Bioinspired Simultaneous Changes in Fluorescence Color, Brightness, and Shape of Hydrogels Enabled by AIEgens

Large Capacity of Data Storage and Information Encryption in Optical Encoder Disk by Integrating Phase Angle and Time Lock Based on Luminescence Metal Nanoclusters

The traditional "matrix" symbol patterns from the luminescence materials are mainly involved in a 2D plane, which seriously limits the information security and storage capacity. Here, a novel strategy is designed to extend two additional dimensions into a 2D plane by integrating time-gated response and phase angle changes of luminescent patterns. The strong orange fluorescence assemblies in an optical encoder disk are obtained after adding metal ions (Zn2+ or Al3+) and ammonia into copper nanoclusters (CuNCs) mainly due to aggregation-induced emission (AIE) behavior. The number of CuNCs-based aggregates is closely related to rotating angle changes. On the contrary, these aggregates can be reversibly dissembled upon exposing to adenosine triphosphate (ATP) in concomitant with their luminescence quenching. Their different quenching rates are on-demand controlled by the coordination reaction kinetics between ATP and metal ions in different pH value, which is conducive to the design of a series of time-locked information. The encoding patterns comprehensively utilize the static and dynamic characteristics of materials by rotating the phase angle at the specific time. The phase angle and time double locks is added into 2D plane to form a 4D storage models, which realizes higher-level information encryption and larger data storage capacity.

A highly stretchable, self-adhesive, anti-freezing dual-network conductive carboxymethyl chitosan based hydrogel for flexible wearable strain sensor

Achieving the integration of multiple properties in a single hydrogel system faces significant challenges. This research presents a simple approach to developing a multifunctional conductive hydrogel with high stretchability (>740 %), electrical conductivity, frost resistance and self-adhesiveness. It serves as a wearable, flexible electronic material, it remains functional even in low-temperature environments. The hydrogel is synthesized by incorporating a uniformly mixed solution of carboxymethyl cellulose (CMC) and aminated carbon nanotubes (NH2-CNTs) into a polyacrylamide (PAM)/gelatin dual-network hydrogel. By adjusting the CMC mass fraction, the optimal composite hydrogel is obtained within a specified gradient. After cross-linking modification with a calcium chloride (CaCl2) solution, enhances its mechanical properties, resulting in a final hydrogel with excellent stretchability (strain = 749 %), strong adhesion, frost resistance, moisture retention, and conductivity. Additionally, this research explores the hydrogel's potential for anti-counterfeiting and salt ion monitoring by analyzing changes in mechanical properties and transparency. The hydrogel exhibits high sensitivity to external strains and effectively monitors human signals such as finger bending, head movement, and speech, even at low temperatures. This research provides new insights into flexible electronic skin, wearable sensors and human-computer interaction, expanding the potential applications of multifunctional conductive hydrogels.

H2O2-Driven Aggregation Induced Emission-Based Nanomotors for the Monitoring and Treatment of Infected Surgical Wound

Post-operative surgical wound monitoring remains a significant clinical challenge in preventing bacterial infection. Current methods rely on indirect observations or costly investigations, often detecting infections only after complications arise. Here the medical sutures coated with Janus-type nanomotors (Pt-MOFs) with infected microenvironment-responsive properties for monitoring and treating surgical site infections are prepared. The Pt-MOFs nanomotors exhibit efficient self-propulsion with enhanced penetration and diffusion in biofilms by catalyzing hydrogen peroxide to produce oxygen bubbles. Copper ions serve dual roles as structural nodes and Fenton-like catalysts, generating antibacterial hydroxyl radicals while forming non-emissive self-aggregates. Here in vitro is shown that Pt-MOFs nanomotors present excellent bacterial imaging and enhanced antibacterial activity against both Gram-positive and Gram-negative bacteria. As a proof of concept, Pt-MOFs nanomotors coated surgical sutures successfully monitor the process of Staphylococcus aureus-infected wounds on mouse model. Furthermore, in vivo studies testify that Pt-MOFs nanomotors play an important role in treating infected surgical wounds through mitigating inflammatory infiltrates, facilitating collagen deposition and accelerating reepithelialization. This combined monitoring and treatment approach offers a promising strategy for surgical wound healing.

Synchronizing Multicolor Changes and Shape Deformation Into Structurally Homogeneous Hydrogels via a Single Photochromophore

The design of synthetic hydrogels that can mimic their biological counterparts in the simultaneous production of multicolor change and shape transformation in response to environmental stimuli is of great importance toward intelligent camouflage, encryption, and actuation. Previous efforts have focused primarily on developing heterogeneous hydrogels that highly rely on respective mechanisms to achieve color and shape changes separately, and synergistically synchronizing such two variations into structurally homogenous hydrogels via a single chromophore has been challenging. Here, the molecular design of a structurally homogenous hydrogel simultaneously exhibiting synchronized multicolor change and shape deformation triggered by a single stimulus of light is reported. The synchronization mechanism originates from a coupled alteration upon irradiation in the fluorescence emission and charge states of a spiropyran photochromophore covalently incorporated into the hydrogel network, thus leading to macroscale color change and shape variation in the hydrogel, respectively. Following this principle, both positive and negative phototropic deformation are obtained concomitantly with synchronized but flexibly tunable multicolor changes upon light illumination and demonstrated the ingenious application of biomimetic actuation, encryption, and camouflage by the rational combination of these two systems. This work represents an innovative molecular design strategy for developing bioinspired materials with synchronized functions via a single compound.

Disturbance-Triggered Instant Crystallization Activating Bioinspired Emissive Gels

Many marine organisms feature sensitive sensory-perceptual systems to sense the surrounding environment and respond to disturbance with intense bioluminescence. However, it remains a great challenge to develop artificial materials that can sense external disturbance and simultaneously activate intense luminescence, although such materials are attractive for visual sensing and intelligent displays. Herein, we present a new class of bioinspired smart gels constructed by integrating hydrophilic polymeric networks, metastable supersaturated salt and fluorophores containing heterogenic atoms. Upon external disturbance, the composite gels undergo an instant and reversible soft-rigid state transition, simultaneously turning on intense fluorescence and activating ultralong afterglow emission with a maximum lifetime of 877.15 ms. The experimental results and molecular dynamics simulations reveal that the disturbance-induced luminescence mainly results from the geometrical confinement of aggregated fluorophores and enhanced molecular interactions to immensely suppress the non-radiative dissipation. Given their versatile and sensitive disturbance-responsiveness, dynamic interactive painting and 3D smart optical displays are demonstrated. This study paves a new avenue to achieve disturbance-sensing soft materials and promotes the development of smart visual sensors and interactive optical displays.

Deformation-Induced Multioptical Morphology Elastomer Constructed from Phosphorescent Nanospheres for Underwater Mechanical Sensing

Combination of multioptical morphology, such as transmission, scattering, fluorescence (FL), and room-temperature phosphorescence (RTP), to build multisignal-integrated devices is highly attractive in future optical devices but extremely difficult owing to the poorly matched material design and construction principles. Here, we report a novel multioptical morphology elastomer (MOME) fabricated by encapsulating monodisperse RTP SiO2 nanoparticles (RTP-SiO2 NPs) with polydimethylsiloxane (PDMS). The switching behavior of optical signals is dependent on the deformation of MOME, such as stretching, bending, and squeezing. The MOME changes from a transparent state to a white scattered state under white light as the deformation increases, while the FL and RTP are significantly enhanced from the original weak state. During deformation, the air voids generated by the separation of RTP-SiO2 NPs and PDMS at the interface result in a refractive index mismatch, leading to a significant enhancement of light scattering and further causing deformation-induced self-scattering enhancement behavior in FL and RTP. Moreover, MOME also has intriguing modulation phenomena, such as dynamic deformation-regulated RTP during the decay process and solvent-deformation synergistically regulated optical switching behavior. On account of the outstanding optical properties, MOME is applied in daily visual monitoring of underwater pipelines, including displacement deviation, leakage, swelling, and localized anomalous protrusions. These findings provide important breakthroughs for the design of multioptical morphology integrated devices, demonstrating great potential for applications.

High-Strength Anisotropic Fluorescent Hydrogel Based on Solvent Exchange for Patterning

Aggregation-induced emission (AIE)-active fluorescent hydrogel materials have found extensive applications in soft robotics, wearable electronics, information encryption, and biomedicine. Nevertheless, it continues to be difficult to create hydrogels that are both highly luminescent and possess strong mechanical capabilities. This study introduces a combined approach of prestretching and solvent exchange to create anisotropic luminous hydrogels made of poly(methacrylic acid-methacrylamide). This method restricts the intrachain rotation of AIE molecules and adjusts the orientation of the polymer network. The increased luminescence and mechanical qualities are determined to be caused by the clustering of AIE molecules, the creation of the associated hydrophobic phase and the asymmetrical polymer network. The fluorescent hydrogels exhibit exceptional mechanical characteristics, including a high fracture stress of 5.97 MPa, an outstanding elastic modulus of 93.97 MPa, and a fracture toughness of 7.21 MJ/m3. Furthermore, the AIE fluorescent hydrogels demonstrate outstanding water retention, antiswelling capabilities, and a writing function for solvent-regulated fluorescent information. This work presents a highly efficient technique for creating anisotropic hydrogels with changeable luminescence properties, which have the potential to be used in several applications, including information encryption, flexible sensors, and soft robots.

Multifunctional Network-Shaped Hydrogel Assemblies

The previously reported hydrogel assemblies carry bulky shapes, for which the unitary assembly form immensely restricted further applications. Yet there are abundant natural examples of network-shaped assemblies constructed by animals, of which it is brought up inspirations for constructing hydrogel assemblies. Herein, the network-shaped assemblies with diverse functions are reported. The precursor solutions are prepared by acrylamide, 4-acryloylmorpholine, choline chloride, and photo-initiators. By means of three dimension (3D) printing, the hydrogel networks are formed driven by hydrogen bonds, and then the prepared jagged hydrogel blocks are assembled into network-shaped hydrogel assembly NSHA-0 by weaving method. Benefitting from the modifiability of hydrogels, hydrogel assemblies with different properties and functions are prepared by incorporating different functional monomers including ion pair acryloyloxyethyl trimethyl ammonium chloride, and sodium p-styrenesulfonate, N-isopropylacrylamide, spiropyran derivative and tetra-(4-pyridylphenyl)ethylene. The incorporation of these monomers bestowed the assemblies self-healing ability, thermo-responsiveness, ultraviolet-responsiveness as well as acid-responsiveness respectively.

Fluorescent robust photoactuator via photo-crosslinking induced single-layered janus polyimide

Advanced smart polymer materials with the ability of reversible deformation under external stimuli hold great potential in robotics, soft machines, and flexible electronics. However, the complexity and low efficiency for fabricating actuators along with their limited functionality hinder further progress. Here an efficient and mild catalyst-free thiol-yne click polymerization was developed to fabricate photosensitive polyimide (PI) films. Then the fluorescent robust photoactuators with single-layered janus structure were directly obtained via UV assisted photo-crosslinking of the films, exhibiting reversible response driven by a pronounced mismatch in expansion between the front and back sides of the films. Achieving selective, non-uniform spatial distribution within the PI films, rapid and reversible complex morphing of the actuators, along with the capabilities for encrypting, reading, and erasing fluorescent information-all through the use of a single UV light source-becomes straightforward. The robust mechanical property and driving ability of these actuators enable the conversion of light energy into obvious motion even under heavy loads and the leaping through the storage and release of energy, ensuring their potential for practical applications that require durability and reliability.

pH-Responsive Fluorescent Switches through Intramolecular Conjugate Addition Reactions and Application in Fluorogenic Bioimaging

We report new photoluminescent switching systems achieved through pH-induced intramolecular oxa-Michael conjugate addition reactions. Ratiometric absorbance and fluorescence emission were observed across conjugate acceptors triggered by pH, resulting in specific pseudo pKa values. The effect of substituents on the pseudo pKa's was investigated, showing increased values from electron-withdrawing to electron-donating groups. Inspired by the physiologically related pKa, a fluorescent probe was designed, successfully distinguishing cancer cells from normal cells through live cellular imaging.

Application of Organic Gel on Skin Realized by Hydrogel/Organic Gel Adhesion

Diversity in solvent selection bestows the organic gel with appealing characteristics embracing antidrying, anti-icing, and antifouling abilities. However, organic gel, subjected to the "toxic" inherent property of solvent, is not able to be manipulated on skin. Herein, introducing the hydrogel layer amid organic gel and skin is envisaged to realize application of organic gel on skin. Hydrogel, inserted as the medium layer, works for the coupling role between skin and organic gel, also avoids the direct contact of organic gel toward skin. First, hydrogel system composed of acrylic acid is fabricated, meanwhile organic gel is prepared employing 2-hydroxyethyl methacrylate, ethylene glycol (EG) as solvent. Organic gel is able to adhere to hydrogel by hydrogen bonding resulting from carboxyl groups of polyacrylic acid chains and hydroxyl groups occurring on 2-hydroxyethyl methacrylate or EG. Additionally, hydrogen bonding enables the hydrogel to be firmly attached to skin, thus organic gel/hydrogel/skin assembly is produced. The further application of organic gel is exploited by incorporating stimuli-responsive dyes including spiropyran and rhodamine derivative.

4D Printing of Reprogrammable Liquid Crystal Elastomers with Synergistic Photochromism and Photoactuation

4D printing of liquid crystal elastomers (LCEs) via direct ink writing has opened up great opportunities to create stimuli-responsive actuations for applications such as soft robotics. However, most 4D-printed LCEs are limited to thermal actuation and fixed shape morphing, posing a challenge for achieving multiple programmable functionalities and reprogrammability. Here, a 4D-printable photochromic titanium-based nanocrystal (TiNC)/LCE composite ink is developed, which enables the reprogrammable photochromism and photoactuation of a single 4D-printed architecture. The printed TiNC/LCE composite exhibits reversible color-switching between white and black in response to ultraviolet (UV) irradiation and oxygen exposure. Upon near-infrared (NIR) irradiation, the UV-irradiated region can undergo photothermal actuation, allowing for robust grasping and weightlifting. By precisely controlling the structural design and the light irradiation, the single 4D-printed TiNC/LCE object can be globally or locally programmed, erased, and reprogrammed to achieve desirable photocontrollable color patterns and 3D structure constructions, such as barcode patterns and origami- and kirigami-inspired structures. This work provides a novel concept for designing and engineering adaptive structures with unique and tunable multifunctionalities, which have potential applications in biomimetic soft robotics, smart construction engineering, camouflage, multilevel information storage, etc.

High-Sensitive Spatiotemporal Distribution Imaging of Compression Stresses Based on Time-Evolutional Responsiveness

Mechanoresponsive materials have been studied to visualize and measure stresses in various fields. However, the high-sensitive and spatiotemporal imaging remain a challenging issue. In particular, the time evolutional responsiveness is not easily integrated in mechanoresponsive materials. In the present study, high-sensitive spatiotemporal imaging of weak compression stresses is achieved by time-evolutional controlled diffusion processes using conjugated polymer, capsule, and sponge. Stimuli-responsive polydiacetylene (PDA) is coated inside a sponge. A mechanoresponsive capsule is set on the top face of the sponge. When compression stresses in the range of 6.67-533 kPa are applied to the device, the blue color of PDA is changed to red by the diffusion of the interior liquid containing a guest polymer flowed out of the disrupted capsule. The applied strength (F/N), time (t/s), and impulse (F·t/N s) are visualized and quantified by the red-color intensity. When a guest metal ion is intercalated in the layered structure of PDA to tune the responsivity, the device visualizes the elapsed time (τ/min) after unloading the stresses. PDA, capsule, and sponge play the important roles to achieve the time evolutional responsiveness for the high-sensitive spatiotemporal distribution imaging through the controlled diffusion processes.

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.

From Fluorescence-Transfer-Lightening-Printing-Assisted Conductive Adhesive Nanocomposite Hydrogels toward Wearable Interactive Optical Information-Electronic Strain Sensors

The interactive flexible device, which monitors the human motion in optical and electrical synergistic modes, has attracted growing attention recently. The incorporation of information attribute within the optical signal is deemed advantageous for improving the interactive efficiency. Therefore, the development of wearable optical information-electronic strain sensors holds substantial promise, but integrating and synergizing various functions and realizing strain-mediated information transformation keep challenging. Herein, an amylopectin (AP) modified nanoclay/polyacrylamide-based nanocomposite (NC) hydrogel and an aggregation-induced-emission-active ink are fabricated. Through the fluorescence-transfer printing of the ink onto the hydrogel film in different strains with nested multiple symbolic information, a wearable interactive fluorescent information-electronic strain sensor is developed. In the sensor, the nanoclay plays a synergistic "one-stone-three-birds" role, contributing to "lightening" fluorescence (≈80 times emission intensity enhancement), ionic conductivity, and excellent stretchability (>1000%). The sensor has high biocompatibility, resilience (elastic recovery ratio: 97.8%), and strain sensitivity (gauge factor (GF): 10.9). Additionally, the AP endows the sensor with skin adhesiveness. The sensor can achieve electrical monitoring of human joint movements while displaying interactive fluorescent information transformation. This research poses an efficient strategy to develop multifunctional materials and provides a general platform for achieving next-generation interactive devices with prospective applications in wearable devices, human-machine interfaces, and artificial intelligence.

Recent Progress in 3D Printing of Polymer Materials as Soft Actuators and Robots

With inspiration from natural systems, various soft actuators and robots have been explored in recent years with versatile applications in biomedical and engineering fields. Soft active materials with rich stimulus-responsive characteristics have been an ideal candidate to devise these soft machines by using different manufacturing technologies. Among these technologies, three-dimensional (3D) printing shows advantages in fabricating constructs with multiple materials and sophisticated architectures. In this Review, we aim to provide an overview of recent progress on 3D printing of soft materials, robotics performances, and representative applications. Typical 3D printing techniques are briefly introduced, followed by state-of-the-art advances in 3D printing of hydrogels, shape memory polymers, liquid crystalline elastomers, and their hybrids as soft actuators and robots. From the perspective of material properties, the commonly used printing techniques and action-generation principles for typical printed constructs are discussed. Actuation performances, locomotive behaviors, and representative applications of printed soft materials are summarized. The relationship between printing structures and action performances of soft actuators and robots is also briefly discussed. Finally, the advantages and limitations of each soft material are compared, and the remaining challenges and future directions in this field are prospected.

Dansylated Nitrile N-Oxide as the Fluorescent Dye Clickable to Unsaturated Bonds without Catalyst

Fluorescent polymeric materials have been exploited in the fields of aesthetical purposes, biomedical engineering, and three-dimensional printing applications. While the fluorescent materials are prepared by the polymerization of fluorescent monomer or the blending a fluorescent dye with common polymer, the covalent immobilization of fluorescent dye onto common polymers is not the practical technique. In this paper, dansylated nitrile N-oxide (Dansyl-NO) has been designed and synthesized to be a stable nitrile N-oxide as the derivative of 2-hydroxy-1-naphthaldehyde. While Dansyl-NO shows good reactivity to an alkene and an alkyne to give fluorescent Dansyl-Ene and Dansyl-Yne, respectively, it hardly reacts to a nitrile. The results indicate that Dansyl-NO serves as a fluorescent dye clickable to alkenes and alkynes. To know the effects of solvent on the fluorescent properties, the UV-vis and fluorescence spectra of Dansyl-Ene are measured in three solvents. Dansyl-Ene shows fluorescent solvatochromism, which appears to be red-shifted along with the increase in solvent polarity. Poly(styrene-co-butadiene) directly reacts with Dansyl-NO to give fluorescent modified SB. The emission spectrum of modified SB is blue-shifted compared with that of Dansyl-Ene. The blue-shift could be possibly attributed to the presence of less polar polymer skeleton around the dansyl moieties of modified SB.

Halloysite nanotubes as nano-support matrix for programming the photo/H2O dual triggered reversible gel actuator

Gel actuators are a kind of soft intelligent material that can convert external stimuli into deformations to generate mechanical responses. The development of gel actuators with advanced structures to integrate multiple responsiveness, programmability, and fast deformation ability is urgently needed. Here, we explored a poly(7-(2-methacryloyloxyethoxy)-4-methylcoumarin-co-acrylic acid-co-glycol) ternary gel network as an actuator with reprogrammable photo/H2O dual responsibilities. In such a design, [2 + 2] photodimerization and photocleavage reactions of coumarin moieties can be realized under 365 and 254 nm light irradiation, respectively, affording reversible photodriven behaviour of the gels. The abundant carboxylic acid in the backbone has the capacity to form additional crosslinks to assist and accelerate the photodriven behaviour. The incorporation and orientation of halloysite nanotubes (HNTs) in gel matrices support an axial direction force and result in a more controllable and programmable actuating behaviour. The synergistic response enables fast grasping-releasing of 5-times the weight of the object in water within 10 min by fabricating HNT-incorporated gels as a four-arm gripper.

Active Materials for Functional Origami

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

A temperature and pressure dual-responsive, stretchable, healable, adhesive, and biocompatible carboxymethyl cellulose-based conductive hydrogels for flexible wearable strain sensor

The study aimed to develop a novel temperature and pressure dual-responsive conductive hydrogel with self-healing, self-adhesive, biocompatible, and stretchable properties, for the development of multifunctional anti-counterfeiting and wearable flexible electronic materials. A conductive hydrogel based on carboxymethyl cellulose (CMC) was synthesized by simple "one pot" free radical polymerization of CMC, acrylamide (AAm) and acrylic acid (AAc). The hydrogel displayed temperature responsiveness and possessed an upper critical solution temperature (UCST) value. In addition, hydrogels also had surprising pressure responsiveness. The synthesized hydrogels were characterized by FTIR, TGA, DSC, and XRD analysis. Importantly, the obtained hydrogels exhibited exceptional mechanical properties (stress: 730 kPa, strain: 880%), fatigue resistance, stretchability, self-healing capability, self-adhesive properties, and conductivity. In addition, valuable insights were obtained into the synthesis and application of flexible anti-counterfeiting and camouflage materials by the temperature and pressure dual-responsive hydrogels. Moreover, the prepared hydrogel, with an electrically sensitive perception of external strain (GF = 2.61, response time: 80 ms), can be utilized for monitoring human movement, emotional changes, physiological signals, language, and more, rendering it suitable for novel flexible anti-counterfeiting materials and versatile wearable iontronics. Overall, this study provided novel insights into the simple and efficient synthesis and sustainable manufacturing of environmentally friendly multifunctional anti-counterfeiting materials and flexible electronic skin sensors.

Stimulus-responsive polymer materials toward multi-mode and multi-level information anti-counterfeiting: recent advances and future challenges

Information storage and security is one of the perennial hot issues in society, while the further advancements of related chemical anti-counterfeiting systems remain a formidable challenge. As emerging anti-counterfeiting materials, stimulus-responsive polymers (SRPs) have attracted extensive attention due to their unique stimulus-responsiveness and charming discoloration performance. At the same time, single-channel decryption technology with low-security levels has been unable to effectively prevent information from being stolen or mimicked. As a result, it would be of great significance to develop SRPs with multi-mode and multi-level anti-counterfeiting characteristics. This study summarizes the latest achievements in advance anti-counterfeiting strategies based on SRPs, including multi-mode anti-counterfeiting (static information) and multi-level anti-counterfeiting (dynamic information). In addition, the promising applications of such materials in anti-counterfeiting labels, identification platforms, intelligent displays, and others are briefly reviewed. Finally, the challenges and opportunities in this emerging field are discussed. This review serves as a useful resource for manipulating SRP-based anti-counterfeiting materials and creating cutting-edge information security and encryption systems.

Bioinspired Multistimuli-Induced Synergistic Changes in Color and Shape of Hydrogel and Actuator Based on Fluorescent Microgels

Fluorescent hydrogels have emerged as one of the most promising candidates for developing biomimetic materials and artificial intelligence owing to their unique fluorescence and responsive properties. However, it is still challenging to fabricate hydrogel that exhibits synergistic changes in fluorescence color and shape in response to multistimulus via a simple method. Herein, blue- and orange-emitting fluorescent microgels (MGs) both are designed and synthesized with pH-, thermal-, and cationic-sensitivity via one-step polymerization, respectively. The two fluorescent MGs are incorporated into transparent doubly crosslinked microgel (DX MG) hydrogels with a preset ratio. The DX MG hydrogels can tune the fluorescent color accompanied by size variation via subjecting to external multistimulus. Thus, DX MG hydrogels can be exploited for multiresponsive fluorescent bilayer actuators. The actuators can undergo complex shape deformation and color changes. Inspired by natural organisms, an artificial morning glory with color and size changes are showcased in response to buffer solutions of different pH values. Besides, an intelligent skin hydrogel, imitating natural calotes versicolor, by assembling four layers of DX MG with different ratios of MGs, is tailored. This work serves as an inspiration for the design and fabrication of novel biomimetic smart materials with synergistic functions.

Manipulation of photoresponsive liquid-crystalline polymers and their applications: from nanoscale to macroscale

We summarize the molecular design of photoresponsive liquid-crystalline polymers, manipulation at multiple scales and various applications based on their intrinsic properties, providing an opportunity for future development in this field.

Flexible Actuators with Hygroscopic Adaptability for Smart Wearables and Soft Grippers

Flexible actuators have garnered significant interest in the domains of biomedical devices, human-machine interfaces, and smart wearables. However, the mechanical properties of existing materials are not sufficiently robust, and the expensive and time-consuming pretreatment process and the ambiguous high-degree-of-freedom deformation mechanism make it difficult to meet the demands of industrialized production. Hence, drawing inspiration from the adaptable movement of living organisms in the natural world, this research created and engineered a fully textile-based humidity-sensitive flexible actuator (TbHs-FA) using high-cost-effective viscose/PET fibers as raw materials. The breakthrough development in actuation performance is covered, including substantial contraction force (92.53 cN), high actuation curvature (16.78 cm-1), and fast response (264 cN s-1 and 46.61 cm-1 s-1). Additionally, the programmable stiffness system and weave structure give TbHs-FAs low hysteresis and fatigue resistance, narrowing the gap between the conceptual laboratory-scale design of existing fully textile-based humidity-sensitive flexible actuators and actual textiles. The high-degree-of-freedom and large bending deformation mechanisms are elucidated for the first time by combining microscopic mechanical structure simulation and macroscopic energy conversion analysis. The novel humidity-sensitive flexible actuator possesses strong mechanical qualities, making it suitable for applications such as flexible robots, medicinal devices, and smart wearables.

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.

Acid-base responsive multifunctional poly(formyl sulfide)s through a facile catalyst-free click polymerization of aldehyde-activated internal diynes and dithiols

Acid-base equilibria play a critical role in biological processes and environmental systems. The development of innovative fluorescent polymeric materials to monitor acid-base equilibria is highly desirable. Herein, a novel catalyst-free click polymerization of aldehyde-activated internal diynes and dithiols was established, and exclusively Markovnikov poly(formyl sulfide)s (PFSs) with high molecular weights and moderate stereoregularity were produced in high yields. Because of the aromatic units and sulfur atoms in their main chains, these polymers possessed high refractive index values. By introducing the fluorene and aldehyde moieties, the resulting PFSs could act as a fluorescent sensor for sensitive hydrazine detection. Taking advantage of the reaction of the aldehyde group and hydrazine, imino-PFSs with remarkable and reversible fluorescence change through alternating fumigation with HCl and NH3 were easily acquired and further applied in multicolor patterning, a rewritable material and quadruple-mode information encryption. Additionally, a test strip of protonated imino-polymer for the tracking of bioamines in situ generated from marine product spoilage was also demonstrated. Collectively, this work not only provides a powerful click polymerization to enrich the multiplicity of sulfur-containing materials, but also opens up enormous opportunities for these functional polysulfides in diverse applications.

Shape memory polymer with programmable recovery onset

Stimulus-responsive shape-shifting polymers1-3 have shown unique promise in emerging applications, including soft robotics4-7, medical devices8, aerospace structures9 and flexible electronics10. Their externally triggered shape-shifting behaviour offers on-demand controllability essential for many device applications. Ironically, accessing external triggers (for example, heating or light) under realistic scenarios has become the greatest bottleneck in demanding applications such as implantable medical devices8. Certain shape-shifting polymers rely on naturally present stimuli (for example, human body temperature for implantable devices)8 as triggers. Although they forgo the need for external stimulation, the ability to control recovery onset is also lost. Naturally triggered, yet actively controllable, shape-shifting behaviour is highly desirable but these two attributes are conflicting. Here we achieved this goal with a four-dimensional printable shape memory hydrogel that operates via phase separation, with its shape-shifting kinetics dominated by internal mass diffusion rather than by heat transport used for common shape memory polymers8-11. This hydrogel can undergo shape transformation at natural ambient temperature, critically with a recovery onset delay. This delay is programmable by altering the degree of phase separation during device programming, which offers a unique mechanism for shape-shifting control. Our naturally triggered shape memory polymer with a tunable recovery onset markedly lowers the barrier for device implementation.

Bioinspired simultaneous regulation in fluorescence of AIEgen-embedded hydrogels

The development of stimuli-responsive functional fluorescent hydrogels is of great significance for the realization of artificial intelligence. In the present work, we design and synthesize a stimulus-responsive hydrogel embedded with an aggregation-induced emission (AIE) monomer, in which the fluorescence brightness and intensity can be tuned. The hydrogel embedded with tetraphenylethene-grafted-poly[3-sulfopropyl methacrylate potassium salt] (TPE-PSPMA) as the functional element is prepared by the radical polymerization method. Among them, the TPE core exhibits adaptive fluorescence ability through the AIE effect, while the PSPMA chain provides tunable hydrophilic properties under an external stimulus. The effect of different cationic surfactants with different lengths of hydrophobic tails on the fluorescence properties of TPE-PSPMA in solution is systematically investigated. With cationic surfactants, such as cetyltrimethylammonium bromide (CTAB), the fluorescence intensity is gradually tuned from 1059 to 4623. And the fluorescence intensities increase with the growth of hydrophobic tails of surfactants, which results from hydrophobicity-induced electrostatic interactions among surfactants and polymer chains. Furthermore, an obvious tunable fluorescence feature of hydrogel copolymerized TPE-PSPMA is realized, resulting from the change of brightness and the dynamic increase of fluorescence intensity (from 1031 to 3138) for the hydrogel immersed in CTAB solution with different soaking times. Such a typical fluorescence-regulated behavior can be attributed to the AIE of the TPE-PSPMA chain and the electrostatic interaction between the surfactant and the anionic polymer chain. The designed TPE-PSPMA-based hydrogel is responsive to stimuli, inspiring the development of intelligent systems such as soft robots and smart wearables.

3D Printing of Multimaterial Contact Lenses

3D printing of multimaterial objects is an emerging field with promising applications. The layer-by-layer material addition technique used in 3D printing enables incorporation of distinct functionalized materials into the specialized devices. However, very few studies have been performed on the usage of multimaterial 3D printing for printable photonic and wearable devices. Here, we employ vat photopolymerization-based 3D printing to produce multimaterial contact lenses, offering enhanced multiband optical filtration, which can be valuable for tackling ocular conditions such as color blindness. A combination of hydroxyethyl methacrylate (HEMA) and polyethylene glycol diacrylate (PEGDA) was used as the base hydrogel for 3D printing. Atto565 and Atto488 dyes were added to the hydrogel for wavelength filtering, each dye suitable for a different type of color blindness. Multimaterial disks and contact lenses, with separate sections containing distinct dyes, were 3D-printed, and their optical properties were studied. The characteristics of multimaterial printing were analyzed, focusing on the formation of a uniform multimaterial interface. In addition, a novel technique was developed for printing multiple dyed materials in complex lateral geometrical patterns, by employing suitable variations in CAD models and the UV curing time. It was observed that the multimaterial printing process does not negatively affect the optical properties of the contact lenses. The printed multimaterial contact lenses offered a combined multi-band color blindness correction due to the two dyes used. The resulting optical spectrum was a close match to the commercially available color blindness correction glasses.

Hydrogel fibers for wearable sensors and soft actuators

Owing to superior softness, wetness, responsiveness, and biocompatibility, bulk hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, optics, and coatings. The one-dimensional (1D) hydrogel fibers possess the metrics from both the hydrogel materials and structural topology, endowing them with extraordinary mechanical, sensing, breathable and weavable properties. As no comprehensive review has been reported for this nascent field, this article aims to provide an overview of hydrogel fibers for soft electronics and actuators. We first introduce the basic properties and measurement methods of hydrogel fibers, including mechanical, electrical, adhesive, and biocompatible properties. Then, typical manufacturing methods for 1D hydrogel fibers and fibrous films are discussed. Next, the recent progress of wearable sensors (e.g., strain, temperature, pH, and humidity) and actuators made from hydrogel fibers is discussed. We conclude with future perspectives on next-generation hydrogel fibers and the remaining challenges. The development of hydrogel fibers will not only provide an unparalleled one-dimensional characteristic, but also translate fundamental understanding of hydrogels into new application boundaries.

Bio-Inspired Electro-Thermal-Hygro Responsive Rewritable Systems with Temporal/Spatial Control for Environment-Interactive Information Display

Utilization of rewritable luminescent materials for secure information storage and delivery has long been envisaged to reduce the cost and environmental wastes. However, it remains challenging to realize a temporally/spatially controlled display of the written information, which is crucial for secure information encryption. Here, inspired by bioelectricity-triggered skin pattern switching in cephalopods, an ideal rewritable system consisting of conductive graphene film and carbon dots (CDs) gel with blue-to-red fluorescence-color changes via water-triggered CDs aggregation and re-dispersion is presented. Its rewritability is guaranteed by using water ink to write on the CDs-gel and employing Joule heat of graphene film to evaporate water. Due to the highly controlled electrical stimulus, temporally/spatially controlled display is achieved, enabling on-demand delivery and duration time regulation of the written information. Furthermore, new-concept environment-interactive rewritable system is obtained by integrating sensitive acoustic/optical sensors and multichannel electronic time-delay devices. This work opens unprecedented avenues of rewritable systems and expands potential uses for information encryption/delivery.

Cholesteric Liquid Crystal Polymeric Coatings for Colorful Artificial Muscles and Motile Humidity Sensor Skin Integrated with Magnetic Composites

Structural colorful cholesterics show impressive susceptibility to external stimulation, leading to applications in electro/mechano-chromic devices. However, out-of-plane actuation of structural colorful actuators based on cholesterics and the integration with other stimulation remains underdeveloped. Herein, colorful actuators and motile humidity sensors are developed using humidity-responsive cholesteric liquid crystal networks (CLCNs) and magnetic composites. The developed colorful actuator can exhibit synergistic out-of-plane shape morphing and color change in response to humidity, with CLCNs as colorful artificial muscles. Through the integration with magnetic control, the motile sensor can be navigated to open and confined spaces with the aid of friction to detect local relative humidity. The integration of multi-stimulation actuation of cholesteric magnetic actuators will expand the research frontier of structural colorful actuators and motile sensors for confined spaces.

Anisotropic Bi-Layer Hydrogel Actuator with pH-Responsive Color-Changing and Photothermal-Responsive Shape-Changing Bi-Functional Synergy

Stimuli-responsive color-changing and shape-changing hydrogels are promising intelligent materials for visual detections and bio-inspired actuations, respectively. However, it is still an early stage to integrate the color-changing performance and shape-changing performance together to provide bi-functional synergistic biomimetic devices, which are difficult to design but will greatly expand further applications of intelligent hydrogels. Herein, we present an anisotropic bi-layer hydrogel by combining a pH-responsive rhodamine-B (RhB)-functionalized fluorescent hydrogel layer and a photothermal-responsive shape-changing melanin-added poly (N-isopropylacrylamide) (PNIPAM) hydrogel layer with fluorescent color-changing and shape-changing bi-functional synergy. This bi-layer hydrogel can obtain fast and complex actuations under irradiation with 808 nm near-infrared (NIR) light due to both the melanin-composited PNIPAM hydrogel with high efficiency of photothermal conversion and the anisotropic structure of this bi-hydrogel. Furthermore, the RhB-functionalized fluorescent hydrogel layer can provide rapid pH-responsive fluorescent color change, which can be integrated with NIR-responsive shape change to achieve bi-functional synergy. As a result, this bi-layer hydrogel can be designed using various biomimetic devices, which can show the actuating process in the dark for real-time tracking and even mimetic starfish to synchronously change both the color and shape. This work provides a new bi-layer hydrogel biomimetic actuator with color-changing and shape-changing bi-functional synergy, which will inspire new strategies for other intelligent composite materials and high-level biomimetic devices.

Recent Advances in Bioinspired Hydrogels: Materials, Devices, and Biosignal Computing

The remarkable ability of biological systems to sense and adapt to complex environmental conditions has inspired new materials and novel designs for next-generation wearable devices. Hydrogels are being intensively investigated for their versatile functions in wearable devices due to their superior softness, biocompatibility, and rapid stimulus response. This review focuses on recent strategies for developing bioinspired hydrogel wearable devices that can accommodate mechanical strain and integrate seamlessly with biological systems. We will provide an overview of different types of bioinspired hydrogels tailored for wearable devices. Next, we will discuss the recent progress of bioinspired hydrogel wearable devices such as electronic skin and smart contact lenses. Also, we will comprehensively summarize biosignal readout methods for hydrogel wearable devices as well as advances in powering and wireless data transmission technologies. Finally, current challenges facing these wearable devices are discussed, and future directions are proposed.

Room-Temperature Self-Healing Glassy Luminescent Hybrid Film

The self-healing of glassy polymer materials on site has always been a huge challenge due to their frozen polymer network. We herein report self-repairable glassy luminescent film by assembling a lanthanide-containing polymer with randomly hyperbranched polymers possessing multiple hydrogen (H) bonds. Because of multiple H bonds, the hybrid film exhibits enhanced mechanical strength, with high glass transition temperature (T_g) of 40.3 °C and high storage modulus of 3.52 GPa, meanwhile, dynamic exchange of multiple H bonds enables its rapid room-temperature self-healing ability. This research provides new insights in preparing mechanical robust yet repairable polymeric functional materials.

Heat-Resistant and Color-Changing Luminescent Polysulfone for Information Encryption and Fire Alarming

An intrinsic difficulty with thermally responsive photoluminescent materials is that high temperatures usually destroy luminance due to the notorious thermal quenching effect. Limited by the vulnerable chemical structure and soft skeleton, most of the existing photoluminescent responsive materials fail to indicate or work at a surging temperature over 100 °C, thus limiting application in display and alarming in harsh conditions. Herein, enlightened by chameleon's adaptive nature to external stimulus, we introduce a topologically optimized electron donor-acceptor (DA) structure and supramolecular interactions of lanthanide ions into the polymer backbone. The emission color determined by the DA structure is stable at high temperatures, and metal-ligand interaction phosphorescence is temperature-adjustable. Owing to the excellent reproducibility and heat resistance of composite films, the sensors can be bent into different three-dimensional structures and adhered to metal surfaces as flexible thermometers with superior display resolution. The polymer composite film could be directly applied as a photoluminescent quick response (QR) code, with patterns simultaneously variable to a temperature from 30 to 150 °C free of manual operation. More importantly, the polymeric composite could be in-situ-oxidized to a "sulfone" structure with an enhanced glass transition temperature of 297-304 °C. The heat- and flame-resistant characteristics of the oxidized films give rise to the application of fire alarming devices since it can locate the fire source and respond exactly depending on the distance from the fire. The unique display, encryption, and alarming functions of the polymeric composite studied in this work bring forward a new concept of developing a great information security and disaster monitoring system with the application of temperature-responsive materials.

Precisely Controllable Artificial Muscle with Continuous Morphing based on "Breathing" of Supramolecular Columns

Skeletal muscles are natural motors executing sophisticated work through precise control of linear contraction. Although various liquid crystal polymers based artificial muscles have been designed, the mechanism based on mainly the order-disorder transition usually leads to discrete shape morphing, leaving arbitrary and precise deformation a huge challenge. Here, one novel photoresponsive hemiphasmidic side-chain liquid crystal polymer with a unique "breathing" columnar phase that enables continuous morphing is presented. Due to confinement inside the supramolecular columnar assembly, the cooperative movements of side-chains and backbones generate a significant negative thermal expansion and lead to temperature-controllable muscle-like elongation/contraction in the oriented polymer strip. The irreversible isomerization of the photoresponsive mesogens results in the synergistic phototunable bending and high-contrast fluorescence change. Based on the orthogonal responses to heat and light, controllable arm-like bending motions of this material, which is applicable in constructing advanced artificial muscles or intelligent soft robotics, are further demonstrated.

Organohydrogel Actuators with Adjustable Stimulus Responsiveness for On-Demand Morphing

Hydrogel actuators showing shape morphing in response to external stimuli are of significant interest for their applications in soft robots, artificial muscles, etc. However, there is still a lack of hydrogel actuators with adjustable stimulus responsiveness for on-demand driving. In this study, an organohydrogel actuator was prepared by a two-step interpenetrating method, resulting in the coexistence of poly(N-isopropylacrylamide-co-4-(2-sulfoethyl)-1-(4-vinylbenzyl) pyridinium betaine) (p(NIPAM-SVBP)) hydrophilic networks and poly(lauryl methacrylate) (pLMA) hydrophobic networks with gradient distribution. In the initial state, the organohydrogel actuator can be driven globally under thermal stimulation. Owing to the unique alkali-chromic performance of SVBP, the organohydrogel actuator can be endowed with photothermal properties and actuate locally under the stimulus of NIR light. More importantly, the organohydrogel will return to the original colorless state after being treated with acid solution. Our work provides a new insight into designing and fabricating novel actuators with adjustable stimulus responsiveness for on-demand morphing.

Aggregation-Induced Emission Luminogen-Encapsulated Fluorescent Hydrogels Enable Rapid and Sensitive Quantitative Detection of Mercury Ions

Hg²⁺ contamination in sewage can accumulate in the human body through the food chains and cause health problems. Herein, a novel aggregation-induced emission luminogen (AIEgen)-encapsulated hydrogel probe for ultrasensitive detection of Hg²⁺ was developed by integrating hydrophobic AIEgens into hydrophilic hydrogels. The working mechanism of the multi-fluorophore AIEgens (TPE-RB) is based on the dark through-bond energy transfer strategy, by which the energy of the dark tetraphenylethene (TPE) derivative is completely transferred to the rhodamine-B derivative (RB), thus resulting in intense photoluminescent intensity. The spatial networks of the supporting hydrogels further provide fixing sites for the hydrophobic AIEgens to enlarge accessible reaction surface for hydrosoluble Hg²⁺, as well create a confined reaction space to facilitate the interaction between the AIEgens and the Hg²⁺. In addition, the abundant hydrogen bonds of hydrogels further promote the Hg²⁺ adsorption, which significantly improves the sensitivity. The integrated TPE-RB-encapsulated hydrogels (TR hydrogels) present excellent specificity, accuracy and precision in Hg²⁺ detection in real-world water samples, with a 4-fold higher sensitivity compared to that of pure AIEgen probes. The as-developed TR hydrogel-based chemosensor holds promising potential as a robust, fast and effective bifunctional platform for the sensitive detection of Hg²⁺.

Self-Assembly of the Tetraphenylethylene-Capped Diserine through a Hierarchical Assembly Process

We report a new peptide-based urchin-shaped structure prepared through two-step self-assembly of tetraphenylethylene-diserine (TPE-SS). Hydrogelation generated nanobelts through the first stage of self-assembly of TPE-SS; these nanobelts further transformed on silicon wafers into urchin-like microstructures featuring nanosized spines. The presence of the TPE moiety in the hydrogelator resulted in aggregation-induced emission characteristics both in the solution and in the gel phases. TPE-SS has the lowest molecular weight of any TPE-capped hydrogelator with β-sheet-like structures under physiological pH. This new design strategy appears to be useful for generating three-dimensional self-assembled microstructures and multifunctional biomaterials. We found that TPE-SS is biocompatible with human mesenchymal stem cells and breast cancer cells, making them potential applications in tissue engineering and biomedical research.

A rapid self-healing glassy polymer/metal-organic-framework hybrid membrane at room temperature

The development of repairable MOF-polymer hybrid materials will greatly extend their service life by repairing fractured parts on the spot; however, it is difficult for robust glassy polymers to self-heal below the glass transition temperature (T_g) as the polymer network is frozen. We herein report glassy polyMOF-RHP hybrid membranes by integrating lanthanide polyMOF (polyLnMOF) with randomly hyperbranched polymers (RHP) bearing a high density of hydrogen bonds. Since crystalline lanthanide MOFs act as multiconnected cross-linking agents and cross-link the interpenetrating polymer network, the obtained polyLnMOF-polymer membrane shows enhanced mechanical strength with a storage modulus of 3.09 GPa and a T_g up to 49 °C. Meanwhile, the high intersegment migration ability of the polyLnMOF-polymer network facilitates the exchange of hydrogen-bonded pairs even in the glassy state, leading to an instantaneous room-temperature self-healing ability. The polyLnMOF-polymer membranes inherit the ratiometric temperature-sensing behavior of pristine lanthanide MOFs, resulting in more processable temperature-sensing membranes. This work provides an appealing strategy for the design of mechanically robust, yet self-healing, MOF-polymer functional materials.

As Fiber Meets with AIE: Opening a Wonderland for Smart Flexible Materials

Aggregation-induced emission luminogens (AIEgens) have recently been developed at a tremendous pace in the area of organic luminescent materials by virtue of their superior properties. However, the practical applications of AIEgens still face the challenge of transforming AIEgens from molecules into materials. Till now, many AIEgens have been integrated into fiber, endowing the fiber with prominent fluorescence and/or photosensitizing capacities. AIEgens and fiber complement each other for making progress in flexible smart materials, in which the utilization of AIEgens creates new application possibilities for fiber, and the fiber provides an excellent carrier for AIEgens towards realizing the conversion from molecule to materials and an ideal platform to research the aggregate state of AIEgens in mesoscale and macroscale. This review begins with a brief summary of the recent advances related to some typical AIEgens with various functions and the technology for the fabrication of AIEgen-functionalized fiber. The most representative applications are then highlighted by focusing on energy conversion, personal protective equipment, biomedical, sensor, and fluorescence-related fields. Finally, the challenges, opportunities, and tendencies in future development are discussed in detail. This review hopes to inspire innovation in AIEgens and fiber from the view of mesoscale and macroscale.

Fluorescent Mechanism and Optical Switching of Fluorophore-Free Organogel

Fluorophore is essential to enable the fluorescence and optical switching in most of polymer gels. Herein, a novel concept is proposed to develop a fluorophore-free organogel that is capable of generation of blue fluorescence at transparent state, while it proceeds with optical switching from blue to purple upon phase transition into non-transparent state in water. Ammonium persulphate (APS) is utilized to initiate co-crosslinking of hydrophilic acrylamide (AM) and hydrophobic 2,2,3,4,4,4-hexafluorobutyl acrylate (HFBA) in dimethyl sulphoxide (DMSO) to give organogel of AM@HFBA at 80 °C. APS decomposes to generate not only radicals, but also ammonium bisulfate (ABS) during heating, in which the elements of ABS produce blue fluorescence (λ = 440 nm), excited by UV light (λ = 365 nm). After the phase transition into non-transparent state, light-reflection behavior at the phase-transitioned surface triggers the optical switching of the organogel from blue to purple under UV light. The optical switching is patternable and reversible, which enables the applications of organogel of AM@HFBA for information encoding/encryption and optical-switchable soft actuators. This method is universal to achieve fluorescence and optical switching for free radical polymerization-based gel systems as long as they are initiated by APS in DMSO.

Engineering viscoelastic mismatch for temporal morphing of tough supramolecular hydrogels

Viscoelasticity is a generic characteristic of soft biotissues and polymeric materials, endowing them with unique time- and rate-dependent properties. Here, by spatiotemporally tailoring the viscoelasticity in tough supramolecular hydrogels, we demonstrate reprogrammable morphing of the gels based on differential viscoelastic recovery processes that lead to internal strain mismatch. The spatial heterogeneity of viscoelasticity is encoded through integrating dissimilar hydrogels or by site-specific treatment of a singular hydrogel. The temporal morphing behavior of tough gels, including a fast deformation process and then a slow shape-recovery process, is related to the kinetics of associative interactions and the entropic elasticity of supramolecular networks after pre-stretching and release, which takes place spontaneously in the absence of external stimuli. Such a kinetically driven morphing mechanism resolves the trade-off between the mechanical robustness and shape-changing speed in tough hydrogels with dense entanglements and physical associations, and should be applicable to other viscoelastic materials. A numerical theory for the temporal morphing of tough supramolecular gels has been formulated by dynamic coupling of viscoelastic recovery and mechanics of deformations, which is further implemented to predict the sophisticated morphed structures. Furthermore, magnetic particles are incorporated into the morphed tough hydrogels to devise versatile soft actuators and robots for specific applications.

Hg(II) immobilization and detection using gel formation with tetra-(4-pyridylphenyl)ethylene and an aggregation-induced luminescence effect

Tetra-(4-pyridylphenyl)ethylene (TPPE), featuring an aggregation-induced luminescence effect (AIE), has been synthesized and used for selective detection of Hg²⁺ in DMF/H₂O (3:7, v/v) binary solutions. There was a color change from colorless to yellow in the detection of the Hg²⁺ ions, in addition to an increased fluorescence emission. This shows that TPPE will function as an excellent "turn-on" fluorescence probe in the detection Hg²⁺. Moreover, the interference of Al³⁺, Ba²⁺, Mn²⁺, Ca²⁺, Fe³⁺, Cu²⁺, Ag⁺, Cd²⁺, Co²⁺, Ni²⁺, Mg²⁺, Pb²⁺, Zn²⁺, and Cr³⁺ ions was found to be negligible under optimized solvent conditions. Cysteine and EDTA were also found to form TPPE-based fluorescent switches with the Hg²⁺ ions. The practical use of the TPPE sensor was also demonstrated by using a specific test kit. Characterization using FT-IR, NMR titration, UV titration, EDS, and HR-MS techniques showed that Hg²⁺ will form a 1:1 complex with TPPE. Also, the observation of a Tyndall effect, in addition to UV absorption and fluorescence spectra, did clearly demonstrate the presence of an AIE. More noteworthy, TPPE and Hg²⁺ were found to form a metal-organic gel (MOG) in the DMF solution. The SEM, TEM, ICP, and Zeta potential analyses confirmed that the fluorescent MOG could further adsorb an excess of Hg²⁺ ions. The BET analyses revealed that the MOG showed a type IV-H3 hysteresis loop according to the International Union of Pure and Applied Chemistry classification. The results of the XRD analysis and of the spectroscopic titrations show that a π-π stacking may be the auxiliary driving force for the gel formation, after that a coordination has taken place. These results indicate that further research on structurally simple metal ion fluorescent probes, which are based on the AIE, is promising for the achievement of a simultaneous fluorescent detection and adsorption of heavy metal pollutants.

High-contrast reversible multiple color-tunable solid luminescent ionic polymers for dynamic multilevel anti-counterfeiting

Dynamic color-tunable luminescent materials, which possess huge potential applications in advanced multilevel luminescence anti-counterfeiting, are of considerable interest. However, it remains challenging to develop simple high-contrast reversible multiple (triple or more than triple) color-tunable high-efficiency solid luminescent materials with low cost, facile synthesis, and good processability. Herein, by simply grafting charged multi-color AIEgen-based chromophores into polymers, a series of high-efficiency multiple color-tunable luminescent single ionic polymers are constructed through tuning feed ratios, counter anions and reaction solvents. Remarkably, some ionic polymers can not only achieve rare high-contrast reversible multiple color-tunable emission in solid states in response to different solvent stimuli, but also could realize excitation-dependent color-tunable emission. To the best of our knowledge, such charming multiple (triple or more than triple) color-tunable solid polymers responding to multiple external stimuli are still rare. Based on comparative studies of emission spectra, excitation spectra and fluorescence lifetimes before and after swelling, it could be inferred that solvent stimuli could induce microstructure changes of these ionic polymers and then change the aggregated-states of their corresponding AIE-active emission centers. Moreover, the different solvent stimuli could induce to produce different degrees of microstructure changes, resulting in their unique multiple color-tunable emission. More significantly, these smart color-tunable ionic polymers show great promise for applications in dynamic multilevel (three-level or even more than three-level) anti-counterfeiting.

Nanomaterials-Functionalized Hydrogels for the Treatment of Cutaneous Wounds

Hydrogels have been utilized extensively in the field of cutaneous wound treatment. The introduction of nanomaterials (NMs), which are a big category of materials with diverse functionalities, can endow the hydrogels with additional and multiple functions to meet the demand for a comprehensive performance in wound dressings. Therefore, NMs-functionalized hydrogels (NMFHs) as wound dressings have drawn intensive attention recently. Herein, an overview of reports about NMFHs for the treatment of cutaneous wounds in the past five years is provided. Firstly, fabrication strategies, which are mainly divided into physical embedding and chemical synthesis of the NMFHs, are summarized and illustrated. Then, functions of the NMFHs brought by the NMs are reviewed, including hemostasis, antimicrobial activity, conductivity, regulation of reactive oxygen species (ROS) level, and stimulus responsiveness (pH responsiveness, photo-responsiveness, and magnetic responsiveness). Finally, current challenges and future perspectives in this field are discussed with the hope of inspiring additional ideas.

Fast and Reversibly Humidity-Responsive Fluorescence Based on AIEgen Proton Transfer

The construction of humidity-responsive fluorescent materials with reversibility, specificity, and sensitivity is of great importance for the development of information encryption, fluorescence patterning, and sensors. Nevertheless, to date, the application of these materials has been limited by their slow response rate and nonspecificity. Herein, a humidity-responsive fluorescence system was designed and assembled to achieve a rapid, reversible, and specific moisture response. The system comprised tetra-(4-pyridylphenyl)ethylene (TPE-4Py) as a fluorescent proton acceptor with an aggregation-induced emission (AIE) effect and poly(acrylic acid) (PAA) as a proton donor with an efficient moisture-capturing ability. The fluorescence color and intensity rapidly changed with increasing relative humidity (RH) because of TPE-4Py protonation, and TPE-4Py deprotonation resulted in recovery of the original fluorescence color in low-humidity environments. The proton transfer between the pyridyl group in TPE-4Py and the carboxyl group in PAA was reversible and chemically stable, and the humidity-responsive fluorescence system showed a high response/recovery speed, an obvious color change, good reversibility, and an outstanding specific moisture response. Because of these advantages, diverse applications of this humidity-responsive fluorescence system in transient fluorescent patterning and the encryption of information were also developed and demonstrated.