Hydrogel-based Actuators Research Articles

Thermoresponsive double-network hydrogel/elastomer hybrid bilayer laminates with high interfacial toughness and controllable shape deformations

Thermoresponsive hydrogel-based actuators have shown great potential in tissue engineering, biosensors and soft robots, while the preparation of temperature-driven bilayer hydrogel actuators with rapid responseness and recovery properties remains a challenge. Inspired by the layered heterostructure of human skin, we developed a Janus bilayer structure actuator consisting of thermoresponsive double network (TDN) hydrogel and polydimethylsiloxane (PDMS) elastomer with tough interface adhesion. Specifically, PNaAMPS/P(NIPAM-co-AAm) TDN hydrogel via photocopolymerization of N-isopropyl acrylamide (NIPAM) and acrylamide (AAm) and reinforced by microgel poly(2-acrylamide-2-methylpropane sulfonate sodium salt) (PNaAMPS) was in situ grafted onto benzophenone-activated PDMS surface, exhibiting strong interface adhesion (628 Jm-2 adhesion energy). The thermoresponsive shape deformations of the double-layer structure actuator were controllable by adjusting hydrogel/elastomer thickness ratio and introduction of metal ions, taking advantage of the different swelling and shrinkage characteristics of the hydrogel layer and the elastomer layer. The TDN/PDMS bilayer structure actuator, which integrates the "Janus" structure, thermal responsiveness and photothermal conversion, provides new ideas for the application and remote control of soft robots and soft actuators.

Preparation of Linear Actuators Based on Polyvinyl Alcohol Hydrogels Activated by AC Voltage.

Currently, the preparation of actuators based on ionic electroactive polymers with a fast response is considered an urgent topic. In this article, a new approach to activate polyvinyl alcohol (PVA) hydrogels by applying an AC voltage is proposed. The suggested approach involves an activation mechanism in which the PVA hydrogel-based actuators undergo extension/contraction (swelling/shrinking) cycles due to the local vibration of the ions. The vibration does not cause movement towards the electrodes but results in hydrogel heating, transforming the water molecules into a gaseous state and causing the actuator to swell. Two types of linear actuators based on PVA hydrogels were prepared, using two types of reinforcement for the elastomeric shell (spiral weave and fabric woven braided mesh). The extension/contraction of the actuators, activation time, and efficiency were studied, considering the PVA content, applied voltage, frequency, and load. It was found that the overall extension of the spiral weave-reinforced actuators under a load of ~20 kPa can reach more than 60%, with an activation time of ~3 s by applying an AC voltage of 200 V and a frequency of 500 Hz. Conversely, the overall contraction of the actuators reinforced by fabric woven braided mesh under the same conditions can reach more than 20%, with an activation time of ~3 s. Moreover, the activation force (swelling load) of the PVA hydrogels can reach up to 297 kPa. The developed actuators have broad applications in medicine, soft robotics, the aerospace industry, and artificial muscles.

Sponge-like, semi-interpenetrating self-sensory hydrogel for smart photothermal-responsive soft actuator with biomimetic self-diagnostic intelligence

Actuators based on hydrogels are good candidates for biomimetic soft robots due to their excellent mechanical properties and flexibility. Despite great efforts in hydrogel-based actuators, their actuation process still differs from the “recognition-judgment-execution” process that conscious intelligent beings go through owing to the lack of perceptive capacity. Herein, to imitate the intelligent actuation process of conscious organisms, a fast photo-thermal responsive and self-sensory soft actuator is proposed based on sponge-like hydrogel with semi-interpenetrating (semi-IPN) polymer network of thermosensitive poly (N-isopropylacrylamide) (PNIPAM) and electronically conductive polyaniline (PANI). The proposed actuator exhibits excellent photo-thermal responsiveness enabling remotely triggered actuation controlled by near infrared light (NIR), covering all azimuths with ultrafast response rate (within 2 s) and recover rate (within 1 s). Meanwhile, the conformational transition of PNIPAM networks during the course of actuation also lead to accompanying migration of the interpenetrated PANI chains and significant variation of local conductivity (>15%), endowing the actuator with self-sensing capacity to monitor its deformations. Eventually, a smart self-adaptive actuator is designed inspired by neuromuscular in intelligent organisms, which can actively perceive external environmental stimuli and consciously regulate its own shape transformation by monitoring and analyzing electrical signal. This sensory actuating hydrogel can provide insights into the development of artificial intelligent soft robotics with higher levels of autonomy and complexity.

Anisotropic aerogel induced gelatin hydrogel actuator with favourable mechanical properties and multiple solvent responsiveness

As an important part of biomimetic/intelligent new materials, the hydrogel actuator can sense changes in the external environment and convert external energy into mechanical energy, therefore attracting extensive attention in many fields. However, the relative weak mechanical properties, small driving force and single responsiveness to external stimuli of hydrogel-based actuators are still the main factor restricting their practical application and future development. In this paper, a heterogeneous hydrogel-based actuator with favourable mechanical properties and multiple solvent responsiveness is constructed via the synergistic action of anisotropic chitosan aerogel as the skeleton and solvent-sensitive gelatin hydrogel as muscle in the actuator framework. The addition of aerogel skeleton and the salting out of gelatin favor the actuator favourable mechanical behavior with maximum tensile stress and compressive stress up to MPa level. Furthermore, the heterogeneous aerogel/hydrogel composite structure greatly improves the driving capability of actuator, presenting multiple solvent responsiveness with the maximum driving amplitude up to 730° in the ion solution. The well-designed actuators can be exploried as intelligen microfluidic valves that recognize and pass specific solutions at different flow rates. This aerogel/hydrogel composite actuator has broad application prospects in microfluidic systems, smart valves, artificial muscles and intelligent human-machine interaction.

An Anisotropic Hydrogel by Programmable Ionic Crosslinking for Sequential Two-Stage Actuation under Single Stimulus

As one of the most important anisotropic intelligent materials, bi-layer stimuli-responsive actuating hydrogels have proven their wide potential in soft robots, artificial muscles, biosensors, and drug delivery. However, they can commonly provide a simple one-actuating process under one external stimulus, which severely limits their further application. Herein, we have developed a new anisotropic hydrogel actuator by local ionic crosslinking on the poly(acrylic acid) (PAA) hydrogel layer of the bi-layer hydrogel for sequential two-stage bending under a single stimulus. Under pH = 13, ionic-crosslinked PAA networks undergo shrinking (-COO-/Fe3+ complexation) and swelling (water absorption) processes. As a combination of Fe3+ crosslinked PAA hydrogel (PAA@Fe3+) with non-swelling poly(3-(1-(4-vinylbenzyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate) (PZ) hydrogel, the as-prepared PZ-PAA@Fe3+ bi-layer hydrogel exhibits distinct fast and large-amplitude bidirectional bending behavior. Such sequential two-stage actuation, including bending orientation, angle, and velocity, can be controlled by pH, temperature, hydrogel thickness, and Fe3+ concentration. Furthermore, hand-patterning Fe3+ to crosslink with PAA enables us to achieve various complex 2D and 3D shape transformations. Our work provides a new bi-layer hydrogel system that performs sequential two-stage bending without switching external stimuli, which will inspire the design of programmable and versatile hydrogel-based actuators.

Heterogeneity Regulation of Bilayer Polysaccharide Hydrogels for Integrating pH- and Humidity-Responsive Actuators and Sensors

Bilayer hydrogel-based actuators have attracted much interest because inhomogeneous structures are easily constructed in hydrogels. We used three kinds of polysaccharides, including anionic carboxymethyl cellulose (CMC), cationic chitosan (CS), and amphoteric carboxymethyl chitosan (CMCS), as both structure-constructing units and actuation-controlling units in this work to fabricate physically crosslinked poly(vinyl alcohol) bilayer hydrogels. The spatial heterogeneity was tuned by changing the types and concentrations of polysaccharides in different layers, to regulate pH- and humidity-driven actions of bilayer hydrogels. Based on the distortion of the ionic channel during the humidity-motivated deformation of bilayer hydrogels, a two-in-one flexible device integrating a humidity-driven actuator and humidity-responsive sensor was then developed, which could detect the alterations of environmental humidity in real time. Moreover, good tensile toughness and interfacial bonding as well as the strain-resistance effect endowed the bilayer hydrogels with the capability of identifying human motion as a strain sensor, unlocking more application scenarios. This work provides an overall insight into the heterogeneity regulation of bilayer hydrogels using polysaccharides as stimulus-responsive units and also proposes an interesting strategy of manufacturing hydrogel-based flexible devices with both actuating and sensing capabilities.

A Biomimetic Bilayer Hydrogel Actuator Based on Thermoresponsive Gelatin Methacryloyl-Poly(N-isopropylacrylamide) Hydrogel with Three-Dimensional Printability.

Development of hydrogel-based actuators with programmable deformation is an important topic that arouses much attention in fundamental and applied research. Most of these actuators are nonbiodegradable or work under nonphysiological conditions. Herein, a temperature-responsive and biodegradable gelatin methacryloyl (GelMA)-poly(N-isopropylacrylamide) hydrogel (i.e., GN hydrogel) network was explored as the active layer of a bilayer actuator. Small-angle X-ray scattering (SAXS) revealed that the GN hydrogel formed a mesoglobular structure (∼230 Å) upon a thermally induced phase transition. Rheological data supported that the GN hydrogel possessed 3D printability and tunable mechanical properties. A bilayer hydrogel actuator composed of active GN and passive GelMA layers was optimized by varying the layer thickness and compositions to achieve large, reproducible, and anisotropic bending with a curvature of ∼5.5 cm-1. Different patterns of the active layer were designed for actuation in programmable control. The 3D printed GN hydrogel constructs showed significant volume reduction (∼25-60% depending on construct design) at 37 °C with the resolution enhanced by the thermo-triggered actuation, while they were able to fully reswell at room temperature. A more intricate 3D printed butterfly actuator demonstrated the ability to mimic the wing movement through thermoresponsiveness. Furthermore, myoblasts laden in the GN hydrogel exhibited significant proliferation of ∼376% in 14 days. This study provides a new fabrication approach for developing biomimetic devices, artificial muscles, and soft robotics for biomedical applications.

A Hydrogel-Based Self-Sensing Underwater Actuator.

Soft robots made of hydrogels are suited for underwater exploration due to their biocompatibility and compliancy. Yet, reaching high dexterity and actuation force for hydrogel-based actuators is challenging. Meanwhile, real-time proprioception is critical for feedback control. Moreover, sensor integration to mimic living organisms remains problematic. To address these challenges, we introduce a hydrogel actuator driven by hydraulic force with a fast response (time constant 0.83 s). The highly stretchable and conductive hydrogel (1400% strain) is molded into the PneuNet shape, and two of them are further assembled symmetrically to actuate bi-directionally. Then, we demonstrate its bionic application for underwater swimming, showing 2 cm/s (0.19 BL/s) speed. Inspired by biological neuromuscular systems’ sensory motion, which unifies the sensing and actuation in a single unit, we explore the hydrogel actuator’s self-sensing capacity utilizing strain-induced resistance change. The results show that the soft actuator’s proprioception can monitor the undulation in real-time with a sensitivity of 0.2%/degree. Furthermore, we take a finite-element method and first-order differential equations to model the actuator’s bending in response to pressure. We show that such a model can precisely predict the robot’s bending response over a range of pressures. With the self-sensing actuator and the proposed model, we expect the new approach can lead to future soft robots for underwater exploration with feedback control, and the underlying mechanism of the undulation control might offer significant insights for biomimetic research.

PH-Responsive Hydrogel Bilayer With Reversible, Bidirectional Bending Behavior

Smart hydrogels are promising materials for shape-shifting structures regarding their large reversible deformation in response to external stimuli in the absence of mechanical loading. Actuators composed of responsive hydrogels have gained significant attention due to their low power consumption, bio-compatibility, fast response, and accessibility. Among these structures, bidirectional hydrogel-based actuators are more fascinating, especially when they have similar reversible bending in both directions. This paper introduces a new design concept of a hydrogel bilayer made of a poly (HEMA-co-DMAEMA) layer and a poly (HEMA-co-AA) hydrogel layer that swells at low and high pH, respectively. This structure is capable of bending in diverse directions while the pH of the aqueous bath alters. The main characteristic of this structure is having reversible bidirectional bending, which has similar behaviors in both directions, unlike previous hydrogel-elastomer bilayers. Then, we develop an analytical method to solve the swelling-induced bidirectional bending of a pH-sensitive hydrogel bilayer. On the other hand, the finite bending of bilayer structure is studied by the finite element method in several cases to demonstrate the validity and accuracy of the proposed analytical solution. Lastly, the impacts of material composition and geometrical factors are investigated to be used for bilayer actuator design and application.

Hydrogel-based strong and fast actuators by electroosmotic turgor pressure.

Hydrogels are promising as materials for soft actuators because of qualities such as softness, transparency, and responsiveness to stimuli. However, weak and slow actuations remain challenging as a result of low modulus and osmosis-driven slow water diffusion, respectively. We used turgor pressure and electroosmosis to realize a strong and fast hydrogel-based actuator. A turgor actuator fabricated with a gel confined by a selectively permeable membrane can retain a high osmotic pressure that drives gel swelling; thus, our actuator exerts large stress [0.73 megapascals (MPa) in 96 minutes (min)] with a 1.16 cubic centimeters of hydrogel. With the accelerated water transport caused by electroosmosis, the gel swells rapidly, enhancing the actuation speed (0.79 MPa in 9 min). Our strategies enable a soft hydrogel to break a brick and construct underwater structures within a few minutes.

Programmable Morphing Hydrogels for Soft Actuators and Robots: From Structure Designs to Active Functions.

Nature provides abundant inspiration and elegant paradigms for the development of smart materials that can actuate, morph, and move on demand. One remarkable capacity of living organisms is to adapt their shapes or positions in response to stimuli. Programmed deformations or movements in plant organs are mainly driven by water absorption/dehydration of cells, while versatile motions of mollusks are based on contraction/extension of muscles. Understanding the general principles of these morphing and motion behaviors can give rise to disruptive technologies for soft robotics, flexible electronics, biomedical devices, etc. As one kind of intelligent material, hydrogels with high similarity to soft biotissues and diverse responses to external stimuli are an ideal candidate to construct soft actuators and robots.The objective of this Account is to give an overview of the fundamental principles for controllable deformations and motions of hydrogels, with a focus on the structure designs and responsive functions of the corresponding soft actuators and robots. This field has been rapidly developed in recent years with a growing understanding of working principles in natural organisms and a substantial revolution of manufacturing technologies to devise bioinspired hydrogel systems with desired structures. Diverse morphing hydrogels and soft actuators/robots have been developed on the basis of several pioneering works, ranging from bending and folding deformations of bilayer hydrogels to self-shaping of non-Euclidean hydrogel surfaces, and from thermoactuated bilayer gel "hands" to electrodriven polyelectrolyte gel "worms". These morphing hydrogels have demonstrated active functions and versatile applications in biomedical and engineering fields.In this Account, we discuss recent progress in morphing hydrogels and highlight the design principles and relevant applications. First, we introduce the fundamentals of basic deformation modes, together with generic structure features, actuation strategies, and morphing mechanisms. The advantages of in-plane gradient structures are highlighted for programmable deformations by harnessing the out-of-plane buckling with bistability nature to obtain sophisticated three-dimensional configurations. Next, we give an overview of soft actuators and robots based on morphing hydrogels and focus on the working principles of the active systems with different structure designs. We discuss the advancements of hydrogel-based soft robots capable of swift locomotion with different gaits and emphasize the significances of structure control and dynamic actuation. Then we summarize versatile applications of hydrogel-based actuators and robots in biomedicines, cargo delivery, soft electronics, information encryption, and so forth. Some hydrogel robots with a built-in feedback loop and self-sensing system exhibit collaborative functions and advanced intelligence that are informative for the design of next-generation hydrogel machines. Finally, concluding remarks are given to discuss future opportunities and remaining challenges in this field. For example, miniature hydrogel-based actuators/robots with therapeutic or diagnostic functions are highly desired for biomedical applications. The morphing mechanisms summarized in this Account should be applicable to other responsive materials. We hope that this Account will inspire more scientists to be involved in this emerging area and make contributions to reveal novel working principles, design multifunctional soft machines, and explore applications in diverse fields.

Bioinspired bilayer hydrogel-based actuator with rapidly bidirectional actuation, programmable deformation and devisable functionality

Smart hydrogel actuators which respond to external stimuli have attracted great attention and shown good application prospects in many fields as their simplicity and reliability, while the challenges remain in terms of the relatively low driving force, responsive velocity and deformation amplitude considering the structure-dependent actuation behavior. Here, we propose a thermosensitive hydrogel-based actuator with bioinspired bilayer structure by applying a textured airlaid paper to induce the formation of heterogeneous construction. This nature-inspired design benefits the hydrogel-based actuator rapidly bidirectional actuation, realizing excellent bending velocity (140.6° s−1 within first 5 s) and bending amplitude (850.0° within 30 s) in hot water, powerful recovery in cool water and remarkable reproducibility upon external environmental stimulus by optimizing the texture orientation and mechanical property of airlaid paper. The excellent bonding between airlaid paper and hydrogel also endows the actuator satisfactory stability for circularly operation under water environment for weeks. The programmable deformation and devisable functionality are further realized by propagable and elaborate design for architecture and component of responsive hydrogel, favoring the hydrogel-based actuator alterable ethanol-sensitive ability, photothermal drivability and regenerability. This bilayer hydrogel-based actuator provides promising potentials for the applications in soft robots, smart actuators, artificial muscles, and intelligent human-machine technologies.

Soft Hydrogel Actuator for Fast Machine-Learning-Assisted Bacteria Detection.

We demonstrate that our bio-electrochemical platform facilitates the reduction of detection time from the 3-day period of the existing tests to 15 min. Machine learning and robotized bioanalytical platforms require the principles such as hydrogel-based actuators for fast and easy analysis of bioactive analytes. Bacteria are fragile and environmentally sensitive microorganisms that require a special environment to support their lifecycles during analytical tests. Here, we develop a bio-electrochemical platform based on the soft hydrogel/eutectic gallium-indium alloy interface for the detection of Streptococcus thermophilus and Bacillus coagulans bacteria in various mediums. The soft hydrogel-based device is capable to support bacteria' viability during detection time. Current-voltage data are used for multilayer perceptron algorithm training. The multilayer perceptron model is capable of detecting bacterial concentrations in the 104 to 108 cfu/mL range of the culture medium or in the dairy products with high accuracy (94%). Such a fast and easy biodetection is extremely important for food and agriculture industries and biomedical and environmental science.

Biomimetic organohydrogel actuator with high response speed and synergistic fluorescent variation

Current hydrogel-based actuators have achieved rapid development due to their excellent performance such as shape-morphing and color-changing for application in fields such as camouflage, biomimetic soft-robotics and so on. However, it is still challenging to fabricate soft robots with the capability of simultaneous changes in shape and fluorescent color at a fast speed when triggered by one single stimulus. Herein, an anisotropic organohydogel actuator made up of rGO-doped hydrophilic poly(N-isopropylacrylamide) (PNIPAM) network and hydrophobic poly(lauryl methacrylate) (PLMA) network is prepared via a two-step interpenetrating method. Bearing fluorescent monomer N-(4-(1,2,2-triphenylvinyl)phenyl)acrylamide (ATPE) as well as fluorescent ligand 6-acrylamidopicolinic acid (6APA), the PLMA network shows fluorescent changes in color or brightness depending on the presence or absence of Eu3+ ions in response to heat/NIR. In a word, the proposed organohydrogel actuator, which exhibits simultaneous fluorescence color variation and fast morphing in response to one stimulus, provides insights in designing and fabricating novel soft robots.

Two-Dimensional MoO2 Nanosheet Composite Hydrogels with High Transmittance and Excellent Photothermal Property for Near-Infrared Responsive Actuators and Microvalves.

Stimulus-responsive intelligent hydrogel actuators have highly promising applications in the fields of soft robotics, smart manipulators, and flexible devices. Near-infrared (NIR) light is considered an ideal method to trigger the response behavior remotely and precisely. In order to realize the excellent optical transmittance and photothermal property of NIR-responsive hydrogels at the same time, two-dimensional nonlayered MoO2 nanosheets (2D-MoO2) with excellent photothermal efficiency (62% under an NIR light irradiation of 808 nm), splendid chemistry stability, and low preparation cost are used as photothermal agents and incorporated into the poly(N-isopropylacrylamide) (PNIPAM) hydrogel network, forming the 2D-MoO2/Laponite/PNIPAM ternary nanocomposite hydrogel (TN hydrogel). It is remarkable that compared with the GO and MXene hydrogels with the same agent content (1.0 mg mL-1) and thickness (1 mm) whose transmittance values are only ∼5% at 600 nm, the TN hydrogel shows a similar NIR-responsive temperature, but much higher optical transmittance (∼53%). Besides, of the three hydrogels with similar transmittance, the TN hydrogel shows a much higher NIR-responsive temperature. The TN hydrogel with a low loading of 2D-MoO2 (1.5 mg mL-1) can produce a significant temperature increase of ∼30 °C after the application of 0.8 W cm-2 NIR light irradiation for 15 s. Impressively, the TN hydrogel exhibits excellent anti-fatigue property, keeping a fast response and temperature rise behavior even after 50 times of heating-cooling cycles. The flexibly controllable and reversible deformation is realized by a well-designed bilayer structure even in harsh environments. The transparent and asymmetric bilayer hydrogel is further used as a soft manipulator to capture objects visually and accurately. The NIR light-controlled microvalve based on this composite hydrogel is also demonstrated. This work provides a novel kind of transparent hybrid NIR response hydrogel for the further development of smart, programmable, reversible hydrogel-based actuators and soft robotics.

3D Printing Hydrogel-Based Soft and Biohybrid Actuators: A Mini-Review on Fabrication Techniques, Applications, and Challenges.

Stimuli-responsive hydrogels are candidate building blocks for soft robotic applications due to many of their unique properties, including tunable mechanical properties and biocompatibility. Over the past decade, there has been significant progress in developing soft and biohybrid actuators using naturally occurring and synthetic hydrogels to address the increasing demands for machines capable of interacting with fragile biological systems. Recent advancements in three-dimensional (3D) printing technology, either as a standalone manufacturing process or integrated with traditional fabrication techniques, have enabled the development of hydrogel-based actuators with on-demand geometry and actuation modalities. This mini-review surveys existing research efforts to inspire the development of novel fabrication techniques using hydrogel building blocks and identify potential future directions. In this article, existing 3D fabrication techniques for hydrogel actuators are first examined. Next, existing actuation mechanisms, including pneumatic, hydraulic, ionic, dehydration-rehydration, and cell-powered actuation, are reviewed with their benefits and limitations discussed. Subsequently, the applications of hydrogel-based actuators, including compliant handling of fragile items, micro-swimmers, wearable devices, and origami structures, are described. Finally, challenges in fabricating functional actuators using existing techniques are discussed.

Tough hydrogels for soft artificial muscles

Mimicking the natural actuating systems artificially provides ample opportunities in the field of soft robotics. In this domain, recent advancements in tough hydrogels along with advanced manufacturing techniques have offered us new tools for creating hybrid and soft artificial muscles that resemble the attributes of natural muscles and biological organisms. This review article provides a summary of the recent developments in the field of hydrogel-based actuators and soft artificial muscles and presents a current account of their emerging applications. Further, the fundamentals behind the actuation of hydrogels are discussed with a focus on hydrogels swelling, diffusion, and mechanical behaviour. The design details of different tough hydrogel systems are highlighted, and their physical and mechanical properties are quantitatively compared with biological tissues. Finally, a brief conclusion is provided with remarks on the remaining challenges in this field.

Highly Bidirectional Bendable Actuator Engineered by LCST-UCST Bilayer Hydrogel with Enhanced Interface.

Thermoresponsive hydrogel-based actuators are highly important for fundamental research and industrial applications, while the preparation of temperature-driven bilayer hydrogel actuators with rapid response to bend and recover properties remains a challenge. To date, most temperature-driven bilayer hydrogel actuators are based on polymers only with a lower critical solution temperature (LCST) or with an upper critical solution temperature (UCST), which need more time to bend and recover just in a small range of bending angle. Herein, we propose a new strategy to design and synthesize a fully temperature-driven bilayer hydrogel actuator, which consists of a poly(N-acryloyl glycinamide) (NAGA) layer with a UCST-type volume phase change and a poly(N-isopropyl acrylamide) (NIPAM)-Laponite nanocomposite layer with an LCST-type volume phase change. Due to the complementary UCST and LCST behavior of the two selected polymers, both layers have opposite thermoresponsive swelling and shrinkage properties at low and high temperatures; this imbues the hydrogel actuator with rapid thermoresponsive bending and recovery ability, as well as a large bending angle. In addition, the incorporation of Laponite nanosheets in PNIPAM layer not only improves the mechanical property of actuators but also provides the excellent bonding ability of the two-layer interface, which prevents delamination caused by excessive local stress on the interface during the bending process. Thanks to high-performance behavior, the actuator can act as an effective and sensitive actuator, such as a gripper to capture, transport, and release an object, or as an electrical circuit switch to turn on and off a light-emitting diode (LED). Overall, such hydrogel actuator may provide new insights for the design and fabrication of artificial intelligence materials.

Actuators assembled from hydrogel blocks of various shapes via condensation reactions

Hydrogel-based actuators have attracted much attention and shown promising applications in many fields. However, it remains a challenge to assemble two hydrogel sheets into a double-layer actuator due to their slippery and non-adhesive nature. Herein, we present a simple and reliable strategy that allows two hydrogel sheets to be bonded together well with chemical bonds formed in condensation reactions, only requiring interfacial contact and then heat-treatment. The results shown that the condensation reactions can occur under heat-treatment and that the two hydrogel layers have good interfacial bonding. The double-layer strip hydrogels prepared in this method showed different deformation rate and bending extent, which can be well tuned by changing internal structures and external stimuli. Complex and controllable three-dimensional shapes of the assembled blocks can be obtained through well-tailored structures. A four-arm gripper prepared in this approach can implement the grasping and releasing an object quickly with different stimuli. This work provides a facile approach in achieving hydrogel bonding and preparing intelligent double-layer soft actuators.

Homogeneous Chitosan/Graphene Oxide Nanocomposite Hydrogel-Based Actuator Driven by Efficient Photothermally Induced Water Gradients

Various kinds of photoresponsive hydrogels have been developed for years; however, the complex synthesis method and high cost limit their broad application. In the current work, a facile strategy f...