Biomedical Engineering Reference
In-Depth Information
The Berry number has four distinct domains.
According to the value assigned to
B
of a poly-
mer, a researcher can determine the likelihood
the polymer will produce nanofibers
[62]
.
Region I, where
B
< 1, is representative of a
very dilute polymer solution with limited
chain entanglement. This results in only poly-
mer droplets being formed. In Region II,
1 <
B
< 3, the fiber diameter increases within the
range of 100-500 nm as
B
increases. This region
is indicative of molecular entanglement that is
just sufficient for fiber formation. Although
fiber formation is observed, there is still some
droplet formation as a result of polymer relax-
ation and surface tension. In Region III, 3 <
B
< 4,
the fiber diameter increases rapidly with
B
and
is in the range of 1,700-2,800 nm. The rapid
increase in fiber diameter is attributed to the
intensive molecular entanglement resulting in
an increase in polymer viscosity. The conse-
quence of increased polymer viscosity also
means that a higher electric field is required to
produce fibers. Finally, in Region IV, where
B
> 4 and there is significant chain entangle-
ment within and among chains fiber diameter
is more dependent on the applied voltage/
electric field and other process parameters
than it is on
B
[62]
.
The two quantitative models for calculating
fiber diameter are a sampling of what research-
ers have produced thus far, with more complex
examples existing. They serve as a suitable start-
ing point for researchers, suggesting parameter
boundaries to achieve desired fibers. There are
also numerous other variations that can be made
to electrospinning parameters to engineer
favorable fibers.
the areas of tissue engineering and regenerative
medicine, aligned fibers can serve as a physical
guide for cellular growth, influence cell adhesion,
and modulate cellular patterns found in native
tissue
[103]
. They are particularly useful when
employed to regenerate tissues that require direc-
tional recruitment and assembly of cells, for
example, in neural tissue engineering. Cooper
et al
.
[103]
demonstrated the benefits of aligned
chitosan-PCL fibers over films and randomly ori-
ented fibers of the same materials in promoting
the attachment and proliferation of Schwann
cells, which are important cells of the peripheral
nervous system. The aligned fibers induced
cellular responses as a result of topographical
and chemical cues for the modulation of neurite
outgrowth
[104]
.
One way to fabricate uniaxially aligned
nanofibers or parallel arrays of fibers is to use a
rapidly rotating drum or cylinder as the collec-
tor. The rotating collector forces fibers to align
in a perpendicular orientation to the axis of rota-
tion of the drum. This method, however, only
produces partially aligned fibers
[62]
. To improve
alignment, researchers modified the drum by
adding a sharp edge.
Another method to fabricate aligned fibers is
to use a pair of split electrodes
[60]
. Two conduc-
tive strips separated by a gap of up to several
centimeters allow for the synthesis of aligned
nanofibers in the gap. Researchers believe that
the insulating gap alters the electrostatic forces
acting on the fibers in the gap. As a result, elec-
trostatic forces act in opposing directions and
fibers are stretched, aligning themselves perpen-
dicular to the edge of the gap. The electrostatic
repulsion between deposited fibers can further
enhance the alignment of the collected fibers.
This technique can also be used to readily stack
the aligned fibers into films or mats for practical
applications
[105]
.
The overarching concept governing the previ-
ously described methods to fabricate aligned fib-
ers is the ability to control the electric field. Some
researchers have modeled the dominant role of
7.2.2.1.1 Electrospinning for uniaxially aligned
nanofibers
Aligning nanofibrous arrays is useful for a vari-
ety of purposes. When fibers are used to impart
additional mechanical integrity as a composite
component, control of the alignment dictates the
degree of structural support the fibers provide. In
