Deformable Organic Nanowire Field-Effect Transistors

Dielectric Design of High Dielectric Constant Poly(Urea-Urethane) Elastomer for Low-Voltage High-Mobility Intrinsically Stretchable All-Solution-Processed Organic Transistors

Stretchable organic transistors for skin-like biomedical applications require low-voltage operation to accommodate limited power supply and safe concerns. However, most of the currently reported stretchable organic transistors operate at relatively high voltages. Decreasing their operational voltage while keeping the high mobility still remains a key challenge. Here, the study presents a new dielectric design to achieve high-dielectric constant poly(urea-urethane) (PUU) elastomer, by incorporating a flexible small-molecular diamine crosslinking agent 4-aminophenyl disulfide (APDS) into the main chain of (poly (propylene glycol), tolylene 2,4-diiso-cyanate terminated) (PPG-TDI). Compared with commercial elastomers, the PUU elastomer as dielectric of the stretchable organic transistors shows the outstanding advantages including lower surface roughness (0.33 nm), higher adhesion (45.18 nN), higher dielectric constant (13.5), as well as higher stretchability (896%). The PUU dielectric enables the intrinsically stretchable, all-solution-processed organic transistor to operate at a low operational voltage down to -10 V, while preserving a substantial mobility of 1.39 cm2 V-1 s-1. Impressively, the transistor also demonstrates excellent electrical stability under repeated switching of 10 000 cycles, and remarkable mechanical robustness when stretched up to 100%. The work opens up a new molecular engineering strategy to successfully realize low-voltage high-mobility stretchable all-solution-processed organic transistors.

Biomimetic wafer-scale alignment of tellurium nanowires for high-mobility flexible and stretchable electronics

Flexible and stretchable thin-film transistors (TFTs) are crucial in skin-like electronics for wearable and implantable applications. Such electronics are usually constrained in performance owing to a lack of high-mobility and stretchable semiconducting channels. Tellurium, a rising semiconductor with superior charge carrier mobilities, has been limited by its intrinsic brittleness and anisotropy. Here, we achieve highly oriented arrays of tellurium nanowires (TeNWs) on various substrates with wafer-scale scalability by a facile lock-and-shear strategy. Such an assembly approach mimics the alignment process of the trailing tentacles of a swimming jellyfish. We further apply these TeNW arrays in high-mobility TFTs and logic gates with improved flexibility and stretchability. More specifically, mobilities over 100 square centimeters per volt per second and on/off ratios of ~104 are achieved in TeNW-TFTs. The TeNW-TFTs on polyethylene terephthalate can sustain an omnidirectional bending strain of 1.3% for more than 1000 cycles. Furthermore, TeNW-TFTs on an elastomeric substrate can withstand a unidirectional strain of 40% with no performance degradation.

Materials-Driven Soft Wearable Bioelectronics for Connected Healthcare

In the era of Internet-of-things, many things can stay connected; however, biological systems, including those necessary for human health, remain unable to stay connected to the global Internet due to the lack of soft conformal biosensors. The fundamental challenge lies in the fact that electronics and biology are distinct and incompatible, as they are based on different materials via different functioning principles. In particular, the human body is soft and curvilinear, yet electronics are typically rigid and planar. Recent advances in materials and materials design have generated tremendous opportunities to design soft wearable bioelectronics, which may bridge the gap, enabling the ultimate dream of connected healthcare for anyone, anytime, and anywhere. We begin with a review of the historical development of healthcare, indicating the significant trend of connected healthcare. This is followed by the focal point of discussion about new materials and materials design, particularly low-dimensional nanomaterials. We summarize material types and their attributes for designing soft bioelectronic sensors; we also cover their synthesis and fabrication methods, including top-down, bottom-up, and their combined approaches. Next, we discuss the wearable energy challenges and progress made to date. In addition to front-end wearable devices, we also describe back-end machine learning algorithms, artificial intelligence, telecommunication, and software. Afterward, we describe the integration of soft wearable bioelectronic systems which have been applied in various testbeds in real-world settings, including laboratories that are preclinical and clinical environments. Finally, we narrate the remaining challenges and opportunities in conjunction with our perspectives.

Oxygen-Induced Barrier Lowering for High-Performance Organic Field-Effect Transistors

Organic field-effect transistors (OFETs) have the advantages of low-cost, large-area processing and could be utilized in a variety of emerging applications. However, the generally large contact resistance (R_c) limits the integration and miniaturization of OFETs. The R_c is difficult to reduce due to an incompatibility between obtaining strong orbit coupling and the barrier height reduction. In this study, we developed an oxygen-induced barrier lowering strategy by introducing oxygen (O₂) into the nanointerface between the electrodes and organic semiconductors layer and achieved an ultralow channel width-normalized R_c (R_c·W) of 89.8 Ω·cm and a high mobility of 11.32 cm² V^-1 s^-1. This work demonstrates that O₂ adsorbed at the nanointerface of metal-semiconductor contact can significantly reduce the R_c from both experiments and theoretical simulations and provides guidance for the construction of high-performance OFETs, which is conducive to the integration and miniaturization of OFETs.

Progress in the Synthesis and Application of Tellurium Nanomaterials

In recent decades, low-dimensional nanodevices have shown great potential to extend Moore's Law. The n-type semiconductors already have several candidate materials for semiconductors with high carrier transport and device performance, but the development of their p-type counterparts remains a challenge. As a p-type narrow bandgap semiconductor, tellurium nanostructure has outstanding electrical properties, controllable bandgap, and good environmental stability. With the addition of methods for synthesizing various emerging tellurium nanostructures with controllable size, shape, and structure, tellurium nanomaterials show great application prospects in next-generation electronics and optoelectronic devices. For tellurium-based nanomaterials, scanning electron microscopy and transmission electron microscopy are the main characterization methods for their morphology. In this paper, the controllable synthesis methods of different tellurium nanostructures are reviewed, and the latest progress in the application of tellurium nanostructures is summarized. The applications of tellurium nanostructures in electronics and optoelectronics, including field-effect transistors, photodetectors, and sensors, are highlighted. Finally, the future challenges, opportunities, and development directions of tellurium nanomaterials are prospected.

Low-Voltage Intrinsically Stretchable Organic Transistor Amplifiers for Ultrasensitive Electrophysiological Signal Detection

Stretchability is a prerequisite for electronic skin devices. However, state-of-the-art stretchable thin-film transistors do not possess sufficiently low operating voltages and good stability, significantly limiting their use in real-world biomedical applications. Herein, a van der Waals-controlling elastomer/carbon quantum dot interfacial polarization methodology is proposed to form a hybrid polymer dielectric with 620% tensile strain and large-area film uniformity (>A4 paper size). Using the hybrid polymer dielectrics, the prepared intrinsically stretchable organic thin-film transistors demonstrate a low operating voltage below 5 V, 100% strain tolerance, and excellent operational stability, as well as a high on-current/off-current ratio of 10⁵ and a steep subthreshold slope of 500 mV dec^-1 . Based on this device technology, an amplifier with a high gain of 90 V V^-1 among the highest values of reported stretchable transistors is realized. This amplifier is at the first time applied to detect human electrophysical signals with an output signal amplitude of over 0.2 V, which even outperforms other types of the state-of-the-art organic amplifiers for human electrophysiology monitoring. This stretchable device technology sufficiently meets the safety and portability requirements of wearable biomedical applications, opening a new opportunity to e-skin with signal control and amplification capabilities.

Stretchable conductors for stretchable field-effect transistors and functional circuits

Stretchable electronics have received intense attention due to their broad application prospects in many areas, and can withstand large deformations and form close contact with curved surfaces. Stretchable conductors are vital components of stretchable electronic devices used in wearables, soft robots, and human-machine interactions. Recent advances in stretchable conductors have motivated basic scientific and technological research efforts. Here, we outline and analyse the development of stretchable conductors in transistors and circuits, and examine advances in materials, device engineering, and preparation technologies. We divide the existing approaches to constructing stretchable transistors with stretchable conductors into the following two types: geometric engineering and intrinsic stretchability engineering. Finally, we consider the challenges and outlook in this field for delivering stretchable electronics.

Ultrathin Hydrogel Films toward Breathable Skin-Integrated Electronics

On-skin electronics that offer revolutionary capabilities in personalized diagnosis, therapeutics, and human-machine interfaces require seamless integration between the skin and electronics. A common question remains whether an ideal interface can be introduced to directly bridge thin-film electronics with the soft skin, allowing the skin to breathe freely and the skin-integrated electronics to function stably. Here, an ever-thinnest hydrogel is reported that is compliant to the glyphic lines and subtle minutiae on the skin without forming air gaps, produced by a facile cold-lamination method. The hydrogels exhibit high water-vapor permeability, allowing nearly unimpeded transepidermal water loss and free breathing of the skin underneath. Hydrogel-interfaced flexible (opto)electronics without causing skin irritation or accelerated device performance deterioration are demonstrated. The long-term applicability is recorded for over one week. With combined features of extreme mechanical compliance, high permeability, and biocompatibility, the ultrathin hydrogel interface promotes the general applicability of skin-integrated electronics.

A low-power stretchable neuromorphic nerve with proprioceptive feedback

By relaying neural signals from the motor cortex to muscles, devices for neurorehabilitation can enhance the movement of limbs in which nerves have been damaged as a consequence of injuries affecting the spinal cord or the lower motor neurons. However, conventional neuroprosthetic devices are rigid and power-hungry. Here we report a stretchable neuromorphic implant that restores coordinated and smooth motions in the legs of mice with neurological motor disorders, enabling the animals to kick a ball, walk or run. The neuromorphic implant acts as an artificial efferent nerve by generating electrophysiological signals from excitatory post-synaptic signals and by providing proprioceptive feedback. The device operates at low power (~1/150 that of a typical microprocessor system), and consists of hydrogel electrodes connected to a stretchable transistor incorporating an organic semiconducting nanowire (acting as an artificial synapse), connected via an ion gel to an artificial proprioceptor incorporating a carbon nanotube strain sensor (acting as an artificial muscle spindle). Stretchable electronics with proprioceptive feedback may inspire the further development of advanced neuromorphic devices for neurorehabilitation.

Printable inks and deformable electronic array devices

Deformable printed electronic array devices are expected to revolutionize next-generation electronics. However, although remarkable technological advances in printable inks and deformable electronic array devices have recently been achieved, technical challenges remain to commercialize these technologies. In this review article a brief introduction to printing methods highlighting significant research studies on ink formation for conductors, semiconductors, and insulators is provided, and the structural design and successful printing strategies of deformable electronic array devices are described. Successful device demonstrations are presented in the applications of passive- and active-matrix array devices. Finally, perspectives and technological challenges to be achieved are pointed out to print practically available deformable devices.

Recent progress of fiber-based transistors: materials, structures and applications

Wearable electronics on fibers or fabrics assembled with electronic functions provide a platform for sensors, displays, circuitry, and computation. These new conceptual devices are human-friendly and programmable, which makes them indispensable for modern electronics. Their unique properties such as being adaptable in daily life, as well as being lightweight and flexible, have enabled many promising applications in robotics, healthcare, and the Internet of Things (IoT). Transistors, one of the fundamental blocks in electronic systems, allow for signal processing and computing. Therefore, study leading to integration of transistors with fabrics has become intensive. Here, several aspects of fiber-based transistors are addressed, including materials, system structures, and their functional devices such as sensory, logical circuitry, memory devices as well as neuromorphic computation. Recently reported advances in development and challenges to realizing fully integrated electronic textile (e-textile) systems are also discussed.

Highly Stretchable High-Performance Silicon Nanowire Field Effect Transistors Integrated on Elastomer Substrates

Quasi-1D silicon nanowires (SiNWs) field effect transistors (FETs) integrated upon large-area elastomers are advantageous candidates for developing various high-performance stretchable electronics and displays. In this work, it is demonstrated that an orderly array of slim SiNW channels, with a diameter of <80 nm, can be precisely grown into desired locations via an in-plane solid-liquid-solid (IPSLS) mechanism, and reliably batch-transferred onto large area polydimethylsiloxane (PDMS) elastomers. Within an optimized discrete FETs-on-islands architecture, the SiNW-FETs can sustain large stretching strains up to 50% and repetitive testing for more than 1000 cycles (under 20% strain), while achieving a high hole carrier mobility, Ion /Ioff current ratio and subthreshold swing (SS) of ≈70 cm2 V-1 s-1 , >105  and 134 - 277 mV decade-1 , respectively, working stably in an ambient environment over 270 days without any passivation protection. These results indicate a promising new routine to batch-manufacture and integrate high-performance, scalable and stretchable SiNW-FET electronics that can work stably in harsh and large-strain environments, which is a key capability for future practical flexible display and wearable electronic applications.

Wafer-scale integration of stretchable semiconducting polymer microstructures via capillary gradient

Organic semiconducting polymers have opened a new paradigm for soft electronics due to their intrinsic flexibility and solution processibility. However, the contradiction between the mechanical stretchability and electronic performances restricts the implementation of high-mobility polymers with rigid molecular backbone in deformable devices. Here, we report the realization of high mobility and stretchability on curvilinear polymer microstructures fabricated by capillary-gradient assembly method. Curvilinear polymer microstructure arrays are fabricated with highly ordered molecular packing, controllable pattern, and wafer-scale homogeneity, leading to hole mobilities of 4.3 and 2.6 cm² V^-1 s^-1 under zero and 100% strain, respectively. Fully stretchable field-effect transistors and logic circuits can be integrated in solution process. Long-range homogeneity is demonstrated with the narrow distribution of height, width, mobility, on-off ratio and threshold voltage across a four-inch wafer. This solution-assembly method provides a platform for wafer-scale and reproducible integration of high-performance soft electronic devices and circuits based on organic semiconductors.

Recent Advances in High-Mobility and High-Stretchability Organic Field-Effect Transistors: From Materials, Devices to Applications

Stretchable organic field-effect transistors (OFETs) are one of the essential building blocks for next-generation wearable electronics due to the high stretchability of OFET well matching with the large deformation of human skin. In recent years, some significant progress of stretchable OFETs have already been made via the strategies of stretchable molecular design and geometry engineering. However, the main opportunity and challenge of stretchable OFETs is still to simultaneously improve their stretchability and mobility. This review covers the recent advances in the research of stretchable OFETs with high mobility. First, the core stretchable materials are summarized, including organic semiconductors, electrodes, dielectrics, and substrates. Second, the materials and healing mechanism of self-healing OFET are summarized in detail. Subsequently, their different configurations and the potential applications are summarized. Finally, an outlook of future research directions and challenges in this area is presented.

Printed Stretchable Single-Nanofiber Interconnections for Individually-Addressable Highly-Integrated Transparent Stretchable Field Effect Transistor Array

Stretchable electronics have been spotlighted as promising next-generation electronics. In order to drive a specific unit device in an integrated stretchable device, the interconnection of the device should be placed in a desired position and addressed individually. In addition, practical stretchable interconnection requires reliable stretchability, high conductivity, optical transparency, high resolution, and fast and large-scale production. This study proposes an approach to meet these requirements. We print the single wavy polymer nanofibers (NFs) in a desired position and convert them into metal NF interconnections. The nanoscale diameter and the wavy cylindrical shape of the metal NFs are the main reasons for the reliable stretchability and the excellent transparency. Using the stretchable metal NFs and the stretchable organic semiconductor NFs, an array of all-stretchable transparent NF-field effect transistors (NF-FETs) is demonstrated. The highly integrated NF-FET array (10 FETs/mm²) shows uniform performance and good stability under repeated severe mechanical deformations.

Self-Healing, Stretchable, Biocompatible, and Conductive Alginate Hydrogels through Dynamic Covalent Bonds for Implantable Electronics

Implantable electronics have recently been attracting attention because of the promising advances in personalized healthcare. They can be used to diagnose and treat chronic diseases by monitoring and applying bioelectrical signals to various organs. However, there are challenges regarding the rigidity and hardness of typical electronic devices that can trigger inflammatory reactions in tissues. In an effort to improve the physicochemical properties of conventional implantable electronics, soft hydrogel-based platforms have emerged as components of implantable electronics. It is important that they meet functional criteria, such as stretchability, biocompatibility, and self-healing. Herein, plant-inspired conductive alginate hydrogels composed of “boronic acid modified alginate” and “oligomerized epigallocatechin gallate,” which are extracted from plant compounds, are proposed. The conductive hydrogels show great stretchability up to 500% and self-healing properties because of the boronic acid-cis-diol dynamic covalent bonds. In addition, as a simple strategy to increase the electrical conductivity of the hydrogels, ionically crosslinked shells with cations (e.g., sodium) were generated on the hydrogel under physiological salt conditions. This decreased the resistance of the conductive hydrogel down to 900 ohm without trading off the original properties of stretchability and self-healing. The hydrogels were used for “electrophysiological bridging” to transfer electromyographic signals in an ex vivo muscle defect model, showing a great bridging effect comparable to that of a muscle-to-muscle contact model. The use of plant-inspired ionically conductive hydrogels is a promising strategy for designing implantable and self-healable bioelectronics.

Interface Design for Stretchable Electronic Devices

Stretchable electronics has emerged over the past decade and is now expected to bring form factor-free innovation in the next-generation electronic devices. Stretchable devices have evolved with the synthesis of new soft materials and new device architectures that require significant deformability while maintaining the high device performance of the conventional rigid devices. As the mismatch in the mechanical stiffness between materials, layers, and device units is the major challenge for stretchable electronics, interface control in varying scales determines the device characteristics and the level of stretchability. This article reviews the recent advances in interface control for stretchable electronic devices. It summarizes the design principles and covers the representative approaches for solving the technological issues related to interfaces at different scales: i) nano- and microscale interfaces between materials, ii) mesoscale interfaces between layers or microstructures, and iii) macroscale interfaces between unit devices, substrates, or electrical connections. The last section discusses the current issues and future challenges of the interfaces for stretchable devices.

Mechanical design of brush coating technology for the alignment of one-dimension nanomaterials

Widespread approaches to fabricate surfaces with aligned nanostructured topographies have been stimulated by opportunities to enhance interface performance by combing physical and chemical effects, in which brush-coating technology (BCT) is a cost-effective and feasible method for aligned film and large-scale production. Here, we reported a BCT process to realize the alignment of various 1D nanostructures through mechanical design that provides a more precise and higher shear force. By regulating the viscosity of dispersion, shear force is proved to be 24 and 20.3 times larger (when the volume ratio of water and glycerol is 1:3) according to the theoretical calculation and ANSYS simulating calculation results respectively, which plays a vital role in brush coating process. The universality was demonstrated by the alignment of one-dimension nanomaterials with different diameters, including silver nanowires (~80 nm), molybdenum trioxide nanobelts (~150 nm), vanadium pentoxide nanobelts (~150 nm) and bismuth sulfide nanobelts (~200 nm), et al., which in consequence have different alignment ratios. Meanwhile, anisotropic and flexible electrical conductors (the resistance anisotropic ratio was 2) and thermoelectric films (Seebeck coefficient was calculated to be 56.7 µV/K) were demonstrated.

Water Passivation of Perovskite Nanocrystals Enables Air-Stable Intrinsically Stretchable Color-Conversion Layers for Stretchable Displays

Conventional organic light-emitting devices without an encapsulation layer are susceptible to degradation when exposed to air, so realization of air-stable intrinsically-stretchable display is a great challenge because the protection of the devices against penetration of moisture and oxygen is even more difficult under stretching. An air-stable intrinsically-stretchable display that is composed of an intrinsically-stretchable electroluminescent device (SELD) integrated with a stretchable color-conversion layer (SCCL) that contains perovskite nanocrystals (PeNCs) is proposed. PeNCs normally decay when exposed to air, but they become resistant to this decay when dispersed in a stretchable elastomer matrix; this change is a result of a compatibility between capping ligands and the elastomer matrix. Counterintuitively, the moisture can efficiently passivate surface defects of PeNCs, to yield significant increases in both photoluminescence intensity and lifetime. A display that can be stretched up to 180% is demonstrated; it is composed of an air-stable SCCL that down-converts the SELD's blue emission and reemits it as green. The work elucidates the basis of moisture-assisted surface passivation of PeNCs and provides a promising strategy to improve the quantum efficiency of PeNCs with the aid of moisture, which allows PeNCs to be applied for air-stable stretchable displays.

Omnidirectionally Stretchable Organic Transistors for Use in Wearable Electronics: Ensuring Overall Stretchability by Applying Nonstretchable Wrinkled Components

With the emergence of wearable human interface technologies, new applications based on stretchable electronics, such as skin-attached sensors or wearable displays, must be developed. Difficulties associated with developing electronic components with the high stretchabilities required for such applications have restricted the range of appearance and utilization of cost- or process-efficient stretchable electronics. Herein, we present omnidirectionally stretchable wrinkled transistors having a shape that replicates human skin, which operates stably on deformable objects or complex surfaces. Our device offers excellent mechanical and electrical stabilities for preserving relative field-effect mobilities within a standard deviation of nearly 5.6%, under a strain level of up to 62%. Even after 10 000 cycles of stretching to 60% strain, the devices exhibited stable operation with little performance changes. These results indicate that the devices display stretchability properties superior to those of organic transistor arrays by utilizing existing nonstretchable device components.

Planar Growth, Integration, and Applications of Semiconducting Nanowires

Silicon and other inorganic semiconductor nanowires (NWs) have been extensively investigated in the last two decades for constructing high-performance nanoelectronics, sensors, and optoelectronics. For many of these applications, these tiny building blocks have to be integrated into the existing planar electronic platform, where precise location, orientation, and layout controls are indispensable. In the advent of More-than-Moore's era, there are also emerging demands for a programmable growth engineering of the geometry, composition, and line-shape of NWs on planar or out-of-plane 3D sidewall surfaces. Here, the critical technologies established for synthesis, transferring, and assembly of NWs upon planar surface are examined; then, the recent progress of in-plane growth of horizontal NWs directly upon crystalline or patterned substrates, constrained by using nanochannels, an epitaxial interface, or amorphous thin film precursors is discussed. Finally, the unique capabilities of planar growth of NWs in achieving precise guided growth control, programmable geometry, composition, and line-shape engineering are reviewed, followed by their latest device applications in building high-performance field-effect transistors, photodetectors, stretchable electronics, and 3D stacked-channel integration.

When Flexible Organic Field-Effect Transistors Meet Biomimetics: A Prospective View of the Internet of Things

The emergence of flexible organic electronics that span the fields of physics and biomimetics creates the possibility for increasingly simple and intelligent products for use in everyday life. Organic field-effect transistors (OFETs), with their inherent flexibility, light weight, and biocompatibility, have shown great promise in the field of biomimicry. By applying such biomimetic OFETs for the internet of things (IoT) makes it possible to imagine novel products and use cases for the future. Recent advances in flexible OFETs and their applications in biomimetic systems are reviewed. Strategies to achieve flexible OFETs are individually discussed and recent progress in biomimetic sensory systems and nervous systems is reviewed in detail. OFETs are revealed to be one of the best systems for mimicking sensory and nervous systems. Additionally, a brief discussion of information storage based on OFETs is presented. Finally, a personal view of the utilization of biomimetic OFETs in the IoT and future challenges in this research area are provided.

Flexible Neuromorphic Electronics for Computing, Soft Robotics, and Neuroprosthetics

Flexible neuromorphic electronics that emulate biological neuronal systems constitute a promising candidate for next-generation wearable computing, soft robotics, and neuroprosthetics. For realization, with the achievement of simple synaptic behaviors in a single device, the construction of artificial synapses with various functions of sensing and responding and integrated systems to mimic complicated computing, sensing, and responding in biological systems is a prerequisite. Artificial synapses that have learning ability can perceive and react to events in the real world; these abilities expand the neuromorphic applications toward health monitoring and cybernetic devices in the future Internet of Things. To demonstrate the flexible neuromorphic systems successfully, it is essential to develop artificial synapses and nerves replicating the functionalities of the biological counterparts and satisfying the requirements for constructing the elements and the integrated systems such as flexibility, low power consumption, high-density integration, and biocompatibility. Here, the progress of flexible neuromorphic electronics is addressed, from basic backgrounds including synaptic characteristics, device structures, and mechanisms of artificial synapses and nerves, to applications for computing, soft robotics, and neuroprosthetics. Finally, future research directions toward wearable artificial neuromorphic systems are suggested for this emerging area.

Intrinsically Stretchable and Shape Memory Conducting Nanofiber for Programmable Flexible Electronic Films

Recently, flexible and stretchable electronic films have been drawing increasing attention but are limited by the nature of elastomeric materials and the embedded structure; thus, these films cannot achieve long-term and stable electrical performance at certain deformation states in practical applications. Here, we report intrinsically stretchable and shape memory polycaprolactone/polyethylene glycol/silver nanowires films (PPAFs) based on a dual-layer network structure of nanofibers that can achieve both shape-fixable and deformation-reversible conductivity in the elongation range. We also demonstrate the resistance characteristic of PPAFs at the same/different deformation rates, which shows the unique memorable resistance and the variable conversion of a "conductive-insulation-conductive" state. Importantly, the change in sheet resistance of the PPAFs fixed at any rate of deformation could sustainably recover the initial sheet resistance even after cyclic thermal responses. Furthermore, we successfully develop the programmable conductivity of PPAFs as a monitoring, switching, and alarming device for shape memory cycles through the ingenious design of a microcircuit and simulation analysis using Proteus software. PPAFs show great potential for changeable characteristics in both shape and resistance for use in flexible electronic films.

Monolithic Integration of Silicon Nanowire Networks as a Soft Wafer for Highly Stretchable and Transparent Electronics

Assembling nanoscale building blocks into an orderly network with a programmable layout and channel designs represents a critical capability to enable a wide range of stretchable electronics. Here, we demonstrate the growth-in-place integration of silicon nanowire (SiNW) springs into highly stretchable, transparent, and quasicontinuous functional networks with a close to unity interconnection among the discrete electrode joints because of a unique double-lane/double-step guiding edge design. The SiNW networks can be reliably transferred to a soft elastomer substrate, conformally attached to highly curved surfaces, or deployed as self-supporting/movable membranes suspended over voids. A high stretchability of >40% is achieved for the SiNW network on an elastomer, which can be employed as a transparent and semiconducting thin-film material endowed with a high carrier mobility of >50 cm²/(V s), I_on/I_off ratio >10⁴, and a tunable transmission of >80% over a wide spectrum range. Reversibly stretchable and bendable sensors based on the SiNW network have been successfully demonstrated, where the local strain distribution within the spring network can be directly observed and analyzed by finite element simulations. This SiNW network has a unique potential to eventually establish a new generically purposed waferlike platform for constructing soft electronics with Si-based hard performances.

2D-Organic Hybrid Heterostructures for Optoelectronic Applications

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low-cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D-organic heterostructures-from their synthesis and fabrication to their state-of-the-art optoelectronic applications-is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

Buckled Structures: Fabrication and Applications in Wearable Electronics

Wearable electronics have attracted a tremendous amount of attention due to their many potential applications, such as personalized health monitoring, motion detection, and smart clothing, where electronic devices must conformably form contacts with curvilinear surfaces and undergo large deformations. Structural design and material selection have been the key factors for the development of wearable electronics in the recent decades. As one of the most widely used geometries, buckling structures endow high stretchability, high mechanical durability, and comfortable contact for human-machine interaction via wearable devices. In addition, buckling structures that are derived from natural biosurfaces have high potential for use in cost-effective and high-grade wearable electronics. This review provides fundamental insights into buckling fabrication and discusses recent advancements for practical applications of buckled electronics, such as interconnects, sensors, transistors, energy storage, and conversion devices. In addition to the incorporation of desired functions, the simple and consecutive manipulation and advanced structural design of the buckled structures are discussed, which are important for advancing the field of wearable electronics. The remaining challenges and future perspectives for buckled electronics are briefly discussed in the final section.

Second Skin Enabled by Advanced Electronics

Electronic second skin is touted as the next interface to expand applications of electronics for natural and seamless interactions with humans to enable smart health care, the Internet of Things, and even to amplify human sensory abilities. Thus, electronic materials are now being actively investigated to construct "second skin." Accordingly, electronic devices are desirable to have skin-like properties such as stretchability, self-healing ability, biocompatibility, and biodegradability. This work reviews recent major progress in the development of both electronic materials and devices toward the second skin. It is concluded with comments on future research directions of the field.

Multi-scale ordering in highly stretchable polymer semiconducting films

Stretchable semiconducting polymers have been developed as a key component to enable skin-like wearable electronics, but their electrical performance must be improved to enable more advanced functionalities. Here, we report a solution processing approach that can achieve multi-scale ordering and alignment of conjugated polymers in stretchable semiconductors to substantially improve their charge carrier mobility. Using solution shearing with a patterned microtrench coating blade, macroscale alignment of conjugated-polymer nanostructures was achieved along the charge transport direction. In conjunction, the nanoscale spatial confinement aligns chain conformation and promotes short-range π-π ordering, substantially reducing the energetic barrier for charge carrier transport. As a result, the mobilities of stretchable conjugated-polymer films have been enhanced up to threefold and maintained under a strain up to 100%. This method may also serve as the basis for large-area manufacturing of stretchable semiconducting films, as demonstrated by the roll-to-roll coating of metre-scale films.

Facile and Microcontrolled Blade Coating of Organic Semiconductor Blends for Uniaxial Crystal Alignment and Reliable Flexible Organic Field-Effect Transistors

The ability to fabricate uniform and high-quality patterns of organic semiconductors using a simple method is necessary to realize high-performance and reliable organic field-effect transistors (OFETs) for practical applications. Here, we report the facile fabrication of chemically patterned substrates in order to provide solvent wetting/dewetting regions and grow patterned crystals during blade coating of a small-molecule semiconductor/insulating polymer blend solution. Polyurethane acrylate is selected as the solvent dewetting material, not only because of its hydrophobicity but also because its patterns are easily produced by selective UV irradiation onto precursor films. 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) crystal patterns are grown on the line-shaped wetting regions of the patterned film, and the crystallinity of TIPS-PEN and alignment of molecules are found using various crystal analysis tools depending on the pattern widths. The smallest width of 5 μm yielded an OFET showing the highest field-effect mobility value of 1.63 cm²/(V·s), which is much higher than the value of the OFET based on the unpatterned TIPS-PEN crystal. Notably, we demonstrate flexible and low-voltage-operating OFETs for practical use of the patterned crystals, and the OFETs show highly stable operation under sustained gate bias stress thanks to the patterned crystals.

Organic crystalline materials in flexible electronics

Flexible electronics have attracted considerable attention recently given their potential to revolutionize human lives. High-performance organic crystalline materials (OCMs) are considered strong candidates for next-generation flexible electronics such as displays, image sensors, and artificial skin. They not only have great advantages in terms of flexibility, molecular diversity, low-cost, solution processability, and inherent compatibility with flexible substrates, but also show less grain boundaries with minimal defects, ensuring excellent and uniform electronic characteristics. Meanwhile, OCMs also serve as a powerful tool to probe the intrinsic electronic and mechanical properties of organics and reveal the flexible device physics for further guidance for flexible materials and device design. While the past decades have witnessed huge advances in OCM-based flexible electronics, this review is intended to provide a timely overview of this fascinating field. First, the crystal packing, charge transport, and assembly protocols of OCMs are introduced. State-of-the-art construction strategies for aligned/patterned OCM on/into flexible substrates are then discussed in detail. Following this, advanced OCM-based flexible devices and their potential applications are highlighted. Finally, future directions and opportunities for this field are proposed, in the hope of providing guidance for future research.

Investigation of Self-Assembly and Charge-Transport Property of One-dimensional PDI₈-CN₂ Nanowires by Solvent-Vapor Annealing

One-dimensional (1D) nanowires have attracted great interest, while air-stable n-type 1D nanowires still remain scarce. Herein, we present solvent-vapor annealing (SVA) made nanowires based on perylene tetracarboxylic diimide (PDI) derivative. It was found that the spin-coated thin films reorganized into nanowires distributed all over the substrate, as a result of the following solvent-vapor annealing effect. Cooperating with the atomic force microscopy and fluorescence microscopy characterization, the PDI₈-CN₂ molecules were supposed to conduct a long-range and entire transport to form the 1D nanowires through the SVA process, which may guarantee its potential morphology tailoring ability. In addition, the nanowire-based transistors displayed air stable electron mobility reaching to 0.15 cm² V-1 s-1, attributing to effective in situ reassembly. Owing to the broader application of organic small-molecule nanowires, this work opens up an attractive approach for exploring new high-performance micro- and nanoelectronics.

Morphological/nanostructural control toward intrinsically stretchable organic electronics

The development of intrinsically stretchable electronics poses great challenges in synthesizing elastomeric conductors, semiconductors and dielectric materials.

Nanomaterials in Skin-Inspired Electronics: Toward Soft and Robust Skin-like Electronic Nanosystems

Skin-inspired wearable electronic/biomedical systems based on functional nanomaterials with exceptional electrical and mechanical properties have revolutionized wearable applications, such as portable Internet of Things, personalized healthcare monitors, human-machine interfaces, and even always-connected precise medicine systems. Despite these advancements, including the ability to predict and to control nanolevel phenomena of functional nanomaterials precisely and strategies for integrating nanomaterials onto desired substrates without performance losses, skin-inspired electronic nanosystems are not yet feasible beyond proof-of-concept devices. In this Perspective, we provide an outlook on skin-like electronics through the review of several recent reports on various materials strategies and integration methodologies of stretchable conducting and semiconducting nanomaterials, which are used as electrodes and active layers in stretchable sensors, transistors, multiplexed arrays, and integrated circuits. To overcome the challenge of realizing robust electronic nanosystems, we discuss using nanomaterials in dynamically cross-linked polymer matrices, focusing on the latest innovations in stretchable self-healing electronics, which could change the paradigm of wearable electronics.

Advanced electronic skin devices for healthcare applications

Electronic skin, a kind of flexible electronic device and system inspired by human skin, has emerged as a promising candidate for wearable personal healthcare applications. Wearable electronic devices with skin-like properties will provide platforms for continuous and real-time monitoring of human physiological signals such as tissue pressure, body motion, temperature, metabolites, electrolyte balance, and disease-related biomarkers. Transdermal drug delivery devices can also be integrated into electronic skin to enhance its non-invasive, real-time dynamic therapy functions. This review summarizes the recent progress in electronic skin devices for applications in human health monitoring and therapy systems as well as several potential mass production technologies such as inkjet printing and 3D printing. The opportunities and challenges in broadening the applications of electronic skin devices in practical healthcare are also discussed.

An Elastic Interfacial Transistor Enabled by Superhydrophobicity

Enabling mechanical responsiveness in field-effect transistors (FETs) offers new technological opportunity beyond the reach of existing platforms. Here a new force-sensing concept is proposed by controlling the wettability of a semiconductor surface, referring to the interfacial field-effect transistors (IFETs). An IFET made by superhydrophobic semiconductor nanowires (NWs) sandwiched between a layer of 2D electron gas (2DEG) and a conductive Cassie-Baxter (CB) sessile droplet is designed. Following the hydrostatic deformation of the CB droplet upon mechanical stress, an extremely small elastic modulus of 820 pascals vertical to the substrate plane, or ≈100 times softer than Ecoflex rubbers, enabling an excellent stress detection limit down to <10 pascals and a stress sensitivity of 36 kPa^-1 is proposed. The IFET exhibits an on/off current ratio exceeding 3 × 10⁴ , as the carrier density profile at the NW/2DEG interface is modulated by a partially penetrated electrostatic field. This study demonstrates a versatile platform that bridges multiple macroscopic interfacial phenomena with nanoelectronic responses.

Low-defectiveness exfoliation of MoS2 nanoparticles and their embedment in hybrid light-emitting polymer nanofibers

Molybdenum disulfide (MoS2) has been attracting extraordinary attention for its intriguing optical, electronic and mechanical properties. Here, we demonstrate hybrid, organic-inorganic light-emitting nanofibers based on MoS2 nanoparticle dopants obtained through a simple and inexpensive sonication process in N-methyl-2-pyrrolidone and successfully encapsulate the nanofibers in polymer filaments. The gentle exfoliation method used to produce the MoS2 nanoparticles results in low defectiveness and preserves the stoichiometry. The fabricated hybrid fibers are smooth, uniform and flawless and exhibit bright and continuous light emission. Moreover, the fibers show significant capability for waveguiding self-emitted light along their longitudinal axes. These findings suggest that emissive MoS2 fibers formed by gentle exfoliation are novel and highly promising optical materials for sensing surfaces and photonic circuits.

Stretchable organic optoelectronic sensorimotor synapse

Emulation of human sensory and motor functions becomes a core technology in bioinspired electronics for next-generation electronic prosthetics and neurologically inspired robotics. An electronic synapse functionalized with an artificial sensory receptor and an artificial motor unit can be a fundamental element of bioinspired soft electronics. Here, we report an organic optoelectronic sensorimotor synapse that uses an organic optoelectronic synapse and a neuromuscular system based on a stretchable organic nanowire synaptic transistor (s-ONWST). The voltage pulses of a self-powered photodetector triggered by optical signals drive the s-ONWST, and resultant informative synaptic outputs are used not only for optical wireless communication of human-machine interfaces but also for light-interactive actuation of an artificial muscle actuator in the same way that a biological muscle fiber contracts. Our organic optoelectronic sensorimotor synapse suggests a promising strategy toward developing bioinspired soft electronics, neurologically inspired robotics, and electronic prostheses.

3D Printing Technologies for Flexible Tactile Sensors toward Wearable Electronics and Electronic Skin

3D printing has attracted a lot of attention in recent years. Over the past three decades, various 3D printing technologies have been developed including photopolymerization-based, materials extrusion-based, sheet lamination-based, binder jetting-based, power bed fusion-based and direct energy deposition-based processes. 3D printing offers unparalleled flexibility and simplicity in the fabrication of highly complex 3D objects. Tactile sensors that emulate human tactile perceptions are used to translate mechanical signals such as force, pressure, strain, shear, torsion, bend, vibration, etc. into electrical signals and play a crucial role toward the realization of wearable electronics and electronic skin. To date, many types of 3D printing technologies have been applied in the manufacturing of various types of tactile sensors including piezoresistive, capacitive and piezoelectric sensors. This review attempts to summarize the current state-of-the-art 3D printing technologies and their applications in tactile sensors for wearable electronics and electronic skin. The applications are categorized into five aspects: 3D-printed molds for microstructuring substrate, electrodes and sensing element; 3D-printed flexible sensor substrate and sensor body for tactile sensors; 3D-printed sensing element; 3D-printed flexible and stretchable electrodes for tactile sensors; and fully 3D-printed tactile sensors. Latest advances in the fabrication of tactile sensors by 3D printing are reviewed and the advantages and limitations of various 3D printing technologies and printable materials are discussed. Finally, future development of 3D-printed tactile sensors is discussed.