Hydrogel Actuators Research Articles

An Electrospinning Anisotropic Hydrogel with Remotely-Controlled Photo-Responsive Deformation and Long-Range Navigation for Synergist Actuation

As a soft/wet intelligent material, hydrogel actuators with multiple stimuli-responsiveness have been widely developed. However, it is still greatly difficult for them to integrate bi/multiple responsiveness together for bio-mimetic synergistic actuation. Here, we have explored a high-strength anisotropic bi-layer hydrogel actuator with P(NIPAM-ABP) layer and Fe3O4/PAN layer via electrospinning technique, which can provide programmable bi-functional synergistic movement. The Fe3O4/PAN layer can provide magnetic responsive navigation for long-range transportation on account of the magnetism of the Fe3O4 nanoparticles. Furthermore, the ultrahigh photothermal conversion efficiency of the Fe3O4 nanoparticles in the Fe3O4/PAN layer, can endow the P(NIPAM-ABP) layer with fast remotely-controlled photothermal-responsive deformation. Most importantly, this hydrogel actuator can achieve complex higher-level programmable movements than before based on the synergy of remotely-controlled deformation and the long-range transportation, which can be utilized to design various novel bio-mimetic soft-robots. Last but not least, the introduction of the electrospinning, not only can achieve high strength (4.59 MPa of tensile strength) of this bi-layer hydrogel, but also can provide both ultrafast (178°/s) and programmable complex photothermal-responsiveness, owing to the ultrahigh specific surface area (ultra-thin and porous structure) and excellent orientation of the thermal-responsive P(NIPAM-ABP) nanofibers respectively. This work will provide a general method via electrospinning for anisotropic hydrogel actuator with bi/multiple-functional synergy and will provide a new strategy for smart actuators and other bio-mimetic intelligent materials/systems.

Grayscale Stereolithography of Gradient Hydrogel with Site‐Selective Shape Deformation

AbstractHydrogels can convert external stimuli into shape deformation and mechanical motion, leading to the potential applications in intelligent devices and soft robotics. However, it is difficult to control the site‐selective shape deformation in a single hydrogel system in comparison with the traditional bilayer actuators with difference in structure or composition across the thickness. Here, a strategy to achieve the programmable shape deformation of hydrogel from a single precursor by grayscale stereolithography is proposed. The designed grayscale patterns can govern the spatially gradient crosslinking density of hydrogels, which leads to the heterogeneity in physicochemical performance, especially swelling ability. As a result, the swelling mismatch inducing the nonuniform internal stresses can drive the printed hydrogels to deform as the expected shape in three dimensions although there is no apparent asymmetric features and bilayer configuration. Grayscale light intensity and exposure time, as the digital printing codes can parametrically regulate the predictable shape deformation. More importantly, hydration and dehydration characterizations can endow the reversible feature of shape deformation hydrogel, benefiting to design the hydrogel actuators and sensitive devices. It is believed that integration of the advanced grayscale stereolithography together with the functional hydrogels uncovers a facile strategy to broad opportunities for sensors, soft robots, actuators, and so on.

Stimuli-responsive hydrogel sponge for ultrafast responsive actuator

With the ability to deform in response to specific stimuli, polymeric hydrogel actuators are important bionic materials. However, because shape deformation is derived from the diffusion of water molecules, the response rate of hydrogel actuators is usually slow, which severely limits their potential applications. In this work, a thermo-responsive PNIPAm hydrogel sponge is developed, and the hydrogel sponge shows ultrafast de-swelling/swelling capacity (equilibrium time 7 s) and large deformation degree (40%) due to the assistance of capillary force. Hydrogel sponge actuator with rapid response performance is further constructed, and the hydrogel sponge actuator could be used to capture moving objects and living creatures. Moreover, light-controlled directional movement can be achieved by incorporating photothermal functional components into the hydrogel sponge actuator. This work would promote the application of hydrogel actuators in soft robots.

Liquid metal enabled continuous flow reactor: A proof-of-concept

Liquid metal enabled continuous flow reactor: A proof-of-concept

Ion-responsive chitosan hydrogel actuator inspired by carrotwood seed pod

Inspired by the gradient hygroscopic structure of carrotwood seed pod, patterned anisotropic structure was created in polysaccharide hydrogel by an anodic electrical writing process. Locally released Fe2+ was oxidized to Fe3+ and chelated with chitosan chains in the written area, resulting in a gradient structure in the hydrogel. The asymmetrical stress generated by the different swelling of the gradient structure enables the hydrogel to bend autonomously. The hydrogel shows opposite bending in deionized water and NaCl solution. The physicochemical properties of the hydrogel are characterized by tensile test, SEM, EDS, XRD, TGA, DTG and FT-IR. SEM and EDS show that the written hydrogel has a structural gradient and a concentration gradient of Fe3+ vertically. Moreover, anodic electrical writing increases the flexibility of chitosan hydrogel due to decreased crystallinity. This controllable electrical writing technique is convenient to create patterned anisotropic structure and provide a novel design concept for natural hydrogel actuators.

Actuation of Hydrogel Architectures Prepared by Electrophoretic Adhesion of Thermoresponsive Microgels.

Owing to their unique properties, hydrogels may be used for preparing soft actuators. Soft actuators are expected to respond quickly; however, the response speed of gels is slow. To study this issue and overcome it, thermoresponsive soft actuators were prepared by the electrophoretic adhesion of cationic and anionic thermoresponsive microgels, comprising poly(diallyldimethylammonium chloride) and poly(styrenesulfonate) sodium salt, respectively. The kinetics of the prepared hydrogel architectures in response to temperature depended on the microgel diameter instead of the architecture size. We also prepared bilayered hydrogel architectures by adhesion of thermoresponsive and/or nonthermoresponsive microgels. These bent rapidly in response to temperature because these architectures consisted of microgel assemblies. In addition, specific bending motion was demonstrated by the adhesion of microgel layers of different sizes. The present study provides not only a guideline for the design of hydrogel actuators with quick response but also presents a method for the free-form fabrication of functional hydrogel materials that undergo complex motions in response to stimuli.

In-situ polymerization for mechanical strong composite actuators based on anisotropic wood and thermoresponsive polymer

Stimuli-responsive hydrogels hold an irreplaceable statue in intelligent actuation materials because of their reversible stretchability and excellent biocompatibility. However, the poor mechanical performance and complicated fabrication process of anisotropic structures severely limit their further applications. Herein, we report a high-strength thermoresponsive wood-PNIPAM composite hydrogel actuator with complex deformations, through a simple in-situ polymerization. In this composite hydrogel actuator, the anisotropic wood and the thermoresponsive PNIPAM hydrogel hydroel can work together to provide bending and even other complex deformations. Owing to strong interfacial interaction, this actuator perfectly realized the combination of good mechanical properties (∼1.1 MPa) and fast actuation speed (∼0.9 s). In addition, by adjusting the orientation direction of wood, this actuator can achieve various complex deformations. Such composite hydrogel actuator could be a good candidate for intelligent applications, such as intelligent actuators, smart valves, manipulators and even soft robots.

Strong, Ultrafast, Reprogrammable Hydrogel Actuators with Muscle-Mimetic Aligned Fibrous Structures

Hydrogel actuators displaying programmable shape transformations promise to be core components in future biomedical and soft robotic devices. However, current hydrogel actuators have shortcomings, including poor mechanical properties, slow response, and lack of shape reprogrammability, which limit their practical applications . Existing molecular designs offer limited efficiency in synergistically addressing these issues in a single hydrogel system. Herein, we propose a strategy to develop hydrogel actuators with muscle-mimetic aligned microfibrillar morphology, combining thermoinduced microphase separation and mechanical alignment. The key to our design is the introduction of metal–phenolic complexes, which not only induce irreversible sol–gel transition via the concentrated coordinate ions above lower critical solution temperature (LCST) but also fix the alignment of bundle network due to dynamic network rearrangement. Our design concept is observed to simultaneously achieve excellent mechanical properties (tensile strength ≈ 1.27 MPa, toughness ≈ 2.0 MJ m–3) and ultrafast actuation (40.1% thermal contraction as short as 1 s), which is a long-lasting challenge in the field. In addition, the dynamic hydrogels can be reprogrammed into spiral, helical, and biomimetic actuators. This work opens new opportunities to realize real-world applications for smart hydrogels as soft machines by fundamentally breaking the current property limit.

Rapid Photothermal Responsive Conductive MXene Nanocomposite Hydrogels for Soft Manipulators and Sensitive Strain Sensors.

Stimulus-responsive hydrogels are of great significance in soft robotics, wearable electronic devices, and sensors. Near-infrared (NIR) light is considered an ideal stimulus as it can trigger the response behavior remotely and precisely. In this work, a smart flexible stimuli-responsive hydrogel with excellent photothermal property and decent conductivity are prepared by incorporating MXene nanosheets into the physically cross-linked poly(N-isopropyl acrylamide) hydrogel matrix. Because of outstanding photothermal effect and dispersion of MXene, the composite hydrogel exhibits rapid photothermal responsiveness and excellent photothermal stability under the NIR irradiation. Furthermore, the anisotropic bilayer hydrogel actuator shows fast and controllable light-driven bending behavior, which can be used as a light-controlled soft manipulator. Meanwhile, the hydrogel sensor exhibits cycling stability and good durability in detecting various deformation and real-time human activities. Therefore, the present study involving the fabrication of MXene nanocomposite hydrogels for potential applications in remotely controlled actuator and wearable electronic device provides a new method for the development of photothermal responsive conductive hydrogels.

Biomimetic 4D‐Printed Breathing Hydrogel Actuators by Nanothylakoid and Thermoresponsive Polymer Networks

AbstractShape‐morphing actuators, which can breathe with the accompany of morphology changes to mimic botanical events, are challenging to fabricate with soft hydrogel materials. Herein, 4D printed‐smart hydrogel actuators are reported that can not only dynamically deform but also generate oxygen (O2) upon external stimulations. The printed breathing actuators featured with spinach leaf‐derived thylakoid membrane (nanothylakoid) for photothermal conversion and catalytical O2 evolution, a poly(N‐isopropylacrylamide) (PNIPA) thermoresponsive polymer network for generating deformation forces by swelling/shrinkage (rehydration/dehydration), and an asymmetric bilayer poly(N‐isopropylacrylamide)/polyacrylamide (PNIPA/PAA) structure to amplify the mechanical motions. Upon thermal stimulation or laser irradiation, the actuator can reversibly bend/unbend because of the photothermal conversion of nanothylakoid and the printed thermoresponsive asymmetric bilayer structure. Additionally, the catalase‐like property of nanothylakoid imparts the actuator with O2 evolution capability to breathe for further mimicking botanical systems. Notably, 4D printing can greatly facilitate and simplify the actuator fabrication process, including adjusting the size and layer compositions. This artificial breathing actuator with photothermal and catalytical properties provides a strategy in designing intelligent hydrogel systems and proves to be a highly promising material candidates in the fields of 3D/4D printing, automated robotics, and smart biomedical devices.

Drosera-Inspired Dual-Actuating Double-Layer Hydrogel Actuator.

Drosera is a small insectivorous plant whose antennae can fold up, encircle, and prey. The rapid movement of the antennae is achieved by the synergistic effect of a double-layer structure with the antennae contracts on the front and expands on the back. In this work, a drosera-inspired dual-actuating double-layer hydrogel actuator is proposed, in which the temperature-responsive poly(N, N-diethyl acrylamide) (PDEAAm) layer acts as the main actuation layer and a moisture-responsive poly(acrylamide) (PAAm) layer acts as the auxiliary actuation layer. In a water environment with low temperature, both the PAAm and PDEAAm layers absorb water and expand with a swelling property. When the temperature exceeds the lower critical solution temperature of PDEAAm, the PDEAAm layer undergoes a hydrophilic-hydrophobic transition and shrinks rapidly. Therefore, the synergistic effect of the double-layer hydrogel enables the double-layer hydrogel to achieve a large bending angle at high temperature. In addition, when designing and fabricating shape-patterned double-layer hydrogels, complex shape changes can be achieved. Due to the physical and chemical properties, the actuator can be used to grab, transport, and release objects. This drosera-inspired double-layer hydrogel actuator has high practical value, which may provide new insights for the design and manufacture of artificial intelligence materials.

Bioinspired Stimuli‐Responsive Hydrogel with Reversible Switching and Fluorescence Behavior Served as Light‐Controlled Soft Actuators

AbstractStimuli‐responsive materials with multiple functions occupy a significant position in the research of bionic artificial intelligence materials. It is a challenge to integrate rapid shape deformation and color change synergistically by a simple method in the same system. Herein, an effective strategy to develop an anisotropic bilayer hydrogel actuator with unified complex deformation and color change is proposed via combining the near‐infrared (NIR) light‐responsive graphene oxide/poly(N‐isopropylacrylamide) (GO‐PNIPAM) active hydrogel layer and an UV light‐responsive fluorescent poly(acrylic acid [4‐(1,2,2‐triphenylvinyl)] benzyl ester/N‐isopropylacrylamide) passive hydrogel layer. The actuator can undergo light‐controlled sapphire fluorescence color change and complex shape deformation simultaneously or separately under light irradiation. It can be remotely manipulated without the requirement for additional reagents, and the irradiation dosage can be accurately modulated through adjusting the irradiation parameters. This work provides new perspectives of intelligent systems with synergistic multiple functions, including smart robots, environmental sensors, and intelligent biomimetic devices.

Magnetically-propelled fecal surrogates for modeling the impact of solid-induced shear forces on primary colonic epithelial cells

The colonic epithelium is continuously exposed to an array of biological and mechanical stimuli as its luminal contents are guided over the epithelial surface through regulated smooth muscle contraction. In this report, the propulsion of solid fecal contents over the colonic epithelium is recapitulated through noninvasive actuation of magnetic agarose hydrogels over primary intestinal epithelial cultures, in contrast to the vast majority of platforms that apply shear forces through liquid microflow. Software-controlled magnetic stepper motors enable experimental control over the frequency and velocity of these events to match in vivo propulsive contractions, while the integration of standardized well plate spacing facilitates rapid integration into existing assay pipelines. The application of these solid-induced shear forces did not deleteriously affect cell monolayer surface coverage, viability, or transepithelial electrical resistance unless the device parameters were raised to a 50× greater contraction frequency and 4× greater fecal velocity than those observed in healthy humans. At a frequency and velocity that is consistent with average human colonic motility, differentiation of the epithelial cells into absorptive and goblet cell phenotypes was not affected. Protein secretion was modulated with a two-fold increase in luminal mucin-2 secretion and a significant reduction in basal interleukin-8 secretion. F-actin, zonula occludens-1, and E-cadherin were each present in their proper basolateral locations, similar to those of static control cultures. While cellular height was unaffected by magnetic agarose propulsion, several alterations in lateral morphology were observed including decreased circularity and compactness, and an increase in major axis length, which align with surface epithelial cell morphologies observed in vivo and may represent early markers of luminal exfoliation. This platform will be of widespread utility for the investigation of fecal propulsive forces on intestinal physiology, shedding light on how the colonic epithelium responds to mechanical cues.

Composite polymer hydrogels with high and reversible elongation under magnetic stimuli

The field of soft actuators is dominated by elastomers that experience mechanical deformations in response to external stimuli. In this context, magnetic stimuli attract considerable interest because of their easy application, tunability, fast response, remote actuation, and safe penetration in biological environments. Since very recently, research interests in the field are being redirected towards hydrogels, which could virtually replace elastomers, overcoming their limitations and expanding the field of application of soft actuators. The mechanical actuation of hydrogels is a nascent field full of challenges, such as achieving reliable and significant responsiveness. Here we demonstrate that the combination of a physical polymer hydrogel with a dispersed phase consisting of clusters of magnetic particles, results in magnetic hydrogel composites that exhibit high and reversible elongation in response to magnetic stimuli. Our analyses show that this response is strongly dependent on the matrix elasticity, the concentration of magnetic particles, and the particle distribution within the network of polymer nanofibres. Our strategy for the maximization of the response of magnetic hydrogels should be a catalyst for the development of novel applications of composite hydrogels, such as a valve remotely actuated by a magnetic field that we also present here as a proof-of-concept.

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.

Liquid Stratification and Diffusion‐Induced Anisotropic Hydrogel Actuators with Excellent Thermosensitivity and Programmable Functionality

Smart hydrogel actuators with programmable anisotropic structures present fascinating prospects considering their distinctive shape transformation and controllable environmental responsiveness under external stimuli. However, the design of anisotropic hydrogels with simple and universal fabrication and programmable functionality is challenging for their valuable applications in smart actuators and soft robots. Herein, a simple, green, and devisable strategy is proposed to construct a heterogeneous porous hydrogel system by the different liquid diffusion (such as amyl alcohol, water, and ethanol) into a monomeric precursor solution of thermosensitive hydrogels. The well‐defined micro/nanoporous gradient and patterned structures related to selective liquid stratification and interfacial diffusion favor the fast response and accurate programmable deformation of hydrogels under temperature stimuli. Inspiringly, this simple diffusion‐driven tactic can be perfectly applicable for different responsive hydrogels with programmable multifunctionality by adding functional nanomaterials into the diffusible liquid. This green, general, and facile diffusion‐driven strategy provides significant guidance for fabricating environmentally responsive hydrogels with tailorable functionality for their multipurpose applications in drug delivery, bioengineering, smart actuators, and soft robots.

Tracking Control of a Miniature 2-DOF Manipulator With Hydrogel Actuators

Due to the nature of the complex spatiotemporal dynamics of stimuli-responsive soft materials, closed-loop control of hydrogel-actuated mechanisms has remained a challenge. This letter demonstrates, for the first time, closed-loop trajectory tracking control in real-time of a millimeter-scale, two degree-of-freedom manipulator via independently-controllable, temperature-responsive hydrogel actuators. A linear state-space model of the manipulator is developed from input-output measurement data, enabling the straightforward application of control techniques to the system. The Normalized Mean Absolute Error (NMAE) between the modeled and measured displacement of the manipulator's tip is below 10%. We propose an Observer-based controller and a robust H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> -optimal controller and evaluate their performance in a trajectory tracking output-feedback framework, compared with and without sinusoidal disturbances and noise. We demonstrate in simulation that the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> -optimal controller, which is computed using Linear Matrix Inequality (LMI) methods, tracks an elliptical trajectory more accurately than the Observer controller and is more robust to disturbances and noise. We also show experimentally that the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> -optimal controller can be used to track different trajectories with an NMAE below 15%, even when the manipulator is subject to a 3 g load, 12.5 times an actuator's weight. Finally, a payload transport scenario is presented as an exemplar application; we demonstrate that an array of four manipulators is capable of moving a payload horizontally by applying the proposed H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> -optimal trajectory-tracking controller to each manipulator in a decoupled manner.

PEG-Induced Controllable Thin-Thickness Gradient and Water Retention: A Simple Way to Programme Deformation of Hydrogel Actuators.

Building the differential growth through the thickness is a promising and challenging approach to design the morphing structures of hydrogel actuators. Besides retaining the size of the hydrogel actuators under environmental stimuli still remains a big challenge. Herein, a facile and universal approach is developed to address both issues by introducing PEG during the polymerization of N-isopropylacrylamide (NIPAm) via one step method using asymmetric mold. Both composition gradient and pore gradient are obtained in micro level along the thickness direction of the final hydrogel, while thin-thickness gradient in macro level. The thickness gradient and water retention can be controllably adjusted by changing PEG concentration. The introduction of PEG effectively improves both responsive and non-shrunken performance by the interaction with PNIPAm. The resultant anisotropic PNIPAm/PEG hydrogel respond quickly and reach maximum deformation (360°) within 10 s at low temperature (40 °C).The various 3D shape and biomimetic movement can be programmed by simply controlling the PEG concentration and mold shape. This strategy can provide new insights into the design intelligent soft materials with 3D morphing for bioinspired and biomedical applications.

Smart Hydrogel Bilayers Prepared by Irradiation.

Environment-responsive hydrogel actuators have attracted tremendous attention due to their intriguing properties. Gamma radiation has been considered as a green cross-linking process for hydrogel synthesis, as toxic cross-linking agents and initiators were not required. In this work, chitosan/agar/P(N-isopropyl acrylamide-co-acrylamide) (CS/agar/P(NIPAM-co-AM)) and CS/agar/Montmorillonite (MMT)/PNIPAM temperature-sensitive hydrogel bilayers were synthesized via gamma radiation at room temperature. The mechanical properties and temperature sensitivity of hydrogels under different agar content and irradiation doses were explored. The enhancement of the mechanical properties of the composite hydrogel can be attributed to the presence of agar and MMT. Due to the different temperature sensitivities provided by the two layers of hydrogel, they can move autonomously and act as a flexible gripper as the temperature changes. Thanks to the antibacterial properties of the hydrogel, their storage time and service life may be improved. The as prepared hydrogel bilayers have potential applications in control devices, soft robots, artificial muscles and other fields.

Robust and rapid responsive organic-inorganic hybrid bilayer hydrogel actuators with silicon nanoparticles as the cross-linker

Hydrogel actuators belong to a type of smart soft materials which can take place anisotropic thrust and/or displacement under various environmental stimuli including thermal, light, pH, humidity etc. Robust strength and rapid response are of critical importance to high-performance hydrogel actuators. Here we report the syntheses and investigations of mechanical properties for series novel organic-inorganic hydrogels by using vinyl functionalized silicon nanoparticles (SiNPs) as the cross-linker. The doped SiNPs significantly enhance the mechanical strength of polyacrylamide (PAAm) hydrogels. Furthermore, the SiNPs composite poly-N-isopropylacrylamide (PNIPAm) hydrogels exhibit rapid response in deswelling and swelling behaviours. The excellent mechanical and responsive properties are attributed to the enhanced rigidity via SiNPs doping into the hydrogel network and the improved water diffusion through mPEG branch chains on the surface of nanoparticles, respectively. As a proof of concept, a bilayer PNIPAm@PAAm hydrogel actuator is facilely synthesized via an in-situ polymerization process. This composite hydrogel actuator shows extremely fast response rate of directional bending in hot water, about 6 times faster than the control without doping of SiNPs. This work demonstrates that these SiNPs composite hydrogels provide enhanced mechanical and responsive performances and may expand the future applications in smart hydrogel materials.