Wearable and Washable Conductors for Active Textiles

Modulus-tunable multifunctional hydrogel ink with nanofillers for 3D-Printed soft electronics

Seamless integration and conformal contact of soft electronics with tissue surfaces have emerged as major challenges in realizing accurate monitoring of biological signals. However, the mechanical mismatch between the electronics and biological tissues impedes the conformal interfacing between them. Attempts have been made to utilize soft hydrogels as the bioelectronic materials to realize tissue-comfortable bioelectronics. However, hydrogels have several limitations in terms of their electrical and mechanical properties. In this study, we present the development of a 3D-printable modulus-tunable hydrogel with multiple functionalities. The hydrogel has a cross-linked double network, which greatly improves its mechanical properties. Functional fillers such as XLG or functionalized carbon nanotubes (fCNT) can be incorporated into the hydrogel to provide tunable mechanics (Young's modulus of 10-300 kPa) and electrical conductivity (electrical conductivity of ∼20 S/m). The developed hydrogel exhibits stretchability (∼1000% strain), self-healing ability (within 5 min), toughness (400-731 kJ/m3) viscoelasticity, tissue conformability, and biocompatibility. Upon examining the rheological properties in the modulated region, hydrogels can be 3D printed to customize the shape and design of the bioelectronics. These hydrogels can be fabricated into ring-shaped strain sensors for wearable sensor applications.

Continuous dual-network alginate hydrogel fibers with superior mechanical and electrical performance for flexible multi-functional sensors

Hydrogel fibers play a crucial role in the design and manufacturing of flexible electronic devices. However, continuous production of hydrogel fibers with high strength, toughness, and conductivity remains a significant challenge. In this study, ion-conductive sodium alginate/polyvinyl alcohol composite hydrogel fibers with an interlocked dual network structure were prepared through continuous wet spinning based on the pH-responsive dynamic borate ester bonds. Owing to the interlocked dual network structure, the resulting hydrogel fibers integrated superior performance of strength (4.31 MPa), elongation-at-break (>1500 %), ion conductivity (17.98 S m-1) and response sensitivity to strain (GF = 3.051). Benefiting from the excellent performance, the composite hydrogel fiber could be applied as motion-detecting sensors, including high-frequency, high-speed reciprocating mechanical motion, and human motion. Furthermore, the superior compatibility for human-computer interaction of the hydrogel fiber was also demonstrated, which a manipulator could be controlled to perform different actions, by a smart glove equipped with the hydrogel fiber sensors.

Flexible hydrogels connecting adhesion and wetting

Raindrops falling on window-panes spread upon contact, whereas hail can cause dents or scratches on the same glass window upon contact. While the former phenomenon resembles classical wetting, the latter is dictated by contact and adhesion theories. The classical Young-Dupre law applies to the wetting of pure liquids on rigid solids, whereas conventional contact mechanics theories account for rigid-on-soft or soft-on-rigid contacts with small deformations in the elastic limit. However, the crossover between adhesion and wetting is yet to be fully resolved. The key lies in the study of soft-on-soft interactions with material properties intermediate between liquids and solids. In this work, we translate adhesion to wetting by experimentally probing the static signature of hydrogels in contact with soft PDMS of varying elasticity of both the components. Consequently, we probe this transition across six orders of magnitude in terms of the characteristic elasto-adhesive parameter of the system. In doing so, we reveal previously unknown phenomenology and a theoretical model which smoothly bridges adhesion of glass spheres with total wetting of pure liquids on any given substrate.

Rubber-glass nanocomposites fabricated using mixed emulsions

Many composites consist of matrices of elastomers and nanoparticles of stiff materials. Such composites often have superior properties and are widely used. Embedding elastomers with nanoparticles commonly necessitates intense shear, using machines like extruders and roll millers, which cut polymer chains and degrade properties. Here, we prepare a rubber-glass nanocomposite by using two aqueous emulsions. Each emulsion is separately prepared with a single species of polymer chains. Each polymer chain is copolymerized with a small amount of silane coupling agent. Upon mixing the two emulsions, as water evaporates, the glassy particles retain the shape, and the rubbery particles change shape to form a continuous matrix. Subsequently, the silane coupling agent condensates, which cross-links the rubbery chains and interlinks the rubbery chains to the glassy particles. The cross-links and interlinks stabilize the nanostructure and lead to superior properties. The nanocomposite simultaneously achieves high modulus (~30 MPa), high toughness (~100 kJ m-2), and high fatigue threshold (~1,000 J m-2). The method of mixed emulsion is environmentally friendly and compatible with various open-air manufacturing processes, such as coat, cast, spray, print, and brush. Additionally, the silane coupling agent can interlink the nanocomposite to other materials. The method of mixed emulsion can be used to fabricate objects of complex shapes, fine features, and prescribed spatial variations of compositions.

Density Functional Theory Approach to Interpret Elastowetting of Hydrogels

Sessile hydrogel drops on rigid surfaces exhibit a wetting/contact morphology intermediate between liquid drops and glass spheres. Using density functional theory, we reveal the contact forces acting between a hydrogel and a rigid glass surface. We show that while transitioning from liquid-like to solid-like hydrogels, there exists a critical hydrogel elasticity that enables a switch from attractive-to-repulsive interaction with the underlying rigid glass surface. Our theoretical model is validated by experimental observations of sessile polyacrylamide hydrogels of varying elasticity on glass surfaces. Further, the proposed model successfully approaches Young's law in the pure liquid limit and work of adhesion in the glassy limit. Lastly, we show a modified contact angle relation, taking into account the hydrogel elasticity to explain the features of a distinct hydrogel foot.

3D spatiotemporally scalable in vivo neural probes based on fluorinated elastomers

Electronic devices for recording neural activity in the nervous system need to be scalable across large spatial and temporal scales while also providing millisecond and single-cell spatiotemporal resolution. However, existing high-resolution neural recording devices cannot achieve simultaneous scalability on both spatial and temporal levels due to a trade-off between sensor density and mechanical flexibility. Here we introduce a three-dimensional (3D) stacking implantable electronic platform, based on perfluorinated dielectric elastomers and tissue-level soft multilayer electrodes, that enables spatiotemporally scalable single-cell neural electrophysiology in the nervous system. Our elastomers exhibit stable dielectric performance for over a year in physiological solutions and are 10,000 times softer than conventional plastic dielectrics. By leveraging these unique characteristics we develop the packaging of lithographed nanometre-thick electrode arrays in a 3D configuration with a cross-sectional density of 7.6 electrodes per 100 µm2. The resulting 3D integrated multilayer soft electrode array retains tissue-level flexibility, reducing chronic immune responses in mouse neural tissues, and demonstrates the ability to reliably track electrical activity in the mouse brain or spinal cord over months without disrupting animal behaviour.

Morphology Control of Electrospun Brominated Butyl Rubber Microfibrous Membrane

Brominated butyl rubber (BIIR) is a derivative of butyl rubber, with the advantage of high physical strength, good vibration damping performance, low permeability, aging resistance, weather resistance, etc. However, it is hard to avoid BIIR fiber sticking together due to serious swelling or merging, resulting in few studies on BIIR electrospinning. In this work, brominated butyl rubber membrane (mat) with BIIR microfiber has been prepared by electrospinning. The spinnability of elastomer BIIR has been explored. The factors influencing the morphology of BIIR microfiber membranes have been studied, including solvent, electrospinning parameters, concentration, and the rheological property of electrospinning solution. The optimal parameters for electrospinning BIIR have been obtained. A BIIR membrane with the ideal microfiber morphology has been obtained, which can be peeled from aluminum foil on a collector easily without being broken. Anti-bacterial property, the electrical conductivity of these membranes, and the mechanical properties of these samples were studied. The optimized BIIR electrospinning solution is Bingham fluid. The results of these experiments show that a BIIR membrane can be used in the field of medical prevention, wearable electronics, electronic skin, and in other fields that require antibacterial functional polymer materials.

Encapsulating eutectogels for stretchable humidity-resistant strain sensors

Eutectogels are stretchable ionic conductors extensively developed in recent years, owing to their distinct advantages of low cost, non-volatility, non-toxicity, and outstanding biocompatibility. However, the susceptibility to humidity caused by the exchange of water molecules between the interiors of eutectogels and the external environment greatly restricts their practical applications. Here, a dip-coating strategy is proposed to fabricate a P(MEA-co-IBA) elastomer-coated P(AAC-co-AAM) eutectogel to achieve satisfactory humidity-resistant capability. The hydrophobic elastomer coating significantly suppresses water exchange without harming the stretchability (>500%) and conductivity of the eutectogel. Strong adhesion forms at the eutectogel-coating interface due to the formation of an interpenetrating layer. The superior electromechanical performances of encapsulated eutectogels enable stretchable ionotronic devices with stable electrical performance (>1 h) and remarkable water-droplet/moist resistances during static/dynamic loadings. A humidity-resistant encapsulated eutectogel-based wearable strain sensor is further demonstrated. The proposed humidity-resistant eutectogels are promising candidates for soft and wearable ionotronics for practical applications.

Intrinsically Nonswellable Multifunctional Hydrogel with Dynamic Nanoconfinement Networks for Robust Tissue-Adaptable Bioelectronics

Developing bioelectronics that retains their long-term functionalities in the human body during daily activities is a current critical issue. To accomplish this, robust tissue adaptability and biointerfacing of bioelectronics should be achieved. Hydrogels have emerged as promising materials for bioelectronics that can softly adapt to and interface with tissues. However, hydrogels lack toughness, requisite electrical properties, and fabrication methodologies. Additionally, the water-swellable property of hydrogels weakens their mechanical properties. In this work, an intrinsically nonswellable multifunctional hydrogel exhibiting tissue-like moduli ranging from 10 to 100 kPa, toughness (400-873 J m^-3 ), stretchability (≈1000% strain), and rapid self-healing ability (within 5 min), is developed. The incorporation of carboxyl- and hydroxyl-functionalized carbon nanotubes (fCNTs) ensures high conductivity of the hydrogel (≈40 S m^-1 ), which can be maintained and recovered even after stretching or rupture. After a simple chemical modification, the hydrogel shows tissue-adhesive properties (≈50 kPa) against the target tissues. Moreover, the hydrogel can be 3D printed with a high resolution (≈100 µm) through heat treatment owing to its shear-thinning capacity, endowing it with fabrication versatility. The hydrogel is successfully applied to underwater electromyography (EMG) detection and ex vivo bladder expansion monitoring, demonstrating its potential for practical bioelectronics.

Liquid metal-based soft, hermetic, and wireless-communicable seals for stretchable systems

Soft materials tend to be highly permeable to gases, making it difficult to create stretchable hermetic seals. With the integration of spacers, we demonstrate the use of liquid metals, which show both metallic and fluidic properties, as stretchable hermetic seals. Such soft seals are used in both a stretchable battery and a stretchable heat transfer system that involve volatile fluids, including water and organic fluids. The capacity retention of the battery was ~72.5% after 500 cycles, and the sealed heat transfer system showed an increased thermal conductivity of approximately 309 watts per meter-kelvin while strained and heated. Furthermore, with the incorporation of a signal transmission window, we demonstrated wireless communication through such seals. This work provides a route to create stretchable yet hermetic packaging design solutions for soft devices.

A Review on Thermal Properties of Hydrogels for Electronic Devices Applications

Hydrogels, as a series of three-dimensional, crosslinked, hydrophilic network polymers, exhibit extraordinary properties in softness, mechanical robustness and biocompatibility, which have been extensively utilized in various fields, especially for electronic devices. However, since hydrogels contain plenty of water, the mechanical and electrochemical properties are susceptible to temperature. The thermal characteristics of hydrogels can significantly affect the performance of flexible electronic devices. In this review, recent research on the thermal characteristics of hydrogels and their applications in electronic devices is summarized. The focus of future work is also proposed. The thermal stability, thermoresponsiveness and thermal conductivity of hydrogels are discussed in detail. Anti-freezing and anti-drying properties are the critical points for the thermal stability of hydrogels. Methods such as introducing soluble ions and organic solvents into hydrogels, forming ionogels, modifying polymer chains and incorporating nanomaterials can improve the thermal stability of hydrogels under extreme environments. In addition, the critical solution temperature is crucial for thermoresponsive hydrogels. The thermoresponsive capacity of hydrogels is usually affected by the composition, concentration, crosslinking degree and hydrophilic/hydrophobic characteristics of copolymers. In addition, the thermal conductivity of hydrogels plays a vital role in the electronics applications. Adding nanocomposites into hydrogels is an effective way to enhance the thermal conductivity of hydrogels.

Enhancing the interfacial binding strength between modular stretchable electronic components

Stretchable electronics are emerging for personalized and decentralized clinics, wearable devices and human-machine interactions. Nowadays, separated stretchable functional parts have been well developed and are approaching practical usage. However, the production of whole stretchable devices with full functions still faces a huge challenge: the integration of different components, which was hindered by the mechanical mismatch and stress/strain concentration at the connection interfaces. To avoid connection failure in stretchable devices, a new research focus is to improve the interfacial binding strength between different components. In this review, recent developments to enhance interfacial strength in wearable/implantable electronics are introduced and catalogued into three major strategies: (i) covalent bonding between different device parts, (ii) molecular interpenetration or mechanical interlocking at the interfaces and (iii) covalent connection between the human body and devices. Besides reviewing current methods, we also discuss the existing challenges and possible improvements for stretchable devices from the aspect of interfacial connections.

Stretchable Ionic Conductors for Soft Electronics

With the rapid development of soft electronics in the era of Internet of Everything (IoE), electrical conductors with stretchability, the indispensable components of soft electronics, have gained new opportunities and also faced increasing challenges. According to the principles of electrical conductivity, stretchable electrical conductors can be divided into electronic conductors and ionic conductors. Different from the stretchable electronic conductors derived from stretchable polymeric matrices integrated with electronically conductive fillers, stretchable ionic conductors are constructed by embedding mobile ions into the crosslinked polymer networks. Therefore, stretchable ionic conductors have received extensive attention and in-depth research in the past decade, thanks to their intrinsic stretchability and electrical conductivity. This review systematically summarizes the achievements on the different categories of stretchable ionic conductors (e.g., hydrogels, ionogels, and liquid-free ion-conductive elastomers), in terms of their design, fabrication, properties, and applications. The advantages and limitations of the different types of stretchable ionic conductors are discussed. Outlooks are also provided to envision the remaining challenges for the further development and practical applications of stretchable ionic conductors. It is expected to arouse inspirations for the design and fabrication of new and high-performance stretchable ionic conductors and advanced soft electronics for the IoE era. This article is protected by copyright. All rights reserved.

A biomimetic elastomeric robot skin using electrical impedance and acoustic tomography for tactile sensing

Human skin perceives physical stimuli applied to the body and mitigates the risk of physical interaction through its soft and resilient mechanical properties. Social robots would benefit from whole-body robotic skin (or tactile sensors) resembling human skin in realizing a safe, intuitive, and contact-rich human-robot interaction. However, existing soft tactile sensors show several drawbacks (complex structure, poor scalability, and fragility), which limit their application in whole-body robotic skin. Here, we introduce biomimetic robotic skin based on hydrogel-elastomer hybrids and tomographic imaging. The developed skin consists of a tough hydrogel and a silicone elastomer forming a skin-inspired multilayer structure, achieving sufficient softness and resilience for protection. The sensor structure can also be easily repaired with adhesives even after severe damage (incision). For multimodal tactile sensation, electrodes and microphones are deployed in the sensor structure to measure local resistance changes and vibration due to touch. The ionic hydrogel layer is deformed owing to an external force, and the resulting local conductivity changes are measured via electrodes. The microphones also detect the vibration generated from touch to determine the location and type of dynamic tactile stimuli. The measurement data are then converted into multimodal tactile information through tomographic imaging and deep neural networks. We further implement a sensorized cosmetic prosthesis, demonstrating that our design could be used to implement deformable or complex-shaped robotic skin.

Chitosan-enhanced nonswelling hydrogel with stable mechanical properties for long-lasting underwater sensing

Existing anti-swelling hydrogels with poor mechanical strength restrict their underwater human monitoring as wearable electronic sensing equipment. Herein, a nonswelling double network (DN) hydrogel with strong self-recoverability (97.22%) was developed by adding chitosan (CS) to poly(acrylic acid-2-methoxyethyl acrylate)-Fe³⁺ [P(AA-MEA)-Fe] network. Owing to the introduction of CS, the hydrogel displayed excellent nonswelling properties under aqueous solutions (pH = 1, 4 and 7), physiological saline, seawater, dodecane, n-hexane and chloroform. Besides, CS improved mechanical properties of hydrogel through non-covalent network (large stretchability of 1199%, tensile strength of 0.462 MPa and toughness of 2.01 MJ/m³). Surprisingly, the hydrogel still reached the extensibility (1072%) and tensile stress (0.467 MPa) even after immersing in water for 7 days. Fabricating hydrogel as flexible strain sensor, periodic real-time signals of human movements (e.g., joint actions and electronic skin touching) were accurately monitored under the water and seawater. The nonswelling P(AA-MEA)-CS-Fe hydrogel shows huge potential in underwater sensing.

Water-Saturated Ion Gel for Humidity-Independent High Precision Epidermal Ionic Temperature Sensor

Although ion gels are attractive sensing materials for deformable epidermal sensors or implantable devices, their sensing performances are highly affected by environmental humidity change, so that their sensing reliability cannot be secured. This study proposes a new concept of maintaining the high-precision temperature sensing performance of highly deformable ion gel sensors. In this approach, a hydrophobic ion gel sensing layer is kept water-saturated by attaching a hydrogel layer, rather than attempting to completely block water penetration. This study performs experimental and theoretical investigation on water concentration in the ion gel, using the analysis of mass transportation at the interface of the ion gel and the hydrogel. By using the charge relaxation time of the ionic molecules, the temperature sensor is not affected by environmental humidity in the extreme range of humidity (30%-100%). This study demonstrates a highly deformable on-skin temperature sensor which shows the same performance either in water or dry state and while exercising with large strains (ε = 50%).

Research Progress on Hydrogel-Elastomer Adhesion

Hydrophilic hydrogels exhibit good mechanical properties and biocompatibility, whereas hydrophobic elastomers show excellent stability, mechanical firmness, and waterproofing in various environments. Hydrogel-elastomer hybrid material devices show varied application prospects in the field of bioelectronics. In this paper, the research progress in hydrogel-elastomer adhesion in recent years, including the hydrogel-elastomer adhesion mechanism, adhesion method, and applications in the bioelectronics field, is reviewed. Finally, the research status of adhesion between hydrogels and elastomers is presented.

Temperature sensing using junctions between mobile ions and mobile electrons

We develop temperature sensors on the basis of charges accumulated at the electrolyte/dielectric interface and dielectric/electrode interface. The accumulated charges make the temperature sensors self-powered, which simplifies circuit design and enables portable sensing. The sensors are stretchable, but deformation does not affect temperature sensing. The sensors have high sensitivity and fast response. They can be made small and transparent. Such temperature sensors open new possibilities to create human–machine interfaces and soft robots in healthcare and engineering.

Sensing technology is under intense development to enable the Internet of everything and everyone in new and useful ways. Here we demonstrate a method of stretchable and self-powered temperature sensing. The basic sensing element consists of three layers: an electrolyte, a dielectric, and an electrode. The electrolyte/dielectric interface accumulates ions, and the dielectric/electrode interface accumulates electrons (in either excess or deficiency). The ions and electrons at the two interfaces are usually not charge-neutral, and this charge imbalance sets up an ionic cloud in the electrolyte. The design functions as a charged temperature-sensitive capacitor. When temperature changes, the ionic cloud changes thickness, and the electrode changes open-circuit voltage. We demonstrate high sensitivity (∼1 mV/K) and fast response (∼10 ms). Such temperature sensors can be made small, stable, and transparent. Depending on the arrangement of the electrolyte, dielectric, and electrode, we develop four designs for the temperature sensor. In addition, the temperature sensor has good linearity in the range of tens of Kelvin. We further show that the temperature sensors can be integrated into stretchable electronics and soft robots.

Soft artificial electroreceptors for noncontact spatial perception

Elasmobranch fishes, such as sharks, skates, and rays, use a network of electroreceptors distributed on their skin to locate adjacent prey. The receptors can detect the electric field generated by the biomechanical activity of the prey. By comparing the intensity of the electric fields sensed by each receptor in the network, the animals can perceive the relative positions of the prey without making physical contact. Inspired by this capacity for prey localization, we developed a soft artificial electroreceptor that can detect the relative positions of nearby objects in a noncontact manner by sensing the electric fields that originate from the objects. By wearing the artificial receptor, one can immediately receive spatial information of a nearby object via auditory signals. The soft artificial electroreceptor is expected to expand the ways we can perceive space by providing a sensory modality that did not evolve naturally in human beings.

Skin-like hydrogel devices for wearable sensing, soft robotics and beyond

Skin-like electronics are developing rapidly to realize a variety of applications such as wearable sensing and soft robotics. Hydrogels, as soft biomaterials, have been studied intensively for skin-like electronic utilities due to their unique features such as softness, wetness, biocompatibility and ionic sensing capability. These features could potentially blur the gap between soft biological systems and hard artificial machines. However, the development of skin-like hydrogel devices is still in its infancy and faces challenges including limited functionality, low ambient stability, poor surface adhesion, and relatively high power consumption (as ionic sensors). This review aims to summarize current development of skin-inspired hydrogel devices to address these challenges. We first conduct an overview of hydrogels and existing strategies to increase their toughness and conductivity. Next, we describe current approaches to leverage hydrogel devices with advanced merits including anti-dehydration, anti-freezing, and adhesion. Thereafter, we highlight state-of-the-art skin-like hydrogel devices for applications including wearable electronics, soft robotics, and energy harvesting. Finally, we conclude and outline the future trends.

ADVANCES IN TECHNIQUES AND APPLICATIONS OF RUBBER SURFACE GRAFTING MODIFICATION

To meet the requirement in the application of medical devices, composites, biomaterials, corrosion resistance, and selective adsorptions, rubber surface modification is usually indispensable. Grafting treatment is one of most significate treatment methods. In this paper, we focus on rubber surface grafting modification, including grafting techniques and applications. Different grafting methods—including monomer grafting polymerization and coupling reaction—are covered and compared briefly. The related applications of surface grafting modification techniques, such as improving compatibility of waste rubber as fillers, hydrophobicity and lipophilicity of sponge rubber for oil–water separation, biocompatibility of rubber in the medical field, and forming surface patterns, are demonstrated in detail. The new research directions of surface grafting techniques as well as main challenges in application are also discussed.

Transduction between magnets and ions

A time-varying magnetic field generates an electric field in an electrolyte, in which ions move. This magnetoionic transduction is studied here in several arrangements. The electrolyte is a hydrogel containing mobile ions, and is in contact with two metallic electrodes. An alternating electric current applied to a metal coil generates a time-varying magnetic field. In response, ions in the hydrogel move. The two hydrogel/electrode interfaces are non-faradaic and accumulate excess ions of opposite signs, which attract and repel electrons in the two electrodes. When the two electrodes are connected to a voltmeter of internal resistance much larger than that of the hydrogel, an open-circuit voltage is measured, linear in the alternating current applied to the metal coil. A metal coil and a hydrogel coil form an ionotronic transformer, in which an alternating electric current in the metal coil induces an alternating ionic current in the hydrogel coil. Such a transformer can be used for noncontact power transmission, with a voltage high enough to turn on many light-emitting diodes in series. The hydrogel is soft, and readily conforms to a curved surface, such as a glove on a human hand. Motion of the hand can be detected by noncontact magnetoionic transduction.

Engineering Hydrogel Adhesion for Biomedical Applications via Chemical Design of the Junction

Hydrogel adhesion inherently relies on engineering the contact surface at soft and hydrated interfaces. Upon contact, adhesion normally occurs through the formation of chemical or physical interactions between the disparate surfaces. The ability to form these adhesion junctions is challenging for hydrogels as the interfaces are wet and deformable and often contain low densities of functional groups. In this Review, we link the design of the binding chemistries or adhesion junctions, whether covalent, dynamic covalent, supramolecular, or physical, to the emergent adhesive properties of soft and hydrated interfaces. Wet adhesion is useful for bonding to or between tissues and implants for a range of biomedical applications. We highlight several recent and emerging adhesive hydrogels for use in biomedicine in the context of efficient junction design. The main focus is on engineering hydrogel adhesion through molecular design of the junctions to tailor the adhesion strength, reversibility, stability, and response to environmental stimuli.

Bioinspired Hydrogel-Polymer Hybrids with a Tough and Antifatigue Interface via One-Step Polymerization

Hydrogel hybrids are one of the key factors in life activities and biomimetic science; however, their development and utilization are critically impeded by their inadequate adhesive strength and intricate process. In nature, barnacles can stick to a variety of solid surfaces firmly (adhesive strength above 300 kPa) using a hydrophobic interface, which inspires us to firmly combine hydrogels and polymers through introducing an adhesive layer. By spreading a hydrophobic liquid membrane directly, tough combination of a hydrogel and a polymer substrate could be achieved after one-step polymerization. The fracture energy of the hydrogel attached to the surface of polyvinyl chloride was up to 1200 J m^-2 and the tensile strength reached 1.21 MPa. Furthermore, the adhesion samples with this method exhibit an antifatigue performance, having withstood large bends and twists. It should be pointed out that this approach can also be applied to a variety of complicated surfaces. This work may expand the application range of hydrogels and provides an inspiration for hydrogel adhesion.

Ionotronic Luminescent Fibers, Fabrics, and Other Configurations

A family of recently developed devices, hydrogel ionotronics, uses hydrogels as ionic conductors, and uses hydrophobic elastomers as dielectrics. This development has posed a challenge: integrate hydrogels and hydrophobic elastomers-in various manufacturing processes-with strong, stretchable, and transparent adhesion. Here, a multistep dip-coating process is described to enable hydrogel ionotronics of diverse configurations. In doing so, a hydrophobic surface is primed to let a hydrophilic precursor wet it, and then polymers of different layers are interlinked with covalent bonds. As a representative example, an ionotronic luminescent fiber that can be lengthened to ≈2.5 times its original length and keeps functioning after 10 000 cycles of stretching is fabricated. A luminescent fabric that displays movable pixels and other configurations is also demonstrated. The proposed method of fabrication expands the design space for hydrogel ionotronics.

Highly Stretchable and Transparent Ionic Conductor with Novel Hydrophobicity and Extreme-Temperature Tolerance

Highly stretchable and transparent ionic conducting materials have enabled new concepts of electronic devices denoted as iontronics, with a distinguishable working mechanism and performances from the conventional electronics. However, the existing ionic conducting materials can hardly bear the humidity and temperature change of our daily life, which has greatly hindered the development and real-world application of iontronics. Herein, we design an ion gel possessing unique traits of hydrophobicity, humidity insensitivity, wide working temperature range (exceeding 100°C, and the range covered our daily life temperature), high conductivity (10⁻³~10⁻⁵ S/cm), extensive stretchability, and high transparency, which is among the best-performing ionic conductors ever developed for flexible iontronics. Several ion gel-based iontronics have been demonstrated, including large-deformation sensors, electroluminescent devices, and ionic cables, which can serve for a long time under harsh conditions. The designed material opens new potential for the real-world application progress of iontronics.

Chemically Coupled Interfacial Adhesion in Multimaterial Printing of Hydrogels and Elastomers

Functional devices that use hydrogels as ionic conductors and elastomers as dielectrics have the advantage of being soft, stretchable, transparent, and biocompatible, making them ideal for biomedical applications. These devices are typically fabricated by manual assembly. Techniques for the manufacturing of soft materials have generally not looked at integrating multiple dissimilar materials. Silane coupling agents have recently shown promise for creating strong bonds between hydrogels and elastomers but have yet to be used in the extrusion printing of complex devices that integrate both hydrogels and elastomers. Here, we demonstrate the viability of silane coupling agents in a system with the rheology and functional composition necessary for three-dimensional (3D) extrusion printing of hydrogel-elastomer materials, specifically polyacrylamide (PAAm) hydrogel and poly(dimethylsiloxane) (PDMS) hydrophobic elastomer. By introducing a charge-neutral surfactant in the PDMS and adjusting silane concentrations in the PAAm, cast material samples demonstrate strong adhesion. We were also able to achieve an interfacial toughness of up to Γ = 193 ± 6.3 J/m² for a fully extrusion printed PAAm hydrogel-on-PDMS bilayer. This result demonstrates that an integration strategy based on silane coupling agents makes it possible for extrusion printing of a wide variety of hydrogel and silicone elastomers.

Stretchable Electrets: Nanoparticle-Elastomer Composites

Manipulating charges is fundamental to numerous systems, and this ability is achieved through materials of diverse characteristics. Electrets are dielectrics that trap charges or dipoles. Applications include electrophotography, microphones, air filters, and energy harvesters. To trap charges or dipoles for a long time, electrets are commonly made of hard dielectrics. Stretchable dielectrics are short-lived electrets. The two properties, longevity and stretchability, conflict; existing electrets struggle to attain both. This work describes an approach to developing stretchable electrets. Nanoparticles of a hard electret are immobilized in a matrix of dielectric elastomer. The composite divides the labor of two functions: the particles trap charges with longevity, and the matrix enables stretchability. The design considerably broadens the choice of materials to enable stretchable electrets. Silica nanoparticles in the polydimethylsiloxane elastomer achieve a charge density ∼ 4 × 10^-5 C m^-2 and a lifetime beyond 60 days. Long-lived, stretchable electrets open extensive opportunities.

Physically Cross-Linked Silk Fibroin-Based Tough Hydrogel Electrolyte with Exceptional Water Retention and Freezing Tolerance

Flexible ionic conductive hydrogel is attracting significant interest as it could be one of the crucial components for multifunctional ionotronic devices. However, their features of inevitably drying out without package and freezing at subzero temperatures may greatly limit the applications of conventional hydrogels in specific situations. Here, we present an ionic conductive hydrogel with water retention and freezing tolerance that consists of silk fibroin, ionic liquid, water, and inorganic salt. It is discovered that the ionic liquid serves multiple purposes to prevent water evaporation, decrease the freezing point, provide the essential conductivity of the hydrogel, etc. As a binary mixed solvent, the ionic liquid/water mixture enhances both water retention and freezing tolerance of the hydrogel electrolyte. Based on the silk fibroin (SF)/1-ethyl-3-methylimidazolium acetate (EMImAc)/H₂O/KCl hydrogel electrolyte, the flexible fiberlike supercapacitor could still function well at a temperature as low as -50 °C and after being stored in the open air for a long time. It is anticipated that this hydrogel will prove useful in developing new applications operating under harsh environments.

Stretchable Lithium-Ion Battery Based on Re-entrant Micro-honeycomb Electrodes and Cross-Linked Gel Electrolyte

Stretchable energy storage devices are of great interest because of their potential applications in body-friendly, skin-like, wearable devices. However, stretchable batteries are very challenging to fabricate. The electrodes must have a degree of stretchability because the active materials occupy most of the volume, and the separator and packaging should also be stretchable. Here, an all-component stretchable lithium-ion battery was realized by leveraging the structural stretchability of re-entrant micro-honeycomb graphene-carbon nanotube (CNT)/active material composite electrodes and a physically cross-linked gel electrolyte, without using an inactive elastomeric substrate or matrix. Active materials interconnected via the entangled CNT and graphene sheets provided a mechanically stable porous network framework, and the inwardly protruding framework in the re-entrant honeycomb structure allowed for structural stretching during deformation. The composite network consisting solely of binder-free, highly conductive materials provided superior electron transfer, and vertically aligned microchannels enabled facile ion transport. Additionally, the physically cross-linked gel electrolyte increased the mechanical stability upon deformation of the electrodes and was effective as a stretchable separator. The resulting stretchable battery showed a high areal capacity of 5.05 mAh·cm^-2, superior electrochemical performance up to 50% strain under repeated (up to 500) stretch-release cycles, and long-term stability of 95.7% after 100 cycles in air conditions.

Multifunctional Conductive Hydrogel/Thermochromic Elastomer Hybrid Fibers with a Core-Shell Segmental Configuration for Wearable Strain and Temperature Sensors

Flexible wearable sensors are emerging as next-generation tools to collect information from the human body and surroundings in a smart, friendly, and real-time manner. A new class of such sensors with various functionality and amenability for the human body is essential for this goal. Unfortunately, the majority of the wearable sensors reported so far in the literature were of a single function (mostly strain sensors) and just a prototype without thinking of continuous mass production. In this paper, we report a series of multifunctional conductive hydrogel/ thermochromic elastomer hybrid fibers with core-shell segmental configuration and their application as flexible wearable strain and temperature sensors to monitor human motion and body/surrounding temperatures. Specifically, a conductive reduced-graphene-oxide-doped poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide (rGO-poly(AMPS-co-AAm)) hydrogel and a thermochromic elastomer containing silicon rubber and thermochromic microcapsules are chosen as strain-sensitive and thermosensitive materials, respectively. A core-shell segmental structure is realized by programming the extrusion of either conductive hydrogel precursor solution or a thermochromic elastomer prepolymer as a core layer via dual-core coaxial wet spinning. Depending on the assembly order and length of the conductive hydrogel and the thermochromic elastomer, the as-prepared hybrid fibers can be used for different purposes, i.e., human-motion monitoring, body or room temperature detection, and color decoration. The strategy described above, i.e., fabrication of core-shell segmental fibers via the wet-spinning method, is especially suitable for mass production in industry and can be further extended to fabricate flexible wearable devices with more components and more functions such as transistors, sensors, displays, and batteries.

Fundamentals and Advances in the Adhesion of Polymer Surfaces and Thin Films

Polymer materials have been widely used in industrial, agricultural, engineering, medical, electronic, and biological fields because of their excellent and diverse properties (e.g., mechanical, optical, electrical, and adhesive properties). The adhesion of polymer materials can affect the stability, alter the surface chemistry, change the surface structure, and influence the performance of the materials. It is of both fundamental and practical importance to understand the adhesion behaviors and interaction mechanisms of polymer surfaces and thin films for the development of new functional polymers and their applications. In this article, the fundamentals of surface energy, adhesion energy, and classical contact mechanics models are presented first, and the commonly used nanomechanical techniques for quantifying the intermolecular and surface interactions of polymers, including the surface forces apparatus (SFA) and atomic force microscope (AFM), are introduced. The advances in the adhesion of surfaces and thin films of various polymers (e.g., elastomers, glassy polymers) are reviewed. The effects of various factors, including the molecular weight, temperature, separation rate, and surface roughness, on the adhesion behaviors of these polymer surfaces and thin films are discussed. Their liquid- to solid-like behaviors during approach and detachment processes are shown. Several commonly applied methodologies used to modulate polymer adhesion are also introduced. Some recent applications based on polymer adhesion, remaining challenging issues, and future perspectives are also presented.

Artificial phototropism for omnidirectional tracking and harvesting of light

Many living organisms track light sources and halt their movement when alignment is achieved. This phenomenon, known as phototropism, occurs, for example, when plants self-orient to face the sun throughout the day. Although many artificial smart materials exhibit non-directional, nastic behaviour in response to an external stimulus, no synthetic material can intrinsically detect and accurately track the direction of the stimulus, that is, exhibit tropistic behaviour. Here we report an artificial phototropic system based on nanostructured stimuli-responsive polymers that can aim and align to the incident light direction in the three-dimensions over a broad temperature range. Such adaptive reconfiguration is realized through a built-in feedback loop rooted in the photothermal and mechanical properties of the material. This system is termed a sunflower-like biomimetic omnidirectional tracker (SunBOT). We show that an array of SunBOTs can, in principle, be used in solar vapour generation devices, as it achieves up to a 400% solar energy-harvesting enhancement over non-tropistic materials at oblique illumination angles. The principle behind our SunBOTs is universal and can be extended to many responsive materials and a broad range of stimuli.

Molecular Staples for Tough and Stretchable Adhesion in Integrated Soft Materials

The integration of soft materials-biological tissues, gels, and elastomers-is a rapidly developing technology of this time. Whereas hard materials are adhered using adhesives of hard polymers since antiquity, these hard polymers are generally unsuited to adhere soft materials, because hard polymers constrain the deformation of soft materials. This paper describes a design principle to use hard polymers to adhere soft materials, such that adhesion remains tough after the adhered soft materials are subject to many cycles of large stretches in the plane of their interface. The two soft materials have stretchable polymer networks, but need not have functional groups for adhesion. The two soft materials are adhered by forming, in situ at their interface, islands of a hard polymer. The adhesion is tough if the islands themselves are strong, and the polymers of the islands are in topological entanglement with the polymer networks of the soft materials. The adhesion is stretchable if the islands are smaller than the flaw sensitivity length. Several methods of forming the hard polymer islands are demonstrated, and the mechanics and chemistry of adhesion are studied. The design principle will enable many hard polymers to form tough and stretchable adhesion between soft materials.

Dual-Core Capacitive Microfiber Sensor for Smart Textile Applications

Wearable sensors for smart textile applications have garnered tremendous interest in recent years and can have enormous potential for human machine interfaces and digital health monitoring. Here, we report a soft capacitive microfiber sensor that can be woven seamlessly into textiles for strain measurement. Comprising a dual-lumen elastomeric microtube and liquid metallic alloy, the microfiber sensor enables continual strain perception even after being completely severed. In addition, our microfiber sensor is highly stretchable and flexible and exhibits tunable sensitivity, excellent linearity, a fast response, and negligible hysteresis. More importantly, the microfiber sensor is minimally affected by train rate and compression during strain sensing. Even under drastic environmental changes, the microfiber sensor presents good electrical stability. By integrating the microfiber sensor imperceptibly with textiles, we devise smart textile wearables to interpret hand gestures, detect limb motion, and monitor respiration rate. We believe that this sensor presents enormous potential in unobtrusive continuous health monitoring.

Hydrogel Paint

For a hydrogel coating on a substrate to be stable, covalent bonds polymerize monomer units into polymer chains, crosslink the polymer chains into a polymer network, and interlink the polymer network to the substrate. The three processes-polymerization, crosslinking, and interlinking-usually concur. This concurrency hinders widespread applications of hydrogel coatings. Here a principle is described to create hydrogel paints that decouple polymerization from crosslinking and interlinking. Like a common paint, a hydrogel paint divides the labor between the paint maker and the paint user. The paint maker formulates the hydrogel paint by copolymerizing monomer units and coupling agents into polymer chains, but does not crosslink them. The paint user applies the paint on various materials (elastomer, plastic, glass, ceramic, or metal), and by various operations (brush, cast, dip, spin, or spray). During cure, the coupling agents crosslink the polymer chains into a network and interlink the polymer network to the substrate. As an example, hydrogels with thickness in the range of 2-20 µm are dip coated on medical nitinol wires. The coated wires reduce friction by eightfold, and remain stable over 50 test cycles. Also demonstrated are several proof-of-concept applications, including stimuli-responsive structures and antifouling model boats.

Stretchable and reflective displays: materials, technologies and strategies

Displays play a significant role in delivering information and providing visual data across all media platforms. Among displays, the prominence of reflective displays is increasing, in the form of E-paper, which has features distinct from emissive displays. These unique features include high visibility under daylight conditions, reduced eye strain and low power consumption, which make them highly effective for outdoor use. Furthermore, such characteristics enable reflective displays to achieve high synergy in combination with wearable devices, which are frequently used for outdoor activities. However, as wearable devices must stretch to conform to the dynamic surfaces of the human body, the issue of how to fabricate stretchable reflective displays should be tackled prior to merging them with wearable devices. In this paper, we discuss stretchable and reflective displays. In particular, we focus on reflective displays that can be divided into two types, passive and active, according to their responses to stretching. Passive displays, which consist of dyes or pigments, exhibit consistent colors under stretching, while active displays, which are based on mechanochromic materials, change their color under the same conditions. We will provide a comprehensive overview of the materials and technologies for each display type, and present strategies for stretchable and reflective displays.

Mechanical characterization of functionally graded soft materials with ultrasound elastography

Functionally graded soft materials (FGSMs) with microstructures and mechanical properties exhibiting gradients across a spatial volume to satisfy specific functions have received interests in recent years. How to characterize the mechanical properties of these FGSMs in vivo/in situ and/or in a non-destructive manner is a great challenge. This paper investigates the use of ultrasound elastography in the mechanical characterization of FGSMs. An efficient finite-element model was built to calculate the dispersion relation for surface waves in FGSMs. For FGSMs with large elastic gradients, the measured dispersion relation can be used to identify mechanical parameters. In the case where the elastic gradient is smaller than a certain critical value calculated here, our analysis on transient wave motion in FGSMs shows that the group velocities measured at different depths can infer the local mechanical properties. Experiments have been performed on polyvinyl alcohol (PVA) cryogel to demonstrate the usefulness of the method. Our analysis and the results may not only find broad applications in mechanical characterization of FGSMs but also facilitate the use of shear wave elastography in clinics because many diseases change the local elastic properties of soft tissues and lead to different material gradients. This article is part of the theme issue 'Rivlin's legacy in continuum mechanics and applied mathematics'.

Hydrogel bioelectronics

Hydrogels have emerged as a promising bioelectronic interfacing material. This review discusses the fundamentals and recent advances in hydrogel bioelectronics.

Stretchable Ionics - A Promising Candidate for Upcoming Wearable Devices

As many devices for human utility aim for fast and convenient communication with users, superb electronic devices are demonstrated to serve as hardware for human-machine interfaces in wearable forms. Wearable devices for daily healthcare and self-diagnosis offer more human-like properties unconstrained by deformation. In this sense, stretchable ionics based on flexible and stretchable hydrogels are on the rise as another means to develop wearable devices for bioapplications for two main reasons: i) ionic currents and choosing the same signal carriers for biological areas, and ii) the adoption of hydrogel ionic conductors, which are intrinsically stretchable materials with biocompatibility. Here, the current status of stretchable ionics and future applications are introduced, whose positive effects can be magnified by stretchable ionics.

Stretchable Seal

Many stretchable electronic devices require stretchable hermetic seals. However, stretchability and permeability are inextricably linked at the molecular level: stretchable, low-permeability materials do not exist. We collect data for the permeation of water and oxygen in many materials and describe the scaling relations for both flat and wrinkled seals. Whereas flat seals struggle to fulfill the simultaneous requirements of stretchability, low stiffness, and low transmissibility, wrinkled seals can fulfill them readily. We further explore the behavior of wrinkled seals under cyclic stretch using aluminum, polyethylene, and silica films on elastomer substrates. The wrinkled aluminum develops fatigue cracks after a small number of cycles, but the wrinkled polyethylene and silica maintain low transmissibility after 10 000 cycles of tensile strain.

Fatigue fracture of nearly elastic hydrogels

Polyacrylamide hydrogels are highly stretchable and nearly elastic. Their stress-stretch curves exhibit small hysteresis, and change negligibly after many loading cycles. Polyacrylamide is used extensively in applications, and is the primary network for many types of tough hydrogels. Recent experiments have shown that polyacrylamide hydrogels are susceptible to fatigue fracture, but available data are limited. Here we study fatigue fracture of polyacrylamide hydrogels of various water contents. We form polymer networks in all samples under the same conditions, and then obtain hydrogels of 96, 87, 78, and 69 wt% of water by solvent exchange. We measure the crack extension under cyclic loads, and the fracture energy under monotonic loading. For the hydrogels of the four water contents, the fatigue thresholds are 4.3, 8.4, 20.5, and 64.5 J m-2, and the fracture energies are 18.9, 71.2, 289, and 611 J m-2. The measured thresholds agree well with the predictions of the Lake-Thomas model for hydrogels of high water content, but not in the case of low water content. It is hoped that further basic studies will soon follow to aid the development of fatigue-resistant hydrogels.

Bioinspired Universal Flexible Elastomer-Based Microchannels

Flexible and stretchable microscale fluidic devices have a broad range of potential applications, ranging from electronic wearable devices for convenient digital lifestyle to biomedical devices. However, simple ways to achieve stable flexible and stretchable fluidic microchannels with dynamic liquid transport have been challenging because every application for elastomeric microchannels is restricted by their complex fabrication process and limited material selection. Here, a universal strategy for building microfluidic devices that possess exceptionally stable and stretching properties is shown. The devices exhibit superior mechanical deformability, including high strain (967%) and recovery ability, where applications as both strain sensor and pressure-flow regulating device are demonstrated. Various microchannels are combined with organic, inorganic, and metallic materials as stable composite microfluidics. Furthermore, with surface chemical modification these stretchable microfluidic devices can also obtain antifouling property to suit for a broad range of industrial and biomedical applications.

Highly Stretchable, Weavable, and Washable Piezoresistive Microfiber Sensors

A key challenge in electronic textiles is to develop an intrinsically conductive thread of sufficient robustness and sensitivity. Here, we demonstrate an elastomeric functionalized microfiber sensor suitable for smart textile and wearable electronics. Unlike conventional conductive threads, our microfiber is highly flexible and stretchable up to 120% strain and possesses excellent piezoresistive characteristics. The microfiber is functionalized by enclosing a conductive liquid metallic alloy within the elastomeric microtube. This embodiment allows shape reconfigurability and robustness, while maintaining an excellent electrical conductivity of 3.27 ± 0.08 MS/m. By producing microfibers the size of cotton threads (160 μm in diameter), a plurality of stretchable tubular elastic piezoresistive microfibers may be woven seamlessly into a fabric to determine the force location and directionality. As a proof of concept, the conductive microfibers woven into a fabric glove were used to obtain physiological measurements from the wrist, elbow pit, and less accessible body parts, such as the neck and foot instep. Importantly, the elastomeric layer protects the sensing element from degradation. Experiments showed that our microfibers suffered minimal electrical drift even after repeated stretching and machine washing. These advantages highlight the unique propositions of our wearable electronics for flexible display, electronic textile, soft robotics, and consumer healthcare applications.

Scalable and Automated Fabrication of Conductive Tough-Hydrogel Microfibers with Ultrastretchability, 3D Printability, and Stress Sensitivity

Creating complex three-dimensional structures from soft yet durable materials enables advances in fields such as flexible electronics, regenerating tissue engineering, and soft robotics. Tough hydrogels that mimic the human skin can bear enormous mechanical loads. By employing a spider-inspired biomimetic microfluidic nozzle, we successfully achieve continuous printing of tough hydrogels into fibers, two-dimensional networks, and even three-dimensional structures without compromising their extreme mechanical properties. The resultant thin fibers demonstrate a stretch up to 21 times of their original length at a water content of 52%, and are intrinsically transparent, biocompatible, and conductive at high stretches. Moreover, the printed robust tough-hydrogel networks can sense strain that are orders of magnitude lower than stretchable conductors by percolations of conductive particles. To demonstrate their potential application, we use printed tough-hydrogel fiber networks as wearable sensors for detecting human motions. The capability to shape tough hydrogels into complex structures by scalable continuous printing opens opportunities for new areas of applications such as tissue scaffolds, large-area soft electronics, and smart textiles.

Fatigue Fracture of Self-Recovery Hydrogels

Hydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress-stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels or even fatigue-free hydrogels. Such a hydrogel typically contains a covalent polymer network, together with some noncovalent, reversible interactions. Here we show that self-recovery hydrogels are still susceptible to fatigue fracture. We study a hydrogel containing both covalently cross-linked polyacrylamide and un-cross-linked poly(vinyl alcohol). For a sample without precut crack, the stress-stretch curve recovers after thousands of loading cycles. For a sample with a precut crack, however, the crack extends cycle by cycle. The threshold for fatigue fracture depends on the covalent network but negligibly on noncovalent interactions. Above the threshold, the noncovalent interactions slow down the extension of the crack under cyclic loads.