Electrospinning Nanofiber Webs
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- Mayurakhi Khan
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Table of Contents
What is Electrospinning?
Electrospinning is a continuous non-heat and non-chemical process where a polymer solution with a positively charged jet produces a Taylor cone through a negatively charged collector and results in uniform ultra-thin fibrous webs. Electrospun fibers are considered nanofibers when they possess a diameter thinner than 500 nm. They can be produced through different polymer solutions or include nanoparticles, drug-impregnated polymers or polymer blends and take their properties when spun. Unlike electrospraying, electrospinning does not break down into droplets and instead forms continuous fibers. The setup for electrospinning involves a polymer solution, a spinning head/jet (spinneret or hypodermic syringe), a high-voltage supply, and a collector, where the spinning heads generally have a positive high-voltage output and the collectors have a low negative voltage.
Nanofiber Webs
Nanofiber webs are created by using a polymer solution with a positive charge and putting them through a syringe, with which a droplet is made. The droplet makes a cone, called the Taylor cone, where the surface tension is overcome by the negatively charged collector. This charged collector is constantly spinning new webs. Electrospinning produces nano-fiber webs that are almost invisible with more than 95% light transmission, extremely lightweight (10-20 mg/m^2), rigid structures and fantastic filtering abilities with ~100% rejection of particles with diameters between 1um and 5um. Different factors affect the performance of nanofibers, such as polymer solution utilized, filter diameter, and flow temperature. Decreasing diameter and increasing temperature make the web more desirable and efficient.
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| Figure 1. Electrospinning process |
Taylor Cone
The Taylor Cone is essential in the electrospinning process, where an hydrodynamic force occurs on the tip of the polymer jet and is pulled thinly by the electrostatic forces to create a thread of webs where it can be collected by a wheel (AKA collector). The surface tension of the electrostatic liquid exposed to an electric field is broken to form a cone with convex sides and a rounded tip. The electric field’s voltage intensity controls the dynamics of the cone due to movement and production of charges between the field and the liquid. With the potential difference between the electric field and liquid, charges in the liquid separate and move towards the droplet. The droplet shape helps in reducing surface area repulsion.
Smart Properties
Electrospun nanofibers with smart properties include stimuli response, shape memory, self-cleaning, superhydrophilic surfaces, superhydrophobic surfaces, self-healing, and sensing.
a) Stimuli response
Nanofibers with stimuli responses can change in volume, shape (physical) or wettability (chemical), depending on external stimulus. Due to the natural porous structure of nanofibers, the response time is fairly quick and is determined by how quickly the stimulus can react with the polymer chains through the pores. Other factors that improve the speed of stimuli response is the surface-to-volume ratio and a smaller diameter, which allow a shortened distance of diffusion. External stimuli include changes in pH, temperature, light, electric fields or magnetic fields. In pH changes, qualities of water absorption, swelling ratio and solubility are exhibited. In temperature changes, nanofiber webs can exhibit differences in surface wettability and volume morphology. This is especially useful in cell culture and drug delivery, where temperature differs frequently. In changes in light, nanofibers undergo changes in light irradiation at various wavelengths. Changes in electric/magnetic fields produce volume changes, swelling/contracting, or specifically with pure iron, nanofibers can exhibit superparamagnetic properties and deflection with strong magnets.
b) Shape memory
Nanofibers have shown the ability to return to their original shape. They possess high strain recovery after deformation in response to external stimulus. Temperature is commonly used to create this effect. Polymers with shape memory properties often contain soft segments with low Tg or Tm, hard segments that provide a stable network structure and are heavily determined by the structure of polymers.
c) Self-cleaning
Self-cleaning coatings have been produced and applied to nanofibers due to their breathability and flexibility. They have been commonly used in clothing, masks and medical bandages. This property is controlled by structure and compositions.
d) Superhydrophilic surfaces
Superhydrophilic properties can be induced through nanofibers containing photocatalytic TiO2 and UV light. It is often used as a self-cleaning coating. Increases in diethanolamine can produce transparent nanoparticles from opaque nanofibers and provide high adhesivity.
e) Superhydrophobic surfaces
Superhydrophobic surfaces are inspired by the rolling motion of water droplets. Such surfaces must meet the requirements of having a high static water contact angle less than 150 degrees, low adhesion to water droplets, and lower adhesion to dust than water. Superhydrophobic surfaces can be created by introducing hydrophobic polymers when electrospinning or by the functional groups in the macromolecular structure of polymers.
f) Self- healing
Self-healing properties involve the idea of being able to detect and independently close up cracks/corrosion in material. This is done through interconnected network-like structures. Cracks are “healed” as integrated healing agents leave the cracked section of the nanofiber, and move to the surrounding matrix, which generates healing materials such as polymers or a cross-linked product to repair the damaged area. Monomer and curing agents are needed to generate healing materials but should not interact unless a composite has been damaged. They are separated with one reagent encapsulated, and the other distributed freely.
Different Techniques
Solution electrospinning
Polymer solutions such as organic polymers that are required to be dissolved and melted without degradation, will be produced through solution or melt electrospinning. Solution electrospinning is more commonly used and more efficient than melt electrospinning. In this technique, the stream of polymer solution is stretched and thinned by whipping instability, where the solvent can then be quickly evaporated and solidified to be collected. Parameters such as high molecular polymer weight and availability of suitable solvent for dissolving the polymer is required.
Emulsion electrospinning
This technique uses an emulsion, which allows the addition of drugs into polymer dissolved organic solvent. The drug is dissolved in an aqueous polymer and then mixed for electrospinning. It can directly integrate hydrophobic/hydrophilic compounds and commonly used into drug delivery with controlled release.
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| Figure 2. Comparison of different electrospinning methods |
Co-axial electrospinning
Co-axial electrospinning involves a dual-solution feed system where there are two pathways for the solution to enter the spinneret tip. Immiscible solutions produce core shell nanofiber structures, using a sheath fluid and core fluid. This method is also commonly used for drug delivery, where drugs are incorporated into the core of the fibers. There are also tri-axial and tetra-axial spinnerets for multiple solutions.
Melt Electrospinning
In melt electrospinning, fibers can electrospun from melted solvents. A heating device such as an electrical heating tape, circulating fluid and laser is present to keep a molten state in the spinneret. When the molten jet is ejected, the surrounding atmosphere/air cools and solidifies the stream. Polymer melts create lower electrical conductivity than polymer solutions, and have higher viscosities than polymer solutions. However, this results in a lower density of surface charges on the molten jet, which decreases the whipping instability efficiency and solvent evaporation, thus producing thicker nanowebs. Polymers used in melt electrospinning should have low melting points and good thermal stability (e.g industrial polymers), since polymers such as organic polymers and proteins will degrade upon heating. Factors such as heat temperature, viscosity and electrical conductivity can change the effectiveness/thinning of nanofiber diameter.
Types of Polymers
Electrospun nanofiber webs have the ability to retain properties from other components, therefore incorporation of varying nanoparticles, nanoscale components with different dimensions and polymers into polymer solutions have been widely used in creating and improving different properties in nanofibers. However with such additions, it is important to ensure continuous and uniform flow during electrospinning. This allows dispersion of different components to be uniform. Certain factors such as conductivity and viscosity may change electrospinning effectiveness.
Organic polymers include natural and synthetic polymers, which can be applied directly to electrospinning for its properties. Natural biopolymers such as DNA, chitin, collagen, gelatin, etc., can be added into polymer solutions and electrospun in nanowebs, as well as conductive polymers such as polyaniline and polypyrrole for conductive properties, functional polymers such as PVDF for piezoelectric properties, biodegradable/biocompatible polymers such as PLC and more. Organic polymers can be applied directly, but must be dissolved and melted without degradation. Methods incorporating organic materials are called solution electrospinning and melt electrospinning.
Colloidal particles
Colloid particles are particles that are suspended in a liquid and range between 10 nm and several microns. Colloids involve a dispersed phase with suspended particles and a continuous phase with a medium of suspension. Colloidal particles must be a specific size and have cross-linking structures for stable electrospinning, where viscosity is a large determinate for nanofiber web diameter.
Composites
Composites are prepared by adding sol-gel precursors or nanoscale components to polymer solutions, where the process of hydrolysis, condensation and gelation of the precursor is produced by contact with surrounding air in the jet. Although precursors such as metal salts, such as nitrates, chlorides and sulfates are commonly used, additives such as acetic acid or hydrochloric acid may be needed to stabilize the solution and process. The addition of a salt aids the conductivity of the solution, thus making electrospinning run smoothly. Also, the modifications of functional groups aid the stability of nanoscale components due its tendency to aggregate. The ability of inorganic polymer solutions with composites are dependent on the viscosity and electrical conductivity of the solution whereas the type, size and concentration of the nano components affect the morphology. The ability of electrospinning with composites can be affected by humidity or saturation of solvent vapor, where minimizing such factors can increase the rates of hydrolysis and gelation.
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| Figure 3. Basic smooth nanofiber webs (left) and beaded nanofiber webs (right) |
Applications
Environment/Sustainability
- Air purifying/Masks
Due to the high porosity, micrometer-sized interstitial space, self-cleaning properties, antibacterial properties and a large surface-to-volume ratio of nanofiber webs, they are ideal for collecting particulate matter (PM2.5)/pollutants. Nanofiber webs utilize the Brownian diffusion and interception for air treatment, resulting in an approximate 99.9% efficiency, ~100% rejection of particles between 1-5μm and filtering particles about 100 nm, where common airborne pathogens range from 0.3-10micrometers and viruses ranging from 0.02-0.3 micrometers. Decreasing the fiber diameter and increasing the flow temperature of air can improve the efficiency, as well as modifying it with active materials. They are commonly used in masks and HEPA filters.
b. Water treatment
The ability to separate and break down pollutants is ideal for water treatment. They are able to filter 3-10μm particles with a >95% rejection, such as toxic ions and organic pollutants through physisorption, chemisorption and electrostatic attraction.
Biomedicine
Due to the variety of structure and properties, nanofiber webs can be used in 2D/3D scaffolds for cell migration, stem cell differentiation (repair or regeneration of numerous types of tissue), cancer diagnosis and 3d tumor model. These functions can be attained through the modification of functional groups, patterning, biodegradability and porosity, as well as using different types of polymer solutions
REFERENCES
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Peter J. Lu (陸述義) and David A. Weitz. (n.d.). Colloidal particles: CRYSTALS, glasses, and gels. Annual Reviews. https://www.annualreviews.org/doi/full/10.1146/annurev-conmatphys-030212-184213#:~:text=Colloidal%20particles%20are%20small%20solid,10%20nm%20and%20several%20microns.&text=Colloidal%20suspensions%20are%20very%20important,to%20flow%20like%20a%20fluid.
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Zhang, C., Feng, F., & Zhang, H. (2018, August 18). Emulsion electrospinning: Fundamentals, food applications and prospects. Trends in Food Science & Technology. https://www.sciencedirect.com/science/article/pii/S0924224418303510.
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