Electrospinning diameter from several microns to 100nm or less

Electrospinning
is a unique spinning fibre production method whereby an electrostatic force is
applied to a polymer solution or melt to produce fibres ranging in diameter
from several microns to 100nm or less for tissue engineering purposes. It is an
efficient, rapid and inexpensive process which is why there is a renewed
interest in electrospinning for applications in tissue engineering. The
high-voltage electrostatic charged applied to the polymer droplet creates a
repulsive force. Once a sufficient voltage is reached, the repulsive force
overcomes the surface tension of the solution in the syringe and the droplet is
stretched, creating a jet which accelerates from the tip of the syringe towards
a grounded metallic collector 39. The point of eruption of this jet is known as the Taylor Cone. The
Taylor cone consists of three phases, which are illustrated in Figure 2.3
below.

Figure 2.3: Formation of
Taylor Cone (A) Dripping Zone, (B) Transition Zone and (C) Jet Forming Zone 40.

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The dripping
zone (A) is when the applied voltage has not yet overcome the effects of
surface tension which causes the drop to accumulate at the needle tip. The
second zone (B) is known as the transition zone. Here, the effects of surface
tension have been partially exceeded. The third zone (C) is known as the jet
forming zone. Here, the effects of surface tension have been exceeded due to a
high voltage applied causing the liquid to jet from the needle. As the jet
accelerates towards the grounded collector, at slow acceleration the jet is
stable and ohmic flow occurs while the solvent begins to evaporate. However, as
the acceleration of the jet increases, the flow becomes convective as the
charge present in the jet moves to the surface of the fibre. The charge present
at the surface then begins to repel itself due to small bends in the fibre,
resulting in a whipping instability which continues until the fibre is finally
deposited on the grounded collector 41. The jet and whipping instability created is illustrated in Figure 2.4
below.

 

Figure 2.4: Schematic of a
typical electrospinning setup whereby fibres are collected onto a rotating drum
42.

Figure 2.4 above
also illustrates the collection of the electrospun fibres onto a rotating drum.
This is to enable the collection of aligned nanofiber matrices whereas randomly
orientated nonwoven fibres are produced when a static flat plate is used as the
collector 43. Additionally, there are several parameter variables which can
change the composition of the electrospun fibres. The molecular weight,
viscosity, surface tension and conductivity of the solution can all affect the
results of the spinning process. Controllable processing parameters which
affect the electrospun fibre include the flow rate of the solution, the
distance between the needle tip and the collector, the electrical voltage
applied to the needle tip and the size of the needle gauge. All of these
factors must be taken into account in order to find the optimal parameters to
produce the required fibre.

As mentioned
previously, electrospinning has been shown to be a
somewhat successful technique in the fabrication of synthetic scaffolds for
vascular tissue engineering.8 Mo et al have reported that PLCL scaffolds
which mimic native ECM have demonstrated favourable interactions with SMCs and
endothelial cells 44. Also, a ‘mechano-active’ small diameter
vascular graft was reported to have been produced via electrospinning by Kwon
et al in 2006 45. Although these results are promising, the
structure and architecture of native blood vessels has still yet to be achieved
which is why a suitable, readily-available TEVG has yet to be produced. This is
due to the dynamic instabilities in the solution-electrospun jet as there is little
control over the deposition of the fibres and the rotational speed of the
collector must be sufficiently high in order to collect aligned fibres from the
jet. In addition, the presence of solvents can be problematic as volatile
solvents used in solution electrospinning can be toxic to cells and tissues.