Electrospinning

Electrospinning

What is Electrospinning?

Electrospinning is a voltage-driven, fabrication process governed by a specific electrohydrodynamic phenomenon where small fibers are yielded from a polymer solution. The most basic setup for this technique involves a solution contained in a reservoir — typically a syringe — tipped with a blunt needle (at least for needle-based electrospinning), a pump, a high voltage power source, and a collector.

The spinning process begins when an electric field is established between the needle tip and collector by applying a specified voltage. While the pump causes the solution to flow at a constant rate, charges accumulate at the surface of the liquid. Soon a point is reached where the electrostatic repulsion is larger than the surface tension that results in the liquid meniscus deforming into a conically shaped structured known as a Taylor cone.

Once the Taylor cone forms, the charged liquid jet ejects towards the collector. Collectors differ by their geometrical configurations that come in many forms: flat plates, rotating drums, mandrels, and disks, but these are not the only variations.

If the solution viscosity is within a certain threshold, the technique generates solid fibers as the solvent evaporates. Violent whipping motion occurs between the cone and the collector, resulting in a non-woven fiber mat deposited on the collector.

Diagram of a Taylor Cone with no voltage applied and with voltage applied.
Taylor cone formation with and without a potential difference.
Diagram of a typical electrospinning setup, including the emittor and collector.
An illustrative schematic of the underlying principles of electrospinning.

Introduction to the Electrospinning Process

Electrospinning fibers

Electrospinning finds use in several industries: life science, biomedical engineering, battery research, and the overall development, production, and commercialization of nanofiber materials. Diameters of the fibers typically range between tens of nanometers to a few micrometers. One of the main advantages of the electrospinning technique over other fabrication methods is its versatility to create fibers with multiple arrangements (ex. aligned fibers, random orientations, or their combinations) and morphological structures (ex. tubular scaffolds, flat surfaces, and asymmetrical configurations).

The popularity of the electrospinning technique has allowed multiple technologies — tissue engineering, regenerative medicine, and encapsulation of bioactive molecules — to emerge and significantly evolve over the past decade.

Nowadays electrospinning is not just exclusive to academic studies; it has received immense recognition and employment as a technique with real commercial applications. Multiple industries around the world have adopted this technique in the development of new product innovations.

Some applications that strongly benefit from and actively use electrospinning are tissue engineering (Phoenix Wound Matrix®, NanoAlignedTM, NanoECMTM, NanoCareTM, NanoMeshTM, Absorv®, TegadermTM), drug delivery (Rivelin® Patch), food encapsulation, insulating materials, energy conversion and storage, and air (Proveil) and water filtration, among others.

Components of Electrospinning

Liquid Feeding Systems

Commonly used syringe pump systems can deliver up to 140 mL of solution per batch at rates ranging from 0.1 µL/h to 6,000 mL/h. Meanwhile, pressurized liquid feeding systems contain multiple volumes of solution that range from only a few milliliters to thousands of milliliters and can furthermore be rendered in a semi-continuous mode.

Polymer melts or solutions implemented in spinning processes are delivered at constant flow rates by means of syringe pumps or pressurized liquid feeding systems. In cases involving syringe pumps, solutions are normally contained in glass or plastic syringes; whereas cases with pressurized liquid feeding systems contain the solution within a reservoir and can consequently be delivered in a semi-continuous manner.

Diagram of a liquid supply system
A schematic illustrative of how a standard reservoir and syringe pump appear.

Power Sources

A high voltage power supply with adjustable voltage is used to start the spinning process by polarizing the emitter. Researchers have used voltages up to +50 kV when creating electrospun fibers in the nano- and microscale.

To collect the electrospun fibers, a collector is typically grounded to attract them into a specific area. While it’s true that grounding a collector works well for small thicknesses, fibers are sometimes collected into unfavorable locations. To improve fiber collection, it is advantageous to apply a negative voltage into the collector. This action maximizes the emitter-collector voltage drop, thus improving fiber yield during the electrospinning process.

Emitters

A wide collection of needles with various gauges and identifying colors

Spinning head emitters are employed in needle-based electrospinning in which multiple configurations can be implemented. These capillary needles can be configured as single, co-axial, and multi-axial phase emitters.

In single phase, only one liquid is flowing through the nozzle to form fibers with a single structure. In co-axial mode, fibers can be developed as core-shell structures — especially when encapsulation is desired — or in split structures known as Janus fibers. Multi-axial phase is the third way to create fibers where more than two solutions can be spun at a time.

Typical electrospinning heads consist of a single capillary needle. However, to increase electrospun fiber throughput, multiple parallel emitters have been implemented in the spinning head. Since all emitters are fed from the same solution feed, production rates and fiber yield are consequently increased.

An additional feature of the emitters during the electrospinning process include the ability to control end product fiber diameter. Shortly, fiber diameter is directly proportional to the emitter inner diameter. Thus, while larger fiber diameters can be obtained with larger inner diameters, smaller fibers can be achieved with smaller inner diameters.

Diagram of different emitters used to electrospin fibers
Many types of fibers can be generated by integrating different emitters in the electrospinning process.
25 Gauge
Color: Red
Outside diameter: 0.52 mm
Inside diameter: 0.25 mm
23 Gauge
Color: Orange
Outside diameter: 0.65 mm
Inside diameter: 0.33 mm
22 Gauge
Color: Blue
Outside diameter: 0.72 mm
Inside diameter: 0.41 mm
20 Gauge
Color: Pink
Outside diameter: 0.91 mm
Inside diameter: 0.61 mm
15 Gauge
Color: Amber
Outside diameter: 1.65 mm
Inside diameter: 1.36 mm
14 Gauge
Color: Olive
Outside diameter: 1.83 mm
Inside diameter: 1.54 mm

Collectors

Once solidified during the spinning process, electrospun fibers can be collected into a wide variety of collectors that mostly consist of stationary and rotating platforms. These collectors will have a significant impact in the final design of the electrospun sample. Stationary collectors are commonly flat plates to collect random fibers where the extent of the area can be tailored to a desired limit. Rotating collectors include disks, mandrels, and drums, all of which have a wide variety of dimensions. Mandrels collect fibers in random orientation due to the short diameter. Conversely, disks and drums are able to collect highly aligned structures if the diameter/length ratio and rotating speed are high enough. Larger linear speeds allow stretching of the fibers during collection and tend to encourage high degrees of alignment.

Collector Type —Flat plateDrumMandrelDiskRoll-to-roll
Dimensions 40 cm x 40 cm (L x W)20 cm x 30 cm (D x L)5 mm x 30 cm (D x L)20 cm x 1 mm (D x L)50 cm x roll capacity (W x L)
Functions Random fibers, particles, and 3D structuresRandom and aligned fibers; collection of particlesBlood vessels and hollow cylindrical samplesAligned fibers and yarnsRandom fibers and particles

Effects of Solution Parameters

In order to obtain reproducible electrospun samples, the polymeric or non-polymeric solution to be processed needs to be reproducible by itself. Otherwise, non-consistent results are obtained, despite maintaining all electrospinning parameters the same between batches. Some of the key parameters to maintain batch-to-batch consistency for electrospun nano- and microfiber development are viscosity, conductivity, and surface tension — all of which are affected by polymer, solvent, and additive selection. Additional factors like solid content and solvent selection are measured to maintain solution reproducibility and manipulate fiber morphology, respectively. Above all else, the solution should be well mixed and homogeneous before being processed through electrospinning.

Viscosity

Viscosity is one of the key properties in electrospinning that is influenced by the molecular weight of the polymer (along with solution concentration and solvent selection of course). In theory, higher molecular weight or higher concentration translates into higher viscosity. Solvents that have more interaction with the polymer structure also tend to result in a solution with higher viscosity.

On one hand, processing with a high viscous solution encourages polymer chain entanglements to occur during the jet formation. These entanglements are the source of the fiber structures characteristic of electrospinning. On the other hand, processing a solution with a low viscosity translates to insufficient polymer entanglements during the process, resulting in the polymer jet breaking and forming particles instead of fibers. This is a different powerful process known as electrospraying.

Conductivity

Solvent and polymer content can affect the overall solution conductivity. Solutions with higher conductivity tend to generate bead-free fibers. The solvent quality and additives will also affect the conductivity of the overall solutions. This is one reason why using the same solvent grade should be maintained to allow for solution reproducibility.

For example, some solvents contain additives or stabilizers that then affect solution conductivity and its spinnability, eventually. Inorganic and organic salts, along with surfactants, can be used to selectively manipulate the solution’s conductivity. When increasing the overall charge density of the solution, jets formed during the electrospinning process can be optimized to promote bead-free fibers.

Solvent Selection

A solvent able to dissolve the polymer(s) and additive(s) while maintaining a homogeneous solution is always desired. Ideally, the solvent in the polymer solution should completely evaporate during the electrospinning process. Otherwise, generated fibers could be soaked wet and cause fused fiber-fiber bonding.

A scientist must be mindful of the solvent properties to avoid undesirable defects. Chief among important solvent properties are boiling point, dielectric constant, and vapor pressure. Using a solvent that has a low boiling point could result in complete evaporation. However, if it evaporates too fast, the polymer will clog the needle, thus disrupting the entire process. To remedy this issue, a solvent with a lower vapor pressure could be selected. Fiber diameter will also be affected by boiling point and vapor pressure. In general, lower boiling point solvents tend to form high fiber diameter. The opposite relation applies for vapor pressure and dielectric constant, however. As an example, polycaprolactone (PCL) fibers have been generated in the nanoscale when high boiling point solvents (like acetic acid) were used. Additionally, microscale PCL fibers formed with low boiling point solvents like dichloromethane (DCM) or 1,1,1,3,3,3-hexafluoroisopropanol (HFIP).

Table of Solvents

Solvent nameSolvent abbreviationBoiling point (°C)Vapor pressure at 20°C (mmHg)Dielectric constantSurface tension at 20°C (mN/m)Density at 20°C (g/mL)
Acetic acidAA11811.46.2271.049
AcetoneAce56185.521.525.20.788
ChloroformCHF611604.827.51.489
DichloromethaneDCM403538.9326.51.326
N,N-DimethylacetamideDMAc1662.2537.836.60.937
N,N-DimethylformamideDMF1522.336.737.10.945
Dimethyl sulfoxideDMSO1890.4246.743.541.096
EthanolEtOH7844.6324.522.10.789
Ethyl acetateEA77736.023.90.901
Formic acidFA10142.9757.937.671.221
GlycerolGly2900.000213.263.41.261
HexaneHX691241.918.430.659
1,1,1,3,3,3-Hexafluoro-2-propanolHFP58120.0115.714.71.596
Isopropyl alcoholIPA823317.9230.786
MethanolMeOH65126.7632.722.70.791
Methyl acetateMA57216.028.024.80.932
N-Methyl-2-pyrrolidonenMP2020.2933.040.791.026
TetrahydrofuranTHF661277.626.40.888
WaterW1001880.372.80.998

References:
Dielectric constant | Vapor pressure | Surface tension I | Surface tension II

Surface Tension

As mentioned before, electrospinning is dependent upon the electrostatic force overcoming the surface tension of the solution. As a consequence, solutions with low surface tensions are usually preferred for electrospun sample development. Surface tension is affected by polymer concentration, solvent selection (single or co-solvent systems), and additives (ex. surfactants). When measuring surface tension, it is recommended to use the same type of needle gauge used during needle-based electrospinning process as it is known that needle gauge itself tends to affect surface tension.

Solid Content

Solid content is also measured to confirm that a solution has been prepared properly. For this, moisture analyzers are usually employed. As a rule of thumb, higher solution concentration (or solid content) translates to higher fiber diameter.

Electrospinning Processing Parameters

Once a homogeneous and well mixed solution has been obtained, the electrospinning process can be initiated. To allow for sample reproducibility, multiple electrospinning parameters must be monitored and recorded to allow for reproducible fiber morphology and batch-to-batch sample consistency. These parameters are as follows.

Voltage

In electrospinning, needles typically carry positive charges which prompt the fibers to accumulate on the negatively charged or grounded collector. Polarity can be reversed so that the needle is rather negatively charged instead of the collector, but this is not a frequent practice. Created with BioRender.com.

Needle Voltage

In order to start the electrospinning process, a voltage needs to be applied to the needle for charges to accumulate and overcome the surface tension. A positive voltage is commonly applied; meanwhile, negative voltage could also be employed to start the process.

Higher positive voltage in the needle typically improves the stretching and the electric field, causing smaller fiber diameter to be formed. Some studies have seen the opposite, or no effect on fiber diameter, as it is dependent on the system to be processed.

Collector Voltage

Although the collector is typically grounded to collect electrospun fibers, researchers are frequently using an opposite charge to the needle in the collector (opposite charges attract each other).

A negative voltage is key to improve sample deposition, decrease sample deposition width, properly collect all generated fibers, and prevent them from going throughout the spinning chamber due to charge repulsion, and improve final sample surface morphology as packing density can be improved (see image below).

An example on how collector voltage influences fiber deposition

The subsequent image conveys three cases of polycaprolactone electrospinning with variable voltages. Each session is consistent by implementations of cylindrical mandrel collectors, sessions differing only in the collector biases of 0 kV, -5 kV, and -15 kV, respectively. With increasing bias, it is evident that thinner and more focused depositions occur.

Voltage and fiber deposition

Needle-Collector Distance

Needle-to-collector distance D

Needle-to-collector distance an important key factor affecting the electric field during electrospinning processes. To obtain dry and defect-free (ex. fiber-fiber bonding) electrospun fibers, the needle-to-collector distance needs to be properly selected for the solvent to completely evaporate through the bending and whipping motion while fibers travel from the needle to the collector.

Decreasing the needle-to-collector distance results in less travel time, though also wet fibers. However, some applications benefit from lesser travel times to induce small fiber-fiber bonding. The aim of fiber-fiber bonding is to improve mechanical properties or adhere two layers between each other and avoid delamination.

Needle Translation and Speed

The majority of researchers perform what’s referred to as ‘static needle-based’ electrospinning. Although this is enough for sample development and optimization, the final sample will not have a homogeneous thickness because a bell-shaped thickness will be formed in its stead (as shown below).

In order to improve sample thickness, needle translation allows for more homogeneous distribution (compared to the right plot). Needle displacement and speed are commonly reported to obtain a well distributed thickness on the final electrospun sample.

Contrast between sample thicknesses corresponding to a static needle and a translating one.

Flow Rate and Syringe Type / Size

Finding an ideal flow rate able to maintain a stable Taylor cone is ideal and is known to be solution dependent. Increasing solution flow rate during the electrospinning process results in larger final fiber diameter. If generating a sample with a syringe pump, it’s ideal to know your syringe material (ex. plastic, Teflon, and glass), type (BD plastic, Hamilton, among others), and diameter (ex. syringe plunger displacement will affect the volume dispensed).

Needle Dimensions

Blunt needles are typically used for needle-based electrospinning. Needle gauges commonly used are 14 to 26 Ga (inside diameter of 1.600 to 0.260 mm; note that by going up in needle gauge, the inside diameter decreases), and with a needle length of 0.5 to 2 inches.

Selecting a proper inside needle diameter is key to prevent needle-clogging due to solvent evaporation. As a rule of thumb, low needle gauges (ex. 14 Ga) are better for solutions with high viscosity, and high needle gauge needle (ex. 26 Ga) are better for solution with low viscosity.

Collector Dimensions and Rotating Speed

Electrospun fibers can be collected in different types of collectors to fulfill application needs. Flat plate collectors are the most commonly used to obtain fibers with random orientation and with a flat surface. Typical applications where this type of collectors are used are for drug delivery, wound healing and energy storage where a large surface area is needed.

Drum collectors (outside diameter typically > 5 cm) are another example to collect fibers through the electrospinning technique. One of the advantages of using a rotating drum is that based on the drum diameter and rotating speed (ex. linear speed), random or aligned fibers can be obtained. At high linear speeds one will typically obtain aligned, or super aligned, fibers that can be used for cell proliferation or to increase sample mechanical properties like tensile strength. On the opposite side, using low linear speed will allow to obtain fibers randomly oriented. Other collectors commonly used with the electrospinning technique are small mandrels to generate artificial blood vessels or capsules for drug delivery, rotating disk to generate aligned fibers and roll-to-roll collectors to produce fibers on a semi-continuous or continuous process.

Sample Collection Substrate

Fiber packing density can be influenced by the type of substrate used in the collector. Aluminum foil is commonly used in the electrospinning field to collect fibers as it is an inexpensive conductive material. One of the main problems from using aluminum foil as the substrate is that sample removal post-processing is not easy and the material can stick to its surface, generating a deformed sample upon removal. Non-conductive collectors can be an option to collect fibers but charges will accumulate over time making the fibers to fly away from the collector and generating lower-packing density. We have found that a combination of conductive and non-conductive material work well to improve sample collection and to allow ease of sample removal like the video below.

Temperature and Relative Humidity

Monitoring processing temperature and relative humidity (RH) is key for electrospinning reproducibility. Solvent evaporation rate and solution viscosity is influenced by temperature. At high temperatures the viscosity will decrease, allowing the user to obtain smaller fibers as the stretching of the jet is increased. Relative humidity influences the solvent evaporation rate, which will have an impact on fiber microstructure and continuous spinnability.

For example, if using a low boiling point solvent in solution and electrospinning at low RH may cause needle clogging. This is not ideal as the Taylor cone will not be maintained over time, causing the process to not be continuous. Meanwhile, if a high RH is used water droplets can accumulate in the surface of the fiber which will lead to fibers with a porous microstructure. The ideal goal is to optimize the environmental conditions to allow for a stable Taylor cone where no needle-clogging occurs over time, the solvent is successfully evaporated during the process and non-desired defects (ex. fiber-fiber bonding, fiber beading, fiber bundles, particles, among others) are encountered in the sample.

Air Flow

In order to improve solvent evaporation, air flow can be introduced in the electrospinning chamber at a desired speed. This parameter, in combination with a slightly negative pressure in the chamber, is more frequently used these days in the field to properly remove evaporated solvents during sample processing. Typical air flows used are 50 to 180 m3/h while constantly maintaining temperature and relative humidity over the minute.

These models convey how various gases can be implemented into electrospinning processes. Created with BioRender.com.

Air flow is also used in coaxial mode where the solution to be electrospun travels through the core needle and a gas (ex. air and nitrogen) flow through the needle. This process is known as gas assisted electrospinning where the pressurized gas aids the electrospinning process by improving solvent evaporation. Applying a hot air is also another alternative known as hot-air gas assisted electrospinning that furthers improve solvent evaporation when needed. A third option is by using solvent vapors around the needle to prevent needle clogging with solutions that easily clog the needle during electrospinning. Finally, blow spinning is an alternative method where no voltage is applied in the needle, but only pressurized air during sample development.

Needleless vs. Needle-Based

There are two ways nanofibers can be made through the electrospinning technique: needle-less and needle-based. Prior to selecting which method it is important to consider the advantages and disadvantages of both.

In needle-less electrospinning, the starting polymer solution is typically transferred to an open vessel where the fibers are generated from a stationary or rotating platform. Mass production of material is one benefit in the needle-less electrospinning process but there are many disadvantages. Fiber morphology and quality are not precisely controlled, the raw materials that can be utilized are limited, which in turn limits versatile fiber production and process parameters such as flow rate, cannot be controlled.

In needle-based electrospinning, the starting polymer solution is typically contained in an air-tight closed reservoir, this minimizes and prevents solvent evaporation. This important difference allows for a wide variety of materials, including high volatile solvents, to be easily processed. There are many advantages of needle-based electrospinning, including flexibility to process different structures like core-shell and multi-axial fibers.

This distinction allows for active pharmaceutical ingredients (API) to be incorporated within a fiber. Additional advantages of needle-based electrospinning is tightly controlled flow rate, number of jets and minimizing solution waste. These advantages of needle-based electrospinning have increased the popularity of this technique, resulting in a high number of published work on electrospun fibers that offer a wide variability of solution recipes.


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