Bioscaffolds: Fabrication and Performance - Engineered Biomimicry - page 174

The most common core molecules are sensitive
biomolecules like the growth factors bone mor-
phogenic protein (BMP) and fiberblast growth
factor (FGF), which elicit favorable responses
from cells. The favorable responses of cells in the
presence of growth factors include enhanced
growth, cell proliferation, and cell differentia-
tion, all of which contribute to tissue growth. It
is necessary that molecules like growth factors
be introduced into the wound or defective area
in a sustained manner to ensure favorable cel-
lular function throughout the wound-healing or
tissue-formation process [113, 114] .
The polymer shell, in addition to protecting
the encapsulated biomolecules and moderating
biomolecular release, can function to make cells
and the implant interact favorably. This behavior
is normally achieved through simple chemical
modifications [115] to the polymer shell's sur-
face. Finally, the core-shell method is applicable
for encasing fibers that are mechanically strong
or have some other benefit but are not benign to
the cells at the site of implantation [115] .
In addition to encasing biomolecules and
other polymers, the core-shell method can be
used to alter the physical properties of electro-
spun nanofibers in an optimal way [114] . The
dynamic evaporation of solvent from the elec-
trospun fluid jets presents an opportunity for
manipulation of the resulting collected nanofib-
ers. As previously explained, electrospinning
has an instability region where the fluid poly-
mer jets are whipped and bent, stretched and
elongated. This is a result of the interactions of
the applied electrostatic force, the solution's
intrinsic viscoelasticity, and its surface tension.
As the fluid jets become increasingly sticky due
to solvent molecule transfer and evaporation,
the electrical forces gradually lose their influ-
ence on the fluid jets because electrons can only
easily interact with fluids. As a result, the elec-
trical drawing process stops when the entire jet,
or sometimes the surface of the jet, solidifies.
The use of surfactant solutions as the shell or
sheath fluid helps remedy this problem. As a

result of using surfactant solutions, the col-
lected fibers have reduced diameters and are
smoother compared to fibers spun with no
sheath solution [114] .
To achieve the core-shell geometry, the outer
solution consisting of a solution or a solvent is
loaded into a syringe and placed into a pump
apparatus. The core solution is then loaded into
another syringe and placed into a pump. A
metallic capillary that has an inner and outer
diameter is attached to the syringes using some
type of polymer piping. The piping from the
inner diameter is connected to the core solution,
and the piping to the outer diameter is con-
nected to the sheath solution. Like traditional
electrospinning, a voltage is applied to the metal-
lic capillary, then a jet forms, and eventually fib-
ers are deposited on a grounded collector [114] .

7.2.2.1.5 Melt electrospinning
Although electrospun nanofibers from polymer
solutions have been successful in producing fib-
ers in the submicron region, the potential clini-
cal use of these nanofibers could be limited due
to the use of harsh solvents and incomplete dry-
ing [116] . Therefore, a need for solvent-free pro-
cessing exists. Zhmayev et al . showed that by
using gas-assisted polymer melt electrospin-
ning, it is possible to achieve fibers with sub-
micrometer diameters [116] . Air drag in the
electrospinning setup, as well as heating pro-
vided by the air stream, aid in thinning the fiber
[116] . This presents a viable alternative to solu-
tions of synthetic polymers such as polylactic
acid (PLA). However, this technique would not
suffice for some natural polymers because heat-
ing could cause structural damage [117] .

7.2.2.2 Thermally Induced Phase Separation
Thermally induced phase separation (TIPS) is
another technique that can be used to produce
nanofibers. TIPS takes advantage of the thermal
instability of polymer solutions [43] and can
readily produce a 3D nanofibrous scaffold with
a five-step process.