Electrospinning offers a unique method for producing nanofibers from polymer solutions, which can be used for advanced application in tissue scaffolding and chemical and biomedical sensors.
History of Electrospinning
Electrospinning is an old technique which was first observed in 1897 by Rayleigh, studied meticulously by Zeleny on electrospraying, and patented by Formhals in 1934. Taylor’s work on electrically driven jets laid the foundation for electrospinning. The term “electrospinning,” which derives from “electrostatic spinning,” has been in common use relatively recently (since around 1994), but its origin can be traced back more than 60 years . From 1934 to 1944, Formhals published a series of patents outlining an experimental setup for the production of polymer filaments utilizing an electrostatic force .
The first patent (US Patent Number: 2116942) on electrospinning was issued for the fabrication of textile yarns and a voltage of 57 kV was utilized for electrospinning cellulose acetate using acetone and monomethyl ether of ethylene glycol as solvents. This process was patented by Antonin Formhals in 1934 who was subsequently granted related patents (U.S. Patents 2116942, 2160962 and 2187306) in 1938, 1939, and 1940.
Formhals’ spinning process consists of a movable thread collecting device to collect threads in an elongated state, like that of a spinning drum in conventional spinning. About 50 patents for electrospinning polymer melts and solutions have been filed in the past 60 years. Vonnegut and Newbauer in 1952 invented a simple device for electrical atomization and produced streams of highly electrified uniform droplets of about 0.1 mm in diameter.
After that, Drozin in 1955 examined the dispersion of a series of liquids into aerosols under high electric potentials, and Simons in 1966 patented an apparatus for the production of non-woven fabrics that were ultra-thin and very light in weight with various patterns using electrical spinning . In 1971, Baumgarten invented an apparatus to electrospin acrylic fibers with diameters in the range of 0.05 – 1.1 μm. Since the 1980s, particularly recently, the electrospinning process has regained more attention probably due to a surging interest in nano technology, as ultrafine fibers or fibrous structures of various polymers with diameters down to submicrons or nanometers can be easily assembled with this process
The surge in popularity of the electrospinning process can be understood by the fact that over 200 universities and research institutes worldwide are studying various aspects of the electrospinning process and the fiber it produces. Also, the number of patents for applications based on electrospinning has grown in recent years. Some companies such as eSpin Technologies, NanoTechnics, and KATO Tech are actively engaged in reaping the benefits of the unique advantages offered by electrospinning, while companies such as Donaldson Company and Freudenberg have been using this process for the last two decades in their air filtration products.
Electrospinning Process
Electrospinning is a method in which materials in solution or melt are formed into nano – or micro – sized continuous fibers (Figure 1) shows a schematic illustration of the basic setup for electrospinning.
It consists of three main components: a high-voltage source, a syringe pump, and a collector. The electrospinning technique can be regarded as a variant of the electrostatic spraying (electrospraying) process, as both techniques use high voltage to induce the formation of liquid jets. Small droplets or particles are formed as a result of the break-up of the electrified jet in electrospraying, whereas a solid fiber is formed as the electrified jet is elongated in electrospinning and this process employs electrostatic forces to stretch the solution or melt it as it solidifies.
The process of electrospinning in general occurs within the atmosphere of a room which implies the temperature and humidity. The electrostatic forces transforming the liquid cause the formation of the droplet to convert from a rounded meniscus into the Taylor cone, a phenomenon observed due to the electrostatic repulsions among similar charges toward the liquid.
At this crucial stage, the applied electric range eventually converts more apparently and succeeds the surface tension of the liquid beginning with a jet of the solution stretched from the top of the Taylor cone. Also, as the jet moves through the air, the current changes from ohmic to convective because of the charges transferring to the surface of the fiber. Furthermore, an unbalanced vortex of the jet occurs in the gap between the tip and the collector.
Types of Electrospinning
Basic Needle Based Electrospinning
Generally, there are vertical and horizontal set ups and these are the two standards for electrospinning as shown in Figure 2.
There are many types of basic needle based and some of these are:
Multi-axial Electrospinning
Modern attempts in electrospinning focus on changing the fibers with structural characteristics like core-sheath, hollow, porous, as well as tri-axial-channel threads for use in different applications.
Coaxial Electrospinning
Core-sheath threads are created by coaxial electrospinning, where a coaxial spinneret is formed outside and the inner needle is generally utilized. Coaxial electrospinning could generate threads from several solution partners, core-sheath, hollow, and practical fibers that may include particles. A hollow fiber is created using coaxial electrospinning usually by a temporary substance as the core and the original material as the shell. Depending on the post-spinning method, oil is usually used as the substitute material as it is almost always easier to remove it than different higher molecular weight substances.
Tri-axial Electrospinning
In triaxial electrospinning, three polymer liquids are poured into a composite Taylor cone utilizing a spinneret.. Triaxial fibers could be produced with differing hydrophobicity and mechanical force.
Bi-component Electrospinning
The picture for the bi-component side by side electrospinning is shown in Figure 2 where the two plastic syringes, each including a polymer liquid, are side-by-side. A typical syringe pump controlled the flow rate of these two polymer solutions. The platinum electrodes immersed in each of these solutions were connected in parallel into a high voltage DC supply, and the free tips of the Teflon needles connected to the syringes were adhered together. The reason for the side-by-side bi-component thread is that the single thread can show the characteristics of all of the components of the fiber.
Multi-needle Electrospinning
The most direct approach to raising productivity is increasing the number of needles, which is connected to multi-needle electro-spinning as shown in Figure 3
Here the polymer solution is applied through multiple needles joined into a high voltage supply, and the syringe pump is used to pump the spinning solvent to the spinneret setup; also, other spinning solutions could be injected individually into two different sets within the same multiple spinneret setups. Due to the large mass of the spinning solvent transfer, a high voltage is required for continuous electrospinning.
The disadvantages of this process include blocking at the top of the needles and cleansing of multiple needles, unsettled electric field force and the difference in fiber size distribution. However, even though a high flow rate (1–18 mL/h) could be obtained in multi-needle methods, the repulsion of adjacent jets in multi-needles is still an issue.
Electroblowing/Gas-assisted/Gas jet Electrospinning
Electroblowing is a method which mixes electrostatic nanofiber products (electrospinning) beside airflow around the spinneret.
Also, the tangential powers of the flowing air working on a drop of mixture add to the creation of the Taylor cone and production of the nanofiber. With the extra elongating force supplied by the gas stream, small diameter fibers are created. A combination of forces of the electric field is applied and the airflow augments the performance of the electrospinning method, and airflow accelerates the vaporization of solvent from the solution. This is the only method in which hyaluronic acid could spin in its native mode.
Magnetic Field Assisted Electrospinning
Juan A. Gonzalez Sanchez et al. described the formation of polymeric nanofibers, including magnetic nanoparticles, using electrospinning supported by a magnetic field.
A magnetic field was employed in situ through the electrospinning process. For this purpose, a combination of Helmholtz Coils, all having 200 turns, providing a current ranging from 1 A to 3 A, were utilized. The coils were parted with a distance equal to the radius of the annular loops (10.5 cm). A step machine was used to rotate the specimen holder during deposition. By observing the electrospinning method, it can be seen that with the utilization of the electromagnetic field via fiber deposition, the polymer stream results were more fixed to the target. The nanofibers produced had a diameter ranging from 100 nm to 700 nm.
Conjugate Electrospinning
The system used in conjugate electrospinning is depicted. It contains two or three very high voltage power supplies with different polarities, two or three spinnerets, and a collector drum. Two or three programmable pumps are employed to control the transfer rate of solutions. Power supplies are connected with spinnerets, sequentially. Spinnerets are arranged in reverse positions on the identical horizontal line. Syringes independently deliver the solution to two or three spinnerets.
The target is a rotating drum controlled by a stepping motor. The fibers of the two or three in reverse become charged electrospinning spinnerets received and elongated via the drum collector at a fixed speed. The nanofiber assemblies, which can be generated by this technique, are dehydrated below vacuum at room temperature.
Centrifugal Electrospinning
During centrifugal electrospinning, the force contributing to the stretching of the solution droplet into fibers is a mixture of centrifugal power and electrostatic energy. Externally, with the application of high voltage, the simple centrifugal spinning of fibers needs the spinneret to revolve at thousands of rpms. However, in centrifugal electrospinning, the revolution speed can be decreased by 50%.
The electric field is provided to stretch the jets to minimal dimensions below the simultaneous drying of the solvent, transmitting a dry nano-fibrous cover on the substrate. With the introduction of the centrifugal strength, a lower voltage is needed in the surface tension of the liquid to succeed in instigating electrospinning. The combination of mechanical revolution and decreased energy makes this a very efficient method for forming aligned nanofibers. Multiple nozzles may be placed around the axis of rotation to raise the generation rate of centrifugal electrospinning.
Needleless Electrospinning
Although conventional electrospinning methods based on the use of a syringe create nanofiber layers in quantities of around 0.1 – 1 gram per hour, needleless electrospinning provides the possibility of industrial-scale manufacture of nanofibers.
Parameters of Electrospinning
It is essential to know the electrospinning operating parameters because these parameters affect fiber morphologies. It is much simpler and more reasonable to obtain required and desired fiber diameters and morphologies by control of those parameters. The typical stages in electrospinning of a polymer to nanofiber are:
a) The diameters for the fibers’ necessity have been compatible plus controllable.
b) The fiber outside should be impurity-free and controllable.
c) Continuous free nanofibers must be collectable.
Fiber diameter is with the numerous essential quantities needed in electrospinning. A complex challenge is to get unity of that fiber’s diameters. The appearance of distortions like beads and pores is the main problem.
The parameters of the electrospinning process can be classified into three parts:
- Solution Parameters: concentration, viscosity, conductivity, molecular weight, surface tension.
- Process Parameters: applied electric field, tip to collector distance, feeding or flow rate.
- Ambient Parameters: humidity and temperature of the surroundings.
Solution Parameters
Concentration
When a solid polymer is dissolved in a solvent, the liquid viscosity is proportionate to the polymer concentration as high viscosity leading to big fiber diameter. Also, higher polymer concentrations will affect the outcome in larger nanofiber diameters. The lowest level of concentration is needed in the electrospinning for fiber production to occur. In very low level of concentrations, electrospray happens instead of electrospinning, because of the lower viscosity. Moreover, considerable surface tensions appear in the solution.
Through low solvent concentrations, a fusion of fibers plus beads is achieved. When the concentration rises, the form of the bead turns from spherical to spindle-like. Eventually, uniformity of fibers with increased diameters is produced. An optimum liquid concentration should be achieved, as at a very low level, beads are generated, whereas, at a very high level of concentration, the production of continuous fibers is prevented because of the inability to manage the flow of the solution.
Molecular Weight
Molecular weight is considered another critical parameter that influences the morphology of the electrospinning nanofiber; likewise, it affects viscosity, surface tension and conductivity. In general, the molecular weight displays the entanglement of polymer strings in solutions, particularly the solution viscosity. Preservation of the set concentration, using the very low molecular weight of polymer leads to the creation of beads more than fibers, raising the molecular weight will produce smooth fibers, whereas using a polymer with extremely high molecular weight outcomes in electrospinning fibers leads to them possessing very wide diameters.
Solution Viscosity
In the electrospinning process, it is necessary to arrange the ideal value for solution viscosity, which is required to get the best result. While using too low viscosity leads to no production of fiber, on the other hand, too high viscosity results in complexity in the ejection of jets of polymer solution. Additionally, viscosity is particularly crucial to fiber morphology. There are three essential factors which are related to each other and those factors are: viscosity, polymer concentration and polymeric molecular weight.
Usually, the solution viscosity can be changed by varying the level concentration of the polymer for the solution. Each viscosity scale for various polymers used for electrospinning is distinct from the other. İncreasing concentration or solution viscosity results in electrospinning nanofibers which include a larger and more uniform diameter. The dominant factor is Surface tension. When the dose in viscosities is low, the result which you obtain will have fibers with beads or beaded fibers.
Surface Tension
Various surface tensions are obtained using different solvents. Decreasing the surface tension causes the generation of nanofibers with no beads, as proposed in previous study, except that low surface tension tends to not always produce typical electrospinning requirements. It is significant in controlling the higher and lower boundaries on which other parameters are set.
Yang et al. studied the influence of surface tensions on poly (vinyl pyrrolidone) (PVP) electrospun nanofiber morphology using ethanol, DMF as well as MC-like solvents. Moreover, they noted that various solvents might produce different surface tensions. They got smoother nanofibers by decreasing the surface tension and preventing the concentration getting set instead of the beaded composition at higher surface tensions.
Conductivity/Surface Charge Density
Solution conductivity is normally prepared by polymer type, solvent used, and the behavior of ionized salts. Usually, higher conductivity of solutions leads to producing a smaller diameter of electrospun nanofibers, increasing the conductivity of the solution and the surface charge density of the solution by the addition of salts to the polymer solution.
In the case of the addition of salt, there are many polymers used for increasing the solution conductivity such as polyamide 6, polyacrylic acid, collagen type IPEO, polyethylene oxide (PEO), etc. Zong et al. have produced fibers with beadless, smaller diameter poly (D-L-lactic acid) (PDLA) nanofibers ranging between 100 into 200 nm via the addition of ionic salts like KH2PO4, NaH2PO4 and NaCl. They determined the influence of ions on the composition compared to fiber structure obtained without adding salt.
Process Parameters
Applied Voltage
Applied voltage is the critical factor in electrospinning as the threshold voltage necessary for the required charged jets has to be drawn from the Taylor cone. Next, the start voltage is transferred, so the fiber production occurs, causing the necessary adjustments on the solution simultaneously with the electric field and starting the electrospinning method. The influence of applied voltage on the electrospinning method has been widely studied. Reneker et al. have noted that applied voltage does not have a critical influence on fiber diameter in the electrospinning of polyethylene oxide.
Zhang et al. proved that there is also more polymer ejection by higher voltages, making it easy for the production of fiber diameter on a larger scale. They studied the impact of voltage on fiber morphology, even diameter distribution by (PVA) poly (vinyl alcohol) /water solution. Some other researchers stated that the increase in applied voltage promoted the reduction of fiber diameter; also, they explained that fiber diameter reduced by increasing voltage because of the increase in electrostatic repulsive power on the charged jet. Bead creation was furthermore discovered through higher voltage .
Feed Rate /Flow Rate
The feed rate is another process for the polymer in a syringe, and the flow rate parameter influences the material transfer rate inside the syringe and on the jet velocity, which is affected directly in the process. A slower flow rate is sustained to 22 allow enough time for the solvent to evaporate. A minimum feed rate should be present. The high feed rate also appeared in the beaded fiber composition.
Distance Between the Tip and the Collector (Tip-To-Collector Distance)
The tip-to-collector distance bears in mind the parameters which are affected, although its influence is less critical on fiber morphology if these nanofibers are compared with the different process parameters. In order to get the evaporation for the solvent from the solution of the polymer, the ideal distance should be selected.
Collectors
Generally, the collector of nanofibers should be selected and modelled as a conductive substrate. The material which is used to collect the nanofiber is aluminum foil whereas, wire mesh, conductive paper or cloth, parallel or gridded and rotating rod are additionally used.
Ambient Parameters
The other parameter also affected in the morphology of nanofibers in the electrospinning process is called ambient parameters, which means the temperature and relative humidity. Using high temperature leads to fibers with decreased diameter, whereas using low relative humidity, the solvent may be completely dry. Additionally, using higher humidity leads to small pores appearing on the fiber surface. Most of the process and solution parameters in this study are based on previous study.
