Biomedical Engineering Reference
In-Depth Information
a Taylor cone will be formed, followed by a liquid jet directed toward the grounded
collector. The jet will experience both solvent evaporation and whipping instability
before it reaches the collector. As a result of stretching by electrostatic repulsion
and whipping, the liquid jet will be continuously reduced in size until it has been
solidified or deposited on the collector. By adjusting experimental parameters such
as the concentration of polymer solution, the voltage, and the distance between
spinneret and collector, fibers with uniform diameters can be routinely produced.
9.2.2
Materials Consideration
Electrospinning has already been successfully applied to generate nanofibers from
more than 100 different types of synthetic and natural polymers [26]. Synthetic
polymers are relatively less expensive and more convenient to work with than
natural polymers. For scaffold fabrication, the most commonly used synthetic
polymers include PCL, PLGA, poly(ethylene oxide) (PEO), and poly(L-lactic) acid
(PLLA). Although these polymers are biocompatible and biodegradable, they may
cause significant inflammation and foreign body reaction when implanted
in vivo
[27]. Natural polymers are therefore more desirable to avoid complications from
severe immune reaction. The most abundant natural polymers are type I and type
III collagens; together they account for almost one-third of the proteins in the
human body [13]. Nanofibers electrospun from collagens will swell when exposed
to the moisture in air and tend to lose their fibrous morphology in a short period of
time [28]. Cross-linking is thus required to maintain the fibrous morphology after
electrospinning [29]. The toxicity associated with some of the cross-linkers may
compromise the usefulness of such nanofibers
in vivo
. Additionally, the mechanical
strength of collagen nanofibers are typically very weak [30]. To improve stability
and mechanical strength, collagens are often mixed with other polymers and then
electrospun into fibers [31]. For example, Boyce and co-workers have demonstrated
electrospinning with a blend of collagen and PCL. Tensile testing indicates that
even inclusion of PCL at a low concentration of 10% could significantly improve
the stability and stiffness of the nanofibers [32].
9.2.3
Incorporation of Bioactive Molecules
Bioactive molecules released from a scaffold at a controlled rate can be used to
stimulate the proliferation and differentiation of seeded cells during
in vitro
cul-
ture, thereby encouraging tissue regeneration after implantation
in vivo
[33]. Many
different types of bioactive molecules have been incorporated into scaffolds of
electrospun nanofibers, including growth factors [34]. Growth factors are endoge-
nous proteins capable of binding to cell receptors and directing cellular activities
[35]. The biggest challenge in incorporating a growth factor into a scaffold is how
to preserve its bioactivity. Several factors in an electrospinning process can lead to
deactivation of a growth factor: the high voltage applied [36], the high density of
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