Ultimate Guide to Electrospinning
While there is no shortage of new and innovative technologies being explored nowadays, we find few of them as exciting as nanofibers. From tissue reconstruction to more effective and compact filters for air pollutants, there is a lot of potential for this up and coming technology. Electrospinning is the most commonly used process to create these nanofibers. What exactly is electrospinning and why is it the method of choice?
In this post, we will explain all the fact with this technology, such as the process, material avaiable.
What is electrospinning?
In electrospinning, a voltage is applied to a controlled flow of a polymer solution. This is typically done by ejecting the polymer solution through a spinneret or a metal syringe needle connected to a power source. When sufficient charge has accumulated on the droplet of liquid, it ejects automatically via electrostatic repulsion. This changes the shape of the droplet into a characteristic “Taylor cone” and stretches the liquid into a very thin fiber. Fibers made via electrospinning typically have diameters in the sub-micrometer to the nanometer scale.
Drying of the polymer solution is achieved almost instantaneously during this phase because of the fiber’s high surface area-to-mass ratio. Further elongation of the fiber happens via continuous electrostatic repulsion until they accumulate on a grounded collector plate or drum.
Several parameters, such as the type of polymer solution used and the applied voltage intensity, can be varied to control the characteristics of the nanofiber created via electrospinning. Today, electrospinning is the most common method for nanofiber production and is practiced in both laboratory and industrial-scale applications.
History of electrospinning
The basic phenomenon behind electrospinning was first observed way back in the 16th century by William Gilbert when he noted how a water droplet formed a characteristic cone shape when it came near a charged surface. Refining this concept further into electrospinning was first done in the late 1800s and patented by 1902.
By the 1930s and 1940s, efforts to scale up the electrospinning process for commercial and industrial use were underway. John Zeleny pioneered the use of metal capillaries while C.L. Norton developed a method for using polymer melts instead of solutions. Patents for the use of electrospinning technology to produce textile yarns were also filed during this period.
The first commercial-scale use of electrospinning produced the “Battlefield Filter,” a smoke filter element developed by the Aerosol Laboratory of the L. Ya. Karpov Institute and manufactured in the Russian city of Tver. Made from a cellulose acetate material, the so-called “BF” filter reached a throughput of 20 million square meters per annum by the 1960s.
The “Taylor cone” was a term coined by Sir Geoffrey Ingram Taylor in the 1960s. By mathematically modeling the response of a fluid droplet to an electric field, Taylor was one of the first few scientists who started developing a theoretical understanding of the electrospinning process.
In the late 1980s, more intensive laboratory studies on electrospinning aimed to use the technology to create fibers in the sub-micrometer and nanometer scales. Early success in the use of electrospinning to cell culture and tissue engineering opened the floodgates for more thorough scientific studies on the topic. Since the early 1990s, it has been claimed that scientific publications about electrospinning seemed to have grown exponentially year to year.
What is electrospinning used for?
Nowadays, work on electrospinning is no longer restricted to laboratory research. Due to their uniquely high porosity and surface-area-to-volume ratio, the nanofibers created through electrospinning have made their way into several biological, commercial, and industrial applications.
1. Drug delivery
Several groups have invested considerable effort into using electrospun materials for enhanced drug delivery systems. Compared to more traditional drug delivery vesicles, electrospun materials provide more degrees of freedom in terms of surface-area-to-volume, allowing for higher drug-loading capacities. In terms of materials, it is possible to use typical biocompatible polymers such as gelatin and alginate for the electrospinning process.
Antibiotic treatment is considered one of the foremost pharmaceutical fields where electrospinning can be valuable. There are two approaches to incorporating drugs into electrospun materials – they can either be coated on the fibers or incorporated directly into the polymer solution.
In any case, the goal is to deliver a higher concentration of the drugs which will then be released over a longer period of time. Initial results have been promising. In a study conducted in 2007, an antibiotic coated on a polycaprolactone (PCL) membrane resulted in an 80% release over the first three hours, with the residual drug releasing over the next 18 hours.
2. Tissue engineering
In the field of tissue engineering, nanospun materials are used to mimic the characteristics of an extracellular matrix. These nanofiber scaffolds act as supports for cell growth, and eventually, tissue regeneration. Again, the compatibility of electrospinning with biocompatible materials makes it ideal for producing scaffolds that can be safely resorbed into the body.
Copious levels of research have been dedicated to developing electrospinning methods to aid the recovery of muscles, ligaments, bone tissue, skin, and blood vessels. There are still some challenges such as the difficulty of creating thick scaffolds via electrospinning. Electrospun meshes also do not provide the best medium for cell infiltration on account of their very small pores.
3. Food packaging
It may seem counter-intuitive to use a porous material created via electrospinning as food packaging. However, research into this particular application has focused on the integration of beneficial active compounds into an electrospun mesh. Thus, packaging material can be enhanced to be less susceptible to deterioration by moisture, oxidation, and temperature. There have also been studies in the incorporation of nutraceuticals or antimicrobials into food packaging, which should also help improve the shelf life of food products.
The biggest challenge for this application is the lack of reproducibility of creating nanofiber meshes, especially for large-scale production. Even when going through the same process, there is an element of randomness in how fibers will be positioned and oriented within the mesh. This lack of predictability in terms of the performance of the final product can be problematic when dealing with matters as finicky as food packaging.
4. Textile treatment
Nanofiber treatment in textiles has been explored as a way to develop a fabric that is waterproof, yet breathable. The idea is to create a densely woven layer of electrospun nanofibers as a treatment for standard fabrics, providing water-resistance without compromising the fabric’s breathability. The small pores provide better resistance to water penetration but maintain better air permeability compared to fabric that has been laminated or coated with a waterproof substance. This technology has a lot of potential applications from camping and outdoor gear to standard, everyday wear.
5. Solar cells and energy storage
Solar power is at the forefront of the energy transition movement nowadays, and nanofiber technology is being explored as a way to address one of its most problematic aspects – efficiency. Electrospun nanofibers integrated into solar panels provide a higher surface area and porosity, thus maximizing light absorption and improving the efficiency of conversion of photoelectric energy.
Energy storage pretty much goes hand-in-hand with solar power generation, given that solar on its own cannot provide baseload energy. Electrospun fibers can also be beneficial in this regard. Studies conducted on the use of carbon nanofibers as cathode material for lithium-air batteries indicate that they can provide higher storage capacities and power densities.
6. Air filtration
Given how electrospun fibers were first commercially used as filter materials, it’s not surprising that this field of application for electrospinning persists today. Compared to both activated carbon and cellulose filter media, nanospun fiber meshes exhibited higher removal of volatile organic compounds and microscopic dust particles. In the medical field, it has also been concluded that the pores of nanofiber filters can effectively reduce the intake of bacteria, viruses, and allergens by up to 99.9%
Advantages of electrospinning technology
Electrospinning may be the most common method to produce nanofibers, but there are also alternatives such as drawing, freeze-drying, phase separation, and self-assembly. While we won’t go into the details of how each method works, we can look at the benefits of electrospinning to understand why it is the method of choice.
- Relatively inexpensive
Compared to other means of nanofiber production, electrospinning is a relatively simple and inexpensive method. The equipment needed to do electrospinning does not need to have a large footprint, making it more friendly for laboratory-scale settings. This is likely also part of the reason why electrospinning has been more thoroughly studied than any other nanofiber technology.
- Produces finer fibers
Electrospinning provides a lot of opportunities for controlling the morphology of the fiber or mesh being produced. By adjusting the voltage bias between the spinneret and the collector, the diameter of the resulting fiber can be increased or decreased. Moreover, the dependence on electrostatic repulsion to pull the polymer material into fibers allows for smaller diameter fibers compared to methods like drawing that relied on more mechanical forces.
- Versatile material compatibility
Unlike other methods, electrospinning can create nanofibers out of just about any type of polymer provided that they have sufficiently high molecular weights. This is in contrast to methods that can work only with high-viscosity solutions.
Despite the benefits, electrospinning is not quite a perfect method. The random manner in which fibers deposit on the grounded collector makes reproducibility a challenge, and it’s very difficult to electrospin thick or large-volume scaffolds.
The electrospinning process is fairly simple but still has room for parameters to be tweaked. These parameters include voltage applied to the spinneret, the flowrate of the polymer solution, the diameter of the opening of the spinneret, the distance between the needle and the collector, the type of grounded collector. Changing any of these parameters can have profound effects on the density of the resulting mesh, the diameter of the fibers, and the geometry by which the individual fibers build the mesh.
In a typical electrospinning setup, the polymer solution is contained inside an enclosed container that extrudes towards a metallic syringe or pipette. A piston controlled to a metering pump provides control over the rate at which the solution is extruded through the pipette.
A power supply connected to the metallic syringe or pipette is responsible for carrying a charge to the polymer solution. Power output is typically high – somewhere in the range of 5 to 30 kV. At the minimum, the charge has to be enough for electrostatic repulsion to overcome the fluid’s surface tension. For most polymer solutions, the value of this surface tension is somewhere between 20 to 40 N/m2.
The electrostatic repulsion results in the gradual drawing of the polymer solution and the liquid droplet taking the characteristic “Taylor cone” shape. The solvent almost instantaneously evaporates during this phase as the polymer elongates at very high strain rates.
This stream initially takes a linear path but is disrupted as it travels closer to the grounded collector. This instability is caused by electrostatic repulsion as charges accumulate on various bends in the fiber. These random movements approximate a spiral shape, which helps further stretch the fibers.
Electrospinning setups can have grounded collectors that are either stationary or rotating. Rotating collectors help in creating meshes that have more aligned fibers, while static collectors maximize the randomness of the fiber alignment. Each option has benefits and drawbacks in terms of the pore size and mechanical integrity of the resultant nanofiber mesh.
Depending on the intended application, nanofibers produced via electrospinning can be created from either natural or synthetic polymers. This versatility in compatible source materials is one of the greatest strengths of the electrospinning process. The following is a short list of the most commonly used polymers in electrospinning:
- Polyglycolic acid (PGA)
- Poly(lactide-co-glycolide) PLGA
- Polyvinyl acetate (PVA)
- Polyurethane (PU)
- Poly (L-lactide) acid (PLLA)
- Polyethylene co-vinyl acetate (PEVA)
- Polycaprolactone (PCL)
Nanofiber technology has proven to be one of the most exciting fields of research in the past few years. As the primary method for creating nanofibers, electrospinning is as equally interesting. Despite being fairly simple in principle, electrospinning is an extremely adaptable technique and is economical for both commercial production and lab-scale studies.