Biomedical Engineering Reference
In-Depth Information
solutions. In figure 4.77, the importance of osmotic pressure is illustrated. It should
be noted that the maximum displacement could be determined when conditions were
switched from pure water to acidic conditions. These findings are important to
describe casual pH hysteric behaviors reported previously.
4.6.7
N
ANOFIBER
E
LECTROSPINNING
IN
G
ENERAL
The electrospinning process is a variation of the better known and understood
electrospraying technique. In the electrospinning process, a high electric field is
provided between a highly viscous polymer solution held by its surface tension at
the end of a capillary tube and a metallic target, as shown in figure 4.83
As the intensity of the electrical field increases, the surface of the liquid hemi-
spherical drop (fig. 4.84a), suspended at equilibrium at the capillary tip, elongates
to form a conical shape, which is known as the
Taylor cone
. The “balancing” of the
repulsive electrostatic force with the surface force of the liquid causes this distortion.
When the electric field reaches a critical value (~0.5 kV/cm), the charge overcomes
the surface tension of the deformed drop and a jet is produced, as shown in figure
4.84b. As a result of the low surface tension in low-viscosity solutions, the jet breaks
apart into a series of droplets. This is the basis of electrospray technology. The
electrically charged jet does not break apart for high-viscosity solutions, but instead
undergoes a series of electrically induced bending instabilities (“necking”) during
its passage to the collection screen that result in hyperstretching of the jet stream.
This stretching process is accompanied by the rapid evaporation of solvent mole-
cules that reduces the diameter of the jet in a “cone-shaped” volume. This is called
the “envelope” cone (fig. 4.83). The as-spun dry fibers accumulate on the surface of
the collection screen. This process results in a porous, nonwoven mesh of nanofibers.
Polymer melts have also been processed into nanofibers and the metallic target used
to quench the as-formed molten fiber mats. Replacing the metal target with a grounded
coagulation bath target leads to instantaneous demixing of the polymer solution, which
leads to the production of continuous nanofiber filaments.
The “under-researched” area of electrostatic fiber spinning is thus a highly
versatile process that offers a potentially valuable tool for creating nanofiber struc-
tures. Since its discovery, this approach has remained little more than a laboratory
Metal target
Fluid droplet
Cone envelope
High voltage
FIGURE 4.83
Schematic of the electrospinning process.
