Biomedical Engineering Reference
In-Depth Information
from the suspended liquid meniscus at the end of the capillary when the applied electric field
overcomes the surface tension of the liquid. Further increase in the electric field causes the hemi-
spherical surface of the droplet at the tip of the capillary tube to elongate and form a conical shape
known as the Taylor cone. When the repulsive electrostatic force overcomes the surface tension of
the fluid, the charged jet is ejected from the tip of the Taylor cone. Within a few centimeters of
travel from the tip, the discharged jet undergoes bending instability (Raleigh instability) and begins
to whip and splits into bundles of smaller fibers. In addition to bending instability, the jet under-
goes elongation (strain
10
3
s
2
1
) which causes it to become very long and
thin (diameter in the range of nanometers to micrometers). The solvent evaporates, leading to the
formation of skin and solidification of the fluid jet followed by the collection of solid charged
polymer fibers on the collector, usually in the form of nonwoven fabric.
Parameters that affect the formation of nanofibers during the electrospinning process include (i)
solution properties—viscosity, elasticity, conductivity, and surface tension, (ii) system properties—
hydrostatic pressure in the capillary, applied voltage, distance between tip and collecting screen,
and (iii) ambient parameters—solution temperature, humidity, and air velocity
[93]
. Comprehensive
reviews on this topic can be found in several monologues
[8,9,13,94]
. Parameters that control fiber
diameter are concentration of the spinning solution, electrical conductivity of the solution, and the
feeding rate of the solution through the nozzle.
The 1D electrospun fibers can be allowed to stack on the electrode to produce a three-dimensional
(3D) fiber mesh. These 3D structures have been used in cell cultures to test for tissue engineering appli-
cations
[95
10
5
and rate of strain
B
B
97]
. The orientation induced during the bending instability experienced by electrospun
fibers have shown to aid in cell growth and differentiation
[98]
. It is also possible to produce core/shell
fiber composite consisting of polymeric core and a low molar mass materials as core. Two dies
arranged in a concentric configuration and are connected to two reservoirs containing different spinning
solutions. The low molar mass of the core (such as water) makes it possible to serve as carriers for bio-
logical materials.
A number of biological molecules can be incorporated into electrospun fibers. Immobilization
of bacteria in electrospun nanofibers has been reported
[99]
. This opens up the possibility of elec-
trospun fibers to serve as carriers for drug and as controlled release agent. Drugs ranging from
antibiotics to anticancer agents and proteins
[92]
have been incorporated into electrospun scaffolds.
Electrospinning of core-shell fibers containing fluorescent proteins or the enzyme bovine serum
albumin has shown
[100]
that many of the functions of these proteins were retained.
While many natural fibers such as chitosan
[101]
, collagen
[102]
, silk
[103]
, hyaluronic acid
[104,105]
, gelatin
[106]
, and fibrinogen
[107,108]
have been electrospun into fibers, there have been
few reports of CNT and natural polymer electrospun composite. Electrospinning silk with CNT
resulted in a sevenfold increase in strength, 35-fold increase in modulus, and a fourfold increase in
electrical conductivity with the addition of 1% CNT
[109]
. At concentrations below percolation
threshold (see discussion on conductivity), addition of CNTs increased the mechanical properties of
PS/CNT electrospun fibers, while at threshold concentrations and above, the properties decline to
level below that of pure PS
[110]
. Significant increase in both modulus and tensile strength was
reported upon the addition of small amounts of MWNT to cellulose-MWNT fibers
[111]
.
Electrospun fibers have been reported to show promise in engineering a number of tissues
[92,98,112,113]
. The micro- and nanoscale features of fabricated fibers are similar to the hierarchi-
cal structure of extracellular matrix. The high surface area
volume ratio of electrospun meshes
