Soft Electronics Research Articles

A self-healing, long-lasting adhesive, lignin-based polyvinyl alcohol organo-hydrogel for strain-sensing applications

Hydrogel-based flexible sensors have garnered considerable interest in the fields of soft electronics, robotics, and human-machine interfaces. For better practical applications, integrating multiple properties—such as self-adhesive, anti-freeze, anti-volatile, self-healing, and antibacterial—into a single gel for flexible sensors remains a challenge. In this paper, a multifunctional lignin-based polyvinyl alcohol gel, containing dynamic covalent bonds, hydrogen bonds, and coordination bonds, is constructed by a simple one-pot method, in which ethylene glycol/water chosen as a binary solvent and KI as a conductive medium. The resulting organogel exhibits self-healing, long-lasting adhesion, UV shielding, antibacterial properties, excellent frost resistance (−20 °C), and volatile resistance properties. In addition, the organogel-based sensor demonstrates satisfactory sensitivity in detecting joint movements and facial expressions. This study provides a new strategy for developing a versatile flexible sensor through the introduction of renewable and bio-based lignin, promising applications in the fields of wearable electronics.

A Soft, Fatigue‐free, and Self‐healable Ionic Elastomer via the Synergy of Skin‐like Assembly and Bouligand Structure

AbstractSoft ionic elastomers that are self‐healable, fatigue‐free, and environment‐tolerant are ideal structural and sensing materials for artificial prosthetics, soft electronics, and robotics to survive unpredictable service conditions. However, most synthetic strategies failed to unite rapid healing, fatigue resistance, and environmental robustness, limited by their singular compositional/structural designs. Here, we present a soft, tough, fatigue‐resistant, and self‐healable ionic elastomer (STFSI elastomer), which fuses skin‐like binary assembly and Bouligand helicoidal structure into a composite of thermoplastic polyurethane (TPU) fibers and a supramolecular ionic biopolymer. The interlocked binary assembly enables skin‐like softness, high stretchability, and strain‐adaptive stiffening through a matrix‐to‐scaffold stress transfer. The Bouligand structure contributes to superhigh fracture toughness (101.6 kJ m−2) and fatigue resistance (4937 J m−2) via mechanical toughening by interlayer slipping and twisted crack propagation path. Besides, the STFSI elastomer is self‐healable through a “bridging” method and environment‐tolerant (−20 °C, strong acid/alkali, saltwater). To demonstrate the versatile structural and sensing applications, we showcase a safety cushion with efficient damping and suppressed rebounding, and a robotic sensor with excellent fatigue crack tolerance and instant sensation recovery upon cutting‐off damage. Our presented synthetic strategy is generalizable to other fiber‐reinforced tough polymers for applications involving demanding mechanical/environmental conditions.

Functional-hydrogel-based electronic-skin patch for accelerated healing and monitoring of skin wounds

Conductive hydrogels feature reasonable electrical performance as well as tissue-like mechanical softness, thus positioning them as promising material candidates for soft bio-integrated electronics. Despite recent advances in materials and their processing technologies, however, facile patterning and monolithic integration of functional hydrogels (e.g., conductive, low-impedance, adhesive, and insulative hydrogels) for all-hydrogel-based soft bioelectronics still poses significant challenges. Here, we report material design, fabrication, and integration strategies for an electronic-skin (e-skin) patch based on functional hydrogels. The e-skin patch was fabricated by using photolithography-compatible functional hydrogels, such as poly(2-hydroxyethyl acrylate) (PHEA) hydrogel (substrate), Ag flake hydrogel (interconnection; conductivity: ∼571.43 S/cm), poly(3,4-ethylenedioxythiophene:polystyrene) (PEDOT:PSS) hydrogel (working electrode; impedance: ∼69.84 Ω @ 1 Hz), polydopamine (PDA) hydrogel (tissue adhesive; shear strength: ∼725.1 kPa), and poly(vinyl alcohol) (PVA) hydrogel (encapsulation). The properties of these functional hydrogels closely resemble those of human tissues in terms of water content and Young's modulus, enabling stable tissue-device interfacing in dynamically changing physiological environments. We demonstrated the efficacy of the e-skin patch through its application to accelerated healing and monitoring of skin wounds in mouse models − efficient fibroblast migration, proliferation, and differentiation promoted by electric field (EF) stimulation and iontophoretic drug delivery, and monitoring of the accelerated healing process through impedance mapping. The all-hydrogel-based e-skin patch is expected to create new opportunities for various clinically-relevant tissue interfacing applications.

A Soft, Fatigue-free, and Self-healable Ionic Elastomer via the Synergy of Skin-like Assembly and Bouligand Structure.

Soft ionic elastomers that are self-healable, fatigue-free, and environment-tolerant are ideal structural and sensing materials for artificial prosthetics, soft electronics, and robotics to survive unpredictable service conditions. However, most synthetic strategies failed to unite rapid healing, fatigue resistance, and environmental robustness, limited by their singular compositional/structural designs. Here, we present a soft, tough, fatigue-resistant, and self-healable ionic elastomer (STFSI elastomer), which fuses skin-like binary assembly and Bouligand helicoidal structure into a composite of thermoplastic polyurethane (TPU) fibers and a supramolecular ionic biopolymer. The interlocked binary assembly enables skin-like softness, high stretchability, and strain-adaptive stiffening through a matrix-to-scaffold stress transfer. The Bouligand structure contributes to superhigh fracture toughness (101.6 kJ m-2) and fatigue resistance (4937 J m-2) via mechanical toughening by interlayer slipping and twisted crack propagation path. Besides, the STFSI elastomer is self-healable through a "bridging" method and environment-tolerant (-20 °C, strong acid/alkali, saltwater). To demonstrate the versatile structural and sensing applications, we showcase a safety cushion with efficient damping and suppressed rebounding, and a robotic sensor with excellent fatigue crack tolerance and instant sensation recovery upon cutting-off damage. Our presented synthetic strategy is generalizable to other fiber-reinforced tough polymers for applications involving demanding mechanical/environmental conditions.

Oxidation-Induced Oxide Shell Rupture and Phase Separation in Eutectic Gallium-Indium Nanoparticles.

Eutectic gallium-indium (EGaIn), a room-temperature liquid metal, has garnered significant attention for its applications in soft electronics, multifunctional materials, energy engineering and drug delivery. A key factor influencing these diverse applications is the spontaneous formation of a native passivating oxide shell that not only encapsulates the liquid metal but also alters the properties from the bulk counterpart. Using environmental scanning transmission electron microscopy, we report in situ observations of the oxidation of EGaIn nanoparticles by ambient air under high-energy electron beam irradiation. Our findings demonstrate that uneven oxide shell growth, driven by inward diffusion of adsorbed O species, creates unbalanced stresses. This compels the liquid metal to deform toward regions with slower oxide growth, resulting in shell rupture and allowing the liquid metal core to flow out. This process initiates top-down self-similar replication of the core-shell liquid metal nanoparticles, causing larger particles to break down into smaller particles. Additionally, internal oxidation triggers phase separation within the liquid core, ultimately leading to the pulverization of the liquid metal into finer solid particles rich in indium. These mechanistic insights into the oxidation behavior of the liquid metal hold practical implications for leveraging this process to reconfigure EGaIn nanoparticles for various applications.

The robustness waterproof ionogel based on the phase separation to form soft hard heterostructures and the interaction of cation-π realizes underwater adhesion and sensing

Flexible ionic conductors hold significant promise for advancing the field of soft ion electronics in the future. However, the creation of flexible conductors that possess mechanical robustness, underwater adhesion and underwater motion monitoring remains a challenge. Drawing inspiration from the multiphase hetero structure found in biological connective tissues, such as high modulus fibers and low modulus water-rich matrices, this article proposes a one-pot polymerization in-situ design strategy to create a hydrophobic ionogel with a hetero structure of soft and hard domains. The mechanical robustness, elasticity, and toughness of the ionogel are achieved through a synergistic combination of a strong skeleton in the hard domain and an elastic matrix in the soft domain. Additionally,hydrophobic aromatic rings and quaternary ammonium cation simulated cation-π interactions in mussel foot silk protein. Upon contact with water, the hydrophobic clusters quickly aggregate, facilitating the expulsion of interfacial water and exposing salicylic acid groups and quaternary ammonium cationic groups, leading to outstanding underwater adhesion between the ionogel and various substrates. The resulting hydrophobic ionogel can be utilized in underwater wearable electronic devices for monitoring human movement and physiological information, underwater repair monitoring, and intelligent soft robotics, demonstrating its vast potential in the field of underwater sensors.

Electrofluids with Tailored Rheoelectrical Properties: Liquid Composites with Tunable Network Structures as Stretchable Conductors.

Flexible and stretchable electronics require both sensing elements and stretching-insensitive electrical connections. Conductive polymer composites and liquid metals are highly deformable but change their conductivity upon elongation and/or contain rare metals. Solid conductive composites are limited in mechanoelectrical properties and are often combined with macroscopic Kirigami structures, but their use is limited by geometrical restraints. Here, we introduce "Electrofluids", concentrated conductive particle suspensions with transient particle contacts that flow under shear that bridge the gap between classic solid composites and liquid metals. We show how Carbon Black (CB) forms large agglomerates when using incompatible solvents that reduce the electrical percolation threshold by 1 order of magnitude compared to more compatible solvents, where CB is well-dispersed. We analyze the correlation between stiffness and electrical conductivity to create a figure of merit of first electrofluids. Sealed elastomeric tubes containing different types of electrofluids were characterized under uniaxial tensile strain, and their electrical resistance was monitored. We found a dependency of the piezoresistivity with the solvent compatibility. Electrofluids enable the rational design of sustainable soft electronics components by simple solvent choice and can be used both as sensor and electrode materials, as we demonstrate.

Polyvinyl alcohol/sodium alginate-based conductive hydrogels with in situ formed bimetallic zeolitic imidazolate frameworks towards soft electronics

Bimetallic zeolitic imidazolate frameworks (BZIFs) have received enormous attention due to their unique physi-chemical properties, but are rarely reported for electrically conductive hydrogel (ECH) applications arising from low intrinsic conductivity and poor dispersion. Herein, we propose an innovative strategy to prepare highly conductive and mechanically robust ECHs by in situ growing Ni/Co-BZIFs within the polyvinyl alcohol/sodium alginate dual network (PZPS). 2-methylimidazole (MeIM) ligands copolymerize with pyrrole monomers, enhancing the electrical conductivity; meanwhile, MeIM ligands act as anchor points for in-situ formation of BZIFs, effectively avoiding phase-to-phase interfacial resistance and ensuring a uniform distribution in the hydrogel network. Due to the synergism of Ni/Co-BZIFs, the PZPS hydrogel exhibits a high areal capacitance of 630.3 mF·cm−2 at a current density of 0.5 mA·cm−2, promising for flexible energy storage devices. In addition, PZPS shows excellent mechanical strength and toughness (with an ultimate tensile strength of 405.0 kPa and a toughness of 784.2 kJ·m−3 at an elongation at break of 474.0 %), a high gauge factor of up to 4.18 over an extremely wide stress range of 0–42 kPa when used as flexible wearable strain/pressure sensors. This study provides new insights to incorporating highly conductive and uniformly dispersed ZIFs into hydrogels for flexible wearable electronics.

Synthesis of Yolk-Shell CoNi2S4 Spheres Modified Flexible Carbon Cloth Electrode for Highly Sensitive Detection of Neurotransmitter Dopamine

Dopamine (DA) is a neurotransmitter that is connected with transitory information in the mammalian central nervous system; changes in its concentration in the body are directly related to mental activity. The absence and/or low level of DA can cause symptoms of several disorders, including Parkinson’s and schizophrenia. As a result, detecting DA in patients is critical for regulating body function. Many efforts have been made to build electrochemical sensing devices to detect DA in human body or biological fluids due to its inherent advantages of high sensitivity, speed, simplicity, and affordability. However, surface modification with nanomaterials, polymers, pretreatments, or simple functionalization are required to improve the performance of these sensing electrodes. Further, exploration of earth-abundant electro-catalysts with high activity and stability is also important for the development of highly selective and sensitive electrochemical sensing interfaces. With these considerations in mind, our study describes the construction of yolk-shell CoNi2S4 spheres modified flexible carbon cloth (CC) electrode for the sensitive detection of DA. The synthesized material's surface and morphological study were examined, and the successful synthesis was confirmed. The yolk-shell CoNi2S4 spheres shown good electrochemical sensing performance towards DA detection due to their unique surface morphology with large specific surface area and synergistic effects. In comparison to unmodified CC, the yolk-shell CoNi2S4 spheres modified CC demonstrated a low detection limit (5.1 nM), great sensitivity, and a wide linear range (0.01 – 5.2 µM). The detection selectivity was also tested in the presence of a high quantity of the common interference ascorbic acid. Selectivity, stability, and repeatability have also been demonstrated to be satisfactory. Furthermore, the fabricated sensor’s flexibility endows it with significant potential in the fabrication of wearable and soft electronics for human healthcare monitoring.

Nanomaterials used in Flexible Electronics: Recent Trends and Future Approaches

Abstract: The latest research in soft electronics reveals a substantial demand for devices that can fold, bend and stretch to suit the requirements of technological advances. Cellulose, silk, and elastomers are employed in making biodegradable, environmentally benign substrates that accommodate nanofibers, nanoparticles, nanotubes, graphene, and biomaterials in their nano-form. Flexible materials can hold circuits and sensors and can substitute conventional substrates. Transient electronics, e-skin, and biosensors are the most sought-after in medical technology, sensors, energy storage devices, and wearables. These stretchable materials lead the way for developing eco-friendly and sustainable technology to attain sustainable development goals. This research work discusses nano species imbibed in printable and flexible electronics. An analysis of the documents extracted from the Scopus database using VOSviewer and patents in the domain of flexible electronics are presented along with altmetrics.

Application of 3D and 4D Printing in Electronics

Nowadays, additive manufacturing technologies have impacted different engineering sectors. Three- and four-dimensional printing techniques are increasingly used in soft and flexible electronics thanks to the possibility of working contemporarily with several materials on various substrates. The materials portfolio is wide, as well as printing processes. Shape memory polymers, together with composites, have gained great success in the electronic field and are becoming increasingly popular for fabricating pH, temperature, humidity, and stress sensors that are integrated into wearable, stretchable, and flexible devices, as well as for the fabrication of communication devices, such as antennas. Here, we report an overview of the state of the art about the application of 4D printing technologies and smart materials in electronics.

Multifunctional PDMS/Schiff base/SiO2 gel assisted fabrication of printed, stretchable and straight copper conductors

Printable, stretchable, and highly conductive metal conductors have emerged as potentially promising interconnects in the field of soft electronics. However, current techniques relying on geometric engineering have been criticized for complicated fabrication process and defective geometric structure. Here, we report a printed, straight, and omnidirectionally stretchable copper conductor, which is facilely fabricated on a flat substrate without requiring prestretching, assisted by a multifunctional gel medium layer. The PSS gel, composited by polydimethylsiloxane, Schiff base and fumed SiO2, offers a unique combination of stretchability, printability, and moldability. Additionally, this gel enables the reduction of silver ions directly on its surface by partially bonded crosslinker molecules. The fully exposed catalyst exhibits excellent activity in catalyzing electroless copper deposition reaction, on which high-quality copper coating is facilely achieved. The produced copper patterns exhibit high conductivity (4.93 × 107 S m−1) and excellent stretchability with fracture strains of ∼ 191 % and ∼ 135 % for the uni- and omni-directionally stretchable patterns respectively, which are superior to the majority of stretchable metals in published literature. The stretchable wrinkles derive form the gel medium layer molded by templates, realizing straight metal lines on flat substrate. This process combines the advantages of traditional in-plane and out-of-plane strategies, demonstrating great potential in practical application.

Stretchable Electrodes for Interconnects in Soft Electronics.

Soft electronics have significantly enhanced user convenience and data accuracy in wearable devices, implantable devices, and human-machine interfaces. However, a persistent challenge in their development has been the disconnection between the rigid and soft components of devices due to the substantial difference in modulus and stretchability. To address this issue, establishing a durable and flexible connection that smoothly links components of varying stiffness to signal-capturing sections with a lower stiffness is essential. In this study, we developed a novel stretchable interconnect that strongly adheres to various materials, facilitating electrical connections effortlessly by applying minimal finger pressure. Capable of stretching up to 1000% while maintaining electrical integrity, this interconnect proves its applicability across multiple domains, including electrocardiogram (ECG), electromyography (EMG), and stretchable light-emitting diode (LED) circuits. Its versatility is further demonstrated through its compatibility with various manufacturing techniques such as 3D printing, painting, and spin coating, highlighting its adaptability in soft electronics.

Lignin Nanosphere‐Modified MXene Activated‐Rapid Gelation of Mechanically Robust, Environmental Adaptive, Highly Conductive Hydrogel for Wearable Sensors Application

AbstractAdvanced conductive hydrogels demonstrate substantial potential for wearable devices. Nevertheless, the transformative advance in soft electronics raises harsh requirements on the hydrogel candidates, such as rapid and on‐site fabrication, mechanical flexibility, high sensitivity, and wide use temperature. Here, this problem is overcome by incorporating a dual catalytic system based on lignin‐modified MXene‐Fe3+ into commercial hydrogels. This system 1) can form a composite hydrogel in a time scale of min at ambient condition without the supply of external energy, 2) incorporates multiple enhanced strategies into polymer chains, and 3) constructs well‐organized hybrid conductive network. The fabricated hydrogel displays an improved and balanced overall performance, including high ductility (2139%), moderate electrical conductivity, and strong temperature tolerance (−70–50 °C). Combined with the great merits of above performance, the hydrogel‐based sensor with good sensing (maximum GF: 2.8), stable repeatability (200% for 200 cycles), and wide work window of 0%–947%, thereby disclosing promising application in physiological movements, such as motion recognition and breathing state detection. Sensationally, even in complex or harsh surroundings, the sensors also produce stable and reliable signal output. Together, the strategy provides a new mentality of designing hydrogel materials for booming and advanced wearable electronics.

Autonomous self‐healing 3D micro‐suction adhesives for multi‐layered amphibious soft skin electronics

AbstractAutonomously self‐healing, reversible, and soft adhesive microarchitectures and structured electric elements could be important features in stable and versatile bioelectronic devices adhere to complex surfaces of the human body (rough, dry, wet, and vulnerable). In this study, we propose an autonomous self‐healing multi‐layered adhesive patch inspired by the octopus, which possess self‐healing and robust adhesion properties in dry/underwater conditions. To implement autonomously self‐healing octopus‐inspired architectures, a dynamic polymer reflow model based on structural and material design suggests criteria for three‐dimensional patterning self‐healing elastomers. In addition, self‐healing multi‐layered microstructures with different moduli endows efficient self‐healing ability, human‐friendly reversible bio‐adhesion, and stable mechanical deformability. Through programmed molecular behavior of microlevel hybrid multiscale architectures, the bioinspired adhesive patch exhibited robust adhesion against rough skin surface under both dry and underwater conditions while enabling autonomous adhesion restoring performance after damaged (over 95% healing efficiency under both conditions for 24 h at 30°C). Finally, we developed a self‐healing skin‐mountable adhesive electronics with repeated attachment and minimal skin irritation by laminating thin gold electrodes on octopus‐like structures. Based on the robust adhesion and intimate contact with skin, we successfully obtained reliable measurements during dynamic motion under dry, wet, and damaged conditions.image

Universal hydrogel adhesives with robust chain entanglement for bridging soft electronic materials

Ensuring stable integration of diverse soft electronic components for reliable operation under dynamic conditions is crucial. However, integrating soft electronics, comprising various materials like polymers, metals, and hydrogels, poses challenges due to their different mechanical and chemical properties. This study introduces a dried-hydrogel adhesive made of poly(vinyl alcohol) and tannic acid multilayers (d-HAPT), which integrates soft electronic materials through moisture-derived chain entanglement. d-HAPT is a thin (~1 µm) and highly transparent (over 85% transmittance in the visible light region) adhesive, showing robust bonding (up to 3.6 MPa) within a short time (<1 min). d-HAPT demonstrates practical application in wearable devices, including a hydrogel touch panel and strain sensors. Additionally, the potential of d-HAPT for use in implantable electronics is demonstrated through in vivo neuromodulation and electrocardiographic recording experiments while confirming its biocompatibility both in vitro and in vivo. It is expected that d-HAPT will provide a reliable platform for integrating soft electronic applications.

Recent advances in laser-induced-graphene-based soft skin electronics for intelligent healthcare

Skin is a rich source of invaluable information for healthcare management and disease diagnostics. The integration of soft skin electronics enables precise and timely capture of these cues at the skin interface. Leveraging attributes such as lightweight design, compact size, high integration, biocompatibility, and enhanced comfort, these technologies hold significant promise for advancing various applications. However, the fabrication process for most existing soft skin electronics typically requires expensive platforms and clean-room environments, potentially inflating production costs. In recent years, the emergence of laser-induced-graphene (LIG) has presented a practical solution for developing soft skin electronics that are both cost-effective and high-performing. This advancement paves the way for the widespread adoption of intelligent healthcare technologies. Here, we comprehensively review recent studies focusing on LIG-based soft skin electronics (LIGS2E) for intelligent healthcare applications. We first outline the preparation methodologies, fundamental properties of LIG, and standard regulation strategies employed in developing soft skin electronics. Subsequently, we present an overview of various LIGS2E designs and their diverse applications in intelligent healthcare. These applications encompass biophysical and biochemical sensors, bio-actuators, and power supply systems. Finally, we deliberate on the potential challenges associated with the practical implementation of LIGS2E in healthcare settings and offer insights into future directions for research and development. By elucidating the capabilities and limitations of LIGS2E, this review aims to contribute to advancing intelligent healthcare technologies.

Programmable Microfluidic‐Assisted Highly Conductive Hydrogel Patches for Customizable Soft Electronics

AbstractThe utilization of hydrogels in soft electronics has led to significant progress in the field of wearable and implantable devices. However, challenges persist in hydrogel electronics, including the delicate equilibrium between stretchability and electrical conductivity, intricacies in miniaturization, and susceptibility to dehydration. Here, a lignin‐polyacrylamide (Ag‐LPA) hydrogel composite endowed with anti‐freeze, self‐adhesive, exceptional water retention properties, and high stretchability (1072%) is presented. Notably, this composite demonstrated impressive electrical conductivity at room temperature (47.924 S cm−1) and extremely cold temperatures (42.507 S cm−1). It is further proposed for microfluidic‐assisted hydrogel patches (MAHPs) to facilitate customizable designs of the Ag‐LPA hydrogel composite. This approach enhances water retention and offers versatility in packaging materials, making it a promising choice for enduring soft electronics applications. As a proof‐of‐concept, soft electronics across diverse applications and dimensions, encompassing healthcare monitoring, environmental temperature sensing, and 3D‐spring pressure monitoring electronics are successfully developed. The scenery of an extremely cold environment is further extended. The conductivity of the embedded Ag‐LPA hydrogel composite unveils the potential of MAHPs in polar rescue missions. It is envisioned that MAHPs will impact the development of sophisticated and tailored soft electronics, thereby forging new frontiers in engineering applications.

Advances in Energy Harvesting Technologies for Wearable Devices.

The development of wearable electronics is revolutionizing human health monitoring, intelligent robotics, and informatics. Yet the reliance on traditional batteries limits their wearability, user comfort, and continuous use. Energy harvesting technologies offer a promising power solution by converting ambient energy from the human body or surrounding environment into electrical power. Despite their potential, current studies often focus on individual modules under specific conditions, which limits practical applicability in diverse real-world environments. Here, this review highlights the recent progress, potential, and technological challenges in energy harvesting technology and accompanying technologies to construct a practical powering module, including power management and energy storage devices for wearable device developments. Also, this paper offers perspectives on designing next-generation wearable soft electronics that enhance quality of life and foster broader adoption in various aspects of daily life.

Emerging Wearable Ultrasound Technology.

This perspective article provides a brief overview on materials, fabrications, beamforming, and applications for wearable ultrasound devices, a rapidly growing field with versatile implications. Recent developments in miniaturization and soft electronics have significantly advanced wearable ultrasound devices. Such devices offer distinctive advantages over traditional ultrasound probes, including prolonged usability and operator independence, and have demonstrated their effectiveness in continuous monitoring, noninvasive therapies, and advanced human-machine interfaces. Wearable ultrasound devices can be classified into three main categories: rigid, flexible, and stretchable, each having distinctive properties and fabrication strategies. Key unique strategies in device design, packaging, and beamforming for each type of wearable ultrasound devices are reviewed. Furthermore, we highlight the latest applications enabled by wearable ultrasound technology in various areas. This article concludes by discussing the outstanding challenges within the field and outlines potential pathways for future advancements.