Electrospun Nanofibers for Regenerative Medicine - Soft Fibrillar Materials: Fabrication and Applications - page 271

9.4.1
Stacked Arrays of Nanofibers
Uniaxially aligned nanofibers can be stacked into multilayered films with a control-
lable, hierarchically porous structure. When a pair of electrodes separated by a void
gap is used as the collector, the uniaxially aligned nanofibers can be sequentially
transferred onto a substrate [76]. By rotating the substrate between the deposition
of different layers, it is fairly straightforward to obtain a multilayered film with the
nanofibers in each layer oriented along a different direction. Alternatively, the void
gap can be replaced by a highly insulating substrate such as quartz or polystyrene.
By patterning the collector into an array of electrodes on an insulating substrate,
the deposition of nanofibers can easily be directed to generate a multilayered film
(Figure 9.4a). In one study, this was achieved by alternating the scheme for applying
the high voltage [78]. The nanofibers in each layer were uniaxially aligned, with their
long axes rotated by 60 ◦ between adjacent layers (Figure 9.4b). We have applied this
type of multilayered scaffold to neural tissue engineering and showed that neurites
derived from ESCs followed the pattern of the underlying scaffold, including at the
intersection of two perpendicular fibers [80]. Wang and co-workers also developed
scaffolds using a layer-by-layer approach [81]. They first deposited a layer of random
fibers mat on a grounded electrode, which was immersed in cell culture medium.
Cells were then seeded onto the first layer of the scaffold, after which the second
layer of fibers was directly electrospun on top of the cells (Figure 9.4c). By repeating
these two steps, scaffolds with a multilayered architecture and embedded with
cells were constructed. Figure 9.4d shows the cross-sectional fluorescent image
of the cell-fiber construct, with the fibers in green and cell nucleus in blue. A
major advantage of this approach is that the composition of the fibers can be easily
tailored. Different growth factors or drugs can be encapsulated in different layers,
and the topography of each layer can also be altered.
9.4.2
Conduits Assembled from Nanofibers
Conduits comprised of random or circumferentially aligned fibers can be readily
fabricated by depositing nanofibers on a rotating mandrel [71b]. However, conduits
with nanofibers aligned parallel to the long axis of the tube are sometimes more
desirable as cells could then be guided to migrate along the tube. To address
this issue, Ramakrishna and co-workers developed a method for fabricating a
tube consisting of diagonally aligned electrospun fibers through a combination
of electrostatic and mechanical forces [83]. A knife-edged auxiliary electrode was
charged with a polarity opposite to the spinneret and placed at a 45-angle relative to
the long axis of the rotating Teflon tube, creating an electrostatic field that promoted
a diagonal alignment for the nanofibers deposited on the collector. By rotating the
Teflon tube, they obtained a tubular conduit with uniform thickness and superior
mechanical strength. Chang and co-workers also developed an electrostatic method
for fabricating multiple, interconnected conduits composed of electrospun fibers