A Primer

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https://doi.org/10.1038/s43586-023-00278-z

Primer

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Electrospinning of nanofibres

Dongxiao Ji ¹, Yagai Lin¹, Xinyue Guo¹, Brindha Ramasubramanian², Rongwu Wang¹, Norbert Radacsi ³, Rajan Jose ^4,5, Xiaohong Qin¹ & Seeram Ramakrishna ²

Abstract

Electrospinning is used to fabricate microscale to nanoscale materials from polymeric solutions based on electrohydrodynamics. Material modifications are achieved through physical and chemical processes, producing diverse material architectures, from laboratory to industrial scales, for conventional and emerging applications. This Primer explains electrospinning technology, encompassing principles, methodologies, equipment, materials, applications, scalability and optimization.

The article begins by elucidating the working principles, providing an overview of electrospinning methods and process parameters at laboratory and industrial scales, and discussing emerging equipment.

Methods are described for tailoring the composition, architecture and properties of electrospun fibres and fibre assemblies. The versatility of these properties makes electrospun materials suitable for diverse applications spanning environmental, energy and medical applications, textiles, wearables, agriculture and advanced materials. The Primer concludes by discussing the constraints of current electrospinning techniques and offers a perspective on the field’s potential future trajectory.

Sections

Introduction Experimentation Results

Applications

Reproducibility and data deposition

Limitations and optimizations Outlook

1Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China. ²Center for Nanotechnology & Sustainability, Department of Mechanical Engineering, College of Design and Engineering, National University of Singapore, Singapore, Singapore. ³School of Engineering, Institute for Materials and Processes, The University of Edinburgh, Edinburgh, UK. ⁴Center for Advanced Intelligent Materials, Universiti Malaysia Pahang Al-Sultan Abdullah, Kuantan, Malaysia. ⁵Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang Al-Sultan Abdullah, Kuantan, Malaysia. e-mail: [email protected]

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of the syringe when the solution is released at high voltage (>10 kV).

As solution delivery continues, an electrified liquid jet initiates from the cone and thins as it travels from the spinneret to the collector, where it is deposited as solid fibres. The critical electrospinning voltage (Vc), where charged polymer material is ejected from the Taylor cone’s tip, is determined by Taylor’s equation, considering the surface tension and viscosity of the solution⁴⁶:



 

 

 



V H

h h

R πRγ

= 4 ln 2

− 1.5 (1.3 ) × 0.09 (1)

c2 2

2

where H represents the distance between the collector and spinneret tip in centimetres, R is the inner radius of the spinneret in centimetres, h is the dimensional length of the syringe containing solution in centimetres and γ is the surface tension of the loaded solution in dyne per centimetre. The constant, 0.09, is incorporated into the formula to give an approximate voltage estimate⁴⁷.

The electrospinning process can be divided into four steps: charging and deformation; jet formation; jet thinning; and solidification and collection. During charging and deformation, a high voltage is applied to a polymer solution or melt to create a charge imbalance between the surface and bulk of the liquid⁴⁸. This voltage can be introduced through a syringe needle, spinneret or additional electrode. Accord- ing to Coulomb’s law, which describes the force (F) between charged objects (q1, q2) at a distance r, the charge imbalance creates an electrostatic force that overcomes the surface tension of the liquid and forms a Taylor cone (Fig. 1b):

F k q q

=( × × )r

1 2 (2)

2

where k is Coulomb’s constant, k = 9 × 10⁹ Nm² C⁻².

In the jet formation step, as the voltage increases, the electrostatic force becomes stronger than the surface tension, causing a jet of liquid to be ejected from the tip of the cone. The initial jet is straight, and the length of the straight segment can be controlled by adjusting the voltage (Fig. 1c). This jet elongation is due to electrostatic repulsion between the like charges on the surface of the liquid, as described by the Rayleigh instability equation:

λ k Q

=ε d×

× ₄² (3)

where λ is the wavelength of the instability, Q is the charge on the droplet, ε is the permittivity of the surrounding medium, d is the diameter of the droplet and k is a constant that depends on the geometry of the system.

As the jet travels through the air, it experiences evaporation and charge dissipation, causing it to thin and become unstable during the jet thinning step. Eventually, the jet solidifies into fibres onto a collection surface in the solidification and collection step. The Taylor cone–jet equation is used to describe the shape and size of the jet as it is ejected from the spinneret⁴⁹:

Q=2π × × ×ε d V

( )

ln r (4)

r²₁

where V is the applied voltage, r1 is the radius of the Taylor cone and r2

is the radius of the jet.

Introduction

Electrospinning enables the production of ultra-fine fibres, with diameters spanning from a few tens of nanometres to micrometres^1,2. Ultra-fine fibres produced by electrospinning have driven advances since their synthetic research inception^3,4, from improving the efficiency of solar cells and batteries^5,6 to advancing biomedical engineering^7,8 and electronics^9,10. The simplicity and scalability of electrospinning make it particularly appealing for manufacturing nanofibres with customized properties to suit a wide array of applications¹¹.

The electrospinning process involves the manipulation of polymer solutions, melts or suspensions with strong electric fields to form distinct structures that have high surface area to volume ratios, tunable morphologies and modifiable compositions. These properties have led to innovations in biomedicine^12,13, environmental remediation^14–16, energy storage^17–19, electronics^20–22, textiles^23,24, food packaging^25–27, agriculture^28,29 and catalysis^30,31.

Electrospinning has emerged as an invaluable research tool, fos- tering international collaborations, knowledge dissemination and technological exchange between scholars, institutions and commercial entities. The widespread implementation of electrospinning in aca- demia and industry has accelerated advances in equipment, materials and techniques, leading to improved control over the resulting fibres^32,33. Consequently, electrospinning has promoted innovation in materials science, engineering and other disciplines, offering a comprehensive, global perspective across fields.

As electrospinning evolves, it is anticipated that integrating advancements in materials science³⁴, chemistry³⁵, physics³⁶, biology³⁷ and engineering³⁸ will produce novel materials and applications.

Emerging research areas include the development of smart and intelligent materials capable of responding to external stimuli.

Combining electrospinning with other fabrication techniques — such as 3D printing³⁹, bioprinting⁴⁰ and microfluidics⁴¹ — and the investigation of new polymers^42,43, composites⁴⁴ and hybrid materials⁴⁵ will improve the properties and performance of electrospun fibres. Additionally, a growing emphasis on sustainability has led to research into environmentally friendly and biodegradable electrospun materials. The unique properties and potential applications of electrospun materials mean it is likely to have increasing impact in diverse disciplines.

This Primer explores electrospinning technology, including the underlying principles, methodologies, materials, applications, scalability and optimization. The article begins by discussing the working principles of electrospinning, with an overview of general spinning methods using standard equipment. By examining the results, a demonstration is provided of how to tailor the composition, structure and properties of electrospun ultra-fine fibres. The versatility of these properties makes electrospun nanomaterials suitable for diverse applications across environmental and medical applications, wearable devices, agriculture, energy, high-performance materials and novel domains. The Primer ends by discussing the constraints of current electrospinning techniques and offers a perspective on the future of the field.

Experimentation

Figure 1a illustrates the basic electrospinning set-up consisting of a spinneret with a syringe and electronics for controlled solution release;

a metallic collector to receive the spun material; and a power supply to create an electric field between the spinneret and the collector. The polymer to be electrospun is either dissolved in a solvent or melted and delivered to the spinneret via the syringe. The process starts with the formation of a cone-shaped solution, known as a Taylor cone, at the tip

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The parameters that collectively govern the electrospinning process and fibre characteristics fall into three categories: operational and instrument variables, such as voltage type, collector type, spinneret design and flow rate; material and solution characteristics, including polymer type, solvent polarity, concentration, viscosity, conductivity, surface tension and additives; and external or environmental conditions, for instance humidity and temperature^50,51.

Role of operational and instrument variables

Spinneret to collector distance and applied voltage. The needle to collector distance (H) determines the electric field strength (E = V/H) in the spinning chamber, which influences the trajectory of the spinning jet, fibre deposition time, morphology and size of the fibres⁵². Alter- ing the distance modifies E and the distribution of electric flux lines, affecting the fibre profile⁵³. The jet continuously thins from the needle to the collector due to increased surface charge density. By increasing H, fibres can be developed with smaller diameters (Fig. 2a).

Increasing the applied voltage affects the jet velocity, leading to shrinkage of the Taylor cone and jet destabilization as more solution is drawn from the needle tip by a stronger electric field. In general, higher voltages result in thinner fibres due to increased stretching and solvent evaporation (Fig. 2a). Low-viscosity solutions generate secondary jets at higher voltages, further reducing the fibre diameter.

Higher voltage reduces the flight time by accelerating the jet stretching to the collector, decreasing the fibre diameter⁵⁰. The choice of voltage source — alternating current (AC) or direct current (DC) — also impacts

fibre formation. DC between 10 and 20 kV is common for low-volume spinning, whereas AC offers lower voltages (~1–10 kV). Whereas DC ensures jet stability and continuous fibre formation, the changing polarity of AC can cause disruptions leading to bead formation and customizable fibre morphology⁵².

Solution feed ratio. The solution feed rate has a substantial effect on fibre diameter and morphology. It is vital to optimize the feed rate for different solutions, usually ranging from 0.05 to 5 ml h^–1, depending on the solution’s viscosity³. Higher feed rates (>10 ml h^–1) cause low-viscosity fluids to splash periodically and produce beaded fibres.

To identify the optimum feed rate, it is advisable to gradually decrease the feed rate from a high threshold. This decreases the size of the beads, eventually resulting in smooth fibres⁵⁴. However, reducing the feed rate too much may stop the spinning process and cause increased variation in the overall fibre diameter. The ideal solution feed ratio for electrospinning depends on the polymer, its molecular weight, the desired fibre properties and application. Factors such as the solvent evaporation rate, polymer concentration and solution viscosity are critical.

The configuration of collectors. Various types of stationary and rotating collectors are shown in Fig. 2b, including stationary collectors — flat plate, ring, parallel plate and electrode array collectors — and rotating collectors — drum with pins, drum with wire, rotating bath, rotating disc, wire drum and knife edge⁵⁵. Flat plate collectors are commonly used to produce non-woven mats and membranes. Parallel plate collectors

Syringe pump

Syringe

Spinneret

Collector Highvoltage

a

c b

Straight segment becomes shorter

–28 ms –8 ms –2 ms

100 µm 0 ms +6 ms +32 ms

+ –

Decreasing voltage

Fig. 1 | Example of a typical electrospinning experimental set-up.

a, Fundamental configuration for electrospinning. b, Visual documentation illustrating progressive transformation of a polyethylene oxide (PEO) solution droplet, transitioning from a conical morphology to a more spherical form, subsequently expelling a jet. c, Real-time monitoring of the jet’s diameter and

linear segment length related to the applied voltage is facilitated by different interference colours observed in the aqueous PEO jet’s straight section. Part b adapted with permission from ref. 46, Elsevier. Part c adapted with permission from ref. 3, American Chemical Society.

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a Parameters affecting electrospinning b Types of collectors

c Types of spinnerets

Fibre diameter (µm)

Parallel Drum with multi-

electrode Bath

Drum with pin Disc

Drum Wire

Wire drum Knife edge

Electrode array

Needle

Disc

Cylindrical Ball

Beaded chain Bowl

Conical

Conical wire coil (spinneret) Solution reservoir

Applied voltage (V)

Unstable jet

Minimum and maximum values, saturation and unstable regimes

Stationary collectors Rotatory collectors

Needle to collector distance (mm)

Saturation regime

Decreased fibre strength

Fibre fusion and agglomeration Brittle fibres

Relative humidity (%)

Temperature (˚C)

Solution conductivity (S m–1)

Surface tension (N m–1)

Fibre diameter (µm) Polymer concentration (wt%) and solution viscosity (m Pa.s)

Better charge dissipation and stretching of jet

Blade

Ring

Plate

Disc spinneret Electrospinning solution

Electrospinning solution Electrospinning

solution Bowl spinneret

Electrospinning solution

High voltage High voltage

High voltage

Beaded chain spinneret

Cylinder

spinneret Ball spinneret

Fig. 2 | Experimental design process and factors affecting electrospinning.

a, Major parameters affecting the fibre diameter: voltage, needle to screen distance, polymer concentration, surface tension and solution conductivity.

The curve represents the trend of electrospun fibre fineness changing with parameters. b, Different types of collectors, including a rotating drum, bath and flat plate. c, Types of spinnerets.

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are designed to generate aligned fibres, and electrode array collectors offer precise control over fibre orientation^56,57. By contrast, ring collectors are suited to circular fibre patterns⁵⁸.

Rotating collectors, including drums with multiple electrodes, pins or wires, are preferred for their ability to enhance throughput and regulate fibre alignment. Rotating a drum with pins gives a large, thin, aligned mat, whereas a drum with knife edge electrodes creates a thick, aligned mat. Wire-wound drums offer control over the alignment area. A rotating disc collector produces alignment but needs speed optimization for nanofibre integrity. A water bath solidifies nanofibres for twisting via a rotating drum⁵⁹. Static collectors, such as plates, produce a random alignment, whereas parallel ring collectors create twisted nanofibre yarns. Parallel electrode collectors provide easy transfer but may not yield thickness. Magnetic nanoparticles can aid unidirectional alignment in some cases.

Minimizing residual charge build-up is critical for improving fibre deposition on low-conducting collectors. Although rotating collectors are frequently used to collect aligned fibres, accumulation of residual charge on the fibre can prevent larger layers of aligned fibres being generated⁶⁰. To address this constraint, an alternative collector design can be used, which has parallel electrodes separated by an air gap, referred to as air blow-assisted electrospinning. The air causes the jet to oscillate, resulting in precisely aligned fibres. Additionally, magnetic fields have been used to engineer and accelerate the jet speed⁶¹. The configuration of spinnerets. Needleless electrospinning has become the preferred method for large-scale nanofibre production (Fig. 2c). It uses various spinneret designs, including rotatory — rotary cylinder, spinal coil spinneret, rotary disc, ball spinneret, rotary cone and moving bead chain — and static types, such as the bowl spinneret.

Needleless electrospinning offers several advantages, including multiple nanofibre jets, elimination of nozzle clogging and faster fibre deposition⁶². The production rate is influenced by the number of spinnerets and the initiation points of the jets. Prominent companies, such as IME Technologies B.V. (recently renamed to Vivolta), Inovenso Ltd and Elmarco, have developed industrial-scale needleless electrospinning facilities that can produce several kilograms per hour. However, the technique requires careful parameter control and considerations of solvent compatibility due to the use of multiple solutions⁶³. Role of material and solution characteristics

Solution characteristics, including viscosity, molecular weight, polymer concentration, conductivity and surface tension, play pivotal roles in fibre production⁵⁴ (Fig. 2a). In the case of lower-viscosity solutions, achieving continuous fibre formation can be challenging, whereas excessively high viscosity can impede expulsion of the jet from the needle tip. The polymer to solution concentration directly influences the fibre diameter, with higher concentrations resulting in larger diameters. However, a minimal concentration threshold is essential, as lower concentrations can lead to bead formation instead of the tangles required to maintain jet equilibrium. The number of entanglements within the solution, which is influenced by polymer molecular weight and viscosity (Fig. 2a), also influences fibre morphology. Higher conductivity leads to smaller-diameter fibres as the jet experiences greater stretching forces (Fig. 2a). Conductivity directly affects the charge build-up within the solution in the electric field.

Holding other parameters constant, surface tension assumes a critical role in fibre generation by controlling the upper and lower boundaries of the applied voltage window⁶⁴.

It is essential to tailor electrospun polymer traits to achieve customizable nanofibre production. For instance, polymethyl meth- acrylate (PMMA) has a lower viscosity range (0.1–0.5 Pa·s), suitable for concentrations between 5 and 20 wt%. By contrast, polyvinyl alcohol (PVA) has a typical viscosity range of 3–25 Pa·s and concentrations of 5–15 wt%^65,66. Solutions with higher molecular weight precursors often have viscosities ranging from 10 to 30 Pa·s or higher. Polymers such as PVA or polycaprolactone (PCL) (~10–20 wt%) can yield highly viscous solutions. Higher molecular weight polymers contain inter- twined, longer chains that form a network-like structure, hindering flow and increasing viscosity. Additionally, these polymers have stronger intermolecular forces, including van der Waals forces and hydrogen bonding, further enhancing the solution’s resistance to flow. Higher polymer concentrations results in more chain entanglement and increased viscosity. Understanding the polymer-specific distinctions and adapting the spinning parameters accordingly is vital for optimal fibre production⁴⁵.

Role of external and environmental conditions

Humidity is a crucial external factor in electrospinning as it affects the surface tension and viscosity of the electrospinning solution. High humidity leads to condensation of water vapour on the fibre surface, causing pores to interconnect due to water droplet coalescence⁶⁷. Pores on the fibre surface impact the mechanical properties, surface area and wettability of the fibres. High humidity also decreases the solution viscosity, reducing the stretching force and causing beads to form on the fibre surface. An increase in temperature reduces solution viscosity, leading to a decrease in fibre diameter due to greater stretching force (Fig. 2a). Excessive heat can cause thermal degradation, affecting the morphology and properties of electrospun fibres. As a result, precise control of temperature and humidity is crucial for achieving the desired fibre diameter and morphology.

Results

Electrospinning can be used across different fields due to the diverse composition and structures produced. This section describes how to characterize electrospinning products and the factors influencing the construction of different materials and structures.

Characterization

Electrospinning products typically range from nanometres to microns, requiring microscopy characterization methods. Scanning electron microscopy and transmission electron microscopy can characterize the physical morphology of electrospun products. Scanning electron microscopy is used to observe the physical shape of the product surface, for example the formation of fibres, beads or particles, if the surface is smooth, and whether the cross-section has a hollow or porous structure. Scanning electron microscopy can quantify the size of the product, whereas transmission electron microscopy is used to observe finer fibre materials (<100 nm); determine the presence of a core–sheath, a wrap or other hierarchical structures; and analyse the elemental composition and crystallization of the material⁶⁸. Charac- terizing the mechanical properties of the nanofibre is challenging, especially for a single fibre. Atomic force microscopy is an alternative mechanical characterization method that can characterize the apparent morphology of fibres and quantify material roughness⁶⁹. A nanoscale universal testing machine has higher sampling and equipment requirements and can directly obtain the mechanical parameters of a single nanofibre, including tensile strength, elongation, modulus

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Flat ribbon Shape

Structure

Alignment andassembly

Twisted ribbon Branch Beaded Circular spiral Shish kebab

Core–shell Nano-branching Janus Hollow Multichannel Porous

Aligned 3D scaffold 3D networks 4D shape memory materials

b a

Polymer

Gel Organic materials Hybrid/precursor

Post treatment Electrospinning

Solution preparation

Living materials

Polymer solution Hybrid solution Sol solution

Inorganic materials Live cell or microorganism solution

Hydrothermal treatment Carbonization Calcination Annealing

Inorganic particles Sol

Live cells or microorganisms

Initial shape Deformed

shape Shape recovery process Recovered shape

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and toughness⁷⁰. The chemical structure of the product can be charac- terized by infrared spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy⁷¹.

Electrospinning products are often collected and applied as nanofibre assemblies, in the form of either non-wovens or yarns. Other characterization techniques are used to study the macroscopic properties of fibre assemblies. For instance, high specific surface area and high porosity are usually determined by gas adsorption and porosity measurements⁷². Further characterization methods may be necessary, such as an evaluation of hydrophilicity, breathability and biocompatibility, through the contact angle⁷³, breathability⁷⁴ and cytotoxicity tests⁷⁵, respectively, depending on the desired application.

Electrospinning of diverse materials

Electrospinning techniques can produce a diverse array of materials (Fig. 3a), which can be categorized into organic, hybrid or composite, inorganic and living materials.

Organic materials. Organic nanofibres, including natural and synthetic polymers, are the main output of electrospinning, with more than 100 options available. Natural polymers include proteins, such as silk protein, collagen, elastin and gelatin; polysaccharides, such as cellulose and its derivatives, chitin, chitosan, hyaluronic acid and glucose; and nucleic acids. Synthetic polymers include water-soluble polymers — such as polyethylene oxide (PEO), PVA and polyvinylpyr- rolidone (PVP) — and polymers that are soluble in organic solvents, such as polyacrylonitrile (PAN), PMMA, polyvinylidene difluoride (PVDF) and polylcaprolactone (PCL). Solvent choice is crucial for successful preparation of organic polymers. Suitable solvents must be chosen to dissolve the polymer, but solvent properties, including conductivity and volatility, affect the stability of the electrospinning process and the morphology of the resulting materials⁷⁶. Some organic polymers have limited solubility in solvents, requiring specialized processing methods to facilitate effective electrospinning. For example, polyimide is difficult to dissolve in any solvent, and different routes have been developed to electrospin it. One approach is to modify the insoluble polymer to improve its solubility⁷⁷. Another method is to perform the spinning with soluble monomers — such as polyacrylic acid (PAA), a precursor of polyimide — and convert it into polyimide nanofibres through thermal or chemical sulfonation⁷⁸. Alternatively, melt electrospinning can be used for applicable materials, which also include polyester (PET) and polypropylene (PP)⁷⁹.

Hybrids or composites. By combining functional nanomaterials with electrospinning technology, products can have more diverse properties, such as optical, electrical, thermal and magnetic characteristics.

These functional nanoparticles are generally inorganic, resulting

in organic–inorganic hybrid materials — including metal/polymer, ceramic/polymer and carbon/polymer nanofibres — where the inorganic component is dispersed within the polymer matrix³. Two widely used methods for preparing composite nanofibre materials are to use sol–gel precursors or to add inorganic nanoparticles into polymer solutions. When using sol–gel solutions, controlling the solution viscosity is crucial for promoting jet stretching. For mixed solutions, achieving optimal electrospinning performance depends on achieving uniform dispersion of the inorganic phase within the polymer solution. Aggre- gation of the inorganic phase can hinder the uniformity and stability of electrospinning. Maintaining a uniform particle size and appropriate concentration improves dispersion. Additionally, a range of physical and chemical techniques, including stirring, grinding, ultrasonic dispersion and surface modification of the inorganic material, can enhance dispersion in polymer solutions. These measures contribute to the formation of a homogeneous and spinnable precursor solution⁸⁰. Inorganic materials. Direct electrospinning of inorganic materials is challenging due to the difficulty of forming a viscous solution from inorganic precursors. Consequently, pure inorganic nanomaterial fabrication typically involves electrospinning of inorganic/polymer composites, followed by specific post-processing techniques, commonly sintering, to eliminate or transform the polymer components. This results in smaller-sized inorganic electrospun materials.

Post-processing is crucial for obtaining inorganic materials. Silver nanoparticles with a diameter of 40 ± 5 nm can be dispersed in ethylene glycol at a concentration of 50 wt% and subsequently electrospun.

Annealing post-treatment at 150 °C produces pure silver nanowires with an average diameter of 338 ± 35 nm (ref. 81). For ceramic inorganic materials, sol–gel spinning precursors can be prepared initially and converted into ceramics or oxides through calcination in the air following electrospinning. Anatase titanium dioxide (TiO₂) nanofibres with an average diameter of 20–200 nm can be produced, for instance, by calcining PVP and amorphous TiO2 composite nanofibres in air at 500 °C (ref. 82). By contrast, carbon materials do not require addition of metal precursors, as many polymers can be directly converted into carbon through carbonization, the process of thermal crack- ing in an oxygen-free environment. PAN, for example, can achieve a carbon yield of greater than 50% and is a commonly used precursor.

The properties of carbon nanofibres are primarily influenced by the diameter and morphology of the precursor nanofibres, as well as the stabilization and carbonization conditions³.

Living materials. Electrospinning can also be used to develop living materials that incorporate live cells and microorganisms. Generally, the solution containing live cells or microorganisms is electrospun by single-nozzle, coaxial, emulsion or other electrospinning methods⁸³. Fig. 3 | Electrospinning of diverse materials and morphology. a, Process

diagram for preparing organic, inorganic, hybrid or composite and living materials through electrospinning technology. b, Shape, structure and assembly of various fibres prepared by electrospinning. Flat ribbon, twisted ribbon and branch reprinted with permission from ref. 88, Wiley. Beaded reprinted with permission from ref. 90. Copyright 2006 American Chemical Society. Circular spiral reprinted with permission from ref. 91, Elsevier. Shish kebab reprinted with permission from ref. 93. Copyright 2008 American Chemical Society.

Society. Janus reprinted with permission from ref. 95. Copyright 2007 American Chemical Society. Hollow reprinted with permission from ref. 97. Copyright 2004 American Chemical Society. Multichannel reprinted with permission from ref. 96.

Copyright 2007 American Chemical Society. Porous reprinted with permission from ref. 108. Copyright 2018 American Chemical Society. Aligned reprinted with permission from ref. 57. Copyright 2003 American Chemical Society. 3D scaffold reprinted with permission from ref. 117, Wiley. 3D networks reprinted with permission from ref. 118, Elsevier. 4D reprinted with permission from ref. 119.

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To maintain the high viability of live cells and microorganisms, appropriate carriers and solvents, spinning parameters and environmental conditions are required. Bio-friendly carriers and solvents are preferred^84,85. In addition, strong electric fields may cause low viability.

Electrospinning must be performed at a controlled temperature to prevent damage to living cells and microorganisms. For example, the favourable growth temperature for most probiotics is from 37 °C to 43 °C (ref. 86). Compared with free-form live cells and microorganisms, the high surface to volume ratio, porosity and biocompatibility of electrospun nanomaterials provide favourable immobilization conditions to promote survival stability and vitality⁸⁷.

Varied morphologies, alignments and assemblies

Morphology control. Beyond cylindrical nanofibres, electrospinning can produce diverse nanofibre geometries (Fig. 3b). Several factors influence the formation of varied fibre shapes, including the fluid mechanical dynamics, the electric charge distribution along the jets and solvent evaporation⁸⁸. If the electrospinning solution has a slow solvent evaporation rate or a swift solute diffusion rate, the electrospinning jet undergoes uniform contraction, producing nanofibres with circular cross-sectional profiles. Conversely, when there are pro- nounced intermolecular interactions in the polymer chains, polymer diffusion is hindered. The solvent rapidly evaporates from the jet surface, forming an outer shell layer with a high concentration of polymer and diverse mechanical properties⁸⁹. As more solvent evaporates, external atmospheric pressure causes an inward collapse of the initially circular jet. The high mechanical strength shell undergoes deformation to give fibres with elliptical cross-sections, whereas regions with lower mechanical strength yield ribbon-like fibres. If the direction of collapse varies, twisted band fibres form. Elevated surface charge densities on the jet surface can perturb the equilibrium between surface tension and electrostatic forces, leading to jet instability. Having an unstable jet causes branching from the principal jet, resulting in branched nanofibres^46,88. A less viscous electrospinning solution and lower surface charge density on the jet generates bead-like nanofibres;

however, decreasing the surface tension gradually diminishes bead formation. Another method of producing bead nanofibres is to add beads to polymer solutions⁹⁰. As the jet interfaces with the collector surface during jet collection, compression occurs, typically causing the jet to buckle. This buckling phenomenon can result in a series of nanofibres interconnected through folds, circular spirals, serra- tions and other complex configurations⁹¹. Other distinctive nanofibre shapes have been documented, including shish kebab-shaped nanofibres, achieved by precisely controlling polymer crystallization during electrospinning⁹² or by inducing directional crystal growth on the nanofibres with post-processing techniques⁹³.

Electrospinning technology has produced an assortment of nanofibre structures, including core–shell, hollow, multi-chamber, Janus and porous configurations. Core–shell structures can be achieved by coaxial electrospinning³. This method involves introducing distinct liquid phases into coaxial needles to form concentrically encapsulated drop- lets that solidify into the core–shell nanofibre structure⁹⁴. Manipulating the spinneret needle configuration into a parallel⁹⁵ or multichannel⁹⁶ arrangement enables Janus structured or multi-component nanofibres to be produced. To further diversify the nanofibre morphology, post-treatment procedures, such as calcination to selectively remove specific components, can transform core–shell and multi-component nanofibres into hollow and multi-chamber structures⁹⁷. Alongside modifying the spinneret configuration, alternative methodologies to

fabricate core–shell nanofibres include emulsion electrospinning⁹⁸ or in situ deposition⁹⁹ of supplementary materials onto the surface of electrospun nanofibres. For Janus structured nanofibres, another method is conjugate electrospinning, where double needles with opposite charges generate Coulombic attraction to form double jets leading to Janus structures¹⁰⁰. Numerous methods can control intra-material pores, such as coaxial electrospinning, where the inner spinneret feeds the target material whereas the outer spinneret contains a sacrificial material or solvent^101,102; spinning an immiscible polymer composite and selectively removing one from the final fibre¹⁰³; spinning an emulsion^104,105; spinning a solution containing porosity generators, such as organic salts^106,107; and solvent evaporation¹⁰⁸. For all these methods, chemical or thermal treatment is required of the post-spun structure to generate intra-material porosity.

Alignment and assembly. Conventional electrospinning produces a random fibre network on a collector surface owing to the asym- metric whipping motion of the jet. In many applications, however, nanostructures with a well-defined alignment are desired. Many strategies can achieve the necessary alignment¹⁰⁹, such as mechanical^110,111, magnetic^112,113 or electrostatic¹¹⁴ approaches. These methods can produce various nanofibre arrangements, such as aligned fibres (Fig. 3b), radial alignment and patterning. The electrospun nanofibres are 1D, but their assembly leads to higher dimensions: 2D, 3D and 4D. Electrospun 2D fibre membranes are typically used in filtration, tissue engineering, photonics and other fields¹¹⁵. Fibres produced by 3D electrospinning are collected on a rotating or moving substrate, enabling fibres to accumulate in multiple directions¹¹⁶. The resulting structure has 3D features, similar to porous scaffolds¹¹⁷ or 3D networks¹¹⁸ (Fig. 3b).

4D electrospinning is used for stimulus-responsive materials, such as shape memory materials^119,120 (Fig. 3b). By using responsive materials, the electrospun structures have dynamic behaviour, including shape transformation, reversible morphological changes or controlled release of encapsulated substances over time.

Influence of novel electrospinning techniques

Near-field electrospinning. Unlike far-field electrospinning — conventional electrospinning performed at H of 5–50 cm and V of 10–30 kV — near-field electrospinning (Fig. 4a) at H of 0.05–5 cm and V of 0.2–12 kV is achieved by reducing H, the distance between the spinneret and the collector^121,122. In far-field electrospinning, the jet undergoes a linear segment followed by an unstable whipping segment, forming random mesh-structured nanofibre membranes.

Near-field electrospinning mitigates or removes the unstable segment, facilitating controlled deposition of stable, linear jets on a stationary or moving collector to build specific 3D configurations, similar to 3D printing. The limited flow rate (0.01–1 ml h^–1 (ref. 3)) restricts output and jet elongation, resulting in reduced production rates and increased diameters.

Melt electrospinning. Melt electrospinning (Fig. 4b) is used to process polymer melts⁷⁵, usually thermoplastics such as PP and PET. The spinning process does not use solvents, making it safer and more environmentally friendly. After being ejected from the spinneret due to heat transfer between the jet and the surrounding medium, the molten jet cools and solidifies to produce fibres. In contrast to solution electrospinning, the low conductivity and high viscosity of the polymer melt decreases the surface charge density on the jet, which suppresses the whipping instability of the jet, resulting in a higher

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output with thicker fibres, usually at the micron level. By adjusting the process parameters — such as lowering the melt flow rate, changing the melt temperature and heating the surrounding air — thinner jets and nanofibres can be formed. In addition, accurately electrospinning fibres onto a specific substrate in the direct write mode is advantageous for substrate design and material performance⁴².

Needleless electrospinning. Restricted electrospinning techniques, such as single-needle and multi-needle/multi-pore methods, have output limitations based on low yields or needle clogging. Unconstrained — often called needleless, free-surface or nozzle-free — electrospinning applies a liquid to the spinneret surface and uses electric fields to create multiple jets. This significantly boosts output and prevents clogs. The key to needleless electrospinning is in the design of the spinnerets.

Stationary spinnerets typically rely on external forces, including magnetic fields, airflow pressure and gravity, to induce disturbances on the surface of charged solutions, forming multiple Taylor cones¹²³. Typical stationary spinnerets (Fig. 4c) include the magnetic fluid¹²⁴, bubble¹²⁵, cleft¹²⁶ and stepped pyramid¹²⁷. Rotating spinnerets (Fig. 4c), such as cylinders¹²⁸, ball¹²⁸, discs¹²⁸ and spiral coils¹²⁹, are partially immersed in the electrospinning solution. The surface is continuously coated with the liquid as they rotate, producing a substantial quantity of jets.

For some stationary modes, continuous liquid supply needs to be considered, whereas for some rotating modes, solvent evaporation due to excessive free liquid surface needs to be avoided³. Some spinnerets have a liquid supply device — mushroom-shaped¹³⁰, annular⁶⁰ and metal wire (Nanospider technology) (Fig. 4c) — which minimizes

evaporation of the spinning liquid by limiting the exposed free liquid surface area and provides a continuous liquid supply. In unconstrained electrospinning, precise control of the liquid flow rate and uniform jet excitation are critical areas for further research.

Electroblowing. Electroblowing (Fig. 4d) is an innovative electrospinning technology that integrates a blowing system with a conventional electrospinning device¹³¹. This approach combines the benefits of electrospinning and solution blowing, also referred to as air-assisted or blowing-assisted electrostatic spinning. Additional airflow in electroblowing results in a strong stretching force that drives the solution from the spinneret tip and accelerates solvent evaporation. This leads to enhanced fibre formation capabilities. Electroblowing enables feed rates up to 150 ml h^–1 and improves yields by nearly 28 times compared with conventional electrospinning^132–134. The airflow action in electroblowing means more viscous spinning solutions can be used and nanofibres can be produced with smaller diameters and fewer beads¹³⁵. Electrospun yarns. By incorporating a series of conventional spinning techniques, continuous fabrication of nanofibre yarns is possible.

Unlike mesh-like structures, this approach harnesses the inherent mechanical strength of yarns to manufacture scaled-up products.

Drawing inspiration from the principles of wet spinning, electrospun nanofibres may be directed into a coagulation bath and subsequently stretched to form yarn¹³⁶. Alternatively, a dynamic water bath system capitalizes on the eddy current flow of water to continuously orient and twist nanofibres into yarn^137,138. However, the twist of nanofibre

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Fig. 4 | Influence of novel electrospinning techniques. Technological breakthroughs in electrospinning. a, Near-field electrospinning. b, Melt electrospinning. c, Needleless electrospinning spinnerets. d, Electroblowing. e, Electrospun yarn.

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yarns collected continuously in a water bath is limited. The conjugate electrospinning (Fig. 4e) set-up used for continuous production of nanoyarns typically comprises two needles, each bearing opposite charges, two power supplies, a rotary collector and a rotating axis dedicated to yarn collection. In this configuration, polymer jets emitted from oppositely charged needles experience mutual attraction, resulting in binding at the rotary collector. Then, the rotary collector and the rotary axis impart a twist to the nanofibre and wind it into yarn. A representative example uses a rotating metal funnel as the collector.

A continuous PVDF nanofibre yarn is produced, extending over tens of kilometres, and is continuously withdrawn and twisted with a winding mechanism. Rotators, such as rotating discs, rotating metal rings and rotating metal hemispheres, can also be used to continuously produce nanofibre yarns¹³⁹. By incorporating yarn supply and tension devices, conjugate electrospinning technology can generate core-spun yarn.

In this configuration, the core-spun yarn is fed directly from the centre of the collector and passes through the spinning triangular cone. In the cone, it converges with the oriented nanofibres at the twisting point and is wrapped into yarn. High-strength and tough nanofibre yarns can be produced by click chemistry¹⁴⁰, but it is challenging to form such yarn at high speeds. This is because needle nozzles have limited yield, and it is difficult to combine emerging electrospinning technology with twisting and take-up devices. As a result, there are low production yields for electrospun yarns. This has limited the majority of nanofibrous yarn to laboratory-based research settings.

Applications

Electrospun nanofibres have 1D ultra-fine morphology, controllable bulk and surface pores, the ability to align fibres in longer lengths or as shorter nanowires vertically on a substrate and a random web structure.

These properties mean electrospun nanofibres can be used in advanced applications, for example in smart sensors, actuators, electrolytes, electrodes, substrates, templates, absorbers and adsorbents¹⁴¹. Additionally, electrospinning could be a viable technology for functional textiles, as various layers could be sequentially deposited through appropriate process modification¹⁴².

Environmental applications

Owing to their electrostatic force and considerable specific surface area, electrospun nanofibres could be used to eliminate harmful ions, particulate matter and contaminants from polluted water and air¹⁴³. The interconnected porous architecture and porosity of nanofibres facilitates unhindered transport of purified solutions or air streams¹⁴⁴. Consequently, electrospun nanofibres are promising for environmental remediation strategies.

Air filtration. The industrial era has brought air pollution challenges, with an estimated annual death toll ranging from four million to nine million due to its direct and indirect consequences¹⁴⁵. Of par- ticular concern is the adverse impact of smaller-diameter pollutants on human health, for example by particulate matter measuring less than 2.5 µm (PM2.5), which can directly enter the respiratory system and accumulate within the lungs^146,147. These particles also include bacteria, viruses and other pathogenic agents as small as several hun- dred nanometres, which further increases morbidity and mortality rates^148,149. Traditional microfibre membranes have limited filtration efficiency for submicron or finer particles due to larger pore size structure¹⁵⁰. By contrast, electrospun nanofibre membranes have a large specific surface area, small pore size and high porosity, which can

effectively mitigate this issue¹⁵¹ (Fig. 5a). The trade-off between high filtration efficiency and low-pressure drop can be addressed by modu- lating the nanofibre membrane’s structure¹⁵², size^153,154, pore size¹⁵⁵ and porosity¹⁵⁶. An example comes from a multiscale nanoarchitectured nanofibre and carbon nanotube network. The network was constructed using a needleless electrospinning and spraying–netting strategy to produce a well-dispersed nanotube network (diameter ~25 nm), which was welded onto charged nanofibre scaffolds (diameter >100 nm) layer by layer. The resulting structure had high filtration efficiency (>99.994% PM0.3 removal) and low resistance (<0.05% atmosphere pressure)¹⁵⁷. More broadly, electrospun nanofibre membranes can offer individual protection and have the potential for source dust removal.

Water purification. Contamination of water sources by toxic heavy metal elements¹⁵⁸, dyes^159–161, fertilizers^162,163, crude oil spills¹⁶⁴ and microorganisms^165,166 threatens the safety of potable water. These pollutants are inherently resistant to degradation under natural conditions and require effective purification methods. Various approaches are available, but adsorption filtration has emerged as a convenient, non-secondary pollution and cost-effective strategy¹⁶⁷. Electrospun nanofibres have a large specific surface area, offering numerous adsorption sites and high adsorption capacity, making them promising for water purification (Fig. 5a). The physical and chemical adsorption efficiency of nanofibres can be enhanced by surface treatment or functionalization. For instance, nanofibre membranes combined with water-stable metal–organic framework particles have higher adsorption capacity for metal ions, such as Cd²⁺ and Zn²⁺ (ref. 168).

Nanofibre membranes can be readily given antimicrobial, antiviral^169,170, self-cleaning¹⁷¹ and catalytic degradation properties^172,173, making them suitable for long-term stable use and recycling. Additionally, seawater desalination is an effective avenue for sustainable freshwater procure- ment, using, for example, nanofibre reverse-osmosis membranes¹⁷⁴ and nanoporous evaporators¹⁷⁵. Atmospheric water harvesting is an alternative approach¹⁷⁶.

Biomedical applications

Tissue engineering. The extracellular matrix (ECM) is found in the majority of human tissues and organs. It has a complex 3D network structure of microfibres and nanofibres¹⁷⁷, meaning electrospun nanofibres can produce a biomimetic representation of the native ECM’s physical architecture¹⁷⁸. Components inherent to the natural ECM, such as growth factors^179,180, therapeutic agents^181,182 and antimicrobial agents¹⁸³, can be readily incorporated into nanofibres using coaxial or emulsion electrospinning. This creates a microenvironment conducive to cellular repair and regeneration. Living cells or materials can be encapsulated into the nanofibre networks using cell electrospinning, enabling bottom-up biological manufacturing^184,185 (Fig. 5b). However, there are challenges, such as stress-induced damage to living materials during the electrospinning process and the need to improve cell viability, metabolic activity and the proliferation rate.

Precise customization of the morphology and size of electrospun nanofibres enables the design of distinct tissue sites, enhancing the versatility and applicability of electrospinning across a wide range of medical scenarios¹⁸⁶. For instance, aligned nanofibres are more suitable for repairing tissues in tendons, ligaments and skeletal muscles than randomly arranged nanofibres^187,188. Anisotropic 3D heart scaffolds can be produced by layering multiple nanofibre layers with specific angular deviations¹⁸⁹. A wide range of naturally occurring, synthetic

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and hybrid polymers, which are biocompatible and biodegradable, can be used in electrospinning, mitigating the risk of immune rejection and secondary surgical removal^190,191.

Wound dressings. An optimal wound dressing should be breathable, antibacterial and biocompatible, absorb wound exudates and promote wound healing¹⁹². The remarkable porosity of electrospun nanofibres

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Fig. 5 | Applications of electrospinning in environmental, biomedical, wearable and other fields. a, Removal of particles with different sizes from polluted air and water by electrospun nanofibre membranes. b, Integration of growth factors, antibacterial agents and live cells or other living materials into nanofibre networks via electrospinning for wound dressings and tissues.

c, Biomimetic electronic skin that uses a surface energy gradient with hydrophilic

and hydrophobic properties, along with a push–pull effect, to achieve directional moisture-wicking. The skin also demonstrates excellent pressure-sensing capabilities, high sensitivity and performance as a single-electrode frictional nanogenerator. Its features make it applicable to self-powered medical sensing, such as precise pulse monitoring, speech recognition, gait recognition or sweat analysis. ECM, extracellular matrix.

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offers breathability for wound dressings, whereas the deformability enables absorption of wound exudates¹⁹³. Furthermore, the small pore size is advantageous for bacterial barrier protection, particularly when the inter-fibre pore size is <800 nm, which can effectively isolate more than 95% of bacteria¹⁹⁴. The potential to functionalize and modify electrospun nanofibre membranes means that antibacterial agents^195,196, anti-inflammatory drugs¹⁹⁷ and other compounds can be incorporated to encourage wound healing. The availability of raw materials and ease of modification means that electrospun nanofibre membranes are an attractive alternative to traditional wound dressings^198,199 (Fig. 5b).

Drug delivery. The high loading efficiency and surface to volume ratio of nanofibres make them ideal for drug delivery²⁰⁰. They can accommodate drug storage through core–shell structured nanofibres²⁰¹ or nanocap- sules. When facilitating controlled drug release, they can be placed on the epidermis^202,203 or implanted within the body, making them promising for cancer therapy applications²⁰⁴. However, it is challenging to control the release rate of hydrophilic drugs for long-term sustained release.

Wearable applications

Wearable devices need to accommodate skin movements²⁰⁵, provide com- fort and breathability²⁰⁶ and demonstrate biocompatibility²⁰⁷, while also having the desired mechanical, electrical and physicochemical properties. Electrospun nanofibres are a promising material for wearable devices as they are soft, lightweight and porous and can be easily modified²⁰⁸. Heat and humidity management. Textiles are important for maintaining human body thermoregulation, as they act as an interface between the body and its surroundings. Three categories of smart temperature regulation have been developed based on electrospun nanofibres.

The first is phase change nanofibres, which are produced through blending²⁰⁹, coaxial²¹⁰ or emulsion electrospinning techniques²¹¹, providing straightforward processing and less leakage of encapsulated phase change materials. The second category is super-material nanofibre membranes with efficient solar reflection. These membranes are fabricated using the electrospinning method with controllable morphology and simple nanoparticle doping, enabling continuous radiative cooling^212,213. The final category is to achieve warmth by enhancing the fabric’s insulation or reducing its infrared emissivity to minimize radiative heat loss²¹⁴. Thermal management alone is insuf- ficient for comprehensive human body thermoregulation. Sweat accumulation between the skin and textiles can cause discomfort due to increased heat and humidity²¹⁵. As a result, it is essential to facilitate rapid evaporation of sweat for additional heat removal. For example, designing textile structures with wettability differences between the two sides of the material enables unidirectional sweat transmission to the external surface²¹⁶ (Fig. 5c). This design feature promotes efficient sweat evaporation, further enhancing the cooling effect. Combined with the radiative cooling function of the electrospun superstructure, the sub-ambient temperature drops to approximately 6.5 °C (ref. 217).

Sensing. The intrinsic properties of electrospun nanofibres — low Young’s modulus, considerable surface area and network structure — make them well-suited for sensing applications²¹⁸. When designing electrophysiological sensors, minimizing the contact impedance between the skin and the electrode is crucial. The ultra-thin and flexible nanonetwork can conform to intricate skin deformations and adheres close to the body surface, facilitating sensitive monitoring of subtle signals, including the electroencephalogram²¹⁹, electrocardiogram²²⁰

(Fig. 5c), electromyogram²²¹, pulse and body movement. For example, ultra-thin nanofibre-based pseudocapacitive sensors with conduc- tive networks on both sides have exceptionally high motion detector sensitivity, up to 79.5 kPa⁻¹, which is attributed to the microstructure of the nanofibre²²². The elevated surface free energy of the nanofibre allows for easy adsorption of sensing components, such as enzymes²²³, antibodies²²⁴ and aptamers²²⁵. The capillary effect of the nanofibre enhances rapid mass transfer of analytes, yielding high sensitivity, specificity and prompt response for biosensing applications. These properties can also be used for wearable plant sensors that monitor a plant’s temperature, humidity and nutrient levels²²⁶. Incorporating functional nanomaterials improves the catalytic activity and sensitivity of sensors²²⁷. Altogether, electrospun nanofibres exhibit considerable potential for non-invasive or implantable deep-level health monitoring.

Energy harvesting and storage devices. Wearable sensing devices with long-term stable operation need to harvest energy from the surrounding environment²²⁸ to avoid cumbersome and inflexible power sources.

Electrospinning has been extensively used to fabricate nanogenerators, such as piezoelectric^229–232, triboelectric^233,234 (Fig. 5c), thermoelectric²³⁵ and hydroelectric generators^236,237, as well as solar cells^238,239, biofuel cells²⁴⁰ and hybrid energy harvesting systems^241,242. Nanostructured electrode materials derived from electrospun nanofibres have a large electrode/electrolyte contact area, small electron to ion transport lengths and adjustable electrolyte wettability. This leads to unique electrochemical reactions that do not occur in bulk or micrometre-sized counterparts²⁴³. Lithium²⁴⁴, zinc²⁴⁵, sodium²⁴⁶ and magnesium²⁴⁷ ion batteries based on electrospun nanofibres have reduced ion diffusion pathways and enhanced ion conductivity. Consequently, they can be used as advanced electrode and battery separator materials^248,249 for high-performance rechargeable batteries. Electrospun materials can be used as flexible electrodes²⁵⁰, enabling production of flexible batteries, which are essential for lightweight wearable devices^251–253. Fuel cells with improved cell voltage and power density can also be produced by electrospinning²⁵⁴. Supercapacitors fabricated from electrospun nanofibres have improved charge density and capacitance, which can be used for enhanced energy storage capabilities^255,256.

For electrical applications, such as charge transport or storage media, the 1D nature of nanofibres allows directed charge transport and cylindrical charge assembly, providing more effective anisotropic behaviour than in 3D isotropic structures. However, as a charge transport medium, the random web structure is undesirable if charges are collected vertically. A longer passage through the length of the fibre increases characteristic resistances and charge recombination, leading to worse performance indicators. In such cases, inorganic fibres could be broken down into shorter nanowires to improve transport efficiency.

Breaking down nanowires can lead to the discovery of band-edge type electron diffusion^257,258. If a photoactive material, such as a dye or perovskite, is dispersed in a suitable polymer or conjugated polymer blend and electrospun into a fibre cloth, the resulting structure will have a lower surface reflection. This enhances the multiple internal reflections in the photoactive layer, producing efficient, lightweight, flexible and stable solar fabrics that can be applied to smart clothing, drones or mini-satellites^259,260. For charge storage, by using electrospun nanofibres as electrodes, the cycling stability can be improved as the larger aspect ratio of the fibres can accommodate volume changes during the charge/discharge process. Using traditional annealing techniques, electrospun nanofibres can crystallize without coalescing and losing their morphology, further improving the cycling stability²⁶¹.