Multi-jet electrospinning methods.
Electrospun nanofibers are being used in a variety of performance apparel applications where their unique properties add to their functionality. Those properties include, small fiber diameter, high surface area, potential to combine chemistry, layer thinness, high porosity, filtration properties, and low basis weight. Electrospinning has been considered as an efficient technique for nanofiber web formation. Polymers have been electrospun into nanofibers mostly after being dissolved in solvent and melted. This chapter presents a comprehensive summary of existing electrospinning methods. Electrospinning methods are classified into different categories depend upon jet formation.
- needleless electrospinning
Nanotechnology denotes to the science and designing concerning materials, structures, and gadgets which at least one of the dimension is 100 nm or less. Electrospun nanofiber webs can be modified to a desired porous structure, and in these are a huge range of polymers that can be used to make nanofibers . The unique combination of high surface area, low weight, flexibility, and porous structure enables us to control the water resistance level, vapor transmission, and air permeability rate. Consumption of nanofibers in the world is shown in Figure 1. Applications of electrospun nanofibers, as shown in Figure 2, include tissue engineering scaffolds , filtration , catalyst and enzyme carriers [4, 5], release control , sensors , energy storage , affinity membranes , recovery of metal ions [10, 11, 12], wound healing , reinforcement , protective clothing [15, 16], and energy conversion and storage .
1.1 Various ways to make nanofibers
Nanofibers can be processed by a number of techniques such as:
Commonly nanofiber fabrication is done by electrospinning method .
In drawing process, single nanofiber is attended by stretching of polymer that is in solution form. With this method only viscoelastic materials have been spun into nanofibers. If polymer is in melt form, then cooling system is necessary to solidify the fiber. On the other hand, if polymer is in solution form, then heating mechanism is necessary to evaporate the solvent. This is a very slow process that is suitable only for lab scale . Process diagram is shown in Figure 3.
1.1.2 Template synthesis
In this method, nonporous membranes are used in which pores are in cylindrical form. Diameters of these pores are uniform. Solid polymers are formed that have diameter equal to size of porous . The process diagram is shown in Figure 4.
1.1.3 Phase separation
In this process, five steps are involved: polymer dissolution, polymer gelation, extraction of solvent, freezing, and freeze-drying. Fiber dimensions are not controllable with this process. This proves only suitable for lab scale . The process diagram is shown in Figure 5.
Instruments used for electrospinning are given below:
High-voltage DC power supply
Spinneret (a small diameter needle connected to the syringe)
The polymer is dissolved in a solvent before electrospinning, and when it is completely dissolved, it forms polymer solution. The polymer fluid is then introduced into the syringe tube for electrospinning. The positive terminal of the DC power supply is connected to the hollow needle , and the negative terminal is connected to the metal collector. With the increase of intensity of the electric field, the repulsive electrostatic force overcomes the surface tension, and the charged jet of the fluid is ejected from the tip of the Taylor cone. The discharged polymer jet undergoes an instability and elongation process, which allows the polymer in the jet to become very long and reduces the diameter of the extruded polymer fiber. The solvent that is used to dissolve the polymer evaporates, and the polymer in the jet is dried. The solvent evaporation depends on the distance between the tip and collector, the solution vapor pressure, and the inside chamber temperature. Stable environmental conditions are therefore important in getting good quality nanofibers. The maximum applied voltage for a needle electrospinning setup is normally less than 30 kV and is also highly humidity dependent . Figure 7 illustrates the schematic diagram of the complete electrospinning setup.
Needleless electrospinning presented as an option electrospinning innovation that deliver nanofibers on a substantial scale. Needleless electrospinning is included as electrospinning of nanofibers straightforwardly from an open fluid surface. Many planes are shaped at the same time from the needleless fiber generator (spinneret) without the impact of capillary effect that is regularly connected with needle electrospinning. Since the fly start in needleless electrospinning is a self-composed process which happens on a free fluid surface, the spinning process is hard to control. In needleless electrospinning process, many shapes of spinneret have been invented that have different levels of production. Figure 8 illustrates a schematic diagram of the complete needleless electrospinning setup.
One of the problems also created with needle electrospinning method is low production rate that is typically less than 0.3 g/h . With needleless electrospinning method, production of nanofibers is 250 times  more than needle electrospinning. Production depends upon the shape of spinneret used in needleless electrospinning. With different shapes of spinnerets, production rates of 2.5–100 g/h can be achieved.
Different needleless setups, like conical wire coil electrospinning spinneret , edge-plate electrospinning setup , splashing electrospinning setup , rotary cone , roller electrospinning process , cylinder , disk , and spiral coil electrospinning processes , were made for large-scale production of nanofibers. In all these needleless setups, the spinneret shape is different. Due to this variation in spinneret shape, nanofiber production rate and fiber morphology is different.
1.2 Production of nanofibers
Based on the jet formation and the way of using the needles, electrospinning methods can be classified as:
Multi-jet electrospinning methods
Multi-needle electrospinning methods
Needleless electrospinning methods
1.2.1 Multi-jet electrospinning methods
In this electrospinning method, multi-jets were used for nanofiber formation. Production of nanofiber increased as compared to needle spinning. Due to multi-jets, uniform web of nanofiber is not formed; this is due to repletion effect between jets. Some multi-jet electrospinning methods are given in Table 1.
1.2.2 Multi-needle electrospinning methods
In multi-needle electrospinning method, a number of needles are used as spinnerets that contain one or different types of polymer solutions. High voltage is applied to the tip of the needle and nanofibers are deposited on collector. The main advantage of multi-needle electrospinning is we can mix different polymers at our required ratio (Table 2).
1.2.3 Classification of electrospinning methods
It may be defined as the method in which fiber jets are produced or generated from the free surface of liquid. It can also be defined as the technique of producing the fibers from open liquid surface. Based on the fiber generating method, the motion of spinneret, and collection direction of fibers, the needleless electrospinning techniques can be classified as:
126.96.36.199 Free surface spinning method
188.8.131.52.1 Bubble spinning methods
In bubble spinning method, air is supplied from porous surface that is placed at the bottom of polymer solution. Bubble is made at the surface of polymer solution, and jet is formed at the charged surface of the bubble. Very fine nanofibers are deposited on collector that is placed at the top of the polymer container (Table 3).
184.108.40.206.2 Free solution spinning methods
In this electrospinning setup, two layers were used: the lower layer was ferromagnetic and the upper layer was polymer solution (Table 4). When electrospinning process was started, a customary electric field was utilized to the gadget; steady vertical spikes of attractive suspension were formed. When high voltage was applied, electrified jets undertake strong stretch by the electric field, solvent evaporates, and solidified nanofibers deposit on the upper counter electrode .
220.127.116.11 Spinneret spinning methods
18.104.22.168.1 Stationary spinnerets
In this needleless electrospinning method, stationary spinneret is used for nanofiber generation. High voltage is applied to spinneret, and there is special mechanism to feed polymer solution on spinneret. Polymer jets are formed on the edges of stationary spinneret that produce nanofibers. Stationary spinnerets are further classified into three categories depending upon the spinneret position:
Horizontal stationary spinnerets.
Downward stationary spinnerets.
Upward stationary spinnerets.
22.214.171.124.1.1 Horizontal stationary spinnerets
In horizontal stationary spinnerets, polymer jets are formed in horizontal direction. Nanofibers are collected on vertical mounted plates. Some of these are given in Table 5.
126.96.36.199.1.2 Downward stationary spinnerets
In downward stationary spinnerets, nanofibers are made in downward direction. Polymer solution is placed in shower-like spinnerets, polymer jets are stretched downward due to high voltage, and nanofibers are collected on the plate that was placed in the bottom. Some of these types of spinnerets are described in Table 6.
188.8.131.52.1.3 Upward stationary spinnerets methods
In this needleless electrospinning setup, a novel spinneret was used that have a stepped pyramid shape (Table 7). When electric field was applied to the system, then nanofibers were generated from the edges of stepped pyramid-shaped spinneret. These nanofibers were collected on the collector that was negatively charged, placed at the top of the spinneret. Nanofiber production increased by increasing applied voltage and keeping working distance and concentration of polymer solution constant .
184.108.40.206.2 Rotating spinnerets
In this needleless electrospinning method, the spinneret is rotated in polymer solution that licks polymer solution into its surface. When high voltage is applied to the spinneret, polymer jets are formed on the surface of spinneret, and nanofibers are formed that are deposited on the collector. Rotating spinnerets are further classified into three categories that are given below:
Horizontal rotating spinnerets
Downward rotating spinnerets
Upward rotating spinnerets
220.127.116.11.2.1 Horizontal rotating spinnerets
In this setup a metal roller was used as spinneret that was connected with high-voltage power supply (Table 8). Polymer solution was splashed onto metal roller through a hole of the solution provider that was placed above the metal roller spinneret. Nanofibers were collected on a metal collector that was placed horizontally .
18.104.22.168.2.2 Downward rotating spinnerets
In this electrospinning setup, polymer solution was continuously fed to the rotating cone by a tube (Table 9). This cone was connected with positively charged applied voltage. When high voltage was applied to cone, nanofibers were generated from the edges of the cone. These nanofibers were collected on a negatively charged collector placed in a downward direction .
22.214.171.124.2.3 Upward rotating spinnerets
This kind of spinnerets is used for more production and uniform nanofiber web formation. Polymer solution is placed in tub and spinneret is rotated in solution. Polymer solution layer is formed on the surface of spinneret. When high voltage is applied to spinneret, then polymer jets are formed that produce nanofiber. These nanofibers are moved in an upward direction and deposited on the collector (Table 10).
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