Biomedical Engineering Reference
In-Depth Information
FIGURE 7.1
Schematic of an electrospinning setup.
the stream's travel toward the collector. During
its flight, the jet undergoes a stretching and
whipping process to draw the fiber into an
ultrafine long filament. The solvent simultane-
ously evaporates and the fibers are deposited on
the grounded collector, thereby creating a non-
woven, randomly aligned fibrous mat
[60]
.
The parameters chosen during electrospin-
ning greatly influence the collected fibers. These
parameters are typically divided into three cat-
egories: polymer parameters, polymer solution
parameters, and parameters of the apparatus.
The type of polymer used and its physical prop-
erties greatly affect the nanofibers. These proper-
ties include the molecular weight, the molecular
weight distribution, and the branching of the
polymer
[12]
. Solution properties found to have
an integral role in fiber formation include viscos-
ity, polymer concentration, conductivity, and
surface tension. Important apparatus parame-
ters are applied flow rate, voltage, distance from
syringe needle tip to collector, type of collector
and whether it is static or dynamic, the type of
needle used, and the ambient conditions during
electrospinning
[61, 94-96]
.
One of the most studied dependent properties
of the fibers is the fiber diameter. Several research-
ers have attempted to sum up the effects of the
many independent variables of electrospinning
on fiber diameter into a succinct mathematical
model. Rutledge
et al
.
[102]
developed a mathe-
matical model that related surface tension
γ
,
static relative permittivity
ǫ
, flow rate
Q
, current
carried by the fiber
I
, and the ratio of initial jet
length to the nozzle diameter
χ
to fiber diameter
d
as follows:
1/3
γǫ
Q
2
I
2
2
π(2
LN
χ
−
3)
(7.1)
D
=
.
This equation was derived by fitting an exponen-
tial model to empirical data. According to this
equation, increasing the current-carrying capa-
bility of fibers by adding more conductive mate-
rials to the polymer solution will significantly
reduce fiber diameter
[62]
. It is also possible to
reduce fiber diameter by manipulating other
independent variables such as a reduction in
either the flow rate
Q
or the nozzle diameter
γ
.
Another model relates fiber diameter to the
molecular weight of the polymer and the con-
centration of the polymer in the spinning solu-
tion. It also uses the dimensionless parameter
called the Berry number
B
. The Berry number is
the product of the intrinsic viscosity
η
and poly-
mer concentration
C,
i.e.,
B
= η
C
.
(7.2)
