Recent progress in the shape deformation of polymeric hydrogels from memory to actuation

Construction of Supramolecular Polymer Network Elastomers Based on Pillar[5]arene/Alkyl Chain Host-Guest Interactions

As a special kind of supramolecular compound with many favorable properties, pillar[n]arene-based supramolecular polymer networks (SPNs) show potential application in many fields. Although we have come a long way using pillar[n]arene to prepare SPNs and construct a series of smart materials, it remains a challenge to enhance the mechanical strength of pillar[n]arene-based SPNs. To address this issue, a new supramolecular regulation strategy was developed, which could precisely control the preparation of pillar[n]arene-based SPN materials with excellent mechanical properties by adjusting the polymer network structures. Specifically, we utilized the host-guest interaction between pillar[5]arene and the alkyl chain of butyl acrylate monomer to form a supramolecular polymer network and achieved the transformation of different states by regulating the cross-linking density of the polymer networks. Additionally, the polymer networks exhibited good stimuli responsiveness as well as excellent dynamic properties and reconfigurable characteristics through a temperature change and the addition of competitive hosts or guests. The research provided new possibilities for the development of polymer materials.

Toward Customizable Smart Gels: A Comprehensive Review of Innovative Printing Techniques and Applications

New production technologies have transformed modern engineering fields, including electronics, mechanics, robotics, and biomedicine. These advancements have led to the creation of smart materials such as alloys, polymers, and gels that respond to various stimuli. This review focuses on smart materials (SMs), including their variety and fabrication techniques, that can be used to construct three- or four-dimensional structures. The mechanisms and designs of smart materials, limitations of current printing technologies, and perspectives for their future uses are also discussed in this review. The printed smart materials are expected to have a major impact on the design of real-world applications.

Constructing Dynamic Macropores in Thermo-Responsive Hydrogel Actuator for Large-Deformable Gripper

Poly(N-isopropyl acrylamide) (PNIPAm)-based smart hydrogels are widely employed in emerging applications such as drug delivery and tissue engineering, because their lower critical solution temperature (LCST) is close to physiological conditions. However, the dense chain collapse during the thermo-responsive phase transition restricts water diffusion, resulting in limited volumetric change. Here, a pure PNIPAm hydrogel that achieves a large-scale volume transition by incorporating PNIPAm microgels, is presented. During its thermo-responsive shrinkage, the microgels contract to 10% of their original volume, generating open macropores that serve as efficient water channels, thereby facilitating volume change of hydrogel bulk. In contrast to conventional PNIPAm hydrogels with static porous structures, these dynamic macropores disappear when the microgels return to their initial state at lower temperatures, preserving the mechanical integrity of the entire hydrogel. This enhanced deformability enables the bilayer hydrogel actuator to achieve bending angles exceeding 1150°, a sixfold increase over traditional PNIPAm-based actuators, allowing it to function as an intelligent gripper capable of capturing small, mobile organisms. This approach, which addresses the inherent challenge of achieving large-scale deformability in conventional bulk PNIPAm hydrogels, is distinct from existing strategies.

Fabrication of PHFPO Surface-Modified Conductive AgNWs/PNAGA Hydrogels with Enhanced Water Retention Capacity toward Highly Sensitive Strain Sensors

Conductive hydrogels, characterized by their unique features of flexibility, biocompatibility, electrical conductivity, and responsiveness to environmental stimuli, have emerged as promising materials for sensitive strain sensors. In this study, a facile strategy to prepare highly conductive hydrogels is reported. Through rational structural and synthetic design, silver nanowires (AgNWs) are incorporated into poly(N-acryloyl glycinamide) (PNAGA) hydrogels, achieving high electrical conductivity (up to 0.88 S m-1), significantly enhanced mechanical properties, and elevated deformative sensitivity. Furthermore, surface modification with polyhexafluoropropylene oxide (PHFPO) has substantially improved the water retention capacity and dressing comfort of this hydrogel material. Based on the above merits, these hydrogels are employed to fabricate highly sensitive wearable strain sensors which can detect and interpret subtle hand and finger movements and enable precise control of machine interfaces. The AgNWs/PNAGA based strain sensors can effectively sense finger motion, enabling the control of robotic fingers to replicate the human hand's gestures. In addition, the high deformative sensitivity and elevated water retention performance of the hydrogels makes them suitable for flow sensing. These conceptual applications demonstrate the potential of this conductive hydrogel in high-performance strain sensors in the future.

Use of Hydrogels in Regenerative Medicine: Focus on Mechanical Properties

Bioengineered materials represent an innovative option to support the regenerative processes of damaged tissues, with the final objective of creating a functional environment closely mimicking the native tissue. Among the different available biomaterials, hydrogels represent the solution of choice for tissue regeneration, thanks to the easy synthesis process and the highly tunable physical and mechanical properties. Moreover, hydrogels are biocompatible and biodegradable, able to integrate in biological environments and to support cellular interactions in order to restore damaged tissues' functionality. This review offers an overview of the current knowledge concerning hydrogel synthesis and characterization and of the recent achievements in their experimental use in supporting skin, bone, cartilage, and muscle regeneration. The currently available in vitro and in vivo results are of great interest, highlighting the need for carefully designed and controlled preclinical studies and clinical trials to support the transition of these innovative biomaterials from the bench to the bedside.

Intelligent Hydrogel-Assisted Hepatocellular Carcinoma Therapy

Given the high malignancy of liver cancer and the liver's unique role in immune and metabolic regulation, current treatments have limited efficacy, resulting in a poor prognosis. Hydrogels, soft 3-dimensional network materials comprising numerous hydrophilic monomers, have considerable potential as intelligent drug delivery systems for liver cancer treatment. The advantages of hydrogels include their versatile delivery modalities, precision targeting, intelligent stimulus response, controlled drug release, high drug loading capacity, excellent slow-release capabilities, and substantial potential as carriers of bioactive molecules. This review presents an in-depth examination of hydrogel-assisted advanced therapies for hepatocellular carcinoma, encompassing small-molecule drug therapy, immunotherapy, gene therapy, and the utilization of other biologics. Furthermore, it examines the integration of hydrogels with conventional liver cancer therapies, including radiation, interventional therapy, and ultrasound. This review provides a comprehensive overview of the numerous advantages of hydrogels and their potential to enhance therapeutic efficacy, targeting, and drug delivery safety. In conclusion, this review addresses the clinical implementation of hydrogels in liver cancer therapy and future challenges and design principles for hydrogel-based systems, and proposes novel research directions and strategies.

Soft Actuators and Actuation: Design, Synthesis, and Applications

Soft actuators are one of the most promising technological advancements with potential solutions to diverse fields' day-to-day challenges. Soft actuators derived from hydrogel materials possess unique features such as flexibility, responsiveness to stimuli, and intricate deformations, making them ideal for soft robotics, artificial muscles, and biomedical applications. This review provides an overview of material composition and design techniques for hydrogel actuators, exploring 3D printing, photopolymerization, cross-linking, and microfabrication methods for improved actuation. It examines applications of hydrogel actuators in biomedical, soft robotics, bioinspired systems, microfluidics, lab-on-a-chip devices, and environmental, and energy systems. Finally, it discusses challenges, opportunities, advancements, and regulatory aspects related to hydrogel actuators.

Biomimetic Multiscale Oriented PVA/NRL Hydrogel Enabled Multistimulus Responsive and Smart Shape Memory Actuator

Shape memory hydrogels provide a worldwide scope for functional soft materials. However, most shape memory hydrogels exhibit poor mechanical properties, leading to low actuation strength, which severely limits their applications in smart biomimetic devices. Herein, a strategy for muscle-inspired shape memory-oriented polyvinyl alcohol (PVA)-natural rubber latex (NRL) hydrogel (OPNH) with multiscale oriented structure is demonstrated. The shape memory function comes from the stretch-induced crystallization of natural rubber (NR), while PVA forms strong hydrogen bonding interactions with proteins and phospholipids on the surface of NRL particles. Meanwhile, the reconfigurable interactions of PVA and NR produce a multiscale-oriented structure during stretch-drying, improving the mechanical and shape memory properties. The resultant OPNH shows excellent interfacial compatibility, exhibiting outstanding mechanical performance (3.2 MPa), high shape fixity (≈80%) and shape recovery ratio (≈92%), high actuation strength (206 kPa), working capacity (105 kJ m- 3), extremely short response time (≈2 s), low response temperature (28 °C) and smart thermal responsiveness. It can even maintain muscle-like working capacity when lifting a load equivalent to 372 times its weight, providing a new class shape memory material for the application in smart biomimetic muscles and multistimulus responsive devices.

Cephalopod-Inspired Chemical-Gated Hydrogel Actuation Systems for Information 3D-Encoding Display

Cephalopods evolve the acetylcholine-gated actuation control function of their skin muscles, which enables their dynamic/static multimode display capacities for achieving perfectly spatial control over the colors/patterns on every inch of skin. Reproduction of artificial analogs that exhibit similar multimodal display is essential to reach advanced information three-dimensional (3D) encoding with higher security than the classic 2D-encoding strategy, but remains underdeveloped. The core difficulty is how to replicate such chemical-gated actuation control function into artificial soft actuating systems. Herein, this work proposes to develop azobenzene-functionalized poly(acrylamide) (PAAm) hydrogel systems, whose upper critical solution temperature (UCST) type actuation responsiveness can be intelligently programmed or even gated by the addition of hydrophilic α-cyclodextrin (α-CD) molecules for reversible association with pendant azobenzene moieties via supramolecular host-guest interactions. By employing such α-CD-gated hydrogel actuator as an analogue of cephalopods' skin muscle, biomimetic mechanically modulated multicolor fluorescent display systems are designed, which demonstrate a conceptually new α-CD-gated "thermal stimulation-hydrogel actuation-fluorescence output" display mechanism. Consequently, high-security 3D-encoding information carriers with an unprecedented combination of single-input multiple-output, dynamic/static dual-mode and spatially controlled display capacities are achieved. This bioinspired strategy brings functional-integrated features for artificial display systems and opens previously unidentified avenues for information security.

Shape Memory Hydrogels for Biomedical Applications

The ability of shape memory polymers to change shape upon external stimulation makes them exceedingly useful in various areas, from biomedical engineering to soft robotics. Especially, shape memory hydrogels (SMHs) are well-suited for biomedical applications due to their inherent biocompatibility, excellent shape morphing performance, tunable physiochemical properties, and responsiveness to a wide range of stimuli (e.g., thermal, chemical, electrical, light). This review provides an overview of the unique features of smart SMHs from their fundamental working mechanisms to types of SMHs classified on the basis of applied stimuli and highlights notable clinical applications. Moreover, the potential of SMHs for surgical, biomedical, and tissue engineering applications is discussed. Finally, this review summarizes the current challenges in synthesizing and fabricating reconfigurable hydrogel-based interfaces and outlines future directions for their potential in personalized medicine and clinical applications.

Hydrogel Rivet with Unidirectional Shape Morphing for Flexible Mechanical Assembly

Integrating diverse materials and functions into highly additive produce has piqued global interest due to the increasing demands of intelligent soft robotics. Nevertheless, existing assembly techniques, especially supramolecular assembly which heavily rely on precise chemical design and specific recognition, may prove inadequate when confronted with diverse external demands. Inspired by the traditional mechanical assembly, rivet connection, herein, a thermo-responsive hydrogel with unidirectional shape-morphing is fabricated and a stable mechanical assembly is constructed by emulating the rivet connection mechanism. This system employed poly(acrylamide-co-acrylic acid) [P(AAm-co-AAc)] to induce continuous swelling and hexylamine-modified polyvinyl alcohol (PVA-C6) as a molecular switch to control the swelling process. The hydrogel rivet, initially threaded through pre-fabricated hollows in two components. Subsequently, upon the disassociation of alkane chains the molecular switch would activate, inducing swelling and stable mechanical assembly via anchor structures. Moreover, to enhance the assembly strength, knots are introduced to enhance assembly strength, guiding localized stress release for programmed deformations. Additionally, the system can be remotely controlled using near-infrared light (NIR) by incorporating photo-thermal nanoparticles. This work presents a universal and efficient strategy for constructing stable mechanical assemblies without compromising overall softness, offering significant potential for the fabrication of integrated soft robots.

Masking Strategy Constructed Metal Coordination Hydrogels with Improved Mechanical Properties for Flexible Electronic Sensors

Metal coordination hydrogels (MC-HGs) that introduce dynamically coordinate bonds together with metal ionic conduction have attracted considerable attention in flexible electronics. However, the traditional soaking method alleged to have technical scalability faces the challenge of forming MC-HGs with a "core-shell" structure, which undoubtedly reduces the whole mechanical properties and ionic stimulation responsiveness required for flexible electronics materials. Herein, a novel strategy referred to as "masking" has been proposed based on the theory of the valence bond and coordination chemistry. By regulating the masking agents and their concentrations as well as pairing mode with the metal ions, the whole mechanical properties of the resulting composites (MC-HGsMasking) show nearly double the values of their traditional soaking samples (MC-HGsSoaking). For example, the fracture stress and toughness of Fe-HGsMasking(SA, 5.0 g/L) are 1.55 MPa and 2.14 MJ/m3, almost twice those of Fe-HGsSoaking (0.83 MPa and 0.93 MJ/m3, respectively). Microstructure characterization combined with finite element analysis, molecular dynamics, and first-principles simulations demonstrates that the masking strategy first facilitating interfacial permeation of metal complexes and then effective coordination with functional ligands (carboxylates) of the hydrogels is the mechanism to strengthen the mechanical properties of composites MC-HGsMasking, which has the potential to break through the limitations of current MC-HGs in flexible electronic sensor applications.

Self-assembled active actomyosin gels spontaneously curve and wrinkle similar to biological cells and tissues

Living systems adopt a diversity of curved and highly dynamic shapes. These diverse morphologies appear on many length scales, from cells to tissues and organismal scales. The common driving force for these dynamic shape changes are contractile stresses generated by myosin motors in the cell cytoskeleton, that converts chemical energy into mechanical work. A good understanding of how contractile stresses in the cytoskeleton arise into different three-dimensional (3D) shapes and what are the shape selection rules that determine their final configurations is still lacking. To obtain insight into the relevant physical mechanisms, we recreate the actomyosin cytoskeleton in vitro, with precisely controlled composition and initial geometry. A set of actomyosin gel discs, intrinsically identical but of variable initial geometry, dynamically self-organize into a family of 3D shapes, such as domes and wrinkled shapes, without the need for specific preprogramming or additional regulation. Shape deformation is driven by the spontaneous emergence of stress gradients driven by myosin and is encoded in the initial disc radius to thickness aspect ratio, which may indicate shaping scalability. Our results suggest that while the dynamical pathways may depend on the detailed interactions between the different microscopic components within the gel, the final selected shapes obey the general theory of elastic deformations of thin sheets. Altogether, our results emphasize the importance for the emergence of active stress gradients for buckling-driven shape deformations and provide insights on the mechanically induced spontaneous shape transitions in contractile active matter, revealing potential shared mechanisms with living systems across scales.

A general route to strong, conductive and antibacterial curdlan-based purely natural eutectohydrogels with self-assembled layer-by-layer network structure

To promote the application extension of curdlan from food industry- dominant to advanced flexible biomaterials, a novel group of purely natural curdlan gels with noticeable performance was developed through a simple heating-cooling approach, i.e., heating the dispersion of pristine curdlan in mixed acidic natural deep eutectic solvents (NADESs) and water at 60-90 °C, and cooling at ambient temperature. The NADESs employed are composed of choline chloride and natural organic acids (lactic acid as the representative). The as-developed gels (called eutectohydrogels) are not only compressible and stretchable but conductive, which traditional curdlan hydrogels are not attainable. The compressive stress at 90 % strain exceeds 2.00 ± 0.03 MPa, the tensile strength and fracture elongation reach 0.131 ± 0.002 MPa and 300 ± 9 % respectively, attributed to the distinctive, reciprocally linked self-assembled layer-by-layer network structure formed during gelation. An electric conductivity up to 2.22 ± 0.04 S‧m^-1 is achieved. The excellent mechanics and conductivity confer them good strain-sensing behavior. Additionally, the eutectohydrogels display high antibacterial activity against S. aureus (a model Gram-positive bacterium) and E. coli (a model Gram-negative bacterium). The outstanding comprehensive performance together with the purely natural attribute makes them broad application prospects in biomedical fields like flexible bioelectronics.

A dual stimuli-responsive smart soft carrier using multi-material 4D printing

This paper proposes a 4D printed smart soft carrier with a hemispherical hollow and openable lid. The soft carrier is composed of a lid with a slot (with a shape of 4 legs), a border, and a hemisphere. The soft carrier is fabricated by 4D printing using smart hydrogels. Specifically, the lid, border, and hemisphere are fabricated using a thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel, a non-responsive polyethylene glycol (PEG) hydrogel with superparamagnetic iron oxide nanoparticles (SPIONs), and a PEG hydrogel, respectively. Since the SPIONs are included in the border, the slot in the center of the lid is opened and closed according to the temperature change caused by near-infrared (NIR) irradiation, and the proposed soft carrier is magnetically driven by an external magnetic field. The hemisphere enables the storage and transport of cargo. The proposed soft carrier can control the opening and closing of the slot and movement to a desired position in water. Several cargo delivery experiments were conducted using various shapes and numbers of cargo. In addition, the proposed soft carrier can successfully handle small living marine organisms. This soft carrier can be manufactured by 4D printing and operated by dual stimuli (NIR and magnetic field) and can safely deliver various types of cargo and delicate organisms without leakage or damage. The flexibility of 4D printing enables the size of the soft carrier to be tailored to the specific physical attributes of various objects, making it an adaptable and versatile delivery approach.

A 3D multistage information encryption platform with self-erasure function based on a synergistically shape-deformable and AIE fluorescence-tunable hydrogel

The traditional stored information is statically shown on single 2D planes, which leads to low information storage capacity and secondary information leakage without the proper handling of decrypted information. Developing a 3D multistage information encryption platform with self-erasure function is highly desirable. Here, a novel bilayer hydrogel with synergistic deformation and fluorescence color (SDFC) change is designed for 3D multistage information encryption. The bilayer hydrogel consisting of a shape-deformable hydrogel layer and a fluorescence hydrogel layer with aggregation-induced emission (AIE) properties can exhibit pH-responsive SDFC change. Fluorescence information can be ionoprinted on the fluorescent hydrogel layer based on electrostatic interactions and dynamic covalent bonds. The 2D bilayer hydrogel encoded with information can synergistically produce predesigned 3D shape configuration and enhanced background fluorescence to wrap information, which is only readable after sequential shape recovery with the disappearance of background fluorescence. Furthermore, multistage information can be further obtained by stepwise decryption due to information with differential fluorescence fading rates. The displayed information is automatically self-erased in the end, avoiding the information secondary leakage. This study paves an avenue for broadening conventional 2D single-level information encryption platforms to 3D multistage counterparts with self-erasure and multi-decryption capabilities based on SDFC change of the bilayer hydrogel.

Swelling-Activated, Soft Mechanochemistry in Polymer Materials

Swelling in polymer materials is a ubiquitous phenomenon. At a molecular level, swelling is dictated by solvent-polymer interactions, and has been thoroughly studied both theoretically and experimentally. Favorable solvent-polymer interactions result in the solvation of polymer chains. For polymers in confined geometries, such as those that are tethered to surfaces, or for polymer networks, solvation can lead to swelling-induced tensions. These tensions act on polymer chains and can lead to stretching, bending, or deformation of the material both at the micro- and macroscopic scale. This Invited Feature Article sheds light on such swelling-induced mechanochemical phenomena in polymer materials across dimensions, and discusses approaches to visualize and characterize these effects.

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.

Thermal-Responsive Hydrogel Actuators with Photo-Programmable Shapes and Actuating Trajectories

Thermal-responsive hydrogel actuators have aroused a wide scope of research interest and have been extensively studied. However, their actuating behaviors are usually monotonous due to their unchangeable shapes and structures. Here, we report thermal-responsive poly(isopropylacrylamide-co-2-(dimethylamino)ethyl methacrylate)/alginate hydrogels with programmable external shapes and internal actuating trajectories. The volume phase transition temperatures of the resulting hydrogels can be tuned in a wide temperature range from 32 to above 50 °C by adjusting the monomer composition. While the formation and photo-dissociation of Fe³⁺-carboxylate tri-coordinates within the entire hydrogel network enable photo-responsive shape memory property, the insufficient dissociation of the tri-coordinates along the irradiation path gives rise to gradient crosslinking for realizing thermal-responsive actuation. Controlling the evolution of the gradient structure facilitates the regulation of the actuating amplitude. Furthermore, we show that the combination of these two types of shape-changing functionalities leads to more flexible and intricate shape-changing behaviors. One interesting application, a programmable hook with changeable actuating behaviors for lifting different objects with specific shapes, is also demonstrated. The proposed strategy can be extended to other types of actuating hydrogels with more advanced actuating behaviors.

Mechanically Ultra-Robust, Elastic, Conductive, and Multifunctional Hybrid Hydrogel for a Triboelectric Nanogenerator and Flexible/Wearable Sensor

Flexibility/wearable electronics such as strain/pressure sensors in human-machine interactions (HMI) are highly developed nowadays. However, challenges remain because of the lack of flexibility, fatigue resistance, and versatility, leading to mechanical damage to device materials during practical applications. In this work, a triple-network conductive hydrogel is fabricated by combining 2D Ti₃ C₂ T_x nanosheets with two kinds of 1D polymer chains, polyacrylamide, and polyvinyl alcohol. The Ti₃ C₂ T_x nanosheets act as the crosslinkers, which combine the two polymer chains of PAM and PVA via hydrogen bonds. Such a unique structure endows the hydrogel (MPP-hydrogel) with merits such as mechanical ultra-robust, super-elasticity, and excellent fatigue resistance. More importantly, the introduced Ti₃ C₂ T_x nanosheets not only enhance the hydrogel's conductivity but help form double electric layers (DELs) between the MXene nanosheets and the free water molecules inside the MPP-hydrogel. When the MPP-hydrogel is used as the electrode of the triboelectric nanogenerator (MPP-TENG), due to the dynamic balance of the DELs under the initial potential difference generated from the contact electrification as the driving force, an enhanced electrical output of the TENG is generated. Moreover, flexible strain/pressure sensors for tiny and low-frequency human motion detection are achieved. This work demonstrates a promising flexible electronic material for e-skin and HMI.

Anion-Induced Reversible Actuation of Squaramide-Crosslinked Polymer Gels

Supramolecular anion binding to squaramide crosslinkers in poly(N,N-dimethylacrylamide) gel networks enhances swelling and allows reversible chemically driven actuation. The volume swelling ratio of the gels is shown to depend on both the type of anion and its concentration. ¹H NMR and UV-vis titrations with the squaramide crosslinkers reveal a relationship between anion binding affinity and the concentration-dependent swelling behavior. Gel swelling is shown to be reversible, and by embedding a solid support into rod-shaped gels, soft actuators are fabricated that undergo forward and backward bending motion in response to changing anion concentration. The swelling and bending process, which is accompanied by intense green coloration of the gel, is achieved by using only low amounts of crosslinker. This macroscopic actuation achieved by anion binding to specific molecular entities in the polymer network will open new opportunities in the field of chemically responsive materials.

Functionalized Cellulose Nanofibers as Crosslinkers to Produce Chitosan Self-Healing Hydrogel and Shape Memory Cryogel

Cellulose nanofibers functionalized with multiple aldehyde group were synthesized as the crosslinker to produce composite self-healing hydrogel and shape memory cryogel from chitosan. The hydrogel possessed effective self-healing (∼100% efficiency) and shear-thinning properties. The cryogel had macroporous structure, large water absorption (>4300%), and high compressibility. Both hydrogel and cryogel were injectable. In particular, the cryogel (nanocellulose/chitosan 1:6) revealed thermally induced shape memory, the mechanism of which was elucidated by in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) as changes in orientation of the induced crystalline structure during the shape memory program. The shape memory cryogel with a large size (15 mm × 10 mm × 1.1 mm) injected through a 16 G syringe needle was recoverable in 37 °C water. Moreover, the cryogel was cytocompatible and promoted cell growth. The nanocellulose-chitosan composite hydrogel and cryogel are injectable and degradable biomaterials with adjustable mechanical properties for potential medical applications.

Light-Guided Growth of Gradient Hydrogels with Programmable Geometries and Thermally Responsive Actuations

Hydrogel actuators have gained considerable interest and experienced significant advancements in recent years. However, the programming of their actuating behaviors is still challenging. Herein, we report the development and regulation of gradient structures of hydrogels for programmable thermally responsive actuating behaviors. The hydrogel actuators are developed by controlling the photoreduction of Fe³⁺ ions coordinated with carboxylate groups from the substrates and their limited diffusion into the precursor solutions to act as both initiators and crosslinkers. The developed hydrogels show well-defined external geometries and controllable thicknesses under spatiotemporal control of ultraviolet irradiation. The shapes and the actuation amplitudes of the hydrogel actuators can be independently regulated by controlling the formation and photodissociation of Fe³⁺-carboxylate coordination in the formed gradient networks. Some interesting applications such as the lifting of an object with a specific shape and directional walking are realized. The proposed method can be extended to other hydrogel actuators with different compositions and stimuli-responsive behaviors.

An UV-photo and ionic dual responsive interpenetrating network hydrogel with shape memory and self-healing properties

Shape memory hydrogels have attracted extensive attention in fields such as artificial tissues, biomimetic devices and diagnostics, and intelligent biosensors. However, the practical applications were hindered by the absence of self-healing capability and multi-stimuli-responsiveness. To address these issues, we developed a shape memory hydrogel with self-healing and dual stimuli-response performance. The hydrogel system was constructed via an interpenetrating network consisting of in situ radical polymerization and host-guest interaction. The hydrogel exhibited rapid self-healing property, which can be stretched after self-healing for 1 min at 25 °C. Besides, the hydrogel displayed varied swelling performance in different light or solvent conditions. Moreover, the hydrogel showed a dual stimuli-responsive shape memory effect to ultraviolet (UV) light and ionic strength in 1 min. Such a shape memory hydrogel with self-healing ability and multi-stimuli-responsive properties will offer an option toward intelligent soft materials for biomedical and bionic research.

Stiffness Variable Polymers Comprising Phase-Changing Side-Chains: Material Syntheses and Application Explorations

Stiffness variable materials have been applied in a variety of engineering fields that require adaptation, automatic modulation, and morphing because of their unique property to switch between a rigid, load-bearing state and a soft, compliant state. Stiffness variable polymers comprising phase-changing side-chains (s-SVPs) have densely grafted, highly crystallizable long alkyl side-chains in a crosslinked network. Such a bottlebrush network-like structure gives rise to rigidity modulation as a result of the reversible crystallization and melting of the side chains. The corresponding modulus changes can be more than 1000-fold within a narrow temperature span, from ≈10²  MPa to ≈10²  kPa or lower. Other important properties of the s-SVP, such as stretchability, optical transmittance, and adhesion, can also be altered. This work reviews the underlying molecular mechanisms in the s-SVP's, discusses the material's structure-property relationship, and summarizes important applications explored so far, including reversible shape transformation, bistable electromechanical transduction, optical modulation, and reversible adhesion.

Multicompartment Hydrogels

Hydrogels belong to the most promising materials in polymer and materials science at the moment. As they feature soft and tissue-like character as well as high water-content, a broad range of applications are addressed with hydrogels, e.g. tissue engineering and wound dressings but also soft robotics, drug delivery, actuators and catalysis. Ways to tailor hydrogel properties are crosslinking mechanism, hydrogel shape and reinforcement, but new features can be introduced by variation of hydrogel composition as well, e.g. via monomer choice, functionalization or compartmentalization. Especially, multicompartment hydrogels drive progress towards complex and highly functional soft materials. In the present review the latest developments in multicompartment hydrogels are highlighted with a focus on three types of compartments, i.e. micellar/vesicular, droplets or multi-layers including various sub-categories. Furthermore, several morphologies of compartmentalized hydrogels and applications of multicompartment hydrogels will be discussed as well. Finally, an outlook towards future developments of the field will be given. The further development of multicompartment hydrogels is highly relevant for a broad range of applications and will have a significant impact on biomedicine and organic devices. This article is protected by copyright. All rights reserved.

Supramolecular Assembly of Shape Memory and Actuating Hydrogels for Programmable Shape Transformation

The deformable diversity of organisms in nature has inspired the development of bionic hydrogel actuators. However, the anisotropic structures of hydrogel actuators cannot be altered after the fabrication process, which restricts hydrogel actuators to provide complex and diverse shape deformations. Herein, we propose a dual programming method to generate numerous anisotropic structures from initial isotropic gelatin-containing hydrogels; the isotropic hydrogel blocks could be first assembled into anisotropic structures based on the coil-triple helix transition of gelatin, and then, the assembled hydrogels could further be fixed into various temporary anisotropies, so that they can produce complex and diverse deformations under the stimulation of pH. In addition, the shape programming and deformation behaviors are reversible. This dual programming method provides more potential for the application of hydrogel actuators in soft robots and bionics.

Photo-Dissociable Fe3+-Carboxylate Coordination: A General Approach toward Hydrogels with Shape Programming and Active Morphing Functionalities

An extendable double network design for hydrogels with programmable external geometries and actuating trajectories is presented. Chemically cross-linked polyacrylamide as the first network penetrated with linear alginate chains is prepared for demonstration. The coordination of Fe³⁺ ions with carboxylate groups in alginate chains acts as the second network, and its dissociation through photoreduction is utilized to realize the photoresponsive shape memory property; the shape fixity ratio and shape recovery ratio both exceed 90%. The gradient dissociation of Fe³⁺-carboxylate coordination under UV facilitates 3D programming of hydrogel geometry. On another aspect, the resulted cross-linking gradient differentiates the extent and rate of solvent-induced volume change of the PAAm network, endowing the hydrogel with photo-programmable solvent-driven actuating behavior. Furthermore, by inducing the formation of Fe³⁺-carboxylate coordination within the entire network for shape programming and cross-linking gradients in specific regions as active joints, hydrogels with designed actuating behaviors based on specific 3D shapes are realized. The shape memory and active morphing functionalities enabled by photo-dissociable Fe³⁺-carboxylate coordination in PAAm hydrogel can be generally extended to other hydrogels.

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.