Electrospinning and Electrospun Nanofibers: Process, Properties and Uses

Last Updated on 22/04/2021

Electrospinning Technology:
There are various processes available to generate nanofibers. These processes include template synthesis, phase separation, and self-assembly. However, electrospinning is the simplest, most straightforward, and cheapest process of producing nano- and micro-sized fibers in a very short period of time with minimum investment. Electrospinning is an experimental setup was outlined for the production of polymer filaments using electrostatic force. When used to spin fibers this way, the process is termed as electrospinning. In other words, electrospinning is a process that creates nanofibers through an electrically charged jet of polymer solution or polymer melt. Electrospinning has emerged as a specialized processing technique for the formation of sub-micron fibers (typically between 100 nm and 1 μm in diameter), with high specific surface areas. Due to their high specific surface area, high porosity, and small pore size, the unique fibers have been suggested for wide range of applications.

Electrospun Nanofibers
Figure 1: Electrospun Nanofibers

Electrospinning technology has been known since twentieth century. Electrospinning is an old but yet immature process which is now used to form nanoscale polymer fibers. It is a relatively simple method to produce submicron fibers from solutions of different polymers and polymer blends. Electrospinning uses an electrical charge to draw very fine fibers from a liquid. Electrospinning shares characteristics of both electrospraying and conventional solution dry spinning of fibers. We can say electrospinning is an established method of producing nanofibers from a wide variety of natural and synthetic polymers.

Conventional fiber spinning techniques such as wet spinning, dry spinning, melt spinning and gel spinning usually produce polymer fibers with diameters down to the micrometer range. If the fiber diameter is reduced from micrometers to nanometers, very large surface area to volume ratios are obtained and flexibility in surface functionalities and better mechanical performance may be achieved.

Because of the small pore size and high surface area inherent in electrospun textiles, these fabrics show promise for use in protective clothing for soldiers filtration applications, membranes, reinforcing fibers in composite materials, optical and electronic applications, biomedical devices (cosmetics, skin healing and skin cleansing, wound dressing, drug delivery and pharmaceuticals, supports for enzymes or catalysts, scaffolds for tissue engineering, and templates for the formation of hollow fibers with inner diameters in the nanometer range.

Important features of electrospinning are:

  1. Suitable solvent should be available for dissolving the polymer.
  2. The vapor pressure of the solvent should be suitable so that it evaporates quickly enough for the fiber to maintain its integrity when it reaches the target but not too quickly to allow the fiber to harden before it reaches the nanometer range.
  3. The viscosity and surface tension of the solvent must neither be too large to prevent the jet from forming nor be too small to allow the polymer solution to drain freely from the pipette.
  4. The power supply should be adequate to overcome the viscosity and surface tension of the polymer solution to form and sustain the jet from the pipette.
  5. The gap between the pipette and grounded surface should not be too small to create sparks between the electrodes but should be large enough for the solvent to evaporate in time for the fibers to form.

Production of Nanofibers by Electrospinning Process:
Electrospinning constitutes a unique technique for the production of nanofibers with diameters down to the range of a few nanometers. Electrospinning is applied predominantly to polymer – based materials including natural and synthetic polymers, but it has been extended towards the production also of metal, ceramic and glass nanofibers exploiting precursor routes. The production either of individual fibers, of random nonwovens or of orientationally highly ordered nonwovens is achieved by an appropriate selection of electrode configurations. Basic features of electrospinning – keeping in mind that the technique is a highly complex process in terms of theory and experiment – can best be introduced using a simple spinning setup and performing simple model experiments with this setup.

The setup:
To do a first electrospinning experiment it just takes

  1. A simple syringe with a metal tip having characteristically an inner diameter of several hundred micrometers;
  2. A solution of a commercially available polymer such as polyethylene oxide (PEO) in a solvent like water choosing, for instance, a concentration of 10 w% PEO in water;
  3. A weight that presses the shaft of the syringe down to slowly discharge the polymer solution through the syringe acting now as a die – or a container with the solution connected via flexible tubing to the syringe so that the container can be positioned well above the syringe using gravity in this case to control the discharge of the polymer solution through the die;
  4. An aluminum foil positioned below the tip of the syringe at a distance of a few centimeters;
  5. A high – voltage appliance able to deliver about 1 kV with the required current well below the microampere range; as shown schematically in Figure 2.
Schematic diagram of electrospinning system
Figure 2: Schematic diagram of electrospinning system

The device described so far will just produce droplets falling off the tip of the syringe and impinging on the aluminum foil if no voltage is applied. However, as a sufficient voltage is applied between the tip of the syringe acting as an electrode and the aluminum foil acting as counterelectrode droplet formation is reduced and the formation of a slim fluid jet sets in that falls towards the counterelectrode. There, it is deposited in collector as a solid fiber due to the evaporation of the solvent.

Properties of Electrospun Nanofibers:
Two forms of electrospun nano fibers were produced by normal and aligned electrospinning methods. The randomly aligned fiber mats were collected on a large, flat, grounded target, while the aligned fibers were created by parallel collecting electrodes. Electron microscopy was used to illustrate the morphology of the electrospun fibers. The results showed that higher concentration could produce larger fibers due to more polymer chains and chain entanglements.

It is difficult to measure mechanical properties of each electrospun single nano-fiber with existing test techniques, because of their very small diameters. Therefore, mechanical tests were performed instead on nano-scale nonwoven webs with conventional testing methods. Mechanical properties of electrospun PU nanowebs were investigated by Pedicini and Farris.

Among the many electrospun polymers reported in the literature are poly (p-phenylene terephthalamide), tri-block copolymers, polyethylene oxide and DNA from solution; and polyethylene and polypropylene from the melt. Nylon was the first commercialized synthetic fiber and is used throughout the world in many applications. It has been widely used as an important engineering plastic and synthetic fiber because of its good mechanical properties. It has been produced by traditional methods such as melt, wet and dry spinning and is available in staple, tow, monofilament and multifilament forms. Fiber diameters produced by these methods range from 10 to 500 µm.

Nylon-6,6 (N6,6), polybenzimidazole (PBI) and poly (tetrafluoroethylene) membranes produced from electrospun fibers as protective layers. Properties of these electrospun membranes, including structural effects upon moisture transport, air convection, aerosol filtration, porosity and tensile strength. N6, the polymer crystalline structure was altered from α to γ form when electrospun.

The ability of the electrospinning process to produce the γ form implies that the fibers are under high stress when they are being formed. Nylon-12 has only one preferred conformation, and the chain conformation is conserved after processing.

Mechanical properties of two widely different molecular weight electrospun N6,6  nanowebs are compared by using conventional test methods. The main objective of this part of the study was to determine whether the use of high molecular weight N6,6 is a viable approach to improve the mechanical properties of electrospun nylon filaments.

Advantages of Electrospun Nanofibers:
There are several advantages to electrospun nanofibers, including:

  1. Many flame-resistant polymers, such as polyetherether ketone (PEEK), polyvinyl chloride (PVC), polyacrylonitrile (PAN), and polystyrene (PS) can be electrospun.
  2. The surface area of nanofibers is 10010,000 times greater than that of conventional fibers.
  3. The noise-absorption rate in nanofibers is expected to be exponentially higher because of the interaction of air molecules of sound waves with the fiber surfaces.
  4. The overall weight of materials used for many industrial applications is less.
  5. Nanofibers can enhance the physical properties of composites.
  6. Nanofibers can be electrospun on both composite and metal surfaces.
  7. Adhesives can be added to polymers to improve the adhesion between the fiber and the surface.
  8. Electrospinning is an economical and technologically mature method for bulk production for different industries.

Uses of Electrospun Nanofiber:
Electrospun nanofibers are usually used for different nanotechnology and biotechnology applications, including cancer and cardiovascular diseases.

Electrospun nanofibers are characterised with large surface area to volume ratio, a beneficial property for application as scaffolds, sensors, filters, membranes, batteries, protective clothing, wound dressing and catalyst.

Electrospun nanofibers have a large surface area per unit mass, these fibers can be used in various fields: filtration and separation of micron-, submicron-, and nanosize organic, inorganic, and biological particles; HF antenna fabrication; light-weight, colorful, and invisible fabric productions; transistor; solar and hydrogen energy; and biomedical applications, such as wound dressings, tissue engineering scaffolds, and artificial blood vessels. Other promising areas of electrospun nano-fibres include nanocomposite fabrications to improve crack resistance and reduce interior aircraft noise.

Recent studies have indicated a number of different ways of making electrospun nanofibres used in textile industries fire-retardant. Using electrospinning methods, different forms (e.g., polymeric, metallic, ceramic, and composite) of nanofibers can be produced and used as flame retardant materials.

Recent studies showed that electrospun nanofibers could be used for the filtration of beverages, drinks, and some other juice products because of the high surface area and porous structure of nanofiber membranes.

References:

  1. Nanofibers and nanotechnology in textiles Edited by P. J. Brown and K. Stevens
  2. An Introduction to Electrospinning and Nanofibers by Seeram Ramakrishna, Kazutoshi Fujihara,
    Wee-Eong Teo.Teik-Cheng Lim & Zuwei Ma
  3. Electrospinning of Nanofibers in Textiles by A. K. Haghi
  4. Synthesis and Applications of Electrospun Nanofibers by Ramazan Asmatulu and Waseem S. Khan
  5. Filtration Properties of Electrospinning Nanofibers, Xiao-Hong Qin, Shan-Yuan Wang
  6. Mechanical and Physical Properties of Electrospun Nanofibers, ZHANG SHU.
  7. Functional Applications of Electrospun Nanofibers Jian Fang, Xungai Wang, and Tong Lin
  8. www.en.wikipedia.org/wiki/Electrospinning

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