Photo-Induced Anomalous Deformation of Poly(N-Isopropylacrylamide) Gel Hybridized with an Inorganic Nanosheet Liquid Crystal Aligned by Electric Field

Liquid-Crystalline Photonic Sandwich: Electroresponsive Colloids of Clay Nanosheets Loading Photofunctional Dyes

Colloidal clay nanosheets obtained by the delamination of layered crystals of smectite-type clay minerals in water form liquid crystals because of their shape anisotropy. Loading of organic dyes onto the liquid crystalline clay nanosheets will enable novel photonic materials, where photofunctions of the loaded dye are controlled by the liquid crystallinity of the clay nanosheets. However, adsorption of organic dyes onto the nanosheets renders the nanosheet surfaces hydrophobic, and consequently, colloidal stability of the nanosheets is lost. In this study, this drawback is overcome by sandwiching cationic stilbazolium dyes between a pair of synthetic fluorohectorite nanosheets. This is realized by the preparation of stilbazolium-clay second-stage intercalation compounds characterized by intercalation of dye cations into every other interlayer space of the hectorite clay, where nonintercalated interlayer spaces are occupied by Na+ ions. The second-stage intercalation compounds are obtained by partial ion exchange of mother clay mineral incorporating Na+ ions in all of the interlayer spaces and delaminated from the Na+-containing interlayer spaces to form clay nanosheets sandwiching the dye molecules. Aqueous colloids of the dye-sandwiching clay nanosheets form colloidal liquid crystals, and the dye-sandwiching liquid crystalline clay nanosheets respond to an applied AC electric field to be aligned parallel to the electric field. The assembled structure of the dye-sandwiching clay nanosheets under the electric field is characterized by aligned discrete clay platelets, which is somewhat different from that of a colloidal liquid crystal of clay nanosheets without dye loading characterized by macroscopic liquid crystalline domains up to submillimeters. The electric alignment of the clay nanosheets induces alteration of light absorption of the sandwiched stilbazolium molecules, which verifies a strategy of constructing stimuli-responsive photonic materials of clay-organic hybrids.

Renewable biomass-based aerogels: from structural design to functional regulation

Global population growth and industrialization have exacerbated the nonrenewable energy crises and environmental issues, thereby stimulating an enormous demand for producing environmentally friendly materials. Typically, biomass-based aerogels (BAs), which are mainly composed of biomass materials, show great application prospects in various fields because of their exceptional properties such as biocompatibility, degradability, and renewability. To improve the performance of BAs to meet the usage requirements of different scenarios, a large number of innovative works in the past few decades have emphasized the importance of micro-structural design in regulating macroscopic functions. Inspired by the ubiquitous random or regularly arranged structures of materials in nature ranging from micro to meso and macro scales, constructing different microstructures often corresponds to completely different functions even with similar biomolecular compositions. This review focuses on the preparation process, design concepts, regulation methods, and the synergistic combination of chemical compositions and microstructures of BAs with different porous structures from the perspective of gel skeleton and pore structure. It not only comprehensively introduces the effect of various microstructures on the physical properties of BAs, but also analyzes their potential applications in the corresponding fields of thermal management, water treatment, atmospheric water harvesting, CO2 absorption, energy storage and conversion, electromagnetic interference (EMI) shielding, biological applications, etc. Finally, we provide our perspectives regarding the challenges and future opportunities of BAs. Overall, our goal is to provide researchers with a thorough understanding of the relationship between the microstructures and properties of BAs, supported by a comprehensive analysis of the available data.

Nanocomposite Hydrogel Actuators with Ordered Structures: From Nanoscale Control to Macroscale Deformations

Flexible intelligent actuators with the characteristics of flexibility, safety and scalability, are highly promising in industrial production, biomedical fields, environmental monitoring, and soft robots. Nanocomposite hydrogels are attractive candidates for soft actuators due to their high pliability, intelligent responsiveness, and capability to execute large-scale rapid reversible deformations under external stimuli. Here, the recent advances of nanocomposite hydrogels as soft actuators are reviewed and focus is on the construction of elaborate and programmable structures by the assembly of nano-objects in the hydrogel matrix. With the help of inducing the gradient or oriented distributions of the nanounits during the gelation process by the external forces or molecular interactions, nanocomposite hydrogels with ordered structures are achieved, which can perform bending, spiraling, patterned deformations, and biomimetic complex shape changes. Given great advantages of these intricate yet programmable shape-morphing, nanocomposite hydrogel actuators have presented high potentials in the fields of moving robots, energy collectors, and biomedicines. In the end, the challenges and future perspectives of this emerging field of nanocomposite hydrogel actuators are proposed.

Structuring gelatin methacryloyl - dextran hydrogels and microgels under shear

Gelatin methacryloyl (GelMA) is a widely used semi-synthetic polymer for a variety of bioapplications. However, the development of versatile GelMA hydrogels requires tuning of their microstructure. Herein, we report the possibility of preparing hydrogels with various microstructures under shear from an aqueous two-phase system (ATPS) consisting of GelMA and dextran. The influence of an applied preshear on dextran/GelMA droplets and bicontinuous systems is investigated by rheology that allows the application of a constant shear and is immediately followed by in situ UV-curing of the GelMA-rich phase. The microstructure of the resulting hydrogels is examined by confocal laser scanning microscopy (CLSM). The results show that the GelMA string phase and GelMA hydrogels with aligned bands can be formed depending on the concentration of dextran and the applied preshear. The influence of the pH of the ATPS is investigated and demonstrates the formation of multiple emulsions upon decreasing the charge density of GelMA. The preshearing of multiple emulsions, following gelation, leads to the formation of porous GelMA microgels. The diversity of the formed structures highlights the application potential of preshearing ATPS in the development of functional soft materials.

Liquid crystalline composite hydrogels with large pH-triggered anisotropic swelling for embolotherapy

Inspired by the anisotropic structure of biological tissues, anisotropic hydrogels have been developed using various nanofillers, however, it remains a big challenge to synthesize hydrogels with large swelling anisotropy. Herein a single molecule filler, α-helical polypeptide, instead of nanoscale fillers, was used to synthesize anisotropic hydrogels. First nematic liquid crystal of poly(γ-benzyl l-glutamate) (PBLG) was prepared by shearing and stabilized by embedding in a crosslinked polymer matrix. The resulting PBLG composite gels were then converted to poly(L-glutamic acid) (PLGA) composite gels by debenzylation. The rigid rod-like structure of α-helical PBLG chains makes them easy to be orientated. The pH-sensitivity of PLGA makes the resulting composite gels pH-sensitive without the need to couple with a stimuli-responsive hydrogel matrix. In response to pH change PLGA composite gels swell anisotropically with a much larger swelling degree in the radial direction than in the axial direction. The swelling anisotropy (3.43) is much higher than most anisotropic hydrogels, particularly the stimuli-responsive ones reported previously. The composite gel also exhibits anisotropic mechanical properties with a larger Young's modulus in the axial direction than that in the radial direction. Preliminary test demonstrated that the composite gels have potential in embolotherapy thanks to its large pH-triggered anisotropic swelling. STATEMENT OF SIGNIFICANCE: Anisotropic hydrogels have important biomedical applications. Introduction of oriented nanofillers has been demonstrated a popular and versatile method for their synthesis, however, it remains a big challenge to achieve large swelling anisotropy. Herein a single molecule filler, α-helical polypeptide, instead of nanoscale fillers, was used to synthesize anisotropic hydrogels. This filler can be easily oriented by shearing. More importantly, as single molecule filler, it can constrain the swelling of hydrogel matrix more effectively. Using this filler, a pH-sensitive hydrogel with large swelling anisotropy (3.43) was successfully synthesized. Thanks to its large pH-triggered anisotropic swelling the hydrogel was successfully used as embolic agent to occlude vessels.

Magnetically anisotropic hydrogels for tissue engineering

Many soft tissues of the human body possess hierarchically anisotropic structures, exhibiting orientation-specific mechanical properties and biological functionality. Hydrogels have been proposed as promising scaffold materials for tissue engineering applications due to their water-rich composition, excellent biocompatibility, and tunable physico-chemical properties. However, conventional hydrogels with homogeneous structures often exhibit isotropic properties that differ from those of biological tissues, limiting their further application. Recently, magnetically anisotropic hydrogels containing long-range ordered magneto-structures have attracted widespread interest owing to their highly controllable assembly strategy, rapid magnetic responsiveness and remote spatiotemporal regulation. In this review, we summarize the latest progress of magnetically anisotropic hydrogels for tissue engineering. The fabrication strategy of magnetically anisotropic hydrogels from magnetic nanofillers with different dimensions is systemically introduced. Then, the effects of magnetically anisotropic cues on the physicochemical properties of hydrogels and the cellular biological behavior are discussed. And the applications of magnetically anisotropic hydrogels in the engineering of different tissues are emphasized. Finally, the current challenges and the future perspectives for magnetically anisotropic hydrogels are presented.

Nanocomposite Hydrogels as Functional Extracellular Matrices

Over recent years, nano-engineered materials have become an important component of artificial extracellular matrices. On one hand, these materials enable static enhancement of the bulk properties of cell scaffolds, for instance, they can alter mechanical properties or electrical conductivity, in order to better mimic the in vivo cell environment. Yet, many nanomaterials also exhibit dynamic, remotely tunable optical, electrical, magnetic, or acoustic properties, and therefore, can be used to non-invasively deliver localized, dynamic stimuli to cells cultured in artificial ECMs in three dimensions. Vice versa, the same, functional nanomaterials, can also report changing environmental conditions-whether or not, as a result of a dynamically applied stimulus-and as such provide means for wireless, long-term monitoring of the cell status inside the culture. In this review article, we present an overview of the technological advances regarding the incorporation of functional nanomaterials in artificial extracellular matrices, highlighting both passive and dynamically tunable nano-engineered components.

Chiral Liquid Crystalline Properties of Cellulose Nanocrystals: Fundamentals and Applications

By using an independent self-assembly process that is occasionally controlled by evaporation, cellulose nanocrystals (CNCs) may create films (pure or in conjunction with other materials) that have iridescent structural colors. The self-forming chiral nematic structures and environmental safety of a new class of photonic liquid crystals (LCs), referred to as CNCs and CNC-embedded materials, make them simple to make and treat. The structure of the matrix interacts with light to give structural coloring, as opposed to other dye pigments, which interact with light by adsorption and reflection. Understanding how CNC self-assembly constructs structures is vital in several fields, including physics, science, and engineering. To constructure this review, the colloidal characteristics of CNC particles and their behavior during the formation of liquid crystals and gelling were studied. Then, some of the recognized applications for these naturally occurring nanoparticles were summarized. Different factors were considered, including the CNC aspect ratio, surface chemistry, concentration, the amount of time needed to produce an anisotropic phase, and the addition of additional substances to the suspension medium. The effects of alignment and the drying process conditions on structural changes are also covered. The focus of this study however is on the optical properties of the films as well as the impact of the aforementioned factors on the final transparency, iridescent colors, and versus the overall response of these bioinspired photonic materials. Control of the examined factors was found to be necessary to produce reliable materials for optoelectronics, intelligent inks and papers, transparent flexible support for electronics, and decorative coatings and films.

Multiresponsive Polymer Nanocomposite Liquid Crystals Having Heterogeneous Phase Transitions for Battery-Free Temperature Maintenance Indicators

Multiresponsive functional materials that respond to more than one external stimulus are promising for novel photonic, electronic, and biomedical applications. However, the design or synthesis of new multiresponsive materials is challenging. Here, this work reports a facile method to prepare a multiresponsive colloidal material by mixing a liquid-crystalline 2D nanocolloid and a functional polymer colloid. For this purpose, electrically sensitive exfoliated α-ZrP 2D nanocolloids and thermosensitive block copolymer colloids that are dispersed well in water are mixed. In the liquid-crystalline nanocomposite, nematic, antinematic, or isotropic assemblies of α-ZrP, nanoparticles can be electrically and selectively obtained by applying electric fields with different frequencies; furthermore, their rheology is thermally and reversibly controlled through thesol-gel-sol transition. The nanocomposite exhibits a solid gel phase within a predesigned gel temperature range and a liquid sol phase outside this range. These properties facilitate the design of a simple display device in which information can be electrically written and thermally stabilized or erased, and using the device, a battery-free temperature maintenance indication function is demonstrated. The proposed polymer nanocomposite method can enrich the physical properties of 2D nanocolloidal liquid crystals and create new opportunities for eco-friendly, reusable, battery-free electro-optical devices.

Magneto-Orientation of Magnetic Double Stacks for Patterned Anisotropic Hydrogels with Multiple Responses and Modulable Motions

Reported here is a multi-response anisotropic poly(N-isopropylacrylamide) hydrogel developed by using a rotating magnetic field to align magnetic double stacks (MDSs) that are fixed by polymerization. The magneto-orientation of MDSs originates from the unique structure with γ-Fe2 O3 nanoparticles sandwiched by two silicate nanosheets. The resultant gels not only exhibit anisotropic optical and mechanical properties but also show anisotropic responses to temperature and light. Gels with complex ordered structures of MDSs are further devised by multi-step magnetic orientation and photolithographic polymerization. These gels show varied birefringence patterns with potentials as information materials, and can deform into specific configurations upon stimulations. Multi-gait motions are further realized in the patterned gel through dynamic deformation under spatiotemporal light and friction regulation by imposed magnetic force. The magneto-orientation assisted fabrication of hydrogels with anisotropic structures and additional functions should bring opportunities for gel materials in biomedical devices, soft actuators/robots, etc.

Manufacturing and post-engineering strategies of hydrogel actuators and sensors: From materials to interfaces

Living bodies are made of numerous bio-sensors and actuators for perceiving external stimuli and making movement. Hydrogels have been considered as ideal candidates for manufacturing bio-sensors and actuators because of their excellent biocompatibility, similar mechanical and electrical properties to that of living organs. The key point of manufacturing hydrogel sensors/actuators is that the materials should not only possess excellent mechanical and electrical properties but also form effective interfacial connections with various substrates. Traditional hydrogel normally shows high electrical resistance (~ MΩ•cm) with limited mechanical strength (<1 MPa), and it is prone to fatigue fracture during continuous loading-unloading cycles. Just like iron should be toughened and hardened into steel, manufacturing and post-treatment processes are necessary for modifying hydrogels. Besides, advanced design and manufacturing strategies can build effective interfaces between sensors/actuators and other substrates, thus enhancing the desired mechanical and electrical performances. Although various literatures have reviewed the manufacture or modification of hydrogels, the summary regarding the post-treatment strategies and the creation of effective electrical and mechanically sustainable interfaces are still lacking. This paper aims at providing an overview of the following topics: (i) the manufacturing and post-engineering treatment of hydrogel sensors and actuators; (ii) the processes of creating sensor(actuator)-substrate interfaces; (iii) the development and innovation of hydrogel manufacturing and interface creation. In the first section, the manufacturing processes and the principles for post-engineering treatments are discussed, and some typical examples are also presented. In the second section, the studies of interfaces between hydrogels and various substrates are reviewed. Lastly, we summarize the current manufacturing processes of hydrogels, and provide potential perspectives for hydrogel manufacturing and post-treatment methods.

The Dispersion and Coagulation of Negatively Charged Ca2Nb3O10 Perovskite Nanosheets in Sodium Alginate Dispersion

Chemically exfoliated nanosheets have been extensively employed as functional nanofillers for the fabrication of polymer nanocomposites due to their remarkable electrical, magnetic and optical properties. However, achieving a good dispersion of charged nanosheets in polymer matrix, which will determine the performance of polymer nanocomposites, remains a challenge. Herein, we investigated the dispersion and aggregation behavior of negatively charged Ca₂Nb₃O₁₀ (CNO) perovskite nanosheets in negatively charged sodium alginate (SA) aqueous dispersion using dynamic light scattering (DLS). When CNO nanosheets meet with SA, aggregation and coagulation inevitably occurred owing to the absorption of SA on nanosheets. By controlling the electrostatic attraction between positively charged poly(ethylene imine) (PEI) and negatively charged SA, the charge density and hydrodynamic size of SA can be tuned to enable the good dispersion of CNO nanosheets in SA. This result may provide a new strategy to achieve the good dispersion of charged nanosheets in charged polymers for the rational design of multifunctional nanocomposites.

Advanced Functional Liquid Crystals

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

Formation of a Giant Anisotropically Ordered Assembled Structure of Inorganic Nanosheets through an Optically Induced Stream

Formation of a desirable submillimeter-scaled assembled structure of particles in the colloid is a difficult subject in colloidal chemistry. Herein, a submillimeter-scaled ordered assembled structure consisting of highly anisotropic two-dimensional plate-like particles, niobate nanosheets, was obtained through an optical manipulation technique that was assisted by a scattering-force-induced stream. A 532 nm continuous wave laser beam with a power of 400 mW was used to illuminate a liquid crystalline niobate nanosheet colloid from the bottom side of a sample cell, inducing the stream of oriented nanosheets toward the upper side of the sample cell. As a result, a 200 μm ordered assembled structure consisting of oriented nanosheets was formed. The assembled structure was also characterized by two-dimensional anisotropy, reflecting that the highly anisotropic morphologies of each nanosheet and the shape of that structure were dependent on the polarization of incident illumination. This study has revealed a new noncontact and on-demand way to obtain submillimeter-scaled ordered anisotropic colloidal assembled structures of nanosized particles such as nanosheets, contributing to fundamental materials science and expanding the utilities of nanosheets.

Mechanical properties of an interpenetrating network poly(vinyl alcohol)/alginate hydrogel with hierarchical fibrous structures

Bioinspired hierarchical fibrous structures were constructed in an interpenetrating poly(vinyl alcohol, PVA)/alginate hydrogel network to improve its mechanical properties. The interpenetrating hydrogel network with hierarchical fibrous structures was prepared by combining the confined drying method and freeze-thaw method. First, Ca2+ cross-linked alginate formed a nano-micro hierarchical fibrous structure via the confined drying method. Then, PVA that was uniformly distributed among the Ca2+-alginate chains was cross-linked by hydrogen bonding via the freeze-thaw method, further dividing the hierarchical fibers into finer fibers. The results of a tensile test demonstrated that both the tensile stress and fracture energy improved by more than double after the introduction of 2 wt% PVA, achieving a combination of high strength (∼12.9 MPa), high toughness (∼13.2 MJ m-3) and large strain (∼161.4%). Cyclic tensile tests showed that a hysteresis loop existed on the loading-unloading curves of the hydrogel along the fibrous directions, and a good self-recovery property emerged after resting for a period of time. The hydrogel with hierarchical fibrous structures constructed by alginate and PVA can be employed in biomedical applications in the future.

Multi-responsive and conductive bilayer hydrogel and its application in flexible devices

Multi-stimuli-responsive hydrogels are intelligent materials that present advantages for application in soft devices compared with conventional machines. In this paper, we prepared a bilayer hydrogel consisting of a poly(2-(dimethylamino)ethyl methacrylate) layer and a poly(N-isopropylacrylamide) layer. The hydrogel responded to temperature, pH, NaCl, and ethanol by undergoing bending deformation. At 40 °C, it only took 23 s for the hydrogel to bend nearly 300°. Carbon black was also introduced into the hydrogel network to render it conductive. Based on its multi-stimuli-responsive properties and conductivity, the hydrogel was used to construct a 4-arm gripper, thermistor, and finger movement monitor. The time required to grip and release an object was 141 s. The resistance changed with temperature, which affected the brightness of an LED. Finger motions were monitored, and the bending angle could be distinguished.

Angular-Independent Structural Colors of Clay Dispersions

Clay mineral nanosheet colloids were found to show angular-independent structural colors after desalting. Naked-eye observation and UV-visible reflectance spectra showed that the color is tuned by varying the average nanosheet size and nanosheet concentration. The low angular-dependence of the structural color was also clarified by these observations, which is the first case for a nanosheet system. The present system is expected as an environmentally benign and low-cost structural color material for various applications.

Multicomponent chiral hydrogel fibers with block configurations based on the chiral liquid crystals of cellulose nanocrystals and M13 bacteriophages

Integrating the advantages unique to CNCs and the M13 virus into blockwise chiral hydrogel fibers, which have block dependent chiral fingerprints, birefringence, (de)swelling behaviors, mechanical strength and stretchability.

Stimuli-Responsive Zinc (II) Coordination Polymers: A Novel Platform for Supramolecular Chromic Smart Tools

The unique role of the zinc (II) cation prompted us to cut a cross-section of the large and complex topic of the stimuli-responsive coordination polymers (CPs). Due to its flexible coordination environment and geometries, easiness of coordination-decoordination equilibria, "optically innocent" ability to "clip" the ligands in emissive architectures, non-toxicity and sustainability, the zinc (II) cation is a good candidate for building supramolecular smart tools. The review summarizes the recent achievements of zinc-based CPs as stimuli-responsive materials able to provide a chromic response. An overview of the past five years has been organised, encompassing 1, 2 and 3D responsive zinc-based CPs; specifically zinc-based metallorganic frameworks and zinc-based nanosized polymeric probes. The most relevant examples were collected following a consequential and progressive approach, referring to the structure-responsiveness relationship, the sensing mechanisms, the analytes and/or parameters detected. Finally, applications of highly bioengineered Zn-CPs for advanced imaging technique have been discussed.

Nonlinear photoelasticity of rubber-like soft materials: comparison between theory and experiment

Photoelasticity often refers to the birefringence effect of materials induced by elastic deformation. Recently, many experiments on the photoelasticity of soft materials have been reported. However, the experimental results are mainly qualitative observations and lack any theoretical analysis. In this paper, we revisit Treloar's and Arruda's models of nonlinear photoelasticity for rubber-like materials. Both models establish the intrinsic relationship between stretch and birefringence, based on the statistics of chain polarizability and a network theory. We discuss the difference of the two models and build an experimental setup to measure the birefringence of PDMS samples as a function of stress/stretch. We vary the curing ratio of PDMS to study the effect of chain density on birefringence and compare with Treloar's theory. We further use experimental data of double-network hydrogels in the literature to compare with theory and find that when the deformation is large compared with the limiting stretch of the material, Arruda's model fits the experimental data much better than Treloar's model. This work presents a basis of using the theory of nonlinear photoelasticity to guide the analysis of experiments.

Perovskite Nanosheet Hydrogels with Mechanochromic Structural Color

Structural color colloidal sols of perovskite nanosheets were synthesized and were immobilized in a polymer hydrogel film by in situ photopolymerization, leading to a novel mechanochromic material. Visible absorption spectroscopy, polarized optical microscopy and small-angle X-ray scattering revealed that the nanosheets are aligned parallel to the film surface with the periodic distance of up to ca. 300 nm, giving the structural color tunable over full color range. The present structural color gel showed reversible mechanochromic response that detects weak stress of 1 kPa with the quick response time less than 1 ms as well as high mechanical toughness (compressive breaking stress of up to 3 MPa). These excellent properties are suitable for applications for mechano-sensors and displays.

Distributed Electric Field Induces Orientations of Nanosheets to Prepare Hydrogels with Elaborate Ordered Structures and Programmed Deformations

Living organisms use musculatures with spatially distributed anisotropic structures to actuate deformations and locomotion with fascinating functions. Replicating such structural features in artificial materials is of great significance yet remains a big challenge. Here, a facile strategy is reported to fabricate hydrogels with elaborate ordered structures of nanosheets (NSs) oriented under a distributed electric field. Multiple electrodes are distributed with various arrangements in the precursor solution containing NSs and gold nanoparticles. A complex electric field induces sophisticated orientations of the NSs that are permanently inscribed by subsequent photo-polymerization. The resultant anisotropic nanocomposite poly(N-isopropylacrylamide) hydrogels exhibit rapid deformation upon heating or photoirradiation, owing to the fast switching of permittivity of the media and electric repulsion between the NSs. The complex alignments of NSs and anisotropic shape change of discrete regions result in programmed deformation of the hydrogels into various configurations. Furthermore, locomotion is realized by a spatiotemporal light stimulation that locally triggers time-variant shape change of the composite hydrogel with complex anisotropic structures. Such a strategy on the basis of the distributed electric-field-generated ordered structures should be applicable to gels, elastomers, and thermosets loaded with other anisotropic particles or liquid crystals, for the design of biomimetic/bioinspired materials with specific functionalities.

Light-steered locomotion of muscle-like hydrogel by self-coordinated shape change and friction modulation

Many creatures have the ability to traverse challenging environments by using their active muscles with anisotropic structures as the motors in a highly coordinated fashion. However, most artificial robots require multiple independently activated actuators to achieve similar purposes. Here we report a hydrogel-based, biomimetic soft robot capable of multimodal locomotion fueled and steered by light irradiation. A muscle-like poly(N-isopropylacrylamide) nanocomposite hydrogel is prepared by electrical orientation of nanosheets and subsequent gelation. Patterned anisotropic hydrogels are fabricated by multi-step electrical orientation and photolithographic polymerization, affording programmed deformations. Under light irradiation, the gold-nanoparticle-incorporated hydrogels undergo concurrent fast isochoric deformation and rapid increase in friction against a hydrophobic substrate. Versatile motion gaits including crawling, walking, and turning with controllable directions are realized in the soft robots by dynamic synergy of localized shape-changing and friction manipulation under spatiotemporal light stimuli. The principle and strategy should merit designing of continuum soft robots with biomimetic mechanisms.

Accelerating the electrical response of solvent-dispersed imogolite nanotubes through structural organisation

Structural organisation of solvent-dispersed imogolite nanotubes accelerated their electrical response, resulting in birefringence variations analogous to a liquid crystal system. Crosslinking, confinement, and helical structuring of the imogolite nanotubes in the solvent led to the cooperative aggregation and dissociation of the dispersed nanotubes, which induced rapid changes in their birefringence.

Digital Programming Graphene Oxide Liquid Crystalline Hybrid Hydrogel by Shearing Microlithography

Designing anisotropic architectures enables the creation of soft materials with rich properties and functions to artificially simulate the evolutionary diversity of biology. In the important liquid crystalline hybrid (LCH) hydrogels, free manipulation of liquid crystalline order in high accuracy and efficiency has been long pursued to design properties and functions but remains a challenge. Here, we realize digital programing LC order in graphene oxide LCH hydrogels in high size resolution (∼20 μm) and efficiency by using shearing microlithography. The localized shear-induced LC order organization is immobilized by cross-linking gelation, and we prepare graphene oxide LCH hydrogels with digital programmed patterns in a large area. The shearing order generates a vertical alignment of graphene oxide sheets in hydrogels and a considerable mechanical anisotropy controlled by the shearing angle and interval spacing. By diversely organizing geometry of LC order, the mechanical response behaviors of LCH hydrogels are designed to exhibit localized deformations, steered cracking, and programmable swelling actuations. Our work offers a versatile avenue to scalably digital program LCH hydrogels in a high efficiency and accuracy. The digital designed hydrogel promises wide uses in actuators, bioscaffolds, biomimetic materials, and soft designer materials.

Imparting Functionality to the Hydrogel by Magnetic-Field-Induced Nano-assembly and Macro-response

Hydrogels are composed of 3D hydrophilic networks with an abundance of water; they are analogous to biological soft tissues. Their unique physico-chemical properties endow hydrogels with great potential in many fields, including tissue engineering and flexible sensing. However, inadequate functionality, such as lack of rapid responsiveness, severely limits practical applications in many areas. Therefore, imparting functionality to the hydrogel is a hot research topic. The magnetic field, as an important physical field, provides a new strategy with a variety of advantages. Magnetic-field-induced ordered nano-assembly brought anisotropic properties and novel performance. Furthermore, the magnetic responsiveness of hydrogels with magnetic nanoparticles can lead to the generation of functionality under magnetic fields. Thus, we aim to systematically describe the significant effect of magnetic fields on the functionality of the hydrogel. In this review, magnetic-field-induced assembly of nanomaterials with different dimensions and resulting functional performance are introduced. The functionalities of hydrogels based on magnetic-field-induced macroscopic responses are also summarized. We believe this review will motivate more exploration of the application of magnetic fields to develop functional hydrogel materials.

Anisotropic hydrogels formed by magnetically-oriented nanoclay suspensions for wound dressings

Anisotropic hydrogels are produced, by magnetic alignment of magnetically sensitized nanoclays followed by polymerization of the hydrogel to freeze the developed oriented structure. The anisotropy in these hydrogels is quantitatively investigated using birefringence and 2D small angle X-ray scattering (SAXS) techniques. The oriented nanoclays being intrinsically birefringent provide optical anisotropy to the hydrogel and this orientation increases with the increase of the applied magnetic field strength. Moreover, 2D SAXS patterns also confirm that the nanoclays are oriented parallel to the permanent magnetic field in the hydrogel with an orientation order parameter of up to 0.67. The field-induced birefringence and 2D SAXS orientation results exhibit a linear correlation over the range of 0 to 9 tesla (T). The resultant anisotropic hydrogels exhibit substantial swelling anisotropy, making them suitable for wound dressings where the out of plane swelling is substantially higher than in-plane swelling to minimize in-plane stress damage to the wounds during healing.

Growth of Colloidal Nanoplate Liquid Crystals Using Temperature Gradients

Controlling colloidal self-assemblies using external forces is essential to develop modern electro-optical and biomedical devices. Importantly, shape anisotropic colloids can provide optical properties such as birefringence. Here we demonstrate that external temperature gradients can be effective in controlling nematic liquid crystalline (LC) order in suspensions of plate-like colloids also known as nanoplates. Nanoplates, in an isotropic suspension, wherein their orientations are random, could be effectively moved using a temperature gradient environment causing a phase transition to LC nematic phase. Such controllably formed nematic phase featured large nematic monodomains and enabled topologically more stable structures that were evident from the absence of hedgehog-type defects which are typically found in nematics formed spontaneously via nucleation and growth mechanism in a sufficiently high concentration suspension of nanoplates. Due to their high surface area-to-volume ratio and excellent thermophoretic properties, nanoplates can prove to be ideal candidates for transport of biomolecules through temperature varying environments.

Nanocomposite hydrogel actuators hybridized with various dimensional nanomaterials for stimuli responsiveness enhancement

Hydrogel actuators, that convert external energy, such as pH, light, heat, magnetic field, and ion strength, into mechanical motion, have been utilized in sensors, artificial muscles, and soft robotics. For a practicality of the hydrogel actuators in a wide range of fields, an establishment of robust mechanical properties and rapid response are required. Several solutions have been proposed, for example, setting porous and anisotropy structures to hydrogels with nanocomposite materials to improve the response speed and deformation efficiency. In this review paper, we focused on hydrogel actuators including various nanocomposite by categorizing the dimensional aspects of additive materials. Moreover, we described the role of diverse additive materials in terms of the improvement of mechanical property and deformation efficiency of the hydrogel actuators. We assumed that this review will provide a beneficial guidance for strategies of developing nanocomposite hydrogel actuators and outlooks for the future research directions.

Electric-Alignment Immobilization of Liquid Crystalline Colloidal Nanosheets with the Aid of a Natural Organic Polymer

Inorganic nanosheets obtained by exfoliation of a layered crystal in water form colloidal liquid crystals, and their alignment can be controlled by an electric field. In order to realize the immobilization of the electrically aligned niobate nanosheets without external forces, an aqueous gelator, agar, is introduced to the niobate nanosheet system to utilize the thermosensitive sol-gel transition property of agar. Alignment of nanosheets in a niobate-agar system is performed by applying an electric field above the sol-gel transition temperature, and then, the sample is cooled down, followed by cooling below the transition temperature with the electric field turned off. The aligned structure is kept for more than 24 h after the removal of the electric field. The concentration of agar is a key parameter for both the orientation of nanosheets and the retention of the orientation.

Liquid Crystalline Colloidal Mixture of Nanosheets and Rods with Dynamically Variable Length

Here, we demonstrate the novel double-component liquid crystalline colloids composed of mesogenic inorganic nanosheets and the rods with dynamically variable length controlled by temperature. As the length-controllable rod, stiff biopolymer microtubule is used, which was successfully polymerized/depolymerized from tubulin proteins through a biochemical process even in the presence of the nanosheets. The mesoscopic structure of the liquid crystal phase was reversibly modifiable as caused by the change of the rod length.

Materials Nanoarchitectonics Using 2D Layered Materials: Recent Developments in the Intercalation Process

Layered inorganic solids as an attractive classification of 2D materials offer material diversity and a wide range of interesting properties. Layered inorganic solids provide an expandable 2D nanospace between each individual layer, the so called interlayer space, to accommodate/arrange guest species such as molecules, nanoparticles, and polymer chains and design unique nanoarchitectures, resulting in the production of intercalation compounds showing different properties in comparison to those of virgin layered materials and guest species. Layered inorganic solids can also be exfoliated to result in nanosheet production. Further ordering of exfoliated nanosheets is also possible via different methods and normally leads to creating soft materials presenting properties and applications different from that of relatively rigid intercalation compounds. Here, the latest studies and up-to-date developments on the possible techniques of designing novel types of materials using layered inorganic solids are specifically highlighted.

Anisotropic Self-Oscillating Reaction in Liquid Crystalline Nanosheet Hydrogels

Anisotropic chemical wave propagation of self-oscillating Belousov-Zhabotinsky (BZ) reaction was demonstrated in the poly( N-isopropylacrylamide) gel films embedded with macroscopically aligned liquid crystalline inorganic nanosheets. Although the average propagation rate of chemical wave v̅ was 3.56 mm min^-1 in the gels without nanosheets, the propagation was retarded in the gels with 1 wt % of nanosheets: [Formula: see text] = 1.89 mm min^-1 and [Formula: see text] = 1.33 mm min^-1 along the direction parallel and perpendicular to the nanosheet planes, respectively. Thus, the wave propagation is anisotropic with the anisotropy ratio [Formula: see text] = 1.42 in these gels and the periodic patterns formed by the BZ reaction were concentric ellipses, different from circles seen in isotropic gels. Furthermore, the propagation rate and degree of anisotropy were controllable by nanosheet concentration. These phenomena can be explained that the diffusion of molecules inside the gel is effectively hindered along the direction perpendicular to the nanosheet planes due to the very large aspect ratio of the aligned nanosheets. The present systems will be applicable for anisotropic self-oscillating soft actuators with one-dimensional motions as well as for ideal model system of BZ reactions.

Bioinspired Nanocomposite Hydrogels with Highly Ordered Structures

In the human body, many soft tissues with hierarchically ordered composite structures, such as cartilage, skeletal muscle, the corneas, and blood vessels, exhibit highly anisotropic mechanical strength and functionality to adapt to complex environments. In artificial soft materials, hydrogels are analogous to these biological soft tissues due to their "soft and wet" properties, their biocompatibility, and their elastic performance. However, conventional hydrogel materials with unordered homogeneous structures inevitably lack high mechanical properties and anisotropic functional performances; thus, their further application is limited. Inspired by biological soft tissues with well-ordered structures, researchers have increasingly investigated highly ordered nanocomposite hydrogels as functional biological engineering soft materials with unique mechanical, optical, and biological properties. These hydrogels incorporate long-range ordered nanocomposite structures within hydrogel network matrixes. Here, the critical design criteria and the state-of-the-art fabrication strategies of nanocomposite hydrogels with highly ordered structures are systemically reviewed. Then, recent progress in applications in the fields of soft actuators, tissue engineering, and sensors is highlighted. The future development and prospective application of highly ordered nanocomposite hydrogels are also discussed.

Graphene and Other 2D Colloids: Liquid Crystals and Macroscopic Fibers

Two-dimensional colloidal nanomaterials are running into renaissance after the enlightening researches of graphene. Macroscopic one-dimensional fiber is an optimal ordered structural form to express the in-plane merits of 2D nanomaterials, and the formation of liquid crystals (LCs) allows the creation of continuous fibers. In the correlated system from LCs to fibers, understanding their macroscopic organizing behavior and transforming them into new solid fibers is greatly significant for applications. Herein, we retrospect the history of 2D colloids and discuss about the concept of 2D nanomaterial fibers in the context of LCs, elaborating the motivation, principle and possible strategies of fabrication. Then we highlight the creation, development and typical applications of graphene fibers. Additionally, the latest advances of other 2D nanomaterial fibers are also summarized. Finally, conclusions, challenges and perspectives are provided to show great expectations of better and more fibrous materials of 2D nanomaterials. This review gives a comprehensive retrospect of the past century-long effort about the whole development of 2D colloids, and plots a clear roadmap - "lamellar solid - LCs - macroscopic fibers - flexible devices", which will certainly open a new era of structural-multifunctional application for the conventional 2D colloids.

Tough and Thermosensitive Poly(N-isopropylacrylamide)/Graphene Oxide Hydrogels with Macroscopically Oriented Liquid Crystalline Structures

Bulk graphene oxide (GO) nanocomposite materials with macroscopically oriented GO liquid crystalline (LC) structures exhibit interesting anisotropic properties, but their facile preparations remain challenging. This work reports for the first time the facile preparation of poly(N-isopropylacrylamide) (PNIPAM)/GO nanocomposite hydrogels with macroscopically oriented LC structures with the assistance of a flow field induced by vacuum degassing and the in situ polymerization accelerated by GO. The hydrogel prepared with a GO concentration of 5.0 mg mL(-1) exhibits macroscopically aligned LC structures, which endow the gels with anisotropic optical, mechanical properties, and dimensional changes during the phase transition. The hydrogels show dramatically enhanced tensile mechanical properties and phase transition rates. The oriented LC structures are not damaged during the phase transition of the PNIPAM/GO hydrogels, and hence their LC behavior undergoes reversible change. Moreover, highly oriented LC structures can also be formed when the gels are elongated, even for the gels which do not have macroscopically oriented LC structures. Very impressively, the oriented LC structures in the hydrogels can be permanently maintained by drying the gel samples elongated to and then kept at a constant tensile strain. The thermosensitive nature of PNIPAM and the angle-dependent nature of the macroscopically aligned GO LC structures allow the practical applications of the PNIPAM/GO hydrogels as optical switches, soft sensors, and actuators and so on.

Hierarchical Layered Heterogeneous Graphene-poly(N-isopropylacrylamide)-clay Hydrogels with Superior Modulus, Strength, and Toughness

Biological composites are renowned for their elaborate heterogeneous architectures at multiple scales, which lead to a unique combination of modulus, strength, and toughness. Inspired by biological composites, mimicking the heterogeneous structural design principles of biological composites is a powerful strategy to construct high-performance structural composites. Here, we creatively transfer some heterogeneous principles of biological composites to the structural design of nanocomposite hydrogels. Unique heterogeneous conductive graphene-PNIPAM-clay hydrogels are prepared through a combination of inhomogeneous water removal processes, in situ free-radical polymerization, and chemical reduction of graphene oxide. The nanocomposite hydrogels exhibit hierarchical layered heterogeneous architectures with alternate stacking of dense laminated layers and loose porous layers. Under tensile load, the stiff dense laminated layers serve as sacrificial layers that fracture at a relatively low strain, while the stretchable loose porous layers serve as energy dissipation layers by large extension afterward. Such local inhomogeneous deformation of the two heterogeneous layers enables the nanocomposite hydrogels to integrate superior modulus, strength, and toughness (9.69 MPa, 0.97 MPa, and 5.60 MJ/m(3), respectively). The study might provide meaningful enlightenments for rational structural design of future high-performance nanocomposite hydrogels.

Monodomain hydrogels prepared by shear-induced orientation and subsequent gelation

Shear-induced orientation of liquid crystalline hydroxypropylcellulose solution with reactants is frozen by subsequent polymerization and gelation process, resulting in monodomain hydrogel with anisotropic optical, swelling, and mechanical properties.