Dynamic Ag-N Bond Enhanced Stretchable Conductor for Transparent and Self-Healing Electronic Skin

Sandwich-Type Self-Healing Sensor with Multilevel for Motion Detection

Real-time detection of various parts of the human body is crucial in medical monitoring and human-machine technology. However, existing self-healing flexible sensing materials are limited in real-life applications due to the weak stability of conductive networks and difficulty in balancing stretchability and self-healing properties. Therefore, the development of wearable flexible sensors with high sensitivity and fast response with self-healing properties is of great interest. In this paper, a novel multilevel self-healing polydimethylsiloxane (PDMS) material is proposed for enhanced sensing capabilities. The PDMS was designed to have multiple bonding mechanisms including hydrogen bonding, coordination bonding, disulfide bonding, and local covalent bonding. To further enhance its sensing properties, modified carbon nanotubes (CNTs) were embedded within the PDMS matrix using a solvent etching technique. This created a sandwich-type sensing material with improved stability and sensitivity. This self-healing flexible sensing material (self-healing efficiency = 70.1% at 80 °C and 6 h) has good mechanical properties (stretchability ≈413%, tensile strength ≈0.69 MPa), thermal conductivity, and electrical conductivity. It has ultrahigh sensitivity, which makes it possible to be manufactured as a multifunctional flexible sensor.

Functional PDMS Elastomers: Bulk Composites, Surface Engineering, and Precision Fabrication

Polydimethylsiloxane (PDMS)-the simplest and most common silicone compound-exemplifies the central characteristics of its class and has attracted tremendous research attention. The development of PDMS-based materials is a vivid reflection of the modern industry. In recent years, PDMS has stood out as the material of choice for various emerging technologies. The rapid improvement in bulk modification strategies and multifunctional surfaces has enabled a whole new generation of PDMS-based materials and devices, facilitating, and even transforming enormous applications, including flexible electronics, superwetting surfaces, soft actuators, wearable and implantable sensors, biomedicals, and autonomous robotics. This paper reviews the latest advances in the field of PDMS-based functional materials, with a focus on the added functionality and their use as programmable materials for smart devices. Recent breakthroughs regarding instant crosslinking and additive manufacturing are featured, and exciting opportunities for future research are highlighted. This review provides a quick entrance to this rapidly evolving field and will help guide the rational design of next-generation soft materials and devices.

Toughening self-healing elastomer crosslinked by metal-ligand coordination through mixed counter anion dynamics

Mechanically tough and self-healable polymeric materials have found widespread applications in a sustainable future. However, coherent strategies for mechanically tough self-healing polymers are still lacking due to a trade-off relationship between mechanical robustness and viscoelasticity. Here, we disclose a toughening strategy for self-healing elastomers crosslinked by metal-ligand coordination. Emphasis was placed on the effects of counter anions on the dynamic mechanical behaviors of polymer networks. As the coordinating ability of the counter anion increases, the binding of the anion leads to slower dynamics, thus limiting the stretchability and increasing the stiffness. Additionally, multimodal anions that can have diverse coordination modes provide unexpected dynamicity. By simply mixing multimodal and non-coordinating anions, we found a significant synergistic effect on mechanical toughness ( > 3 fold) and self-healing efficiency, which provides new insights into the design of coordination-based tough self-healing polymers.

One-step rapid colorimetric detection of K+ using silver nanoparticles modified by crown ether

Urinary potassium is an important parameter in clinical health diagnosis. Rapid and convenient detection of potassium ions (K⁺) in urine is essential for personal healthcare and health management. Here, crown ether (4-aminodibenzo-18-crown-6, ADC) modified silver nanoparticles (ADC-Ag NPs) were successfully prepared for one-step rapid colorimetric detection of urinary potassium. The detection mechanism is as follows: due to the matching sizes of the diameter of K⁺ and the cavity in crown ether 6, K⁺ is encapsulated between the cavities of two crown ethers, resulting in the clumping of ADC-Ag NPs and the color of the solution being altered. The colorimetric detection method has a fast response and is completed within 20 minutes. It also shows good selectivity and interference immunity. The lowest detectable concentration is 20 μM with the naked eye and 2.16 μM for UV-vis absorption spectroscopy. A good linear relationship (R² = 0.9931) between the absorption intensity ratio and K⁺ concentration (0-100 μM) indicates that this colorimetric probe can be used to detect K⁺. The method was also applied for quantitative analysis of K⁺ in real urine samples with recovery between 116 and 120%.

A portable architectonics of Al/carbon nitride/metal-organic frameworks anchored Ag nanoparticles for SERS detection and photocatalytic degradation of fungicide

In recent years, it is urgent to develop bi-functional materials for highly sensitive SERS detection and photocatalytic degradation of contaminants in water of fish pond. Herein, using 5-mercapto-1-methyltetrazole as the ligand, the tree-trunk like zeolitic imidazolate framework (ZIF-8) is induced and in-situ grown on the surface of aluminum/flower carbon nitride (Al/f-C₃N₄). Then, AgNPs are tightly anchored in ZIF-8 of Al/f-C₃N₄/ZIF-8 by strong Ag-N and Ag-S bonds, and a portable architecture of Al/f-C₃N₄/ZIF-8/Ag is successfully prepared. Results indicate that the Al/f-C₃N₄/ZIF-8/Ag architecture exhibits excellent SERS activity and the detection limit can as low as 2.15 × 10^-11 mol⋅L^-1 for crystal violet (CV, a typical fungicide). Also, the Al/f-C₃N₄/ZIF-8/Ag substrate presents good photocatalytic activity for CV molecule, and the degradation efficiency reaches 98.58% after illumination for 90 min. This is mainly due to the good adsorption capacity of ZIF-8 which can enrich more CV molecules and pull them to "hot spots" generated by Ag in Al/f-C₃N₄/ZIF-8/Ag, and thus SERS response are enhanced significantly. Besides, the strong synergistic effect of f-C₃N₄, ZIF-8 and AgNPs is also important which facilitates the separation of photogenerated electrons and holes. Thus, the designed portable and bi-functional substrate could be used as a potential material for the detection and removal of CV in practical application.

The development of stretchable and self-repairing materials applied to electronic skin

Flexible electronic devices play a key role in the fields of flexible batteries, electronic skins, and flexible displays, which have attracted more and more attention in the past few years. Among them, the application areas of electronic skin in new energy, artificial intelligence, and other high-tech applications are increasing. Semiconductors are an indispensable part of electronic skin components. The design of semiconductor structure not only needs to maintain good carrier mobility, but also considers extensibility and self-healing capability, which is always a challenging work. Though flexible electronic devices are important for our daily life, the research on this topic is quite rare in the past few years. In this work, the recently published work regarding to stretchable semiconductors as well as self-healing conductors are reviewed. In addition, the current shortcomings, future challenges as well as an outlook of this technology are discussed. The final goal is to outline a theoretical framework for the design of high-performance flexible electronic devices that can at the same time address their commercialization challenges.

Lithium Bonds Enable Small Biomass Molecule-Based Ionoelastomers with Multiple Functions for Soft Intelligent Electronics

Lipoic acid (LA), which originates from animals and plants, is a small biomass molecule and has recently shown great application value in soft conductors. However, the severe depolymerization of LA places a significant limitation on its utilization. A strategy of using Li-bonds as both depolymerization quenchers and dynamic mediators to melt transform LA into high-performance ionoelastomers (IEs) is proposed. They feature dry networks while simultaneously combining transparency, stretchability, conductivity, self-healing ability, non-corrosive property, re-mouldability, strain-sensitivity, recyclability, and degradability. Most of the existing soft conductors' drawbacks, such as the tedious synthesis, non-renewable polymer networks, limited functions, and single-use only, are successfully solved. In addition, the multi-functions allow IEs to be used as soft sensors in human-computer interactive games and wireless remote sports assistants. Notably, the recycled IE also provides an efficient conductive filler for transparent ionic papers, which can be used to design soft transparent triboelectric nanogenerators for energy harvesting and multidirectional motion sensing. This work creates a new direction for future research involving intelligent soft electronics.

A Self-Supporting, Conductor-Exposing, Stretchable, Ultrathin, and Recyclable Kirigami-Structured Liquid Metal Paper for Multifunctional E-Skin

Electronic skin (E-skin) is a crucial seamless human-machine interface (HMI), holding promise in healthcare monitoring and personal electronics. Liquid metal (LM) has been recognized as an ideal electrode material to fabricate E-skins. However, conventional sealed LM electrodes cannot expose the LM layer for direct contact with the skin resulting in the low performance of electrophysiological monitoring. Furthermore, traditional printed LM electrodes are difficult to transfer or recycle, and fractures easily occur under stretching of the substrate. Here, we report a kind of LM electrode that we call a kirigami-structured LM paper (KLP), which is self-supporting, conductor-exposing, stretchable, ultrathin, and recyclable for multifunctional E-skin. The KLP is fabricated by the kirigami paper cutting art with three types of structures including uniaxial, biaxial, and square spiral. The KLP can act as an E-skin to acquire high-quality electrophysiological signals, such as electroencephalogram (EEG), electrocardiogram (ECG), and electromyogram (EMG). Upon integration with a triboelectric nanogenerator (TENG), the KLP can also operate as a self-powered E-skin. On the basis of the self-powered E-skin, we further developed a smart dialing communication system, which is applied on human skin to call a cellphone. Compared with conventional sealed or printed LM electrodes, the KLP can simultaneously achieve self-supporting, conductor-exposing, stretchable, ultrathin, and recyclable features. Such KLP offers potential for E-skins in healthcare monitoring and intelligent control, as well as smart robots, virtual reality, on-skin personal electronics, etc.

Inherently Conductive Poly(dimethylsiloxane) Elastomers Synergistically Mediated by Nanocellulose/Carbon Nanotube Nanohybrids toward Highly Sensitive, Stretchable, and Durable Strain Sensors

With the rapid development of soft electronics, flexible and stretchable strain sensors are highly desirable. However, coupling of high sensitivity and stretchability in a single strain sensor remains a challenge. Herein, a kind of conductive elastomer is constructed with poly(dimethylsiloxane) (PDMS) and silylated cellulose nanocrystal (SCNC)/carbon nanotube (CNT) nanohybrids through a facile one-pot solution-casting method. The hydrophobic SCNCs can effectively facilitate the dispersion of CNTs in PDMS and synergistically improve the interfacial compatibility between CNTs and the PDMS matrix, resulting in favorable stress and electron transfer in the polymer network. Due to the outstanding electrical conductivity of CNTs and the excellent dispersity and high mechanical performance of SCNCs, combined with the good compatibility between SCNC-mediated carbon nanotubes (SCNC-CNTs) and PDMS, the resulting composite elastomer (SCNC-CNT/PDMS) shows high electrical conductivity (∼2.77 S m^-1), tensile strength (∼5.72 MPa), and fatigue resistance properties. The strain sensor assembled by SCNC-CNT/PDMS demonstrates a high strain range above 100%, appealing strain sensitivity with a gauge factor of 37.11 at 50-100% strain, and long-term stability and durability, which is capable of monitoring both real-time human motions and acoustic vibrations. This work paves a new way for the design and controllable preparation of flexible and stretchable conductive elastomers, demonstrating promising applications in wearable devices and intelligent electronics.

Self-Healing Functional Electronic Devices

Electronic devices with various functions bring great convenience and revolutionize the way we live. They are inevitable to degrade over time because of physical or chemical fatigue and damage during practical operation. To make these devices have the ability to autonomously heal from cracks and restore their mechanical and electrical properties, self-healing materials emerged as the time requires for constructing robust and self-healing electronic devices. Here the development of self-healing electronic devices with different functions, for example, energy harvesting, energy storage, sensing, and transmission, is reviewed. The new application scenarios and existing challenges are explored, and possible strategies and perspectives for future practical applications are discussed.

A Lamellibranchia-inspired epidermal electrode for electrophysiology

The capability to accurately monitor electrophysiological signals and instantly provide feedback to users is crucial for wearable healthcare. However, commercial gel electrodes suffer from drying out and irritation on skin with time, severely affecting signal quality for practical use. Toward a gel-free electrophysiology, epidermal electrodes that can accurately detect biosignals and simultaneously achieve the multifunctional properties of on-skin electronics needs are highly desirable. In this work, inspired by Lamellibranchia, which can adhere tightly to various surfaces using their extensible, adhesive and self-healing byssal threads, we developed a gel-free epidermal electrode to acquire high-quality electrophysiological signals based on a novel polymer substrate design. This polymer (STAR) features extreme stretchability (>2300% strain), high transparency (>90% transmittance at λ = 550 nm), gentle adhesion (adhesion strengths: tens of kPa), and rapid self-healing ability (95% healing efficiency in 10 min). Combined with silver nanowires as conductors, STAR was employed as a self-healing, stretchable and adhesive epidermal electrode for electrophysiological signal recording, showing a signal-to-noise ratio (SNR) even higher than that of commercial electrodes, and being able to control an artificial limb as an intermediate for human-machine interface. We believe our Lamellibranchia inspired STAR will pave a new way to design multifunctional polymers for epidermal electronics, accelerating the development of emerging wearable healthcare.

Stretchable Electronics Based on PDMS Substrates

Stretchable electronics, which can retain their functions under stretching, have attracted great interest in recent decades. Elastic substrates, which bear the applied strain and regulate the strain distribution in circuits, are indispensable components in stretchable electronics. Moreover, the self-healing property of the substrate is a premise to endow stretchable electronics with the same characteristics, so the device may recover from failure resulting from large and frequent deformations. Therefore, the properties of the elastic substrate are crucial to the overall performance of stretchable devices. Poly(dimethylsiloxane) (PDMS) is widely used as the substrate material for stretchable electronics, not only because of its advantages, which include stable chemical properties, good thermal stability, transparency, and biological compatibility, but also because of its capability of attaining designer functionalities via surface modification and bulk property tailoring. Herein, the strategies for fabricating stretchable electronics on PDMS substrates are summarized, and the influence of the physical and chemical properties of PDMS, including surface chemical status, physical modulus, geometric structures, and self-healing properties, on the performance of stretchable electronics is discussed. Finally, the challenges and future opportunities of stretchable electronics based on PDMS substrates are considered.

3D Printed Flexible Strain Sensors: From Printing to Devices and Signals

The revolutionary and pioneering advancements of flexible electronics provide the boundless potential to become one of the leading trends in the exploitation of wearable devices and electronic skin. Working as substantial intermediates for the collection of external mechanical signals, flexible strain sensors that get intensive attention are regarded as indispensable components in flexible integrated electronic systems. Compared with conventional preparation methods including complicated lithography and transfer printing, 3D printing technology is utilized to manufacture various flexible strain sensors owing to the low processing cost, superior fabrication accuracy, and satisfactory production efficiency. Herein, up-to-date flexible strain sensors fabricated via 3D printing are highlighted, focusing on different printing methods based on photocuring and materials extrusion, including Digital Light Processing (DLP), fused deposition modeling (FDM), and direct ink writing (DIW). Sensing mechanisms of 3D printed strain sensors are also discussed. Furthermore, the existing bottlenecks and future prospects are provided for further progressing research.

Application of a super-stretched self-healing elastomer based on methyl vinyl silicone rubber for wearable electronic sensors

A super-stretched self-healing elastomer for flexible electronic devices by introducing quadruple hydrogen bonds.

Waterproof, thin, high-performance pressure sensors-hand drawing for underwater wearable applications

Wearable sensors, especially pressure sensors, have become an indispensable part of life when reflecting human interactions and surroundings. However, the difficulties in technology and production-cost still limit their applicability in the field of human monitoring and healthcare. Herein, we propose a fabrication method with flexible, waterproof, thin, and high-performance circuits - based on hand-drawing for pressure sensors. The shape of the sensor is drawn on the pyralux film without assistance from any designing software and the wet-tissues coated by CNTs act as a sensing layer. Such sensor showed a sensitivity (~0.2 kPa^-1) while ensuring thinness (~0.26 mm) and flexibility for touch detection or breathing monitoring. More especially, our sensor is waterproof for underwater wearable applications, which is a drawback of conventional paper-based sensors. Its outstanding capability is demonstrated in a real application when detecting touch actions to control a phone, while the sensor is dipped underwater. In addition, by leveraging machine learning technology, these touch actions were processed and classified to achieve highly accurate monitoring (up to 94%). The available materials, easy fabrication techniques, and machine learning algorithms are expected to bring significant contributions to the development of hand-drawing sensors in the future.

Tannic Acid-Silver Dual Catalysis Induced Rapid Polymerization of Conductive Hydrogel Sensors with Excellent Stretchability, Self-Adhesion, and Strain-Sensitivity Properties

The application of conductive hydrogels in intelligent biomimetic electronics is a hot topic in recent years, but it is still a great challenge to develop the conductive hydrogels through a rapid fabrication process at ambient temperature. In this work, a versatile poly(acrylamide) @cellulose nanocrystal/tannic acid-silver nanocomposite (NC) hydrogel integrated with excellent stretchability, repeatable self-adhesion, high strain sensitivity, and antibacterial property, was synthesized via radical polymerization within 30 s at ambient temperature. Notably, this rapid polymerization was realized through a tannic acid-silver (TA-Ag) mediated dynamic catalysis system that was capable of activating ammonium persulfate and then initiated the free-radical polymerization of the acrylamide monomer. Benefiting from the incorporation of TA-Ag metal ion nanocomplexes and cellulose nanocrystals, which acted as dynamic connecting bridges by hydrogen bonds to efficiently dissipate energy, the obtained NC hydrogels exhibited prominent tensile strain (up to 4000%), flexibility, self-recovery, and antifatigue properties. In addition, the hydrogels showed repeatable adhesiveness to different substrates (e.g., glass, wood, bone, metal, and skin) and significant antibacterial properties, which were merits for the hydrogels to be assembled into a flexible epidermal sensor for long-term human-machine interfacial contact without concerns about the use of external adhesive tapes and bacterial breeding. Moreover, the remarkable conductivity (σ ∼ 5.6 ms cm^-1) and strain sensitivity (gauge factor = 1.02) allowed the flexible epidermal sensors to monitor various human motions in real time, including huge movement of deformations (e.g., wrist, elbow, neck, shoulder) and subtle motions. It is envisioned that this work would provide a promising strategy for the rapid preparation of conductive hydrogels in the application of flexible electronic skin, biomedical devices, and soft robotics.

Ultrafast Photoinduced Interconnection of Metal-Polymer Composites for Fabrication of Transparent and Stretchable Electronic Skins

The high interest sparked by the foldable smartphones recently released on the market is gradually shifting to the next generation of flexible electronic devices, such as electronic skins in the form of stretchable thin films. To develop such devices, good mechanical flexibility of all components (including the substrate, electrode, and encapsulant) is critical. Various technologies have been developed to enhance the flexibility of these components; however, progress in developing interconnection methods for flexible and stretchable devices has been limited. Here, we developed an ultrafast photoinduced interconnection method that does not require any adhesive or surface treatment. This method is based on heating metal nanostructures using intense pulsed light (IPL) and the reversible cross-linking of polymers. First, we synthesized a stretchable, transparent, and free-standing polymer substrate that can be reversibly cross-linked, and then Ag nanowire (AgNW) networks were formed on its surface. This electrode was irradiated with IPL, which locally heated the AgNWs, followed by decomposition of the polymer via the retro-Diels-Alder reaction and recross-linking. Independently fabricated AgNW/polymer films were layered and irradiated three times with IPL to form a bonded sample with excellent joint quality and no increase in electrical resistance compared to a single electrode. Furthermore, the interconnected electrodes were stretchable and optically transparent. Even when more than 200% strain was applied in a peel test, no breakage at the joint was observed. This allowed us to successfully produce a stretchable, transparent, and bending-insensitive pressure sensor for various applications such as motion detectors or pressure sensor arrays.

An antibacterial and biocompatible multilayer biomedical coating capable of healing damages

Besides the excellent biocompatibility and high antibacterial property, multifunctional biomedical coatings with a long service time is highly desirable for extended applications, which is still an ongoing challenge. The self-healing property enables new directions for effectively prolonging their service life and significantly improving their reliability. Herein, an efficient and simple method is used to facilely prepare antibacterial, biocompatibile multilayer polyelectrolyte coatings, which are capable of healing damages. The synthetic strategy involves the alternate deposition of Chitosan (CS) and sodium carboxymethyl cellulose (CMC) via the layer-by-layer (LBL) self-assembly technique. The CS/CMC multilayer polyelectrolyte coating features high antibacterial property, fast and efficient self-healing property, and excellent biocompatibility. These features allow the CS/CMC polyelectrolyte coating to have extended lifespan and to be highly promising for novel functional stent coating applications.

A CS/CMC multilayer polyelectrolyte coating was developed, which features fast and efficient self-healing property, high antibacterial property, and excellent biocompatibility.