Electrospun nanofiber-based soft electronics

Electrospun nanofibers have received considerable attention in the field of soft electronics owing to their promising advantages and superior properties in flexibility and/or stretchability, conductivity, and transparency; furthermore, their one-dimensional nanostructure, high surface area, and diverse fibrous morphologies are also desirable. Herein, we provide an overview of electrospun nanofiber-based soft electronics. A brief introduction of the unique structure and properties of electrospun nanofiber materials is provided, and assembly strategies for flexible/stretchable electronics are highlighted. We then summarize the latest progress in the design and fabrication of representative flexible/stretchable electronic devices utilizing electrospun nanofibers, such as flexible/stretchable conductors, sensors, energy harvesting and storage devices, and transistors. Finally, a conclusion and several future research directions for electrospun nanofiber-based soft electronics are proposed. The development of low-cost, efficient, and large-scale methods for fabricating ‘soft’ electronics, conducting materials with improved flexibility and stretchability, increases the range of possible applications. Flexible electronics are useful for foldable displays, healthcare monitoring, artificial skins and implantable bioelectronics. One approach to fabricating these devices is to construct them from conductive nanofibers. Takao Someya from the University of Tokyo and colleagues review recent advances in constructing nanofiber-based soft electronics using a technique called electrospinning. Electrospinning works by drawing a molten material through a nozzle into an electric field to produce strands much finer than a human hair. The authors review the structure and properties of electrospun nanofiber materials and the various strategies for assembling flexible and stretchable electronic devices such as sensors, transistors, and components for energy harvesting and storage. This review introduce the structure and properties of electrospun nanofiber materials and the various strategies for assembling soft electronic devices such as sensors, transistors, and components for energy harvesting and storage.


Introduction
Flexible and stretchable electronics have attracted worldwide attention in recent years due to their promising features, such as thinness, lightness, flexibility, stretchability, conformability, and compatibility 1,2 . Accordingly, they have been applied widely in the development of flexible displays 3,4 , healthcare monitoring 5,6 , sensors [7][8][9] , artificial skins 10,11 , energy harvesting and storage devices 12,13 , and implantable bioelectronics 14,15 . Tremendous effort has been applied to improve the performance of soft electronics by designing flexible/stretchable structures and/or finding intrinsically soft materials [16][17][18][19] . To this end, several advanced nanomaterials (e.g., graphene, carbon nanotubes (CNT), nanoparticles, nanowires (NW)/nanofibers, nanomembranes) have been developed to manufacture soft electronics due to their large specific surface area and excellent mechanical and electrical properties [20][21][22] . There are several configurations to fabricate nanomaterial-enabled soft electronics, such as wavy, prestrain, composite, and transfer 23,24 . Considering repeatability and large-scale fabrication, it is becoming obvious that both physical and economic factors will limit their further advances with the aforementioned techniques, in addition to the complex procedures required during the fabrication of nanomaterials 25,26 . However, electrospinning provides a low-cost, efficient, and large-scale method for preparing one-dimensional nanofibers with exciting mechanical and electrical characteristics, including high porosity, surface area, conductivity, transparency, and ultrahigh flexibility (Fig. 1). In addition, diverse, viable nanofiber assemblies are pivotal in devising thin, soft, lightweight, breathable, and conformable electronic devices. Moreover, the applicability of biocompatible polymers makes electrospinning ideal to fabricate substrate materials for flexible/stretchable electronic devices that directly interface with tissues, organs, or cells [27][28][29] .
With the rapid development of electrospinning techniques, significant progress has been made toward functional applications of electrospun nanofibers in soft electronics ( Fig. 1) [30][31][32][33] . Organic polymers, small molecules, colloidal particles, composites, organic/inorganic, and organic/organic systems have been successfully electrospun into nonwoven mats through modified electrospinning techniques [34][35][36] , further extending the application of electrospinning for flexible/stretchable electronic devices. Although electrospun nanofibers have been widely investigated for several decades, their use as building blocks for flexible and stretchable electronics is still at the early stage despite their advantageous features 37,38 . This review aims to introduce readers to the current development of electrospun nanofiber-based soft electronics and to suggest possibilities for several exciting developments. State-of-the-art applications in soft electronics, including flexible/stretchable conductors, transparent electrodes, strain sensors, pressure sensors, energy harvesting and storage devices, transistors, and optoelectronics, are presented in detail. Finally, an outlook on future prospects is also provided.

Electrospinning
Among the different methods for producing onedimensional nanostructures, electrospinning is the simplest and most economically viable; furthermore, electrospinning has been a commercially successful process for the large-scale production of nanofibers with adjustable diameters (from nanometers to micrometers) 39,40 . Electrospinning was first studied in the 1930s and began to draw noticeable attention from the late 20th century due to growing industrial and academic interest 41 . To date, more than 100 different types of organic polymers, both natural and synthetic, have been successfully explored for solution electrospinning to directly produce continuous nanofibers 27 .
Technically, electrospinning is a process that uses a strong electrical field to draw a polymer solution or melt into fine filaments. The basic electrospinning setup, shown in Fig. 2, consists of a needle nozzle, high-voltage power supply, container for the spinning fluid, and an electrode collector. When a viscous fluid is charged at a high voltage, the electrostatic force draws the fluid into a liquid jet. Because of the interaction between the jet and the external electric field and the charge repulsion inside the jet, the charged jet undergoes bending or whipping instability to stretch it thinner. Solvent evaporation from the filaments results in solid fibers 42,43 . In most cases, the as-spun fibers deposit randomly on the electrode collector, forming a nonwoven, entangled nanofiber mat. While the diameters and morphologies (e.g., parallel and crossed fiber arrays, helical or wavy fibers, twisted fiber yarns, patterned fiber webs, and 3D fibrous stacks) of these electrospun nanofibers can be well adjusted by a variety of parameters, such as the intrinsic properties of the solution (e.g., the type of polymer and solvent, the conformation of the polymer chain, viscosity/concentration, elasticity, electrical conductivity, and the polarity and surface tension of the solvent), the operational conditions  57 , copyright 2019 Wiley-VCH). Pressure sensors (modified from ref. 135 , copyright 2017 Wiley-VCH). Chemical sensors (modified from ref. 197 , copyright 2020 Elsevier). Nanogenerators (modified from ref. 139 , copyright 2018 Royal Society of Chemistry). Batteries (modified from ref. 202 , copyright 2019 Wiley-VCH). Supercapacitors (modified from ref. 186 , copyright 2015 Elsevier). Photodetectors (modified from ref. 196 , copyright 2014 Springer Nature). Light-emitting diodes (LEDs) (modified from ref. 203 , copyright 2018 American Chemical Society). Transistors (modified from ref. 54 , copyright 2020 Royal Society of Chemistry). Conductors (modified from ref. 96 , copyright 2019 American Chemical Society). Transparent electrodes (modified from ref. 83 , copyright 2014 American Chemical Society).
(e.g., the strength of the applied electric field, the distance between the spinneret and collector, and the feeding rate of the polymer solution), as well as the humidity and temperature of the surroundings 44,45 . Therefore, the electrospinning process should be optimized in regard to the above parameters to obtain a uniform, long, continuous fibers with a desired diameter, structure, composition, morphology, porosity, and orientation for use in developing a variety of electronic, environmental, and biomedical applications 41,46,47 .

Strategies for electrospun nanofibers in soft electronics
Diverse nanomaterials, such as organic polymer nanofibers, carbon nanofibers, metal nanofibers, ceramic nanofibers, and inorganic hybrid nanofibers, have been fabricated directly or indirectly by electrospinning. We have seen their increasingly growing applications in soft electronics, including transparent electrodes, conductors, transistors, optoelectronics, sensors, and energy devices 48 . To this end, different methods have been employed to engineer electrospun nanofibers as passive components (such as functional substrates, templates, and precursors) and active components (such as friction layers for nanogenerators, electrolyte separators for energy storage devices, and sensing layers for chemical sensors) for flexible/stretchable electronic devices 49 . In the following, we will introduce six main strategies to assemble electrospun nanofibers in soft electronic systems (Fig. 3).

Entangled porous nanofiber sheet
Flexible/stretchable electronic devices are generally fabricated by depositing electrical materials onto elastomeric substrates or by embedding them into elastomeric matrixes 16,55 . The elastomeric materials used for soft electronics are mainly polydimethylsiloxane (PDMS), polyimide (PI), polyethylene terephthalate (PET), Ecoflex, Parylene, latex rubber, paper, etc. 17 . However, these polymeric substrates have inherent mechanical mismatches with commonly used electrical materials such as metals, metallic nanomaterials, and carbon-based materials 18 . In addition, the limited gas/ liquid permeability will restrain their application in onskin electronics for long-term continuous monitoring 56 . In contrast, elastomer substrates prepared from electrospun nanofiber sheets can not only provide notable mechanical flexibility but also breathability for wearable skin electronics. Furthermore, the large surface area and high pore volume of nanofiber-based substrates can benefit the loading of large amounts of electrically active materials 44 , resulting in highly sensitive devices. Wearable electronic devices can be lightweight, conformable, highly sensitive, flexible, and/or stretchable and breathable while using nonconducting electrospun nanofiber sheets as supporting materials 57,58 . Recently, a high-adhesion, biocompatible, ultrathin PDMS nano/ microfiber mat was developed by using electrospun core-sheath PDMS-polyvinylpyrrolidone (PVP) nanofibers. The PDMS nano/microfiber mat was suggested to be an ideal candidate as a supporting substrate for epidermal electronics 59 . In addition, nonconducting electrospun nanofiber sheets can be used as other components in soft electronic devices, such as electrolytes and separators for batteries and insulating/passivation layers 60,61 .

Aligned nanofiber array
Highly aligned nanofibers can be easily achieved by adjusting conditions regarding the flow rate and collector during the electrospinning process 68 . Apart from the single semiconducting nanofibers previously mentioned in "Single semiconducting nanofiber", aligned nanofibers from conjugated polymers have also been used as channel materials. It has been reported that the maximum oncurrent values can be manipulated by the number of aligned P3HT electrospun nanofibers connecting the source (S)/drain (D) electrodes 52 . Specifically, a transistor based on a single P3HT nanofiber has an on-current value  53 , copyright 2018 Wiley-VCH). Entangled porous nanofiber sheet (modified from ref. 108 , copyright 2016 Wiley-VCH). Aligned nanofiber arrays (modified from ref. 69 , copyright 2013 IOPscience). Yarns (modified from ref. 76 , copyright 2020 American Chemical Society). Template (modified from ref. 56 , copyright 2017 Springer Nature). Reinforcement (modified from ref. 90 , copyright 2019 American Chemical Society). of 0.7 mA, and the value increases to 3.8 mA when there are 10 aligned nanofibers. Therefore, this approach is promising for fabricating transistor arrays with high density.
A stretchable platinum-based transparent electrode has been made with a sheet resistance of 131 Ω/sq at T = 90% by using aligned electrospun nanofibers as templates 69 . Ultrafine and well-aligned PVDF-based nanofibers can be utilized to fabricate high-performance piezoelectric pressure sensors due to the enhanced piezoelectric coefficient 68,70,71 . Moreover, a multidirectional strain sensor was developed by orthogonally stacking two aligned carbon nanofiber layers with a remarkable selectivity of 3.84 72 .

Yarn
Notably, e-textiles are of great importance for wearable electronics because of their potential use in portable electronics, smart fabrics, and wearable displays 73 . Though other spinning methods (dry, force, melting, and wet spinning) have been developed to produce nano/ microfibers for e-textile applications, they are not suitable for large-scale manufacturing 74,75 . To this end, diverse three-dimensional porous microfibers or yarn-shaped flexible/stretchable conductors have been prepared by wrapping conductive membranes based on electrospun nanofibers. Typically, entangled nanofibers are loaded with electrically conducting materials, such as CNTs, graphene, and silver nanoparticles (AgNPs) [76][77][78][79] . The asprepared yarns are generally used as highly sensitive strain and pressure sensors with a large maximum strain (>50%) and good conductivity [79][80][81][82] .

Temporary template
A typical approach to fabricate transparent electrodes is to use electrospun polymer nanofiber mats as temporary templates 69,83,84 . After the removal of nanofibers by annealing or dissolving in solution, the porous mesh structures of the conductive metal layers are maintained. Characteristics such as the transparency and sheet resistance of the transparent electrode can be well tuned by changing the morphology of the electrospun nanofiber sheet. Furthermore, a substrate-free, breathable, on-skin conductor can be readily obtained by dissolving polyvinyl alcohol (PVA) nanofiber sheets with a water mist after attachment 56,85 .

Reinforcement
Inspired by reinforcement wires in concrete, numerous nanotube or nanowire-based materials have been widely used as reinforcement materials in composites to enhance mechanical performence 86,87 . Upon incorporating electrospun nanofibers into an elastomer matrix, the as-received elastomer substrate becomes a skin-like substrate even though the thickness is on the micrometer scale 88,89 . The good mechanical properties originate from the existence of randomly distributed nanofibers in the polymer matrix. In addition, the fiber reinforcing effect enhances the toughness and suppresses crack growth due to the embedded nanofibers in the composite conductor 90 .

Soft electronic applications of electrospun nanofibers
Because the impressive properties of electrospun nanofibers in one-dimensional nanostructures, namely, the porosity, flexibility, conductivity, transparency, and diverse fibrous morphologies are vital to designing high-performance soft electronics, a number of recently developed flexible/stretchable electronic devices are discussed and summarized in the following sections, including conductors, transparent electrodes, strain sensors, pressure sensors, nanogenerators, supercapacitors, batteries, transistors, and other applications.

Conductors
Flexible and/or stretchable conductors are indispensable components for soft electronic integrations to develop next-generation wearable electronics and soft robotics 20,91,92 . In this section, we introduce flexible/ stretchable nontransparent conductors and transparent electrodes made from electrospun nanofibers.

Nontransparent conductors
The basic feature of a flexible and/or stretchable conductor is that it retains high conductivity under certain mechanical deformations [93][94][95] . Thanks to the excellent flexibility of the nanofiber-networked porous structure from electrospinning, a plethora of flexible/stretchable conductive films or yarns have been fabricated 63,[76][77][78]96,97 . For example, carbon nanofiber (CNF)/graphene nanosheet (GNS) composite paper can be directly prepared by electrospinning after adding GNS to the electrospun polymer precursor. The flexible paper shows a specific capacitance up to 197 F/g, which is 24% higher than that of pure CNF paper, revealing its promise for application in high-performance flexible capacitors 98 .
By using water-dissolvable PVA electrospun polymers, Someya et al. reported substrate-free on-skin electronics 56,85 . The fabrication process was simple and effective. In brief, a PVA nanofiber sheet was prepared by electrospinning. Then, a 70-100-nm-thick Au layer was deposited on top using a shadow mask. When the Au/PVA conductors were placed on skin and sprayed with water, the PVA nanofibers easily dissolved, and the Au nanomesh conductor adhered to the skin (Fig. 4a). Figure 4b shows the fine ridges of the human finger, demonstrating the good conformability of the material due to the ultrathin adhesive layer produced by the dissolved PVA. Compared to conventional planar substrates, substrate-free nanomeshes offer notable gas permeability and a significant decrease in stuffiness, rashes, and other inflammatory skin reactions. Furthermore, these nanomesh conductors can survive 10,000 cycles of finger bending (~40% strain) (Fig. 4c, d). A recently published work has shown that with heat-press treatment at 180°C, the resistance of the nanomesh conductors decrease by 76%, and the stretchability increases from 25 to 80% strain 85 .
Flexible/stretchable conductors have also been fabricated when electrospun nanofibers are used as supporting scaffolds for electrical materials. Someya et al. presented an ultrasoft Au/Parylene/PU conductor and employed it to monitor dynamically pulsing cardiomyocytes 99 . The PU nanofiber (diameter: 800-900 nm) sheet was ultrathin and flexible and had a coverage of 10-20%. The Parylene layer (200-nm-thick) was deposited via chemical vapor deposition to uniformly cover the PU nanofibers. Finally, the conductor was completed by depositing a 100-nmthick Au layer on top (Fig. 5a). The effective Young's modulus of the PU nanosubstrate was estimated to be 0.274 ± 0.039 MPa, which was two to three orders of magnitude lower than that of a uniform pure Parylene layer. The Au/Parylene/PU nanomesh conductor could be stretched to 30% strain while remaining highly conductive (211.6 ± 63.4 Ω) (Fig. 5b). Moreover, this conductor exhibited stable electrical responses under repetitive stretching (up to 20% strain) and releasing despite their ultrasoftness (Fig. 5c). To increase stretchability and conductivity, electrospun polymer nanofibers can also Fig. 4 Nontransparent flexible/stretchable conductors from electrospun nanofibers. a Schematic of substrate-free nanomesh conductors. The lamination process of Au nanomesh onto the skin is as follows: first, the Au layer is deposited onto electrospun PVA nanofibers; then, PVA is dissolved by being sprayed with water; final, after the removal of PVA, the nanomesh conductor adheres to the skin. b Picture of a nanomesh conductor attached to a fingertip. Scale bar, 1 mm. c SEM image of a nanomesh conductor after PVA dissolution. Scale bar, 5 µm. c Photographs showing the Au nanomesh adhered to fingers in relaxed and stretched states. d Conductance changes during 10,000 cycles of finger bending (modified from ref. 56 , copyright 2017 Springer Nature). function as reinforcements 57,90 . A porous AgNW/PU nanofiber elastic conductor can be prepared with a layerby-layer structure of nanofibers/nanowires due to interfacial hydrogen bonding 57 . The resultant nanofiber-based conductor exhibits a conductivity of up to 9190 S/cm, good stretchability of 310% strain, and high cycling durability at 70% strain. The as-prepared conductor also demonstrates good breathability for continuous on-skin electrophysiological signal recording.

Transparent electrodes
Flexible transparent electrodes, which combine high electrical conductivity, optical transparency, and mechanical flexibility, play a fundamental role in soft electronic applications, including transparent heaters, photovoltaics, supercapacitors, light-emitting diodes, and batteries 100 . Although transparent conductive oxides are universally used as conducting materials, their brittleness and required high-pressure vacuum processes during fabrication limit their applications in flexible/stretchable electronics 101 .
Considerable efforts have been directed toward manufacturing flexible transparent electrodes with onedimensional nanowires or nanofibers owing to their outstanding physical properties 48,102 . Owing to electrospinning, a low-cost and high-throughput method, diverse materials such as metals, polymers, and composites have been manufactured directly or with other techniques to construct flexible/stretchable transparent electrodes 69,83,103-106 .
A representative example is a metal nanotrough network-based transparent electrode developed by Cui et al. 84 . First, water-soluble PVA and PVP are electrospun into polymer nanofibers as templates. Then, a thermal evaporator is employed to deposit metals onto the porous nanofiber sheet. Finally, the transparent electrode is obtained by dissolving the underlying polymer nanofiber support (Fig. 6a). Various materials have been used via this approach to fabricate transparent electrodes, including silicon, indium tin oxide (ITO) and metals, such as gold, silver, copper, platinum, aluminum, chromium, nickel, and their alloys. After the polymer nanofiber is removed, the nanotroughs are found to be connected at junctions and exhibit concave crosssections (Fig. 6b). The sheet resistance (Rs) values of copper nanotrough networks have the best performance: 2 Ω/sq at T = 90%, 10 Ω/sq at T = 95%, and 17 Ω/sq at T = 97%, comparable to that of state-of-the-art devicegrade ITO (Fig. 6c). Furthermore, the nanotrough networks show good adhesion after attaching to flexible substrates such as PET plastic and paper (Fig. 6d) 84 . After introducing an in-plane sinusoidal wavy structure, an Au nanotrough networks can be stretched to more than 300% strain 107 . This buckled Au nanotrough electrode exhibits fatigue-free (>100% strain) properties and presents high optical transparency of 91% with a sheet resistance of 10 Ω/sq. Transparent self-junctioned metal electrodes can also be made by seeding metal ions or nanoparticles onto electrospun polymer nanofibers and subsequently performing electroplating or oxidation 105,[108][109][110] . In addition to thermal evaporation and electroless deposition, the direct coating of functional materials onto electrospun polymer nanofiber scaffolds is also effective in creating percolation networks for transparent electrodes 106,111,112 . After immersion in a highly concentrated dispersion of single-walled CNTs, the electrospun nylon 6 membrane becomes a conductive transparent film with a sheet resistance of 2100 Ω/sq at T = 77.4% 106 6 Flexible/stretchable transparent electrodes from electrospun nanofibers. a Schematic of the polymer-nanofiber templating process for fabricating nanotrough networks. b SEM images of a junction between two nanotroughs (right) and the cross-section of a single gold nanotrough (left). c Sheet resistance versus optical transmission (at 550nm) for copper, gold, silver, and aluminum nanotrough networks. d Photographs of 'conducting paper', demonstrating the reversible flexibility of nanotrough network-based transparent electrodes (modified from ref. 84 , copyright 2013 Springer Nature).

Sensors
Various highly sensitive flexible/stretchable sensors have been developed using electrospun nanofibers owing to their high surface area and ultraflexibility. Strain sensors and pressure sensors, the two most studied sensors using electrospun nanofibers, are reviewed in this section.

Strain sensors
Stretchable strain sensors have been extensively developed over the past two decades and have been employed in various applications, such as human activity monitoring and soft robotics 116,117 . The working mechanism is that mechanical deformations induce electrical responses in terms of resistance, capacitance or voltage 118,119 . Electrospun nanofiber-based train sensors exhibit lightness, flexibility/stretchability, and gas permeability, all of which are significant in wearable electronic applications. An effective way to fabricate strain sensors from electrospun nanofibers is to decorate or deposit functional materials onto nanofiber scaffolds due to their high surface area 79,[120][121][122][123][124] . A typical example is the use of a hybrid ZnNW/polysteryne (PS) nanofiber network as the functional material of a strain sensor 125 . The ZnNW/PS nanofiber porous structure is fabricated by attaching ZnO seeds onto electrospun PS nanofibers and subsequently growing ZnO NWs. The fabricated sensor can be stretched up to 50% strain with a gauge factor of~116 (Fig. 7a-c).
To continuously monitor skin strain without disturbing natural skin motions, Someya et al. 126 developed a breathable, ultrathin, and soft nanomesh skin-strain gauge. The nanomeshes were fabricated by reinforcing electrospun PU nanofibers formed by an electrospinning method with extremely thin PDMS. The sensor was ultralightweight (0.012 mg/cm 2 ) and extremely thin (430 nm), which allowed it to adhere to the skin and detect minute skin strains (Fig. 7d). The ultraconformability was also demonstrated by the distinguishable sweat pores shown in Fig. 7e while laminating the nanomeshes onto the surface of artificial skin. The as-fabricated sensor exhibited high mechanical durability (a resistance degradation of <3% after 5000 repetitive stretching and releasing cycles under 60% strain) and excellent sustainability (under 40% strain for 12 h with a <0.053 electrical drift error). The reliable electrical responses were independent of the stretch frequencies, and its thin geometry and softness provided minimum mechanical interference on natural skin motions. Hence, the skin strain could be quantitatively measured without affecting the natural movement of the skin. By attaching the developed nanomesh sensor to the face, they succeeded in measuring mapping facial strains during speech (Fig. 7f). The measured strain from the attached sensors on the right side of the face was asymmetric to that of the left side of the face with black markers. They also succeeded in mapping facial skin strains during a speech even after 3.5 h of wearing the device. This nanomesh sensor is expected to be used in smart facial masks that can accurately recognize facial microexpressions such as joy, anger, sadness, and fear.
Other strategies to fabricate stretchable strain sensors include the following: (1) directly electrospinning polymer nanofibers and carbonizing 72,127 ; (2) directly electrospinning functional nanofibers, such as PVDF nanofibers 64,71,128 ; and (3) adding functional materials to the polymer solution for electrospinning 80 . When any of these three strategies are used, the alignment, density, and diameter of the electrospun nanofibers are tuned to obtain the best performance of the stretchable strain sensor. Table 1 illustrates examples of electrospun nanofiber-based stretchable strain sensors using the aforementioned strategies.

Pressure sensors
The measurement of small normal pressures is critical to accurately evaluate external stimuli on curvilinear and dynamic surfaces such as human skin and natural tissues [129][130][131] . Electrospun nanofiber mats have high tolerance for repetitive external pressing due to their porous structures and high flexibility. Therefore, a large number of wearable pressure sensors have been constructed using electrospun nanofibers. Among these sensors, there are four types of working mechanisms: capacitive 132 , resistive 66 , triboelectric 133 , and piezoelectric 134 . Based on this, electrospun nanofibers are applied as dielectric layers, conductive layers, or electrode materials in pressure sensor assemblies. For example, Zhang et al. 135 used carbonized silk nanofibers as a conductive layer and fabricated a resistive pressure sensor. The developed pressure sensor had a rapid response time of <16.7 ms, a low detection limit of only 0.8 Pa, and a sensitivity of up to 34.47 kPa −1 . These outstanding performances were attributed to the unique N-doped silk nanofiber mats. To optimize piezoelectric responses, a highly aligned P(VDF-TrFe) nanofiber array has been fabricated into a flexible and freestanding sheet by electrospinning on a fastrotating collector 68 . A flexible pressure sensor can be simply formed by adding two electrical contacts at the ends of the as-fabricated sheet, and the resulting pressure sensor can detect pressure signals that are as low as 0.1 Pa. In addition, the device exhibits an excellent response under an extremely low-pressure regime, which suggests its potential utility as a sensor-energy harvesting component for applications in human motion detection and robotics 68 . Table 2 provides a summary of recently reported electrospun nanofiber-based flexible/stretchable pressure sensors.

Energy harvesting and storage devices
Wearable energy generation and storage devices are prerequisites to implement independent wearable electronic platforms. In this section, we present the development of soft energy harvesting devices (e.g., nanogenerators, batteries) and storage devices such as supercapacitors based on electrospun nanofibers.

Nanogenerators
There has been an increasing demand for renewable energy due to the rapid consumption of fossil fuels as well as environmental issues. Hence, mechanical energy harvesting systems that can convert small-scale energy from vibrations and motions into electrical power have been developed to solve this problem 49 . To date, there are two major types of nanogenerators: piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs). PENGs have attracted extensive attention since 2006 and are mainly fabricated from PVDF-based materials due to their large piezoelectric coefficients 136 . TENGs were first demonstrated in 2012 by Wang et al., in which they coupled the triboelectric effect and electrostatic induction 137,138 . With the introduction of piezoelectric, triboelectric, and/or pyroelectric electrospun nanofibers, a flexible/stretchable electronic system can realize self-powered operation with attractive mechanical and electrical performances [139][140][141][142][143][144] .
For example, a breathable, all-nanofiber TENG has been fabricated for effective mechanical energy harvesting and whole-body physiological signal detection 145 . The device is composed of a top polylactic-co-glycolic acid (PLGA) triboelectric layer, a bottom PVA nanofiber substrate, and an AgNW layer in between. This one-electrode TENG shows a high power density of 130 mW/m 2 and pressure sensitivity of up to 0.011 kPa −1 owing to the high surface area provided by the three-dimensional micro-to-nano hierarchical pores. TENG-based e-skin can be used to monitor whole-body physiological signals with good breathability, biodegradability, and antibacterial properties 145 . By anchoring piezoelectric barium titanate nanoparticles onto P(VDF-TrFE) nanofibers, the as-achieved PENG exhibits a significantly enhanced output of 6 V and 1.5 μA. The values for the PENG made from only P(VDF-TrFE) membranes are only 1.25 V and 0.6 μA 146 . Moreover, various pressure/tactile sensors have been developed by piezoelectric and/or triboelectric electrospun nanofibers for manufacturing wearable energy-sustainable electronic devices for monitoring human motion 103,134,140,[147][148][149][150][151][152] .

Batteries
Lithium (Li)-ion batteries play a pivotal role in energy storage systems for fast-growing demands in hybrid/full electric vehicles, portable devices, and power tools 153,154 . A Li-ion battery consists of a cathode, electrolyte, separator, and Li anode 155 . The separator is sandwiched between the two electrodes and immersed in the electrolyte. Li is the lightest metal in electrochemical energy storage and has the lowest reduction potential and an excellent theoretical specific capacity of 3860 mAh/g 60 . Owing to their large surface area, high porosity, and good conductivity, electrospun nanofibers have shown great potential for the development of flexible/stretchable Liion batteries 44 . To date, various electrodes, separators, and electrolytes have been developed using electrospun nanofibers to improve the electrochemical, mechanical, and safety performances of Li-ion batteries [156][157][158][159][160] .
Free-standing Li 2 S paper electrodes based on electrospun carbon nanofibers have been fabricated for flexible high-energy Li-ion batteries 161 . The free-standing conductive electrode also largely lowers the cost and manufacturing complexity of Li 2 S cathodes. The flexible paper electrode is composed of N-doped carbon nanofibers (NCNFs) decorated with ultrafine Li 2 S nanoparticles (Fig. 8a-c). These mechanically robust paper electrodes allow a high content of Li 2 S loading through layer-by-layer stacking. This highly opened but microscopically confined design enhances structural stability, reaction kinetics, and areal specific capacities. In addition, high performance with a high capacity of 520 mAh/ g (4.68 mAh/cm 2 ) is maintained with a stable capacity retention of over 65% for 200 cycles when utilizing a very high Li 2 S loading of 9.0 mg/cm 2 (Fig. 8d); additionally, an approximately 100% coulombic efficiency is achieved. Electrodes with a Li 2 S loading of 6.0 mg/cm 2 can deliver high capacities of 650, 540, 505, and 490 mAh/g (3.9, 3.24, 3.03, and 2.94 mAh/cm 2 , respectively) at current rates of 0.2, 0.5, 0.1, and 2.0C, respectively. The high rate capacity is retained with multilayered Li 2 S@NCNF electrodes (Fig. 8e, f). The high initial capacity of Li 2 S@NCNF//Fe 3 O 4 full cells can power an LED array consisting of 11*44 diodes (Fig. 8g). Figure 8h, i shows that the as-obtained prototype battery exhibits excellent flexibility while powering an LED 161 . Recently, Jeong et al. reported an intrinsically stretchable Li-ion battery that exhibited a high capacity of 91.8 mAh/g at 100% strain using an electrospun poly(styrene-b-butadiene-bstyrene) (SBS) block copolymer nanofiber mat as the separator membrane 162 . The developed stretchable rechargeable battery was successfully used to power stand-alone intrinsically stretchable electronic device platforms.
In addition to Li-ion batteries, other batteries, such as sodium-ion and zinc-air batteries, can also be achieved with electrospun nannofibers [163][164][165] . For example, by loading S 1−x Se x on porous carbon nanofibers, a flexible cathode electrode is achieved, which delivers a high reversible capacity of 840 mAh/g after 100 cycles at 0.1 A/g for Li-S batteries and 762 mAh/g after 100 cycles at 0.1 A/g for Na-S batteries 166 .

Supercapacitors
Supercapacitors, i.e., electrochemical capacitors, are energy storage devices that possess advantageous properties, such as fast charging/discharging capacities, high power and energy densities, long-term cycling, and safe operation 167,168 . A supercapacitor consists of two electrodes with a sandwiched ion-permeable electrolyte layer/ seperator 169,170 . To integrate a supercapacitor into a soft electronic system, it should maintain high performance under repeated mechanical deformations 171,172 . Following this target, the development of flexible/stretchable supercapacitors mainly focuses on designing highly flexible/stretchable capacitive materials for electrodes 173,174 . As a novel class of electrode materials, porous electrospun capacitive nanofibers are able to provide (1) mechanical flexibility, (2) a large surface area accessible to the electrolyte, (3) efficient ion transport kinetics, and (4) structural stability 62,160 . Consequently, high capacitive performance can be readily realized by engineering the pore size distribution of interconnected nanofibers 175,176 .
Electrospun carbon nanofiber-based electrodes are one of the most important components of supercapacitors because of their large specific surface area and good conductivity and chemical stability 177 . Strategies such as dopants, hollow structures, and composites have been applied to enhance electrochemical activity [178][179][180] . For instance, vanadium (V)/cobalt monoxide (CoO) has been introduced with conductive carbon nanofibers to produce extra reaction sites, accelerate electron transfer, and stabilize the electrode structure with good mechanical flexibility and accessibility for real applications 181 . VCO/ carbon nanofibers are achieved by electrospinning PAN, which is subsequently annealed (carbonization process) (Fig. 9a). The above electrode yields a high areal capacitance of 1.83 F/cm 2 at a current density of 8 mA/cm 2 . A binder-free, nonpolar asymmetric flexible supercapacitor is then assembled with PVA/KOH gel as the electrolyte and separator between the two binder-free VCO/carbon nanofiber electrodes (Fig. 9b). The supercapacitor yields an areal energy density up to 44.2 μWh/cm 2 at a power density of 2.8 mW/cm 2 (Fig. 9c). The device also exhibits good mechanical flexibility and is able to survive a high bending angle of 180°with a small capacitance decay of 5.5% 181 .
Electrospun nanofiber mats can also be employed as a separator in a supercapacitor 182,183 . Wei et al. demonstrated a fully stretchable supercapacitor with an electrospun membrane made of PU nanofibers as a separator, buckled SWNT films as electrodes, and an organic electrolyte consisting of tetraethylammonium tetrafluoroborate in propylene carbonate 182 . The resultant supercapacitor exhibited full stretchability up to 31.5% strain and retained 94.6% capacitance even after 2521 cycles of dynamic stretching and release under a high strain rate of 4.46% strain/s (Fig. 9d-f).
In addition, porous entangled nanofibers can enhance the capacitive performance by loading a high content of active capacitive materials due to the high surface area of these materials 184,185 . For example, an SWCNT-coated electrospun SBS nanofiber mat has been employed to fabricate a high-performance fully stretchable supercapacitor. The as-obtained supercapacitor delivers a volumetric capacitance of 15.2 F/cm 3 at 0.021 A/cm 3 and can maintain this performance under strains of up to 40% 185 . Specifically, an all-textile-based flexible supercapacitor has been fabricated by using electrospun PEDOT nanofibers as electrodes, a PAN electrospun nanofibrous membrane as a separator, and carbon fabric cloths as current collectors 186 . The as-prepared supercapacitor is totally flexible and demonstrates high cyclability under ambient conditions.

Transistors
A field-effect transistor (FET) is generally regarded as an electrical switch that uses an external electric field to control the charge transport behaviors in a device. It consists of three electrodes (source, drain, and gate); a semiconductor, i.e., an active channel; and a dielectric utilized as a thin insulating layer between the channel and gate electrode 187,188 . When a voltage is applied at the gate electrode, charge carriers move from the source to the drain to generate a source-drain current. Therefore, by adjusting the gate voltage, the source-drain current can be well controlled 189 . Flexible/stretchable transistors are of high importance in soft electronic integrations, and are capable of effectively manipulating and processing electrical signals 190,191 . However, polymer semiconductors made from film-type materials or nanofibril composites usually suffer from degraded charge carrier transport and limited flexibility. These issues can be overcome by the application of electrospun organic semiconducting nanofibers, where charge carrier transport can be enhanced by the preferential orientation of the polymer chains along the longitudinal direction of the fibers. It has been reported that the charge carrier mobility of P3HT electrospun nanofibers is dramatically improved by over 3 orders of magnitude 192 . In addition, electrospun semiconductors can avoid complex lithography-based patterning procedures by being directly aligned on polymeric substrates.
For instance, a single electrospun semiconducting NW with a diameter of 675 ± 40 nm has been used as an active channel, which is composed of a fused thiophene diketopyrrolopyrrole-based polymer semiconductor and high-molecular-weight polyethylene oxide as both the molecular binder and deformability enhancer (Fig. 10a) 53 . SBS elastomer films are applied as stretchable substrates for the transistor. CNT percolating networks are transferred to SBS films to fabricate S/D electrodes. Then, a straight semiconducting NW was transferred on top of the S/D electrodes. The transistor fabrication is completed by subsequently transferring nonionic SEBS dielectric and eutectic gallium-indium gate electrodes on the NW. The original mobility and maximum drain current of the device are 0.78 ± 0.21 cm 2 /Vs and −6.7 ± 1.88 nA at a gate/drain voltage of −100 V, respectively. When stretching along the channel length structure to 100% strain, the elongated NWs become wrinkled after release. Thus, both the mobility and drain current vary by <20% during cyclic stretching. In contrast, along the perpendicular direction, both the mobility and drain current remain almost the same up to a strain of 100%. The reason is that NWs do not actually experience stress but wrinkle in the channel length direction. By applying prestrain to the NWs to obtain a serpentine geometry, the mechanical durability of the transistor can be largely enhanced (Fig. 10b). The electrical performance can be easily maintained when stretched in both the length and width directions to ≥100% strain (Fig. 10c-e). When attached to a rubber balloon, the deformable transistor can survive a three-dimensional volume change of 1735% with a constant current output.
In addition to single nanofibers 50,51,54,193 , entangled or aligned electrospun semiconductor nanofibers are applicable as channel materials 52,194 . An entirely stretchable transistor has been developed by using electrospun SBS nanofibers as the substrate, P3HT nanofibers as the active channel, an ion-gel dielectric layer, S/D electrodes, and gate materials made of Au-nanosheet networks (Fig. 11a) 65 . The SBS nanofiber mat has a thickness of 500 μm, and the average diameter of a nanofiber is 300 nm. Au nanosheets are first transferred to PDMS and then to SBS membranes to form S/D electrodes with a size of 100*800 μm. The active channel is fabricated by directly collecting electrospun P3HT nanofiber (diameter: 2.5 μm) sheets between the S/D electrodes. The dielectric layer is also made from stretchable materials, namely, ion gels, and bridges the S/D electrodes. Finally, the Aunanosheet network is transferred to the ion-gel dielectric layer to form a gate electrode (Fig. 11b). Figure 11c shows that a 15*17 transistor array can be stably operated up to 70% strain. The device mobility experiences a light decrease to 15.5 cm 2 /Vs after 500 cycles under 70% strain and recovers to 18 cm 2 /Vs after 1500 cycles of stretching and releasing under 70% strain (Fig. 11d). This decrease may be because of the change in position of P3HT fibers under repeated stretching. Notably, the on-off current and threshold voltage remain the same during the cycling test.

Other applications
Other applications, such as flexible/stretchable heaters 195 , photodetectors 196 , chemical sensors 88,197 , optically encoded sensors 198 , solar cells 199 , and organic LEDs 114,115 , have also been developed with electrospun nanofibers. For example, a fully transparent and flexible ultraviolet-visible photodetector has been fabricated using the photoelectric effect of electrospun ZnO/CdO (cadmium oxide) nanofibers 200 . ZnO/CdO nanofibers have been fabricated by fusing ZnO and CdO nanoparticles into individual nanofibers. These highly aligned ZnO/CdO nanofibers, i.e., channel materials, can be exposed to light and induce a change in conductivity. The hybrid photodetector shows a transparency of up to 95% in the spectral range of 400-800 nm and a good mechanical durability of 200 bending cycles, as shown in Fig. 12a-c. The as-achieved fibrous membranes can be used as chemiresistors for wearable chemical sensing applications by incorporating CNTs or graphene on electrospun fibrous scaffolds 197,201 . A stretchable and highly sensitive chemical sensor has been manufactured by utilizing multilayered PU nanofiber/RGO networks (Fig. 12d) 201 . The conductance of the device increases upon exposure to NO 2 (oxidizing gas) and decreases upon exposure to NH 3 (reducing gas). Here, RGO is the p-type semiconductor, while NO 2 molecules are the electron acceptors and NH 3 molecules are the electron donors. The device exhibits a responsivity of~40% toward a 2.5 ppm exposure of NO 2 (Fig. 12e). The current responses to NO 2 exposure show very small changes even after 5000 cyclic stretching/ releasing tests at 50% strain ( Fig. 12f-h) 201 . e Electrical responses of a rigid RGO device under exposure to 2.5 ppm NO 2 and 25 ppm NH 3 diluted in dry air. f Electrical response of the chemical sensor under 2.5 ppm NO 2 at strains of 0, 30, and 50%. Test C denotes 5000 cycles of 50% strain. g Electrical responses at different concentrations of NO 2 at room temperature. h Electrical responses at different concentrations of NO 2 under 50% strain after 5000 stretching cycles (modified from ref. 201 , copyright 2017 IOPscience).

Summary and outlook
In this review, we first introduced the basic electrospinning technique for nanofiber fabrication. The advantageous electrical and mechanical properties of electrospun nanofibers include high transparency, good conductivity, a large surface area, diverse porous structures, excellent flexibility and/or stretchability, a onedimensional nanostructure, and easy scale-up, all of which provide strong benefits in the development of highperformance flexible/stretchable electronics. Generally, there are six strategies for the design of soft electronic devices: (1) single semiconducting nanofibers, (2) entangled porous nanofiber sheets, (3) aligned nanofiber arrays, (4) yarn, (5) temporary templates, and (6) reinforcement. Both single nanofibers and assembled nanofibers have been proven to be superior candidates for use in soft electronic applications. We then presented recent advances in electrospun nanofiber-based soft electronic devices, such as flexible/stretchable conductors, transparent electrodes, strain sensors, pressure sensors, nanogenerators, supercapacitors, batteries, and transistors. Nevertheless, to approach cost-effective, daily life applications, there are several remaining challenges regarding nanofiber fabrication and assembly, device performance, and system integration.
(1) It is difficult to electrospin organic conducting and semiconducting polymers, which are commonly used for conventional transistors, energy devices, and optoelectronics. Although some electrospun conducting (e.g., PPy and PANI) and functional (e.g., PEDOT: PSS and P3HT) nanofibers have been utilized for soft electronic devices, spinnable active polymers are still limited. The basic requirements for solution electrospinning are a sufficiently high molecular weight of the polymer and an appropriate solvent. The electrical conductivity of the polymer also influences the electrospinnability. Hence, advances in lowering electrospinning requirements and developing novel polymers and polymer blends are highly desired to achieve electrospinnable organic functional polymers for flexible/stretchable electronic applications. (2) It is still crucial to improve the flexibility and stretchability of electrospun nanofibers. Existing electrospun nanofiber-based soft electronics endow a certain degree of mechanical flexibility owing to the inherent flexibility of nanofibers. However, some electrospun nanomaterials, such as carbon nanofibers and metallic oxide nanofibers, have limited mechanical flexibility, resulting in inferior mechanical properties of the as-fabricated devices. It is necessary to precisely control the density, orientation, and porosity of the nanofibers in an effort to improve the flexibility/stretchability while maintaining the electrical performance of the devices. In addition, different nanofiber assembly methods should be combined, and new device assembly techniques are needed to realize soft electronics with desirable flexibility/stretchability. (3) As fundamental breakthroughs in individual electrospun nanofiber-based soft electronic devices have already been achieved over the past 10 years, these successes will lead to particularly promising achievements when combining two or more of the as-developed devices. Due to tremendous advancements in the flexible/stretchable electronic field, it is highly necessary to develop integrated and portable soft electronics. An appropriate combination of sensors and energy harvesting and storage devices can achieve a fully flexible/stretchable system based on electrospun nanofibers. For instance, a textile format, self-powered integrated wearable system can be prepared by using asfabricated yarn sensors 80 , all-textile flexible supercapacitors 186 , and all-nanofiber-based ultralight TENG 151 . We believe that the assembly of existing components into smart soft systems will bring new opportunities both in the market and in academic research.