There are various methods of nanofibers production. One of the most flexible and cost-effective is electrospinning, allowing formation under the influence of high electrical voltage of long, continuous fibres with diameter ranging from few nanometers to several micrometers. Electrospun nanofibers are created from electrically charged jets of polymer solution or polymer melt. Unusual properties of resulting fibers predispose them for plenty of applications with the most promising being nowadays tissue engineering.
History of Electrospinning
Observation of electrospinning process started at the end of XIX century and continues during XX century (Reyleigh, Zeleny, Formhals, Baumgarten). Since the 1980s and especially in recent years, electrospinning process has regained more attention probably due to a surging interest in nanotechnology. In fact, name for this process ,,electrospinning'' derived from ,,electrostatic spinning'', is relatively new term, which appeared around 1994.
How does it work?
Source: Ning Zhu, Xiongbiao Chen ,,Biofabrication of Tissue Scaffolds'', in book entitled ,,Advances in Biomaterials Science and Biomadical Applications''
Electrospinning involves the use of a high voltage (commonly from 5 to 30 kV) applied classicaly between a needle of a syringe, filled with moleculary entangled polymer solution. The solution is pumped through a syringe at a constant rate, forming a droplet at the end of the needle. When the surface tension is overcome by electrostatic repulsion of charges within solution, jet is ejected, moving then toward metallic collector which is usually grounded. The solvent evaporates during jet travelling, while the jet is elongated at very high strain rates, ranging from 100 to 1000 s-1. The structure of nanofibers collected finally is far from thermodynamic equilibrium (metastable structures).
The most important forces acting first on a drop and then on a travelling jet are:
- external electrostatic field which acts directly on charges within polymer jet but also results in additional electric polarization of flowing material
- electrostatic repulsion among the charges
- surface tension, trying to reduce repulsion between charges
- the forces related to viscoelasticity of polymer.
Formation of nanofibres is mainly caused by repulsion occuring among charges resulting in high stretching of the polymer stream. Stream initially flows in straight. On the way to the collector, the various instabilities occur. The jet is seriously elongated by a bending and whipping processes caused by electrostatic repulsion initiated at small bends in the fiber, until it is finally deposited on the collector. The elongation and thinning of the fiber resulting from this bending/whipping instabilities leads to the formation of uniform fibers. The spiral movement of the stream substantially increases the path between the needle and the collector, resulting in a significant stretching (nanometer diameter).
Currently, there are two standard electrospinning setups, vertical and horizontal. The process can also be carried out using two several independent syringes/needles with different solutions.
Using a rotating collector (drum) we can affect the arrangement of fibers
Parameters affecting the process of electrospinning
The most significant effect for the process of electrospinning have the properties of polymer solution, i.e:
- solution viscosity depending mostly on polymer molecular weight as well as polymer concentration,
- solution surface tension,
- solution electric conductivity,
- dielectric constant of solvent.
Initialization of electrospinning process needs overcoming of surface tension of solution by electric interactions between charges. The surface tension of commonly used solvents does not vary seriously, being for most solvents between 20 and 40 mN/m2, with exception for very high surface tension of water. One of the conditions for electrospinning is that the viscosity should be above some critical value to prevent the breakage of the jet. The higher molecular weight of polymer (viscosity of solution) results in more entanglements, preventing the breakage of the fibers and formation of beads.
The second group of parameters which affect electrospinning process are processing conditions:
- applied voltage,
- flow rate of solution,
- type of a collector,
- diameter of a needle,
- distance between a needle and a collector.
The third group are environmental parameters:
Among nanofibers from polymers, being very important class of materials for medical applications (scaffolds), both natural like collagen, gelatin, chitosan, fibrine, etc., and synthetic like PCL, PLLA, PLA, PGA, PLGA, PU etc. should be mentioned.
Main advantages of electrospinning method are:
- simple and low cost equipment,
- possibility of scalling the process,
- possibility to control fiber morphology,
- practically all kind of polymers with high enough molecular weight can be processed by electrospinning.
Main disadvantages of electrospinning method are:
- used solvents can be toxic,
- problematic to obtain 3D structures as well as sufficeint size of pores needed for biomedical applications,
- process depends on many variables.
Nanofibres produced by electrospinning can be used for analytical chemistry, tissue enginnering, filtration techniques, electronic or enviromental engineering. In tissue enginnering, electrospinning of nanofibers is used for fabrication of scaffolds for tissue regeneration, mimicking natural ECM. Thanks to this material and method, we can create for instance artificial skin, bone and cartilage implants. Nanofibres also can be used for drug delivery system or vascular surgery.
Nanofiber mats used for medical dressings
Source: M. Mahfuzur Rahman Chowdhury, ,,Electrospinning Process. Nanofiber and their application'', http://www.cottonbangladesh.com/January2009/ElectroSpinning.htm
Source: Sarah Young, ,, New fiber nanogenerators could lead to electric clothing'', Media Relations, February 12, 2010
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