gels showed not only highly tunable stiffness and gelation times, but excellent self-healing ability and injectability with different sized needles, and accelerated degradation resulting from chemical signal-induced coacervation disruption. This work is expected to be a first step in the realization of a new class of signal-responsive injectable materials.

Autonomous Soft Robots Empowered by Chemical Reaction Networks

Hydrogel actuators are important for designing stimuli-sensitive soft robots. They generate mechanical motion by exploiting compartmentalized (de)swelling in response to a stimulus. However, classical switching methods, such as manually lowering or increasing the pH, cannot provide more complex autonomous motions. By coupling an autonomously operating pH-flip with programmable lifetimes to a hydrogel system containing pH-responsive and non-responsive compartments, autoonenomous forward and backward motion as well as more complex tasks, such as interlocking of "puzzle pieces" and collection of objects are realized. All operations are initiated by one simple trigger, and the devices operate in a "fire and forget" mode. More complex self-regulatory behavior is obtained by adding chemo-mechano-chemo feedback mechanisms. Due to its simplicity, this method shows great potential for the autonomous operation of soft grippers and metamaterials.

Temporary Actuation of Bilayer Polymer Hydrogels Mediated by the Enzymatic Reaction

Most soft actuators have the ability of monotonic responsiveness. That is, there is only one response action after being stimulated once. In this work, a temporarily responsive bilayer hydrogel actuator is designed and fabricated by combining a tertiary amine-containing pH-responsive layer and a urease-containing non-responsive layer. The hydrogel actuator can achieve programed deformation and recovery driven by chemical fuels (i.e., acidic urea solutions), which is essentially regulated by rapid acidification and slow enzymatic production of ammonia for recovering the pH of the system. The actuation extent and duration can be simply controlled by the fuel levels, and the repeated actuations are also possible via refueling. Furthermore, we fabricate several hydrogel devices that can display specific patterns or lift an item. This enzymatic method shows new possibilities to control the temporary actuation of polymer hydrogels potentially useful in many fields such as soft robotics, biomimetic devices, and environmental sensing.

Bioinspired Self-Resettable Hydrogel Actuators Powered by a Chemical Fuel

The movements of soft living tissues, such as muscle, have sparked a strong interest in the design of hydrogel actuators; however, so far, typical manmade examples still lag behind their biological counterparts, which usually function under nonequilibrium conditions through the consumption of high-energy biomolecules and show highly autonomous behaviors. Here, we report on self-resettable hydrogel actuators that are powered by a chemical fuel and can spontaneously return to their original states over time once the fuels are depleted. Self-resettable actuation originates from a chemical fuel-mediated transient change in the hydrophilicity of the hydrogel networks. The actuation extent and duration can be programmed by the fuel levels, and the self-resettable actuation process is highly recyclable through refueling. Furthermore, various proof-of-concept autonomous soft robots are created, resembling the movements of soft-bodied creatures in nature. This work may serve as a starting point for the development of lifelike soft robots with autonomous behaviors.

Bioinspired Hydrogels as Platforms for Life-Science Applications: Challenges and Opportunities

Hydrogels, as interconnected networks (polymer mesh; physically, chemically, or dynamic crosslinked networks) incorporating a high amount of water, present structural characteristics similar to soft natural tissue. They enable the diffusion of different molecules (ions, drugs, and grow factors) and have the ability to take over the action of external factors. Their nature provides a wide variety of raw materials and inspiration for functional soft matter obtained by complex mechanisms and hierarchical self-assembly. Over the last decade, many studies focused on developing innovative and high-performance materials, with new or improved functions, by mimicking biological structures at different length scales. Hydrogels with natural or synthetic origin can be engineered as bulk materials, micro- or nanoparticles, patches, membranes, supramolecular pathways, bio-inks, etc. The specific features of hydrogels make them suitable for a wide variety of applications, including tissue engineering scaffolds (repair/regeneration), wound healing, drug delivery carriers, bio-inks, soft robotics, sensors, actuators, catalysis, food safety, and hygiene products. This review is focused on recent advances in the field of bioinspired hydrogels that can serve as platforms for life-science applications. A brief outlook on the actual trends and future directions is also presented.

Perpetuating enzymatically-induced spatiotemporal pH and catalytic heterogeneity of a hydrogel by nanoparticles

The attainment of spatiotemporally inhomogeneous chemical and physical properties within a system is gaining attention across disciplines due to the resemblance to environmental and biological heterogeneity. Notably, the origin of natural pH gradients and how they have been incorporated in cellular systems is one of the most important questions in understanding the prebiotic origin of life. Herein, we have demonstrated a spatiotemporal pH gradient formation pattern on a hydrogel surface by employing two different enzymatic reactions, namely, the reactions of glucose oxidase (pH decreasing) and urease (pH increasing). We found here a generic pattern of spatiotemporal change in pH and proton transfer catalytic activity that was completely altered in a cationic gold nanoparticle containing hydrogel. In the absence of nanoparticles, the gradually generated macroscopic pH gradient slowly diminished with time, whereas the presence of nanoparticles helped to perpetuate the generated gradient effect. This behavior is due to the differential responsiveness of the interface of the cationic nanoparticle in temporally changing surroundings with increasing or decreasing pH or ionic contents. Moreover, the catalytic proton transfer ability of the nanoparticle showed a concerted kinetic response following the spatiotemporal pH dynamics in the gel matrix. Notably, this nanoparticle-driven spatiotemporally resolved gel matrix will find applicability in the area of the membrane-free generation and control of spatially segregated chemistry at the macroscopic scale.

This work reports perpetuating effect in enzymatically generated spatiotemporal pH gradient across a hydrogel in presence of cationic gold nanoparticle; showing a new route in spatially resolved chemistry in a membrane-free environment.