Biomedical Engineering Reference
In-Depth Information
solution. The thickness of the fi ber tends to increase with increasing concentration of the solu-
tion, and it can be reduced signifi cantly by increasing the electric conductivity, for instance (e.g.,
by adding salts). Thicker fi bers result if the feeding rate, that is, the amount of spinning solution
fl owing through the die is increased. The magnitude of the applied electric fi eld may also affect
the fi ber diameter (see Figure 5.5). Fibers with circular cross-section are in most cases formed by
electrospinning, yet ribbon-like fi bers with a rectangular cross-section have also been manufac-
tured. It is assumed that their formation is the result of a rapid evaporation of the solvent causing
skin formation followed by a collapse of the fi bers toward a rectangular cross-section. Hollow
and porous fi bers can also be produced by electrospinning. The electrospinning process itself is
characterized by a swift and physically powerful elongational deformation of the spinning jet due
to a specifi c volatility, the so-called whipping instability, which takes place in the course of fi ber
formation [9].
The whipping mode corresponds to long wavelength oscillations of the centerline of the jet,
that is, the jet is subjected to bending modes. The whipping mode tends to dominate for large static
charge densities because high charge densities at the surface simultaneously tend to suppress both
the decomposition of the jet into individual droplets resulting from the Rayleigh and the axisym-
metric modes of instabilities. The corresponding large deformation of the jet gives rise to nanofi -
bers displaying a strong orientation of the chain molecules as well as of the crystals in the fi ber,
as evident, for instance, from electron diffraction studies [10]. Such orientations cause signifi cant
increase in the mechanical stiffness and strength of the fi bers. A particular advantage of nanofi bers
is its high strength due to the low probability of defects on the fi ber surface acting as nucleating
sites for cracks. A tensile study done on single electrospun PCL fi bers revealed that both stress at
break and yield stress decreased with increasing fi ber diameter whereas strain at break was found
to decrease with increasing diameter [11]. To further enhance stiffness and strength, biodegradable
polymers are spun to nanofi bers displaying liquid crystalline phases.
Molecular self-organization
effects characterize such phases that are already in the fl uid phase, and their signature is the forma-
tion of spontaneous orientational orders.
Nanofi bers with surface nanopores can also be produced along various routes. Mixed solvents
or single solvents causing phase separations into solvent-rich and solvent-poor areas within the jet
or ternary solutions containing two different polymers, which phase separate in the fi bers, have
been used for this purpose. Pore formation can also be induced in highly humid conditions where
condensation processes lead to the formation of water islands within the fi bers.
Many publications report various polymeric fi bers electrospun from solutions, both degradable
and nondegradable, and from organic and aqueous solutions. The collected fi bers may demonstrate
various morphologies, with beads and fused structures, and some with distinct nanotopographies.
Nonwoven scaffolds have been fabricated from a large number of polymeric materials, and many
different cell types have been deposited onto these with positive results. In addition, some cells
seeded on oriented fi brous mats result in oriented cell growth, or demonstrate some guided axonal
growth in the case of neurons.
One issue with the use of solvents, which are typically volatile, in electrospinning for TE appli-
cations is their toxicity. Electrospun scaffolds from such polymer solutions therefore require solvent
removal prior to cell seeding. Electrospun polymeric aqueous solutions necessitate airborne cross-
linking prior to collection onto any aqueous-based target to avoid redissolution, with the traveling
time between the spinneret and collector being in the order of milliseconds.
5.2.2 E
LECTROSPINNING
OF
N
ATUR AL
P
OLYMERS
5.2.2.1 Collagen/Gelatin
Two methods have been currently adopted for collagen electrospinning. The fi rst method involves
the electrospinning of highly concentrated pure collagen dissolved in a highly volatile solvent, com-
monly utilized 1,1,1,3,3,3-hexafl uoro-2-propanol (HFP) and 2,2,2-trifl uoroethanol (TFE), and the
