Ionotronic Luminescent Fibers, Fabrics, and Other Configurations

Fibres-threads of intelligence-enable a new generation of wearable systems

Fabrics represent a unique platform for seamlessly integrating electronics into everyday experiences. The advancements in functionalizing fabrics at both the single fibre level and within constructed fabrics have fundamentally altered their utility. The revolution in materials, structures, and functionality at the fibre level enables intimate and imperceptible integration, rapidly transforming fibres and fabrics into next-generation wearable devices and systems. In this review, we explore recent scientific and technological breakthroughs in smart fibre-enabled fabrics. We examine common challenges and bottlenecks in fibre materials, physics, chemistry, fabrication strategies, and applications that shape the future of wearable electronics. We propose a closed-loop smart fibre-enabled fabric ecosystem encompassing proactive sensing, interactive communication, data storage and processing, real-time feedback, and energy storage and harvesting, intended to tackle significant challenges in wearable technology. Finally, we envision computing fabrics as sophisticated wearable platforms with system-level attributes for data management, machine learning, artificial intelligence, and closed-loop intelligent networks.

Self-Adhesive, Stretchable, and Thermosensitive Iontronic Hydrogels for Highly Sensitive Neuromorphic Sensing-Synaptic Systems

Artificial sensory afferent nerves that emulate receptor nanochannel perception and synaptic ionic information processing in chemical environments are highly desirable for bioelectronics. However, challenges persist in achieving life-like nanoscale conformal contact, agile multimodal sensing response, and synaptic feedback with ions. Here, a precisely tuned phase transition poly(N-isopropylacrylamide) (PNIPAM) hydrogel is introduced through the water molecule reservoir strategy. The resulting hydrogel with strongly cross-linked networks exhibits excellent mechanical performance (∼2000% elongation) and robust adhesive strength. Importantly, the hydrogel's enhanced ionic conductance and heterogeneous structure of the temperature-sensitive component enable highly sensitive strain information perception (GFmax = 7.94, response time ∼ 87 ms), temperature information perception (TCRmax = -1.974%/°C, response time ∼ 270 ms), and low energy consumption synaptic plasticity (42.2 fJ/spike). As a demonstration, a neuromorphic sensing-synaptic system is constructed integrating iontronic strain/temperature sensors with fiber synapses for real-time information sensing, discrimination, and feedback. This work holds enormous potential in bioinspired robotics and bioelectronics.

Structural design of light-emitting fibers and fabrics for wearable and smart devices

Flexible light-emitting fibers and fabrics serve to bridge human-machine interactions. The desire for practical applications and the commercialization of flexible light-emitting fibers has accelerated structural progress and improvements. This review focuses on the structural design of light-emitting fibers and fabrics, starting with a summary of design principles, emission mechanisms, and structural evolution of coaxial structured light-emitting fibers. Subsequently, we explore recent advances in the helical structure design strategies that boost the mechanical sensitivity of light-emitting fibers. Following that, we analyze continuous preparation processes and the development of large-area intelligent light-emitting fabrics based on interwoven structures. Examples based on stiff and rigid inorganic-based light-emitting diodes integrated into flexible systems are also presented. Finally, we discuss the current challenges and future opportunities for light-emitting applications in the field of wearable and smart devices.

Cellulose nanocrystals-based optical organohydrogel fiber with customizable iridescent colors for strain and humidity response

Stimuli-responsive optical hydrogels are widely used in various fields including environmental sensing, optical encryption, and intelligent display manufacturing. However, these hydrogels are susceptible to water losses when exposed to air, leading to structural damage, significantly shortened service lives, and compromised durability. This study presents mechanically robust, environmentally stable, and multi-stimuli responsive optical organohydrogel fibers with customizable iridescent colors. These fibers are fabricated by incorporating tunicate cellulose nanocrystals, alginate, and acrylamide in a glycerol-water binary system. The synthesized fibers exhibit high strength (1.38 MPa), moisture retention capabilities, and elastic properties. Furthermore, a sensor based on these fibers demonstrates high- and low-temperature resistance along with stimuli-responsive characteristics, effectively detecting changes in environmental humidity and strains. Moreover, the fiber sensor demonstrates continuous, repeatable, and quantitatively predictable moisture discoloration responses across a humidity range of 11 % and 98 %. During strain sensing, the optical-organohydrogel-based sensor demonstrates a large working strain (50 %) and excellent cycling stability, underscoring its potential for effectively monitoring a wide range of intricate human motions. Overall, the synthesized fibers and their simple fabrication method can elicit new avenues for numerous related applications including the large-scale implementation of advanced wearable technology.

Alternating current electroluminescence devices: recent advances and functional applications

Wearable smart devices and visualisation sensors based on alternating current electroluminescence (ACEL) have received considerable attention in recent years. Due to the unique properties of ACEL devices, such as high mechanical strength, adaptability to complex environments, and no need for energy level matching, ACEL is suitable for multifunctional applications and visualisation sensing platforms. This review comprehensively outlines the latest developments in ACEL devices, starting with an analysis of the mechanism, classification, and optimisation strategies of ACEL. It introduces the functional applications of ACEL in multicolour displays, high-durability displays, stretchable and wearable displays, and autonomous function displays. Particularly, it emphasises the research progress of ACEL in sensory displays under interactive conditions such as liquid sensing, environmental factor sensing, kinetic energy sensing, and biosensing. Finally, it forecasts the challenges and new opportunities faced by future functional and interactive ACEL devices in fields such as artificial intelligence, smart robotics, and human-computer interaction.

Spider-silk-inspired strong and tough hydrogel fibers with anti-freezing and water retention properties

Ideal hydrogel fibers with high toughness and environmental tolerance are indispensable for their long-term application in flexible electronics as actuating and sensing elements. However, current hydrogel fibers exhibit poor mechanical properties and environmental instability due to their intrinsically weak molecular (chain) interactions. Inspired by the multilevel adjustment of spider silk network structure by ions, bionic hydrogel fibers with elaborated ionic crosslinking and crystalline domains are constructed. Bionic hydrogel fibers show a toughness of 162.25 ± 21.99 megajoules per cubic meter, comparable to that of spider silks. The demonstrated bionic structural engineering strategy can be generalized to other polymers and inorganic salts for fabricating hydrogel fibers with broadly tunable mechanical properties. In addition, the introduction of inorganic salt/glycerol/water ternary solvent during constructing bionic structures endows hydrogel fibers with anti-freezing, water retention, and self-regeneration properties. This work provides ideas to fabricate hydrogel fibers with high mechanical properties and stability for flexible electronics.

Alternating Current Electroluminescence for Human-Interactive Sensing Displays

The development of stimuli-interactive displays based on alternating current (AC)-driven electroluminescence (EL) is of great interest, owing to their simple device architectures suitable for wearable applications requiring resilient mechanical flexibility and stretchability. AC-EL displays can serve as emerging platforms for various human-interactive sensing displays (HISDs) where human information is electrically detected and directly visualized using EL, promoting the development of the interaction of human-machine technologies. This review provides a holistic overview of the latest developments in AC-EL displays with an emphasis on their applications for HISDs. AC-EL displays based on exciton recombination or impact excitations of hot electrons are classified into four representative groups depending upon their device architecture: 1) displays without insulating layers, 2) displays with single insulating layers, 3) displays with double insulating layers, and 4) displays with EL materials embedded in an insulating matrix. State-of-the-art AC HISDs are discussed. Furthermore, emerging stimuli-interactive AC-EL displays are described, followed by a discussion of scientific and engineering challenges and perspectives for future stimuli-interactive AC-EL displays serving as photo-electronic human-machine interfaces.

Fiber Crossbars: An Emerging Architecture of Smart Electronic Textiles

Smart wearables have a significant impact on people's daily lives, enabling personalized motion monitoring, realizing the Internet of Things, and even reshaping the next generation of telemedicine systems. Fiber crossbars (FCs), constructed by crossing two fibers, have become an emerging architecture among the accessible structures of state-of-the-art smart electronic textiles. The mechanical, chemical, and electrical interactions between crossing fibers result in extensive functionalities, leading to the significant development of innovative electronic textiles employing FCs as their basic units. This review provides a timely and comprehensive overview of the structure designs, material selections, and assembly techniques of FC-based devices. The recent advances in FC-based devices are summarized, including multipurpose sensing, multiple-mode computing, high-resolution display, high-efficient power supply, and large-scale textile systems. Finally, current challenges, potential solutions, and future perspectives for FC-based systems are discussed for their further development in scale-up production and commercial applications.

Autonomous Electroluminescent Textile for Visual Interaction and Environmental Warning

Visual interaction is a promising strategy for the externalized expression and transmission of information, having wide application prospects in wearable luminous textiles. Achieving an autonomous luminous display and dynamic light response to environmental stimuli is attractive but attracts little attention. Herein, we propose a liquid responsive structure based on alternating-current electroluminescent fibers and demonstrate conductive-liquid-bridging electroluminescent fabrics with high integration and personalized patterns. Impressively, our electroluminescent fibers and textiles could afford a sensitive response and high robustness to water, glycerol, ethanol, and sodium chloride solution. The final electroluminescent textiles show an excellent luminescence performance of 149.08 cd m-2. On the proof of concept, a rain-sensing umbrella, luminous sportswear, and liquid response glove are fabricated to demonstrate water detection, visual interaction, and environmental warning. The textile-type visualizing-responding strategy proposed in this work may open up new avenues for the application of ACEL devices in the field of visual interaction.

LEGO-like Assembly of Fibrous Modules for Display Textiles

Due to their promising advantages over classical rigid devices, the development of display textiles has exciting potential for various fields, including sensor technology, healthcare, and communication. To realize display textiles, it is necessary to prepare light-emitting building blocks at the fiber level and then weave or knit them to form the desired textile structures. However, from a practical viewpoint, it is difficult to continuously weave functional fibers containing light-emitting devices using conventional textile technologies. To address this issue, we introduced fibrous modules that can be assembled like LEGO blocks to realize textile displays. A unique feature of this work is that the light-emitting pixels are generated through a simple contact between modular electrochemiluminescent (ECL) fibers. Each fiber is composed of a single metallic wire coated with a gel-type ECL electrolyte that is formed by using a simple dip-coating method in ambient air. The sticky nature of the gel electrolyte enables the construction of light-emitting pixels through the simple physical contact of two or more fiber modules without the need for external pressure or heating. The diversity of this technology offers in terms of fibrous module arrangements and assembly can provide various options for designing the geometries of light-emitting pixels. We have implemented this technique to demonstrate not only a 1 × 1 pixel but also 3 × 3 pixels with an irregular shape. These results demonstrate that the unique strategy for display devices developed in this work provides a feasible approach for various electronic and optical textile applications.

Biomimetic Spun Silk Ionotronic Fibers for Intelligent Discrimination of Motions and Tactile Stimuli

Innovation in the ionotronics field has significantly accelerated the development of ultraflexible devices and machines. However, it is still challenging to develop efficient ionotronic-based fibers with necessary stretchability, resilience, and conductivity due to inherent conflict in producing spinning dopes with both high polymer and ion concentrations and low viscosities. Inspired by the liquid crystalline spinning of animal silk, this study circumvents the inherent tradeoff in other spinning methods by dry spinning a nematic silk microfibril dope solution. The liquid crystalline texture allows the spinning dope to flow through the spinneret and form free-standing fibers under minimal external forces. The resultant silk-sourced ionotronic fibers (SSIFs) are highly stretchable, tough, resilient, and fatigue-resistant. These mechanical advantages ensure a rapid and recoverable electromechanical response of SSIFs to kinematic deformations. Further, the incorporation of SSIFs into core-shell triboelectric nanogenerator fibers provides outstanding stable and sensitive triboelectric response to precisely and sensitively perceive small pressures. Moreover, by implementing a combination of machine learning and Internet of Things techniques, the SSIFs can sort objects made of different materials. With these structural, processing, performance, and functional merits, the SSIFs prepared herein are expected to be applied in human-machine interfaces.

Flexible Wet-Spun PEDOT:PSS Microfibers Integrating Thermal-Sensing and Joule Heating Functions for Smart Textiles

Multifunctional fiber materials play a key role in the field of smart textiles. Temperature sensing and active thermal management are two important functions of smart fabrics, but few studies have combined both functions in a single fiber material. In this work, we demonstrate a temperature-sensing and in situ heating functionalized conductive polymer microfiber by exploiting its high electrical conductivity and thermoelectric properties. The conductive polymer microfibers were prepared by wet-spinning the PEDOT:PSS aqueous dispersion with ionic liquid additives, which was used to enhance the electrical and mechanical properties of the final microfibers. The thermoelectric properties of these microfibers were further studied. Due to their excellent flexibility and mechanical properties, these fibers can be easily integrated into commercial fabrics for the manufacture of smart textiles through knitting. We further demonstrated a smart glove with integrated temperature-sensing and in situ heating functions, and further explored thermoelectric fiber-based temperature-sensing array fabric. These works combine the thermoelectric properties and heating function of conductive polymer fibers, providing new insights that enable further development of high-performance, multifunctional wearable smart textiles.

Water-dispersible and stable polydopamine coated cellulose nanocrystal-MXene composites for high transparent, adhesive and conductive hydrogels

High conductive and transparent hydrogels with adhesion function are ideal candidates for soft electronic devices. However, it remains a challenge to design appropriate conductive nanofillers to endow hydrogels with all these characteristics. The 2D MXene sheets are promising conductive nanofillers for hydrogels due to excellent electricity and water-dispersibility. However, MXene is quite susceptible to oxidation. In this study, polydopamine (PDA) was employed to protect the MXene from oxidation and meanwhile endow hydrogels with adhesion. However, PDA coated MXene (PDA@MXene) were easily flocculated from dispersion. 1D cellulose nanocrystals (CNCs) were employed as steric stabilizers to prevent the agglomeration of MXene during the self-polymerization of dopamine. The obtained PDA coated CNC-MXene (PCM) sheets display outstanding water-dispersible and anti-oxidation stability and are promising conductive nanofillers for hydrogels. During the fabrication of polyacrylamide hydrogels, the PCM sheets were partially degraded into PCM nanoflakes with smaller size, leading to transparent PCM-PAM hydrogels. The PCM-PAM hydrogels can self-adhere to skin, and possess high transmittance of 75 % at 660 nm, superior electric conductivity of 4.7 S/m with MXene content as low as 0.1 % and excellent sensitivity. This study will facilitate the development of MXene based stable, water-dispersible conductive nanofillers and multi-functional hydrogels.

Polyelectrolyte elastomer-based ionotronic sensors with multi-mode sensing capabilities via multi-material 3D printing

Stretchable ionotronics have drawn increasing attention during the past decade, enabling myriad applications in engineering and biomedicine. However, existing ionotronic sensors suffer from limited sensing capabilities due to simple device structures and poor stability due to the leakage of ingredients. In this study, we rationally design and fabricate a plethora of architected leakage-free ionotronic sensors with multi-mode sensing capabilities, using DLP-based 3D printing and a polyelectrolyte elastomer. We synthesize a photo-polymerizable ionic monomer for the polyelectrolyte elastomer, which is stretchable, transparent, ionically conductive, thermally stable, and leakage-resistant. The printed sensors possess robust interfaces and extraordinary long-term stability. The multi-material 3D printing allows high flexibility in structural design, enabling the sensing of tension, compression, shear, and torsion, with on-demand tailorable sensitivities through elaborate programming of device architectures. Furthermore, we fabricate integrated ionotronic sensors that can perceive different mechanical stimuli simultaneously without mutual signal interferences. We demonstrate a sensing kit consisting of four shear sensors and one compressive sensor, and connect it to a remote-control system that is programmed to wirelessly control the flight of a drone. Multi-material 3D printing of leakage-free polyelectrolyte elastomers paves new avenues for manufacturing stretchable ionotronics by resolving the deficiencies of stability and functionalities simultaneously.

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.

Aqueous spinning of robust, self-healable, and crack-resistant hydrogel microfibers enabled by hydrogen bond nanoconfinement

Robust damage-tolerant hydrogel fibers with high strength, crack resistance, and self-healing properties are indispensable for their long-term uses in soft machines and robots as load-bearing and actuating elements. However, current hydrogel fibers with inherent homogeneous structure are generally vulnerable to defects and cracks and thus local mechanical failure readily occurs across fiber normal. Here, inspired by spider spinning, we introduce a facile, energy-efficient aqueous pultrusion spinning process to continuously produce stiff yet extensible hydrogel microfibers at ambient conditions. The resulting microfibers are not only crack-insensitive but also rapidly heal the cracks in 30 s by moisture, owing to their structural nanoconfinement with hydrogen bond clusters embedded in an ionically complexed hygroscopic matrix. Moreover, the nanoconfined structure is highly energy-dissipating, moisture-sensitive but stable in water, leading to excellent damping and supercontraction properties. This work creates opportunities for the sustainable spinning of robust hydrogel-based fibrous materials towards diverse intelligent applications.

Carbon Dots-Based Ultrastretchable and Conductive Hydrogels for High-Performance Tactile Sensors and Self-Powered Electronic Skin

Smart tactile sensing materials have excellent development prospects, including wearable health-monitoring equipment and energy collection. Hydrogels have received extensive attention in tactile sensing owing to their transparency and high elasticity. In this study, highly crosslinked hydrogels are fabricated by chemically crosslinking polyacrylamide with lithium magnesium silicate and decorated with carbon quantum dots. Magnesium lithium silicate provides abundant covalent bonds and improves the mechanical properties of the hydrogels. The luminescent properties endowed by the carbon dots further broaden the application of hydrogels for realizing flexible electronics. The hydrogel-based strain sensor exhibits excellent sensitivity (gauge factor 2.6), a broad strain response range (0-2000%), good cyclicity, and durability (1250). Strain sensors can be used to detect human motions. More importantly, the hydrogel can also be used as a flexible self-supporting triboelectric electrode for effectively detecting pressure in the range of 1-25 N and delivering a short-circuit current (I_SC ) of 2.6 µA, open-circuit voltage (V_OC ) of 115 V, and short-circuit transfer charge (Q_SC ) of 29 nC. The results reveal new possibilities for human-computer interactions and electronic robot skins.

Asymmetrically Enhanced Coplanar-Electrode Electroluminescence for Information Encryption and Ultrahighly Stretchable Displays

Traditional alternating-current-driven electroluminescent (AC-EL) devices adopting a sandwich structure are commonly used in solid-state lighting and displays, while the emerging coplanar-electrode alternating-current-driven light-emitting variants manifest excellent application prospects in intelligent, multifunctional, and full-color displays, and sensing purposes. In this work, an asymmetrically enhanced coplanar-electrode AC-EL device with a universal and straightforward architecture is designed based on the impedance adjustment strategy. This newly devised asymmetric structure extends the functionalities of the coplanar-electrode AC-EL devices by overcoming the bottlenecks of complicated patterning procedures and high driving voltages of symmetric configuration. The developed device design enables a new type of information encryption and ultrahighly stretchable patterned displays. Notably, the novel encryption appliances demonstrate feasible encryption/decryption features, multiple encryptions, and practical applicability; the biaxially stretchable display devices achieve the highest tensile performance in the field of stretchable electroluminescent pattern displays, and outperform the ultrahighly stretchable sandwich devices in terms of simple patterning process, higher brightness, wider color gamut, and long-term stability. The proposed configuration opens up new avenues for AC-EL devices toward a plethora of smart applications in wearable electronics with intelligent displays, dynamic interaction of human-machine interface, and soft robotics.

Water Washable and Flexible Light-Emitting Fibers Based on Electrochemiluminescent Gels

Herein, a new concept of device architecture to fabricate fibrous light-emitting devices is demonstrated based on an electrochemiluminescence (ECL) material for an electronic textile system. A unique feature of this work is that instead of conventional semiconductor materials, such as organics, perovskites, and quantum dots for fibrous light emitting devices, a solid-state ECL electrolyte gel is employed as a light-emitting layer. The solid-state ECL gel is prepared from a precursor solution composed of matrix polymer, ionic liquid, and ECL luminophore. From this, we successfully realize light-emitting fibers through a simple and cost-effective single-step dip-coating method in ambient air, without complicated multistep vacuum processes. The resulting fiber devices reliably operated under applied AC bias of ±2.5 V and showed luminance of 47 cd m^-2. More importantly, the light-emitting fibers exhibited outstanding water resistance without any passivation layers, owing to the water immiscible and hydrophobic nature of the ECL gel. In addition, because of their simple structure, the fiber devices can be easily deformed and woven together with commercial knitwear by hand. Therefore, these results suggest a promising strategy for the development of practical fiber displays and contribute to progress in electronic textile technology.

Electrically Programmable Interfacial Adhesion for Ultrastrong Hydrogel Bonding

Adjustable interfacial adhesion is of great significance in smart-hydrogel-related engineering fields. This study presents an electroadhesion strategy for universal and ultrastrong hydrogel bonding with electrically programmable strength. An ionic hydrogel containing lithium ions is designed to achieve hydrated-ion-diffusion-mediated interfacial adhesion, where external electric fields are employed to precisely control spatiotemporal dynamics of the ion diffusion across ionic adhesion region (IAR). The hydrogel can realize a universal, ultrastrong, efficient, tough, reversible, and environmentally tolerant electroadhesion to diverse hydrogels, whose peak adhesion strength and interfacial adhesion toughness are as high as 1.2 MPa and 3750 J m^-2 , respectively. With a mechanoelectric coupling model, the dominant role of the hydrated ions in IAR played in the interfacial electroadhesion is further quantitatively revealed. The proposed strategy opens a door for developing high-performance adhesion hydrogels with electrically programmable functions, which are indispensable for various emerging fields like flexible electronics and soft robotics.

Simulation of the peel of hydrogels with stiff backing

In this work, the peel of hydrogels under a stiff backing constraint was studied using a finite element method. The finite element method was first validated by comparing the simulation results to theoretical predictions and experimental measurements. Then, the method was used to investigate the effects of adhesion thickness, adhesion length and backing thickness on the peel behaviors, as well as the stress distribution within the adhesion layer. The results indicated that the peel force-displacement curve has a constant profile when the adhesion thickness and backing thickness are prescribed so long as the adhesion length is sufficiently long. The peak peel force increases with the adhesion length and then plateaus. The larger the intrinsic peak stress or the thicker the backing, the higher the plateau. The steady-state peel force is independent of the backing thickness, while positively correlated with the strain energy storage of the hydrogel adhesion layer. The critical vertical displacement corresponding to the peak peel force increases with the hydrogel thickness and decreases with the backing thickness. However, the critical vertical displacement corresponding to the steady-state peel force increases with the backing thickness. The present work puts forward an effective numerical approach to probe the peel of hydrogels, which is beneficial for the design of relevant structures.

3D Printing of Conductive Hydrogel-Elastomer Hybrids for Stretchable Electronics

Electronically conductive hydrogels integrated with dielectric elastomers show great promise in a wide range of applications, such as biomedical devices, soft robotics, and stretchable electronics. However, one big conundrum that impedes the functionality and performance of hydrogel-elastomer-based devices lies in the strict demands of device integration and the requirements for devices with satisfactory mechanical and electrical properties. Herein, the digital light processing three-dimensional (3D) printing method is used to fabricate 3D functional devices that bridge submillimeter-scale device resolution to centimeter-scale object size and simultaneously realize complex hybrid structures with strong adhesion interfaces and desired functionalities. The interconnected poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) network endows the PAAm hydrogel with high conductivity and superior electrical stability and poly(2-hydroxyethyl acrylate) functions as an insulating medium. The strong interfacial bonding between the hydrogel and elastomer is achieved by incomplete photopolymerization that ensures the stability of the hybrid structure. Lastly, applications of stretchable electronics illustrated as 3D-printed electroluminescent devices and 3D-printed capacitive sensors are conceptually demonstrated. This strategy will open up avenues to fabricate conductive hydrogel-elastomer hybrids in next-generation multifunctional stretchable electronics.

Dual-Encryption in a Shape-Memory Hydrogel with Tunable Fluorescence and Reconfigurable Architecture

Materials capable of shape-morphing and/or fluorescence imaging have practical significances in the fields of anti-counterfeiting, information display, and information protection. However, it's challenging to realize these functions in hydrogels due to the poor mechanical properties and lack of tunable fluorescence. A tough hydrogel with good shape-memory ability and phototunable fluorescence is reported here, which affords reprogrammable shape designing and information encoding for dual-encryption. This hydrogel is prepared by incorporating donor-acceptor chromophore units into a poly(1-vinylimidazole-co-methacrylic acid) network, in which the dense intra- and interchain hydrogen bonds lead to desirable features including high stiffness, high toughness, and temperature-mediated shape-memory property. Additionally, the hydrogel shows photomediated tunable fluorescence through a unimer-to-dimer transformation of the chromophores. By combining photolithography and origami/kirigami designs, hydrogel sheets encoded with fluorescent patterns can deform into specific 3D configurations. The geometrically encrypted fluorescent information in the architected hydrogels is readable only after sequential shape recovery and UV light irradiation. As demonstrated by proof-of-concept experiments, both the fluorescent pattern and the 3D configuration are reprogrammable, facilitating repeated information protection and display. The design of tough hydrogels with rewritable fluorescent patterns and reconfigurable shapes should guide the future development of smart materials with improved security and wider applications in aqueous environments.

Direct-ink-write printing of hydrogels using dilute inks

Direct-ink-write (DIW) printing has been used in myriad applications. Existing DIW printing relies on inks of specific rheology to compromise with printing process, imposing restrictions on the choice of printable materials. Reported ink viscosity ranges from 10-1 to 103 Pa·s. Here we report a method to enable DIW printing that is compatible with dilute ink (10-3 Pa·s) by manipulating the interactions between ink and substrate. By exemplifying hydrogel printing, we build a printing system and show that dilute ink of appropriate surface energy, once extruded, can spontaneously wet and reside within the region of higher surface energy on a substrate of lower surface energy, while resisting gravity and maintaining shape before solidification. We demonstrate the diversity for printing various materials on various substrates and three deployments immediately enabled by the proposed method. The method expands the range of printable materials for DIW printing and the toolbox for additive manufacturing.

Fatigue Damage-Resistant Physical Hydrogel Adhesion

Strong adhesion between hydrogels and various engineering surfaces has been achieved; yet, achieving fatigue-resistant hydrogel adhesion remains challenging. Here, we examine the fatigue of a specific type of hydrogel adhesion enabled by hydrogen bonds and wrinkling and show that the physical interactions–based hydrogel adhesion can resist fatigue damage. We synthesize polyacrylamide hydrogel as the adherend and poly(acrylic acid-co-acrylamide) hydrogel as the adhesive. The adherend and the adhesive interact via hydrogen bonds. We further introduce wrinkles at the interface by biaxially prestretching and then releasing the adherends and perform butt-joint tests to probe the adhesion performance. Experimental results reveal that the samples with a wrinkled interface resist fatigue damage, while the samples with a flat interface fail in ~9,000 cycles at stress levels of 70 and 63% peak stresses in static failure. The endurance limit of the wrinkled-interface samples is comparable to the peak stress of the flat-interface samples. Moreover, we find that the nearly perfectly elastic polyacrylamide hydrogel also suffers fatigue damage, which limits the fatigue life of the wrinkled-interface samples. When cohesive failure ensues, the evolutions of the elastic modulus of wrinkled-interface samples and hydrogel bulk, both in satisfactory agreements with the predictions of damage accumulation theory, are alike. We observe similar behaviors in different material systems with polyacrylamide hydrogels with different water contents. This work proves that physical interactions can be engaged in engineering fatigue-resistant adhesion between soft materials such as hydrogels.