Electrospinning of Nanofibers | Applications of Electrospun Nanofibers

Last Updated on 30/05/2021

Electrospinning of Nanofibers | Applications of Electrospun Nanofibers

Ashish Kumar Dua
M.Tech, Dept. of Textile Engineering,
Indian Institute of Technology (IIT), Delhi.
Email: ashisdua@gmail.com

 

What is Nanotechnology?
Nanotechnology is the engineering at the atomic or molecular level. It is the collective term for a wide range of technologies, processing techniques, modeling, and measurements that involve the manipulating of matter at the smallest scale (from 1_100 nm). Nanotechnology is highly integrated with our society and will continue to be in the next few decades. It will have a more significant impact on our society than other technologies. Generally, nanotechnology involves components and structures with nanosize, and entails developing, creating, or modifying devices, systems, and materials within that length scale. Nanotechnology is concerned with the creation of fibers, particles, and materials at nanoscale dimensions.

These fibers, particles, and materials are referred to as nanofibers, nanoparticles, and nanomaterials, respectively, and they exhibit unusual and exotic properties that are not present in traditional bulk materials. In this article I will discuss electrospinning of nanofibers and applications of electrospun nanofibers.

What is a Nanofiber?

  • A nanofiber is a continuous fiber which has a diameter in the range of billionths of a meter.
  • The smallest nano-fibers made today are between 1.5 and 1.75 nanometers.
  • At the right a human hair (80,000 nanometers) is place on a mat of nano-fibers.
nanofiber (Image from Brian P. Sautter, University of Illinois at Chicago)
Fig: Nanofiber (Image from Brian P. Sautter, University of Illinois at Chicago)

Nanofibers are a continuous filament much like a fishing line. The material produced from nanofibers forms mats of non-woven material on the collection plates or devices. Threads from garments are different in that they have been twisted into yarns and then woven into fabrics. Nanofibers are easily made from manmade polymers such as nylon and natural polymers such as proteins.

nanofiber measuring unit

Unique Properties of Nanofibers

  1. Nanofibers are very small which gives them unique physical and chemical properties and allows them to be used in very small places.
  2. Nano-fibers have a huge surface area compared to their volume
  3. Low basis weight
  4. High porosity

High axial strength Scientist study both the chemical and physical properties of nanofibers. Because these fibers are so small, they are much more influenced by intra and intermolecular forces (things like electrical charge and magnetic fields). The huge surface area available on a nanofibre makes it very suitable for our new technologies which require smaller and smaller environments for chemical reactions to occur. Increasing the surface area speeds up a chemical reaction. Students may ask questions about how we see, and work in such a small environment. Much work is done using high speed cameras with enhanced light and magnification. Scientists also use SEM (scanning electron microscope) and TEM (transmission electron microscopes) to see surfaces of nanofibres and structure.

Surface-to-Volume Comparison:

Surface-to-Volume Comparison

Neglecting spaces between the smaller boxes, the volumes of the box on the left and the boxes on the right are the same but the surface area of the smaller boxes added together is much greater than the single box.

ABC blocks help students to understand the concept of surface to volume ratios. They need to see the additional surfaces.

Methods of Producing the Nanofibers:
Basically there are three methods:

  1. Drawing of Nano-fibers
  2. Template Synthesis
  3. Electrospinning

When:

  • 1934 Formhals
  • In the last few years the work of several researchers have made the electrospinning process suitable for the solutions of wide variety of polymers, ceramics and composite materials to produce nanofibers.

Principle:

  • Use of electrostatic and mechanical force to spin fibers from the tip of a fine spinneret.
  • When the electrostatic Force overcomes the surface tension, a charged jet is ejected, which is then elongated in the electrostatic field. After a variety of jet destabilizations occurring simultaneously with solvent evaporation, the ultra-thinned jet is solidified and then deposited on the collector to form an overlaid nano-fiber mat.
Principle of electrospinning nanofiber
Fig: Schematic diagram of electrospinning process

Construction:
It consists of four major components:

  1. High voltage power supply
  2. Heating assembly (for melt- electrospinning)
  3. Syringe and
  4. Collector

High voltage power supply

  • DC (more feasible) or AC
  • Very high voltage (usually in the range of 10 – 30 kV)
  • The polarity of the electrospinning system is arbitrary and can be reversed depending upon the polymer type and final product

Heating assembly

1. To melt the polymer to suitable viscosity, this can be electrospun easily.

2. Various sources such as:

  • Heating element
  • Heating gun
  • Laser heating
  • Ultrasound heating

Syringe or capillary tube:

  • Very fine capillary tube which holds the polymer melt into which a metal electrode is inserted.
  • It is mounted horizontally or vertically on an adjustable electrically insulating stand. A spinneret is connected to the syringe at one end for the production of nano-fibers.
  • A syringe pump is used to supply the polymer at a constant and controllable rate .

Collector:

  • Used to collect the electrospun fibers.
  • The collector is mounted on an insulating stand so that its potential can be controlled.
  • The collector may also be in the form of a rotating and translating mandrel.

Process:

  • Solution is prepared by dissolving the polymer in a suitable solvent in a particular weight ratio (typically about 15 to 20% polymer).
  • The solution is loaded into a syringe and the syringe is placed on a syringe pump which pumps the solution at a fixed flow rate to a needle.

electrospinning process

  • Jet initiation
  • Jet thinning
  • Jet stabilization

Experimental investigation of the governing parameters in the electrospinning of polymer solutions

  1. A high voltage is used to create an electrically charged jet of polymer solution or melt out of the syringe.
  2. One electrode is placed into the spinning solution/melt and the other attached to the collector which is simply grounded.
  3. The electric field is subjected to the end of the capillary tube that contains the solution fluid held by its surface tension.
  4. This induces a charge on the surface of the liquid. Mutual charge repulsion and the contraction of the surface charges to the counter electrode cause a force directly opposite to the surface tension.
  5. As the intensity of the electric field is increased, the hemispherical surface of the fluid at the tip of the capillary tube elongates to form a conical shape known as the Taylor cone.
  6. Further increasing the electric field, a critical value is attained with which the repulsive electrostatic force overcomes the surface tension and the charged jet of the fluid is ejected from the tip of the Taylor cone.
  7. The discharged polymer solution jet undergoes an instability and elongation process, which allows the jet to become very long and thin.
  8. Solvent evaporates, leaving behind a charged polymer fiber or In the case of the melt the discharged jet solidifies when it travels in the air.

electron flow

Important Features of Electrospinning:

  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.

Solution Conductivity:

  • Higher the conductivity of the solution more charges on the surface and hence easier to stretch
  • Polymers have less conductivity can be increased by the addition of the salts or electrolytes which produces ions and hence voltage required to produce smooth fibers reduces.
Solvents Conductivity (mS/m)
Acetone 0.034
Butanol 0.0202
Distilled Water 0.447
Ethanol 0.0554
Methano 0.1207
Propanol 0.0385
Dimethylformamide 1.090

Taylor Cone:

conic angle

Conic angle for electrospinning PAN/DMF solutions at different PAN concentrations (wt%) under a constant electric field of 80 kVm−1

Conic angle for electrospinning

Conic angle for PAN/DMF solutions (16 wt%) under varied electric field (needle inner diameter: 0.48 mm)

Conic angle for PAN DMF solutions

Types of Electrospinning:
It is two types; solution electrospinning and melt-electrospinning.

  1. In the melt-electrospinning as no solvent is used to dissolve the polymer, it is free from the problem of solvent recycling or removal.
  2. The productivity is higher due to no loss of mass due to solvent evaporation in melt-electrospinning.
  3. The polymers without appropriate solvents at room temperature such as polyethylene and polypropylene can be easily melt-electrospun.
  4. Melt-electrospinning favours the production of multi-component systems such as blends and composites as in many cases no common solvent for all the components may be found.
  5. The melt-electrospinning results the fibers, which are thicker than those produced from solution electrospinning.

Polymer-solvent used in solvent electrospinning:

Polymer

  • Nylon 6 and nylon 66
  • Polyacrylonitrile
  • PET
  • PVA
  • Polystyrene
  • Nylon-6-co-polyamide
  • Polybenzimidazole
  • Polyramide
  • Polyimides

Solvents

  • Formic Acid
  • Dimethyl formaldehyde
  • Trifluoroacetic acid
  • Water
  • DMF/Toluene
  • Formic acid
  • Dimethyl acetamide
  • Sulfuric acid
  • Phenol

Effect of Parameters:

1. Effect of solution concentration on fiber morphology:

  • Evaporation of solvent
  • Beads formation at lower concentration
SEM micrographs of Nylon 6 electrospun fibre at a voltage of 15 kV, collector distance 8 cm for different polymer concentration
SEM micrographs of Nylon 6 electrospun fibre at a voltage of 15 kV, collector distance 8 cm for different polymer concentration (a) 15 wt.%, (b) 20 wt.% and (c) 25 wt.%

Physical properties of Nylon 6/formic acid at different concentration:

Nylon Conc. (wt%) Viscosity (pa.s) Surface tension (mN/m) Electric conductivity (S/cm) Fiber Dia (nm)
15 2.845 44.8 .0044 250-700
20 3.358 78.5 .00294 750-1200
25 4.856 53 .00116 750-1550
Formic acid .0002 28 .09

2. Effect of applied electric field:

  • Higher electric field values are obtained either through decreasing the distance between the tip and collector or by applying higher voltages.
  • Increasing the electric field strength will increase the electrostatic repulsive force on the fluid results in more stretching hence slight reduction in diameter
Effect of voltage on morphology with 20 wt.% Nylon 6 polymer solution, tip to target distance
Effect of voltage on morphology with 20 wt.% Nylon 6 polymer solution, tip to target distance 8 cm, a) 12 kV, b) 15 kV, c) 18 kV.

3. Effect of distance from tip to collector:
The wider gap allowed more time for the fluid jet to stretch and for the solvent to evaporate results in reduction of diameter.

Scanning electron microscopy (SEM) images of electrospun
Scanning electron microscopy (SEM) images of electrospun 20 wt.% Nylon 6 fibres obtained from formic acid solution at different collecting gap distance, (a) 5 cm, (b) 8 cm and (c) 11 cm and electric power 15 kV constantly

4. Effect of flow rate:
At the low flow rate (about 0.20-0.25 ml/hr) the electrospun fiber is cylindrical and uniform. At higher flow rates (about 0.26- 0.300 ml/hr) the fiber surface is rougher.

Scanning electron microscopy of elctrospun fibres (Nylon 6 solution from formic acid)
Scanning electron microscopy of elctrospun fibres (Nylon 6 solution from formic acid), a) 0.200 ml/hr, b) 0.2500 ml/hr, c) 0.2600 ml/h and the solution was 20 wt.% and electric field was 15 kV and tip to collector distance was 8 cm.

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

1. Electrospun nanofibers for drug delivery: Electrospun nanofibers are used for drug delivery, Gene Delivery, DNA Delivery etc. it is also used in cancer treatment.

2. Electrospun nanofibers for fire-retardant fabrics: Electrospun nanofibers are used in flame retardant finishing of fabric. Fire retardancy is a process of preventing fire from spreading to adjacent locations, while fire retardant is a substance that is employed to stop or slow down the fire spreading by reducing its intensity. It is usually accomplished by reducing chemical reactions of the flames or delaying combustion. Organic substances, flammability, porosity, and oxygen level in the environment can substantially accelerate the fire and dense smoke and suspended particles. Generally, fire retardant materials reduce the flammability through two actions: (1) physical action where fire is physically blocked and (2) chemical action where a chemical reaction is initiated to stop the fire.

Recent studies have indicated a number of different ways of making electrospun nanofibers 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 . They can be nanofibrous membranes or coatings to advance the flame resistance, thermal stability, and other mechanical and electrical properties through mixing additives and fillers to the polymeric structures. The prevention of ignition and flame through noncombustible fillers and enhancing the heat capacity of the nanofibers are frequently employed approaches.

3. Filtration: Filter media is used to protect people and precision equipment from dust particles, smog, evaporate water, virus etc.

Applications of electrospun nanofibers in filtration
Fig: Applications of electrospun nanofibers in filtration

4. Medical application:

  • Electrospun biocompatible polymer nanofibers can be deposited as a thin porous film onto a hard tissue prosthetic device designed to be implanted into the human body. This coating film is expected to efficiently reduce the stiffness mismatch at the tissue interphase and hence prevent the device failure after the implantation
  • Nanofiber, spun from compounds naturally present in blood, can be used as bandages or sutures that ultimately dissolve into body. This nanofiber minimize infection rate, blood loss and is also absorbed by the body.
  • Artificial blood vessels, artificial organs, and medical face masks.
  • Nanofiber nonwoven fabric for Soft-tissue biomedical applications.
Nanofiber nonwoven fabric for Soft-tissue biomedical applications
Fig: Nanofiber nonwoven fabric for Soft-tissue biomedical applications

5. Protective clothing: Protective clothing is becoming more important for some specific work environments, including hospitals, battlefields, police stations, fire departments, and other occupational environments in which the risk of exposure to fires, chemicals, pathogens, and nanoparticles exists. Nanofibers with high flexibility, surface area, functionalization, light-weight, porosity, and pore size make them ideal candidates for protective clothing. It is easy to prevent the inhalation of micron-sized particles above 2.5 μm; nevertheless, nanoparticles between 1 and 100 nm are potentially dangerous as they can get directly absorbed through the skin and digestive systems. Electrospun nanofibers have been shown to effectively filter microfiber and nanofibers into different contaminated water sources, such as water jet cutting, suspended nanoparticle dispersion, and lake water.

nanofiber as a protective clothing
Fig: Nanofibers are used as a protective clothing

6. Electrospun nanofibers for agriculture and food industries: Because of the tailoring properties, high porosity and surface areas, ease of active ingredient additions, and flexibility of electrospun nanofibers, many researchers and scientists have been investigating the major applications of nanofibers in food industry and agricultural applications. The other advantages of using electrospun nanofibers over the other fiber-spinning applications are that electrospun nanofiber spinnerets can be pointed out in any directions with extended distance to cover a greater surface area. This will have a lot of flexibility for food and plant protection when the electrospinning process is cost-effective. The active ingredients into the electrospun nanofibers, including pheromones, fungicides, herbicides, insecticides, pesticides, and hormones can be adjusted based on the need.

Electrospun nanofibers are used in packaging industry because the nanofibers can keep many properties of foods and beverages, including taste, flavor, color/appearance, texture, consistency, as well as barrier, mechanical, and antimicrobial properties during transport and storage.

7. Electrospun nanofibers for tissue engineering

8. Electrospun nanofibers for photonics and electronics applications

References:

  1. Synthesis and Applications of Electrospun Nanofibers by Ramazan Asmatulu and Waseem S. Khan
  2. An Introduction to Electrospinning and Nanofibers by Kazutoshi Fujihara, Seeram Ramakrishna, and Teik-Cheng Lim
  3. International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 10 No: 06
  4. S.A. Therona, E. Zussmana,  A.L.. Yarina, Experimental investigation of the governing parameters in the electrospinning of polymer solutions, Polymer 45 (2004) 2017–2030
  5. Melt electrospinning for production of nanofibres, The Indian textile journal, sept. – 2012
  6. Phys. D: Appl. Phys. 44 (2011) 435401 (6pp)
  7. www.electro-spinning.com by Nabond

You may also like:

  1. Electrospinning and Electrospun Nanofibers: Process, Properties and Uses
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