Biomimetic Color-Changing Hierarchical and Gradient Hydrogel Actuators Based on Salt-Induced Microphase Separation

Solvent-Triggered, Ultra-Adhesive, Conductive, and Biocompatible Transition Gels for Wearable Devices

The development of robust adhesive, conductive, and flexible materials has garnered significant attention in the realm of human-machine interface and electronic devices. Conventional preparation methods to achieve these exceptional properties rely on incorporating highly polar raw materials, multiple components, or solvents. However, the overexposure of functional groups and the inherent toxicity of organic solvents often render gels non-stick or potentially biocompatible making them unsuitable for human-contact devices. In this study, a straightforward three-step strategy is devised for preparing responsive adhesive gels without complex components. Structurally conductive poly(N-(2-hydroxyethyl)-acrylamide-co-p-styrene sulfonate hydrate) (PHEAA-NaSS) gels are synthesized by integrating ionic and hydrophilic networks with distinct solvent effects. Initially, the in-suit formed PHEAA-NaSS networks are activated by dimethyl sulfoxide, which substantially increases intramolecular hydrogen bonding and enhances the matrix stretchability and interfacial adhesion. Subsequently, ethanol exchange reduced solvent impact and led to a compact network that limited surface exposure of ionic and hydrophilic groups, resulting in nonstick, robust for convenient storage. Finally, upon contacting with water, the network demonstrates rehydration, resulting in favorable adhesion, biocompatibility, and conductivity. The proposed PHEAA-NaSS/W gels can stably and reliably capture joint motion and electrophysiological signals. Furthermore, this uncomplicated gel preparation method is also applicable to other electrolyte monomers.

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

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

Bionic ordered structured hydrogels: structure types, design strategies, optimization mechanism of mechanical properties and applications

Natural organisms, such as lobsters, lotus, and humans, exhibit exceptional mechanical properties due to their ordered structures. However, traditional hydrogels have limitations in their mechanical and physical properties due to their disordered molecular structures when compared with natural organisms. Therefore, inspired by nature and the properties of hydrogels similar to those of biological soft tissues, researchers are increasingly focusing on how to investigate bionic ordered structured hydrogels and render them as bioengineering soft materials with unique mechanical properties. In this paper, we systematically introduce the various structure types, design strategies, and optimization mechanisms used to enhance the strength, toughness, and anti-fatigue properties of bionic ordered structured hydrogels in recent years. We further review the potential applications of bionic ordered structured hydrogels in various fields, including sensors, bioremediation materials, actuators, and impact-resistant materials. Finally, we summarize the challenges and future development prospects of bionic ordered structured hydrogels in preparation and applications.

Edible Origami Actuators Using Gelatin-Based Bioplastics

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

Photoresponsive hydrogel-based soft robot: A review

Soft robots have received a lot of attention because of their great human-robot interaction and environmental adaptability. Most soft robots are currently limited in their applications due to wired drives. Photoresponsive soft robotics is one of the most effective ways to promote wireless soft drives. Among the many soft robotics materials, photoresponsive hydrogels have received a lot of attention due to their good biocompatibility, ductility, and excellent photoresponse properties. This paper visualizes and analyzes the research hotspots in the field of hydrogels using the literature analysis tool Citespace, demonstrating that photoresponsive hydrogel technology is currently a key research direction. Therefore, this paper summarizes the current state of research on photoresponsive hydrogels in terms of photochemical and photothermal response mechanisms. The progress of the application of photoresponsive hydrogels in soft robots is highlighted based on bilayer, gradient, orientation, and patterned structures. Finally, the main factors influencing its application at this stage are discussed, including the development directions and insights. Advancement in photoresponsive hydrogel technology is crucial for its application in the field of soft robotics. The advantages and disadvantages of different preparation methods and structures should be considered in different application scenarios to select the best design scheme.

Amphibious Nastic Hydrogel Based on the Tropic Movement of Gelatin and Its Opposite Phase Transition to PNIPAm

Mimicking the anisotropic structure and environmental adaptation of organisms in nature remains a key objective in the field of hydrogels. However, it has been very challenging due to complex fabrication and confined application only in water. Here, we demonstrate a new strategy of spontaneous fabrication of an anisotropic hydrogel based on our finding in the tropic movement of gelatin toward the Teflon template. The obtained hydrogel exhibits fast response and recovery under temperature stimuli both in aqueous and non-aqueous environments, making use of the approximate transition temperature and opposite phase transition behavior of gelatin and poly(N-isopropylacrylamide) (PNIPAm). Its recovery performance in water is more than 50 times faster than that of the PNIPAm hydrogel. Furthermore, the PNIPAm/gelatin hydrogel can achieve 3D complex deformations, stealth deformation, erasable and reprogrammed surface patterning, and multistage encryption by simply modulating the location and shape of gelatin to achieve an anisotropic structure. The work provides a simple and versatile way to obtain an anisotropic hydrogel with a definite and predictable structure, which is demonstrated across a range of different monomers. It improves the responsive performance and broadens the hydrogel application to the non-aqueous environment. Additionally, this tropic movement of gelatin can be extended for the design of new types of anisotropic materials and thus endows the materials with diverse functionality.

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.

Fabrication of Temperature- and Alcohol-Responsive Photonic Crystal Hydrogel and Its Application for Sustained Drug Release

Herein, crack-free photonic crystal templates with enhanced color contrast were first demonstrated by the coassembly of polystyrene (PS) microspheres and graphene oxide (GO). Then, photonic crystal hydrogels (PCHs) with quick responses to temperature and alcohol solution concentration changes were fabricated by photopolymerization of monomers in the gaps of the self-assembled colloidal crystal templates. The structural color of the PCHs changed from yellow to blue within 120 s as the temperature rose from 25 to 40 °C, whereas upon a decrease in temperature from 40 to 25 °C, the structural color changed from blue to yellow. The structural color of the PCHs also shows an obvious response with the concentration of alcohol solution ranging from 40 to 100 wt %. The quick responses of the PCHs' structural color to changes in temperature and alcohol solution concentration are attributed to the temperature sensitivity of poly(N-isopropylacrylamide) and preferential adsorption and swelling of the alcohol solution for the polymer chains. Furthermore, moxifloxacin (Mox) was loaded into PCHs by hydrogel swelling and exhibited sustained released by increasing the temperature. The sustained release process was facilely monitored by observing the corresponding color changes in real time. The rapid and visible response offers the fabricated PCHs great potential application prospects in the semiquantitative analysis of alcohol concentration and intelligent drug delivery.

One-step programmable electrofabrication of chitosan asymmetric hydrogels with 3D shape deformation

Programmable asymmetric hydrogels with tunable structure/shape or physical/chemical properties in response to external stimuli show particular significance in smart systems, but there is lack of simple, rapid, and cheap strategy to design such hydrogel systems. Herein, we report a one-step electrodeposition method to construct chitosan asymmetric hydrogels with tunable thickness and pore size that can be conveniently modulated by the process parameters. Our approach greatly simplifies the process of hydrogel preparation with complex shapes and asymmetric structure organization. The formation mechanism of asymmetric structure has been proposed, based on gelation behavior and entanglement of chitosan chains in the hydrogel-solution system under the electric field. By changing the shape of the electrodes, hydrogels with the morphology of strip, tube, flower, etc. can be obtained precisely and conveniently. They can perform programmable 2D to 3D smart dynamic deformation under pH and metal ions stimulation, indicating the broad application potential in soft robot and biosensor areas.

The Dynamic Mortise-and-Tenon Interlock Assists Hydrated Soft Robots Toward Off-Road Locomotion

Natural locomotion such as walking, crawling, and swimming relies on spatially controlled deformation of soft tissues, which could allow efficient interaction with the external environment. As one of the ideal candidates for biomimetic materials, hydrogels can exhibit versatile bionic morphings. However, it remains an enormous challenge to transfer these in situ deformations to locomotion, particularly above complex terrains. Herein, inspired by the crawling mode of inchworms, an isotropic hydrogel with thermoresponsiveness could evolve to an anisotropic hydrogel actuator via interfacial diffusion polymerization, further evolving to multisection structure and exhibiting adaptive deformation with diverse degrees of freedom. Therefore, a dynamic mortise-and-tenon interlock could be generated through the interaction between the self-deformation of the hydrogel actuator and rough terrains, inducing continual multidimensional locomotion on various artificial rough substrates and natural sandy terrain. Interestingly, benefiting from the powerful mechanical energy transfer capability, the crawlable hydrogel actuators could also be utilized as hydrogel motors to activate static cargos to overstep complex terrains, which exhibit the potential application of a biomimetic mechanical discoloration device. Therefore, we believe that this design principle and control strategy may be of potential interest to the field of deformable materials, soft robots, and biomimetic devices.

Thermo- and Mechanochromic Camouflage and Self-Healing in Biomimetic Soft Actuators Based on Liquid Crystal Elastomers

In nature, many mysterious creatures capable of deformation camouflage, color camouflage, and self-healing have inspired scientists to develop various biomimetic soft robots. However, the systematic integration of all the above functionalities into a single soft actuator system still remains a challenge. Here we chemically introduce a multi-stimuli-responsive tetraarylsuccinonitrile (TASN) chromophore into a liquid crystal elastomer (LCE) network through a facile thiol-ene photoaddition method. The obtained TASN-LCE soft actuators not only exhibit reversible shape-morphing and reversible color-changing behavior in response to heat and mechanical compression, but also show excellent self-healing, reprogramming and recycling characteristics. We hope that such a TASN-LCE actuator system endowed with dynamic distortion, thermo- and mechano-chromic camouflage, and self-healing functionalities would pave the way for further development of multifunctional biomimetic soft robotic devices.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Shape- and Color-Switchable Polyurethane Thermochromic Actuators Based on Metal-Containing Ionic Liquids

Many creatures have excellent control over their form, color, and morphology, allowing them to respond to the interaction of environmental stimuli better. Here, the bioinspired synergistic shape-color-switchable actuators based on thermally induced shape-memory triethanolamine cross-linked polyurethane (TEAPU) and thermochromic ionic liquids (ILs) were prepared. The thermochromic ILs with various metalized anions, including bis(1-butyl-3-methylimidazolium) tetrachloro nickelate ([Bmim]₂[NiCl₄]) and bis(1-butyl-3-methylimidazolium) tetrachloride cobalt ([Bmim]₂[CoCl₄]), are investigated. The actuators exhibit thermochromic response, as evidenced by a shift in the color of the composites, which is due to the formation of the tetrahedral complex MCl₄^2- (M = Ni and Co) after dehydration. The shape-color-switchable thermochromic actuators have strong molecular interaction between TEAPU and ILs and can mimic natural flowers and change the color and shape quickly in a narrow temperature range (30-70 °C). In addition, these thermochromic actuators can lift more than 50 times their weight and withstand strains of more than 1100%. The results represent the potential application in artificial muscle actuators and intelligent camouflages.

Promotion of Color-Changing Luminescent Hydrogels from Thermo to Electrical Responsiveness toward Biomimetic Skin Applications

The active color-changing ability of many living species has inspired scientists to replicate the optical property into soft wet and tissue-like hydrogel materials. However, the color-changing processes of most reported examples are controlled by the traditional stimuli (e.g., pH, temperature, and ions), which may suffer from the residual chemical product accumulation, and have difficulty in achieving local control and integration into the commercial robots, especially when applied as biomimetic skins. Herein, inspired by the nervous (bioelectricity) control of skin color change in cephalopods, we present an electrically powered multicolor fluorescent hydrogel system with asymmetric configuration that couples thermoresponsive fluorescent hydrogel with stacked graphene assembly (SGA)-based conductive paper through luminous paint as the middle layer. Owing to the highly controllable electrical stimulus in terms of amplitude and duration, the Joule heat supplied by SGA film can be regulated locally and in real time, leading to precise and local emission color control at low voltage. It also avoids the addition of any chemicals. Furthermore, the electrically powered color-changing hydrogel system can be conveniently integrated into the commercial robots as biomimetic skins that help them achieve desirable camouflage, display, or alarming functions.

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.

Hydrogen-bonding-assisted toughening of hierarchical carboxymethyl cellulose hydrogels for biomechanical sensing

Herein we present a CMC based hydrogel that can be engineered for texture by applying a soaking strategy after freeze-thaw. These two processes lead to a hierarchal structure within the CMC polymer hydrogels. Such hydrogels can be strain hardening; that is the gel modulus increases with higher strain levels. Both the failure stress and energy are high (1.54 MPa and 9.94 kJm^-2 respectively). At sub-failure strains, the hydrogels show a high degree of recoverability as evidenced by their minimal hysteresis in a stress-strain test. Furthermore, these hydrogels are electrically conductive making them good candidates for tough sensors and wearable electronic devices.

Tough Adhesion of Freezing- and Drying-Tolerant Transparent Nanocomposite Organohydrogels

Tough hydrogels with strong wet adhesion have drawn extensive attention for various applications. However, it is still challenging to achieve both excellent wet adhesion and freezing- and drying-tolerance in hydrogels. In this study, we present tough transparent nanocomposite organohydrogels based on the glycerol-water binary solvent system in the presence of Al(OH)₃ nanoparticles as a cross-linker. The resultant organohydrogels exhibited excellent tensile strength (∼0.9 MPa), high transparency (97%), superior anti-drying and anti-freezing properties, and good ionic conductivity. In particular, polyacrylic acid (PAA) was chosen as the bridging polymer to endow the organohydrogels with strong wet adhesion. The interfacial adhesion energy exceeded 2200 J m^-2, which was ascribed to the synergy of ionic coordination and hydrogen bonds between the nanoparticles and carboxyl groups in PAA chains. Interestingly, based on the strong wet adhesion, the transparent organohydrogels can be assembled into hydraulically driven soft variable-focus lenses with long-term ambient stability. This work will provide a new insight into controlled wet adhesion ̵of hydrogel and have great potential for hydrogel-based functional devices with long-term ambient stability.

Electric-Field-Induced Gradient Ionogels for Highly Sensitive, Broad-Range-Response, and Freeze/Heat-Resistant Ionic Fingers

Human fingers exhibit both high sensitivity and wide tactile range. The finger skin structures are designed to display gradient microstructures and compressibility. Inspired by the gradient mechanical Young's modulus distribution, an electric-field-induced cationic crosslinker migration strategy is demonstrated to prepare gradient ionogels. Due to the gradient of the crosslinkers, the ionogels exhibit more than four orders of magnitude difference between the anode and the cathode side, enabling gradient ionogel-based flexible iontronic sensors having high-sensitivity and broader-range detection (from 3 × 10² to 2.5 × 10⁶  Pa) simultaneously. Moreover, owing to the remarkable properties of the gradient ionogels, the flexible iontronic sensors also show good long-time stability (even after 10 000 cycles loadings) and excellent performance over a wide temperature range (from -108 to 300 °C). The flexible iontronic sensors are further integrated on soft grips, exhibiting remarkable performance under various conditions. These attractive features demonstrate that gradient ionogels will be promising candidates for smart sensor applications in complex and extreme conditions.

Salt-responsive polyampholyte-based hydrogel actuators with gradient porous structures

A polyampholyte-based hydrogel actuator with water-responsive shape deformation was fabricated, and the gradient distribution of chemical composition was proved by micro-FTIR.

Mechanically Strong, Tough, and Shape Deformable Poly(acrylamide-co-vinylimidazole) Hydrogels Based on Cu2+ Complexation

Shape deformable hydrogels have drawn great attention due to their wide applications as soft actuators. Here we report a novel kind of mechanically strong, tough, and shape deformable poly(acrylamide-co-vinylimidazole) [poly(AAm-co-VI)] hydrogel prepared by photoinitiated copolymerization and the followed immersing in a Cu²⁺ aqueous solution. Strong Cu²⁺ complexation with imidazole groups dramatically enhances the mechanical properties of the hydrogels, whose tensile strength, elastic modulus, toughness, and fracture energy reach up to 7.7 ± 0.76 MPa, 15.4 ± 1.2 MPa, 23.2 ± 2.5 MJ m^-3, and 22.1 ± 2.3 kJ m^-2, respectively. More impressively, shape deformation (bending) can be easily achieved by coating Cu²⁺ solution on one side of hydrogel strips. Furthermore, precise control of the shape deformation from 1D to 2D and 2D to 3D can be achieved by adjusting Cu²⁺ concentration, coating time, region, and one or two side(s) of hydrogel samples. The Cu²⁺ complexation provides a simple way to simultaneously improve the mechanical properties of hydrogels and enable them with shape deformability. The mechanically strong, tough, and shape deformable hydrogels might be a promising candidate for soft actuators.

Gradient Hydrogels-The State of the Art in Preparation Methods

Gradient hydrogels refer to hydrogel materials with a gradual or abrupt change in one or some of their properties. They represent examples of more sophisticated gel materials in comparison to simple, native gel networks. Here, we review techniques used to prepare gradient hydrogels which have been reported in literature over the last few years. A variety of simple preparation methods are available, most of which can be relatively easily utilized in standard laboratories.