Electrospinning is a fiber production method that uses electrical force (based on electrohydrodynamic principles) to draw charged threads of polymer solutions for producing nanofibers with diameters ranging from nanometers to micrometers. Electrospinning shares characteristics of both electrospraying and conventional solution dry spinning of fibers.Ziabicki, A. (1976) Fundamentals of fiber formation, John Wiley and Sons, London, . 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 voltage is applied to a liquid droplet, the body of the liquid becomes charged, and electrostatic repulsion counteracts the surface tension and the droplet is stretched; at a critical point a stream of liquid erupts from the surface. This point of eruption is known as the Taylor cone. If the molecular cohesion of the liquid is sufficiently high, stream breakup does not occur (if it does, droplets are electrosprayed) and a charged liquid jet is formed.High speed video of the taylor cone formation and electrospinning. youtube.comSingle nozzle electrospinning process nanofiber formation video. youtube.com
As the jet dries in flight, the mode of current flow changes from ohmic to convective as the charge migrates to the surface of the fiber. The jet is then elongated by a whipping process caused by electrostatic repulsion initiated at small bends in the fiber, until it is finally deposited on the grounded collector.High speed video of the whipping instability. youtube.com The elongation and thinning of the fiber resulting from this bending instability leads to the formation of uniform fibers with nanometer-scale diameters.
Parameters Source: 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.ParametersEffect on fibersLinked toSolution ParametersViscosityIf 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 WeightPolymer ConcentrationIncrease 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, ViscosityMolecular weightReflects the number of entanglements of polymeric chains in solution, thus its viscosity. Viscosity, Surface tension, ConductivityConductivityDirectly related to the accumulation of charges under the electric field. Higher conductivity results in stronger stretching of the jet producing smaller diameter fiber.VoltageSurface tensionWith all other parameters fixed, it determines the upper and lower boundaries of the electrospinning window. Processing ParametersVoltageFiber 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 rateTip to collector distanceAffects the traveling time of the polymer jet; should be high enough to allow complete evaporation of the solvent. Voltage, Feed rateFeed rateDetermines 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, ViscosityAmbient ParametersHumidityHigh humidity may result in pores on fiber surfaceTemperatureAn 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 spinneret (typically a hypodermic syringe needle) connected to a high-voltage (5 to 50 kV) direct current power supply, a syringe pump, and a grounded collector. A polymer solution, sol-gel, particulate suspension or melt is loaded into the syringe and this liquid is extruded from the needle tip at a constant rate by a syringe pump. 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 Needleless (also known as nozzle-free) electrospinning Electrospun nanofibers are widely investigated for wound healing applications because their high surface area, interconnected porosity, and structural similarity to the extracellular matrix can support cell adhesion, proliferation, and tissue regeneration. Multiplying the needles High-throughput roller electrospinning Wire electrospinning Bubble electrospinning Ball electrospinning High speed electrospinning Plate edge electrospinning Bowl electrospinning Hollow tube electrospinning Rotary cone electrospinning Spiral coil electrospinning Electroblowing
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. 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.
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. 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 Emulsions can be used to create core shell or composite fibers without modification of the spinneret. However, these fibers are typically more difficult to produce compared to coaxial spinning due to the greater number of variables which must be accounted for in creating the emulsion. A water phase and an immiscible solvent phase are mixed in the presence of an emulsifying agent to form the emulsion. Any agent which stabilizes the interface between the immiscible phases can be used. Surfactants such as sodium dodecyl sulfate, Triton X-100 and nanoparticles have been used successfully. During the electrospinning process the emulsion droplets within the fluid are stretched and gradually confined leading to their coalescence. If the volume fraction of inner fluid is sufficiently high, a continuous inner core can be formed.
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.
Melt electrospinning
Electrospinning of polymer melts eliminates the need for volatile solvents in solution electrospinning. 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.
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.
History
In the late 16th century William GilbertGilbert, W. (1628) De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies, and on That Great Magnet the Earth), London, Peter Short. set out to describe the behavior of magnetic and electrostatic phenomena. He observed that when a suitably electrically charged piece of amber was brought near a droplet of water it would form a cone shape and small droplets would be ejected from the tip of the cone: this is the first recorded observation of electrospraying.
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". 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 1900Cooley, J.F. Patent GB 06385 "Improved methods of and apparatus for electrically separating the relatively volatile liquid component from the component of relatively fixed substances of composite fluids" 19 May 1900 and February 1902Cooley, J. F. et al. "Apparatus for electrically dispersing fluids" Issue date: February 4, 1902 and by W.J. Morton in July 1902.Morton, W.J. et al. "Method of dispersing fluids" Issue date: July 29, 1902
In 1914 John Zeleny, published work on the behavior of fluid droplets at the end of metal capillaries. 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 1934Formhals, Anton et al. "Process and apparatus for preparing artificial threads" Issue date: October 2, 1934 to 1944Formhals, Anton et al. "Method and apparatus for spinning" Issue date: May 30, 1944 for the fabrication of textile yarns. Electrospinning from a melt rather than a solution was patented by C.L. Norton in 1936Norton, C.L. "Method and apparatus for producing fibrous or filamentary material" Issue date: July 21, 1936 using an air-blast to assist fiber formation.