Biomimetic Nanofi brous Scaffolds for Bone Tissue Engineering Applications - Biomimetics: Advancing Nanobiomaterials and Tissue Engineering - page 76

potential (typically 5-30 kV) between the needle and target draws a thin
stream of the polymer solution towards the target located 5-30 cm away.
Electrostatic repulsion in the polymer stream overcomes any surface ten-
sion and draws the stream into an extremely thin fi lament, which impacts
the collector. The result is a mat of fi bers 10 nm to several microns in diam-
eter (Figure 4.2b) with high porosity. The fi nal fi ber diameter is affected by
the type of polymer used, the choice of solvent, and parameters such as
the strength of the electrical fi eld, polymer solution fl ow rate, and distance
to the collector [53, 54].
A variety of both natural and synthetic materials, mostly polymers,
have been successfully used to fabricate nanofi brous electrospun mate-
rials. These include the biological materials collagen [55], chitosan [56],
silk fi brin [57], cellulose [58], dextran [59], fi brinogen [60], and DNA
[61]. Synthetic materials have included degradable polymers such as
PLA [62], PGA [63], Poly (e-caprolactone) (PCL) [64], and nondegrad-
able biocompatible materials such as nylon [65], and polyurethane [66],
among others. Reinforced composites have been investigated, primarily
hydroxyapatite-reinforced polymers [67-70]. Polymer-polymer blends
such as collagen with PEO (Poly-ethelene oxide), chitosan with PCL, or
even fi bers reinforced with aligned rod-shaped viruses [71] or carbon
nanotubes [69] are possible. A recently developed technique allows for
the creation beds of metallic TiO nanofi bers by mineralizing polyvinyl-
pyrrolidone (PVP) electrospun nanofi bers [72] through precipitation fol-
lowed by burning off the polymer template—a technique that should
work effectively with calcium phosphate minerals as well. By utilizing
multiple needles [73], or needles inside of other needles (coaxial elec-
trospinning) [74], composite scaffolds can be produced of two differing
fi ber compositions, giving even more control to the properties of the fi nal
nanofi brous mats. For a more complete listing of the various types of
materials capable of being used, see the reviews by Huang et al. [54], and
Li et al. [75].
In a typical electrospinning setup, a random mat of fi bers is deposited
onto a surface with a resulting porosity of 85-90%. By modifi cations to the
orientation of the collector electrode, or the use of additional electrically
charged rings or plates to “focus” the polymer fi lament, greater control
can be achieved, although precise control of where the fi lament impacts
the target has not been demonstrated. A simple and effective technique to
produce highly aligned nanofi bers scaffolds is to collect the fi bers on the
edge of a rotating disk, ensuring that nearly all fi bers aligned parallel to
the direction of rotation [76]. This technique is interesting in particular for
neural tissue engineering, as aligned scaffold fabricated in this manner
appear capable of directing the growth of neurons along a single axis [77,
78]. Another technique deposits a bed of fi bers onto a rotating cylinder,
creating a seamless tube for potential use as a vascular graft material. For