Electrospinning

Electrospinning is a fiber production method that uses electric force to draw charged threads of polymer solutions or polymer melts up to fiber diameters in the order of some hundred nanometers. Electrospinning shares characteristics of both electrospraying and conventional solution dry spinning of fibers.^[1] The process does not require the use of coagulation chemistry or high temperatures to produce solid threads from solution. This makes the process particularly suited to the production of fibers using large and complex molecules. Electrospinning from molten precursors is also practiced; this method ensures that no solvent can be carried over into the final product.

Process

When a sufficiently high

Parameters

• Molecular weight, molecular-weight distribution and architecture (branched, linear etc.) of the polymer
• Solution properties (viscosity, conductivity and surface tension)
• Electric potential, flow rate and concentration
• Distance between the capillary and collection screen
• Ambient parameters (temperature, humidity and air velocity in the chamber)
• Motion and size of target screen (collector)
• Needle gauge

Effects of Electrospinning parameters.

Parameters	Effect on fibers	Linked to
Solution Parameters
Viscosity	If too low, no continuous fiber formation will occur; if too high, the ejection of the jet from the needle tip will be impeded.	Polymer concentration, Molecular Weight
Polymer Concentration	Increase in concentration results in increased diameter. A minimum concentration is required: if too low, there will not be enough entanglements to sustain the jet (beads).	Surface tension, Viscosity
Molecular weight	Reflects the number of entanglements of polymeric chains in solution, thus its viscosity.	Viscosity, Surface tension, Conductivity
Conductivity	Directly related to the accumulation of charges under the electric field. Higher conductivity results in stronger stretching of the jet producing smaller diameter fiber.	Voltage
Surface tension	With all other parameters fixed, it determines the upper and lower boundaries of the electrospinning window.
Processing Parameters
Voltage	Fiber formation occurs only after a certain threshold voltage. Higher voltages cause greater stretching of the solution with reduction of fiber diameters; but if the voltage is too high, it may cause instability of the jet and increase the fiber diameter.	Tip to collector distance, Conductivity, Feed rate
Tip to collector distance	Affects the traveling time of the polymer jet; should be high enough to allow complete evaporation of the solvent.	Voltage, Feed rate
Feed rate	Determines the amount of solution available per unit of time. Influences the jet velocity and the material transfer rate. Increasing the rate causes more polymer to be processed at a given instant, thus increasing fiber diameter.	Tip to collector distance, Voltage, Viscosity
Ambient Parameters
Humidity	High humidity may result in pores on fiber surface
Temperature	An increase in temperature results in a decrease in fiber diameter thanks to a decrease in viscosity.	Viscosity

Apparatus and range

The standard laboratory setup for electrospinning consists of a

Alternatively, the droplet at the tip of the spinneret can be replenished by feeding from a header tank providing a constant feed pressure. This constant pressure type feed works better for lower viscosity feedstocks.

Scaling-up possibilities

• Alternating current electrospinning^{[7][8][9][10][11][12]}
• Needleless (also known as, nozzle-free) electrospinning ^[13][14]
• Multiplying the needles^[15]
• High-throughput roller electrospinning^[16]
• Wire electrospinning
• Bubble electrospinning^[17]
• Ball electrospinning^[18][19]
• High speed electrospinning^[20]
• Plate edge electrospinning^[21]
• Bowl electrospinning^[22]
• Hollow tube electrospinning^[23]
• Rotary cone electrospinning^[24]
• Spiral coil electrospinning^[25]
• Electroblowing^[26]

Other techniques

Modification of the spinneret and/or the type of solution can allow for the creation of fibers with unique structures and properties. Electrospun fibers can adopt a porous or core–shell morphology depending on the type of materials being spun as well as the evaporation rates and miscibility for the solvents involved. For techniques which involve multiple spinning fluids, the general criteria for the creation of fibers depends upon the spinnability of the outer solution.^[27] This opens up the possibility of creating composite fibers which can function as drug delivery systems or possess the ability to self-heal upon failure.^[28][29]

Co-axial electrospinning

A coaxial setup uses a dual-solution feed system which allows for the injection of one solution into another at the tip of the spinneret. The sheath fluid is believed to act as a carrier which draws in the inner fluid at the Taylor Cone of the electrospinning jet.^[27] If the solutions are immiscible then a core shell structure is usually observed. Miscible solutions however can result in porosity or a fiber with distinct phases due to phase separation during solidification of the fiber. For more advanced setups, a triaxial or quadaxial (tetra-axial) spinneret can be used with multiple solutions.

Emulsion electrospinning

Electrospinning of blends is a variation of this technique which uses the fact that polymers are generally immiscible with each and can phase segregate without the use of surfactants. This method can be simplified further if a solvent which dissolves both polymers is used.[32]

Melt electrospinning

Electrospinning of polymer melts eliminates the need for volatile solvents in solution electrospinning.^[33] Semi crystalline polymer fibers such as PE, PET and PP, which would otherwise be impossible or very difficult to create using solution spinning, can be created. The setup is very similar to that employed in conventional electrospinning and includes the use of a syringe or spinneret, a high voltage supply and the collector. The polymer melt is usually produced by heating from either resistance heating, circulating fluids, air heating or lasers.^[34]

Due to the high viscosity of polymer melts, the fiber diameters are usually slightly larger than those obtained from solution electrospinning. The fiber uniformity upon achieving stable flow rates and thermal equilibrium, tends to be very good. The whipping instability which is the predominant stage in which the fiber is stretched for spinning from solutions can be absent from the process due to the low melt conductivity and high viscosity of the melt. The most significant factors which affect the fiber size tend to be the feed rate, the molecular weight of the polymer and the diameter of the spinneret. Fiber sizes ranging from ~250 nm to several hundreds of micrometers have been created thus far with the lower sizes being achieved using low molecular weight polymers.^[35]

History

In the late 16th century

In 1887 C. V. Boys described “the old, but little known experiment of electrical spinning”. Boys’ apparatus consisted of “a small dish, insulated and connected with an electrical machine”.^[37] He found that as his stock liquid reached the edge of the dish, that he could draw fibers from a number of materials including shellac, beeswax, sealing-wax, gutta-percha and collodion.

The process of electrospinning was patented by J.F. Cooley in May 1900^[38] and February 1902^[39] and by W.J. Morton in July 1902.^[40]

In 1914 John Zeleny, published work on the behavior of fluid droplets at the end of metal capillaries.^[41] His effort began the attempt to mathematically model the behavior of fluids under electrostatic forces.

Further developments toward commercialization were made by Anton Formhals, and described in a sequence of patents from 1934^[42] to 1944^[43] for the fabrication of textile yarns. Electrospinning from a melt rather than a solution was patented by C.L. Norton in 1936^[44] using an air-blast to assist fiber formation.

In 1938 Nathalie D. Rozenblum and Igor V. Petryanov-Sokolov,

Simon, in a 1988 NIH SBIR grant report, showed that solution electrospinning could be used to produce nano- and submicron-scale polystyrene and polycarbonate fibrous mats specifically intended for use as in vitro cell substrates. This early application of electrospun fibrous lattices for cell culture and tissue engineering showed that various cell types would adhere to and proliferate upon the fibers in vitro. Small changes in the surface chemistry of the fibers were also observed depending upon the polarity of the electric field during spinning.[52]

In the early 1990s several research groups (notably that of Reneker and Rutledge who popularised the name electrospinning for the process)^[53] demonstrated that many organic polymers could be electrospun into nanofibers. Between 1996 and 2003 the interest in electrospinning underwent an explosive growth, with the number of publications and patent applications approximately doubling every year.^[5]

Since 1995 there have been further theoretical developments of the driving mechanisms of the electrospinning process. Reznik et al. described the shape of the Taylor cone and the subsequent ejection of a fluid jet.^[54] Hohman et al. investigated the relative growth rates of the numerous proposed instabilities in an electrically forced jet once in flight^[55] and endeavors to describe the most important instability to the electrospinning process, the bending (whipping) instability.

Uses

The size of an electrospun fiber can be in the nano scale and the fibers may possess nano scale surface texture, leading to different modes of interaction with other materials compared with macroscale materials.

Filtration

The use of nanofiber webs as a filtering medium is well established. Due to the small size of the fibers London-Van Der Waals forces are an important method of adhesion between the fibers and the captured materials. Polymeric nanofibers have been used in air filtration applications for more than seven decades.^[47][57] Because of poor bulk mechanical properties of thin nanowebs, they are laid over a filtration medium substrate. The small fiber diameters cause slip flows at fiber surfaces, causing an increase in the interception and inertial impaction efficiencies of these composite filter media. The enhanced filtration efficiency at the same pressure drop is possible with fibers having diameters less than 0.5 micrometer. Since the essential properties of protective clothing are high moisture vapor transport, increased fabric breath-ability, and enhanced toxic chemical resistance, electrospun nanofiber membranes are good candidates for these applications.^[58]

Textile manufacturing

The majority of early patents for electrospinning were for textile applications, however little woven fabric was actually produced, perhaps due to difficulties in handling the barely visible fibers. However, electrospinning has the potential to produce seamless non-woven garments by integrating advanced manufacturing with fiber electrospinning. This would introduce multi-functionality (flame, chemical, environmental protection) by blending fibers into electrospinlaced (using electrospinning to combine different fibers and coatings to form three-dimensional shapes, such as clothing)^[59] layers in combination with polymer coatings.^[60]

Medical

Electrospinning can also be used for medical purposes.

Cosmetic

Electrospun nanomaterials have been employed to control their delivery so they can work within skin to improve its appearance.^[74] Electrospinning is an alternative to traditional nanoemulsions and nanoliposomes.

Pharmaceutical manufacturing

The continuous manner and the effective drying effect enable the integration of electrospinning into continuous pharmaceutical manufacturing systems.^[75] The synthesized liquid drug can be quickly turned into an electrospun solid product processable for tableting and other dosage forms.

Composites

Ultra-fine electrospun fibers show clear potential for the manufacture of long fiber composite materials.^[76]

Application is limited by difficulties in making sufficient quantities of fiber to make substantial large scale articles in a reasonable time scale. For this reason medical applications requiring relatively small amounts of fiber are a popular area of application for electrospun fiber reinforced materials.

Electrospinning is being investigated as a source of cost-effective, easy to manufacture wound dressings, medical implants, and scaffolds for the production of artificial human tissues. These scaffolds fulfill a similar purpose as the extracellular matrix in natural tissue. Biodegradable polymers, such as polycaprolactone^[77] and polysaccharides,^[78] are typically used for this purpose. These fibers may then be coated with collagen to promote cell attachment, although collagen has successfully been spun directly into membranes.^[79]

Catalysts

Electrospun fibers may have potential as a surface for enzymes to be immobilized on. These enzymes could be used to break down toxic chemicals in the environment, among other things.^[5]

Mass production

Thus far, at least eight countries in the world have companies which provide industrial-level and laboratory-scale electrospinning machines: three companies each in Italy and Czech Republic, two each in Iran, Japan, and Spain, and one each in the Netherlands, New Zealand^[80] and Turkey.^[81]

References