Introductory Chapter: The Electrospinning

1. Introduction

The production of nanofibers from a polymer solution or melt may be accomplished using an electric field called electrospinning. Due to the peculiar characteristics of the nanofibers produced by this procedure, there has been a significant interest in various areas, including medicine, biotechnology, and materials science [1]. Originally conceived as a theoretical concept, electrospinning has evolved into a versatile and widely used process with diverse applications across several sectors. Due to its ability to produce nanofibers with exact control over their structure and composition, this method benefits researchers and industries [2].

2. History of electrospinning

L.J. Cohn and J.A. Sprague first proposed the process of electrospinning in 1934, and they also secured a patent for it. At the same time, Formhals also obtained a separate patent for a method that closely resembled electrospinning [3, 4]. However, the first applications and understanding of electrospinning were limited, and the technique did not get much attention from the research community at that time. Taylor’s work in the 1960s had a significant role in the progress of electrospinning by developing a model for the hopper configuration, where an electric field produces liquid droplets [5, 6]. His partnership with JR Melcher resulted in the creation of the “Leaky Dielectric Model” for conductive liquids [7].

Electrospinning gained practical importance in the 1990s as a consequence of researchers creating more controlled techniques and beginning to investigate applications in a range of sectors [8, 9]. In particular, Darrel Reneker and Gregory were able to effectively decrease a variety of organic polymers to the nanoscale by the use of electrospinning [10, 11]. In order to manufacture ultrafine fibers with dimensions that are smaller than 5 micrometers, Darrel Reneker was the pioneer who introduced the method of submitting a polymer dispersion to high voltage levels. This was the beginning of the technology’s rise to prominence. In addition, during this time period, advancements in polymer science and materials engineering led to the creation of electrospinning as a method that had the potential to be regarded as practical [12].

Electrospinning is an intriguing technique used to produce fibers with advantageous structural characteristics, as seen in Figure 1.

Figure 1.

Different types of fibers synthesized via electrospinning method.

3. Fibers manufactured by electrospinning

3.1 Porous fibers

These fibers are known as porous fibers because they have holes or empty areas within their structure. The size and form of the pores further define their practical applicability, but in general, porous fibers are used in filtering because of their capacity to trap particles [13].

3.2 Core-shell fibers

The core of these fibers is made of a distinct substance, while the shell is made of a different material. Due to the fact that each component may contribute its own unique traits, this structure makes it possible to create one-of-a-kind combinations of attributes [14].

3.3 Helical fibers

A twisted or coiled structure is characteristic of fibers that are spiral or helical in form. It is possible for the fibers to acquire intriguing mechanical and optical characteristics as a result of this winding [15].

3.4 Hollow fibers

The structure of hollow fibers is characterized by the presence of voids or empty areas. The empty area may be filled with a particular material, making this design ideal for applications, such as medicine administration, where it can be filled with specific substances [16].

3.5 Multichannel fibers

Multi-channel fibers, often known as fibers having several routes or channels, provide unique capabilities that are not found in other fiber kinds. Due to their versatility, multichannel fibers provide a promising and adaptable foundation for many technological applications. This is especially accurate in fields where meticulous regulation of fluid flow or sensing is crucial. Researchers persist in exploring and developing innovative uses for these fibers in many applications and areas of research [17].

4. Basic electrospinning methods

There are a few distinct methods that are used in the manufacturing of fibers from polymers. These methods include wet spinning, dry spinning, liquefying, and gel spinning. Every approach has its own set of qualities, benefits, and applications that are unique to him. Following is a summary of the distinctions that exist between these various spinning processes:

4.1 Wet-spinning

Extruding a polymer solution through a spinneret and into a coagulation bath is the process involved in wet spinning. It is the presence of a non-solvent for the polymer in the coagulation solution that causes the polymer to solidify and form fibers. The use of wet spinning is advantageous because it may be used for polymers that are difficult to dissolve in conventional solvents. The manufacture of fibers with high strength is made possible as a result. Wet spinning is often used in the production of rayon and acrylic fibers, among other applications [18].

4.2 Dry spinning

The process of dry spinning involves the extrusion of a polymer solution into a heated air chamber by use of a spinneret. The fibers become hardened when the solvent evaporates, leaving behind the fibers. In the case of polymers that are capable of being dissolved in volatile solvents, dry spinning is an advantageous technique. It enables the manufacture of fibers that have certain qualities, such as porosity, among other characteristics. In the context of applications, dry spinning is often used for the production of acetate fibers and some kinds of polyesters [19].

4.3 Melt spinning

Melting a polymer and then extruding it via a spinneret is the process that is involved in the liquefying process, which is also referred to as melt spinning. When the polymer is allowed to cool, it becomes solid and forms fibers. A number of advantages include the fact that liquefying is appropriate for thermoplastic polymers that may be melted without suffering any chemical transformations. This is a basic procedure that has relatively minimal costs associated with manufacturing. Molten spinning is often used in the production of nylon, polyester, and polypropylene fibers, among other applications [20].

4.4 Gel spinning

Gel spinning is a process that includes the creation of a gel from a polymer solution or melt. The gel generated by this process is then pulled or stretched in order to align the polymer chains. High-strength fibers are formed by the gel that has been aligned and then hardened. Gel spinning results in fibers that have good strength and modulus, which is a significant advantage. It is possible to improve the mechanical characteristics of the material by aligning the polymer chains during the drawing process. The production of high-performance fibers such as aramid fibers (e.g., Kevlar) is often accomplished by the use of gel spinning [21].

5. Variation in electrospinning

Electrospinning may be categorized into various variations and types, each designed for specific purposes or desired outcomes. The selection of the electrospinning technology is based on the desired characteristics of the nanofibers, including their composition, structure, shape, and specific parameters relevant to the intended application.

5.1 Conventional electrospinning

This is the most basic kind of electrospinning, in which a polymer solution or melt is delivered to a spinneret using a syringe pump. A high voltage is delivered between the spinneret and a grounded collector, causing nanofibers to develop [22].

5.2 Coaxial electrospinning

The process of coaxial electrospinning includes the simultaneous spinning of several fluids with the use of needles that are either concentric or coaxial. Through the use of this technique, it is possible to create core-shell structures, in which one component acts as the core and another substance encloses it as a shell [23].

5.3 Needleless electrospinning

Needleless electrospinning employs a spinneret-free setup, such as a rotating disk or drum, instead of the conventional spinneret utilized in the process. This approach is highly suitable for large-scale production and has the capacity to streamline the electrospinning process [24].

5.4 Emulsion electrospinning

When one liquid is suspended in another in the form of tiny droplets, the result is an emulsion. One method for creating droplets within a continuous phase is emulsion electrospinning. This technique makes use of emulsions, which are combinations of two liquids that do not readily blend together. As the liquid evaporates, the electric field induces the formation of fibers [25].

5.5 Rotary jet-spinning

The production of nanofibers is accomplished by the use of a spinning spinneret in this method. A centrifugal force is generated as a result of the spinneret’s spinning, which makes it possible to deposit fibers on a collector in a regulated manner [26].

5.6 Bubble electrospinning

The electrospinning process is altered by the incorporation of a gas using the bubble electrospinning technology. Depending on the gas and polymer solution mixture, this process might potentially lead to the formation of porous structures or hollow fibers [27].

5.7 Melt electrospinning

The use of molten polymer rather than a polymer solution is what is involved in the process of melt electrospinning. As the molten polymer reaches the collector, it undergoes the process of electrospinning, which results in the formation of fibers [28].

5.8 Side-by-side electrospinning

Side-by-side electrospinning is a technique that involves electrospinning two or more polymer solutions concurrently via separate channels, yet the resulting solutions are collected together. This enables the development of composite fibers that include components that are unique from one another [29].

5.9 Multi-jet electrospinning

A multi-jet electrospinning setup uses a number of nozzles or jets to electrospin several polymer solutions simultaneously. Complex fiber structures may be created using this technique [30].

6. Electrospinning process configuration

An electrospinning setup consists of three primary components: (i) a voltage source, (ii) a metal fiber, and (iii) a semiconductor collector [31]. Both of these components are essential to the process. It is possible to further subdivide the semiconductor collector into three primary groups, which are as follows: (a) fixed flat plate collectors, (b) rotating drum collectors, and (c) rotating disk collectors [32].

7. Working principle

At the location of the liquid droplets created at the needle’s tip, an adequate voltage is applied. The liquid gets charged. Electrostatic repulsion counteracts the surface tension. Thus, the droplet is stretched, and at a critical point, a stream of liquid bursts from the surface and spirals into a cone-like structure termed a “Taylor cone.” Once the Taylor cone is constructed, the fluid jet is directed to the metal collector. The formation of solid fibers may be attributed to the liquid’s thickness, cooling, or evaporation. The solvent’s swirling action causes the Taylor cone to evaporate off the collector during flight, coating the collector with a substance other than fiber [33, 34, 35].

8. Parameter optimizations for the electrospinning process

The quality of electrospun fibers generated from polymer solutions may be influenced by critical parameters, which can be classified into three subclasses as given in Figure 2 [36]:

1. Solution parameters

2. Process parameters

3. Environmental parameters

Figure 2.

Optimizing parameters for electrospinning of fibers.

8.1 Solution parameters

A lot of things about the solution affect the properties of electrospun fibers made from polymer solutions. These include how fast the solvent evaporates, how thick the polymer solution is, how concentrated it is, and how high the surface tension is [37]. The concentration and viscosity of the produced fiber have a direct proportional relationship and have comparable effects. The creation of a bead is attributed to a polymer solution with low viscosity or low concentration, while a polymer solution with greater viscosity and concentration results in fibers with a larger diameter. Typically, raising the polymer solution concentration leads to a larger fiber diameter. The polymer solution exhibits bead formation due to its elevated surface tension, whereas a decrease in surface tension promotes the creation of smooth fibers. The polymer’s concentration, surface tension, and solvent viscosity influence the electrons’ spin rate and the fibers’ shape [38].

8.2 Process parameters

The surface tension of the polymer solution has a significant impact on the critical voltage, which is the voltage at which the charged jet begins the electrospinning process. The diameter of the electrospun fibers exhibited an inverse relationship with the applied voltage, such that lowering the voltage resulted in an increase in diameter, and vice versa. The primary factor influencing fiber solidification is the distance between the tip and the collector since it determines the amount of time the threads have to dry before reaching the collector. An insufficient gap encourages the development of beads, while a greater length yields bead-free fibers [39].

8.3 Environmental parameters

Humidity and temperature are regarded as factors that are distinctive to the environment. The characteristics of electrospun fibers are influenced indirectly by these factors. By raising the temperature, the rate of evaporation rises, leading to an elevation in viscosity and concentration. Consequently, this promotes the creation of fibers with a greater diameter. Furthermore, the viscosity of the solution often reduces as the temperature rises. With an increase in humidity, there is a corresponding rise in the average diameter of the fibers [40].

9. Applications of electrospun nanofibers

Electrospun nanofibers have been used in a broad variety of sectors as a result of their exceptional characteristics, which include a large surface area, a tiny diameter, and a shape that can be adjusted. The many domains in which electrospun nanofibers are used are outlined in Figure 3.

Figure 3.

Applications of electrospun nanofibers.

Electrospun nanofibers have a number of significant uses some notable applications of electrospun nanofibers include:

9.1 Biomedical

Electrospun nanofibers have the potential to serve as drug delivery systems, offering precise and regulated release of medications while enhancing their bioavailability. Nanofibrous scaffolds are made to look like the extracellular matrix. This makes it easier for cells to attach and for tissues to grow back in areas like bone tissue engineering and wound healing. Electrospun nanofibers may be used to generate wound dressings that are both permeable and very porous. These dressings help encourage healing by maintaining a moist environment and avoiding bacterial infection.

9.2 Textile

Nanofibers may be introduced into textiles to offer new capabilities, such as resistance to water, antibacterial characteristics, and better breathability. These functionalities can further enhance the textiles’ overall performance. Electrospun nanofibers have the potential to improve the protective characteristics of clothing, making it more resistant to the effects of environmental conditions, diseases, and toxins.

9.3 Defense

Electrospun nanofibers’ unique qualities make them useful in defense and military technology. Ballistic textiles made using nanofibers are lightweight, flexible, and projectile-resistant. Electrospun nanofibers may be functionalized to make chemical- and biological-resistant garments. Gas filtration using nanofibrous membranes protects military troops from harmful gasses and chemical warfare weapons. Electrospun nanofibers enhance air quality in confined military areas by filtering particulates. Nanofibers can make lightweight, flexible communication antennas. They may be used as sensors to provide real-time environmental data. Electronic warfare methods like signal jamming and interference may use electrospun nanofibers. Nanofibers having radar-absorbing or deflecting qualities may be used in military stealth equipment. Electrospun nanofibers may be utilized to make antibacterial wound dressings for speedier field injury recovery.

9.4 Filtration

Nanofibrous membranes are well-suited for air and water filtration applications due to their expansive surface area and minuscule pore size. These membranes have the ability to extract particles and impurities from both the air and water. Electrospun nanofibers may be used for the specific separation of oil and water, making them potentially valuable in environmental cleanup and industrial applications.

9.5 Sensor

Nanofibers may be functionalized for sensing applications, which can detect changes in temperature, humidity, or certain chemical compounds. This technology applies to the field of sensor technology. Flexible Electronics: Electrospun nanofibers have the potential to be used as components in electronic devices that are both flexible and lightweight. These devices include capacitors and sensors.

9.6 Energy storage

Nanofibers have the potential to be exploited in energy storage devices, such as fuel cells and batteries, in order to enhance the performance of electrodes and the efficiency with which they conduct energy. Nanofibrous materials synthesized via the electrospinning approach have the potential to increase the efficiency of solar cells by offering a large surface area for the absorption of light and by enhancing the movement of electrons.

Nanofibers that have been electrospun are becoming more versatile, and current research efforts are helping to broaden their uses across a variety of sectors. Because they may be modified in terms of their qualities and usefulness, they are very useful in tackling unique difficulties that are present in a variety of sectors.

10. Future prospects

Electrospinning is a method for creating a wide range of one-dimensional materials with adjustable structural properties. A lot of ground has been covered, but we still have a ways to go. Attaining consistent size and shape in nanofibers, while simultaneously satisfying the demands of end-users continues to pose a significant challenge. There are fewer diverse uses for natural polymers because they lack the mechanical and chemical characteristics of synthetic polymers. Researchers are working on hybrid polymer systems that include synthetic and natural polymers to address this. These systems are electrospun and provide improved functionality, particularly in biotechnology. In tissue engineering, adding nanoparticles after electrospinning therapy will increase its effectiveness. Studying the interactions between nanofiber designs and various drug-related release patterns is crucial. There are many unanswered questions and untapped potential applications for electrospun nanofibers in the energy sector, in filtering applications, and as sensors. To be useful, nanofiber membranes need upgrades to their surface area, pore diameters, and other properties.