Biomedical Engineering Reference
In-Depth Information
Polymer solution/melt
Taylor cone
High-voltage
power supply
Bending instability
region
Collection screen
FIGURE 5.3
Schematic drawing of the electrospinning process. As the metal capillary is charged by a high-
voltage bias, polymer solution or melt is ejected from the tip forming the Taylor cone. The polymer jet spirals
downward while bending and stretching, creating ultrathin fi bers. Processing conditions, such as working
distance, voltage, and concentration, can be easily adapted to allow for the fabrication of different fi ber diam-
eters and orientation. Orientation is changed simply by using different collection devices such as dual rings,
rapidly rotating drums, etc.
to the electric fi eld is imparted on the polymer jet, and a spiraling of the jet results. The diameter
of the spiral is eventually balanced by the surface tension of the polymer solution or melt; however,
the polymer jet undergoes magnitudinal (up to fi ve orders) reduction in diameter as it travels toward
the collector. Solvent evaporation or cooling of the polymer prior to landing at the collector results
in electrospun fi bers with sizes typically between 200 nm and 5 µm, depending on the conditions.
The effects of aerodynamics and gravity are small, and electrospinning can be performed with
the spinneret(s) and collector(s) in either the vertical or the horizontal axis [5]. Several methods of
nanofi ber collection systems are shown in Figure 5.4 below.
The path of the polymer jet and the forces involved with it are extremely dynamic and the col-
lection is rapid. It has been determined that the initial polymer jet reaches a speed of 15 m/s before
velocimetry data become diffi cult to interpret due to the bending instabilities. Such high speeds,
coupled with long spiraled traveling distances, make accurately controlled deposition of the elec-
trospun fi ber technically challenging.
Electrospinning parameters such as applied electric voltage, polymer solution or melt fl ow rate,
needle-to-collector distance, solution concentration, and solvent type affect the resulting fi ber mor-
phology and dimensions. Figure 5.5 shows the effect of changes in electric voltage in the resulting
dimensions of a poly(caprolactone) (PCL) and collagen (PCL/Col) blend system. It was observed
that increasing the voltage from 10 to 20 kV decreased the average diameter of the fi bers whereas
doubling the fl ow rate from 0.75 to 1.50 mL/h did not affect the diameter signifi cantly. However, it
was reported elsewhere that the relationship might not be linear. Baumgarten revealed that increas-
ing the applied fi eld up to a certain threshold might instead cause incremental increase
in the fi ber
diameter. It was proposed that increasing the electric fi led strength causes an increase in volumetric
fl ow rate of the polymer out of the capillary [6].
The effect of polymer concentration in morphology and dimensions of electrospun collagen type I
fi bers can be observed in Figure 5.6. Bovine collagen was dissolved at varying solution concentration
