Biomedical Engineering Reference
In-Depth Information
two types of electrostatic forces. The splitting of electric field with electric field lines
pointing toward the two electrodes produces the first force. This force will pull the
fiber toward the electrodes and further induce opposite charges on the surfaces of
the electrodes when the fiber travels to their vicinity. This gives rise to a second
force, which stretches the nanofiber across the gap to render it perpendicular to
the edges of the electrodes. These two types of electrostatic forces work together to
produce a uniaxially aligned array of nanofibers. If the fibers spanning across the
void gap discharge very slowly and repel each other, the extent of alignment will be
improved with deposition time [76].
One of the remarkable features associated with the gap technique is that it is
convenient to transfer the aligned fibers onto other solid substrates for further
applications. Single fibers collected across the gap can be easily picked up and
tested without transferring to other substrates. As single fibers are very sensitive
to stress and strain, avoiding transferring the single fibers is highly desirable in
mechanical testing assays [77]. In particular, it has been established that uniaxially
aligned fibers could be directly deposited on an insulating substrate on which the
pair-wise electrodes can be patterned. This variant is commonly used to fabricate
multilayered constructs by controlling the scheme or the configuration for applying
high voltage [78]. The detailed explanation will be provided in Section 9.4.1.
In addition to uniaxial alignment, it is sometimes desirable to align the fibers
into other patterns. For example, scaffolds made of radially aligned nanofibers
may improve wound healing by providing contact guidance for cell migration [68].
In this case, radially aligned nanofibers will encourage cells to migrate from the
peripheral healthy tissue toward the central, injured site. Radial alignment can
be achieved using a ring collector with a point electrode in the center. In order
to ensure that all the fibers pass through the central point electrode, the needle
electrode should be slightly higher than its peripheral ring collector (Figure 9.3c).
Since cells tend to migrate along the fibers, cells seeded around the periphery of a
radially aligned scaffold would follow the nanofibers and migrate inward to cover
the whole scaffold at a speed faster than if they were seeded on a scaffold made of
random nanofibers [68].
Another useful type of alignment can be found in scaffolds that contain arrays of
microwells. These scaffolds can be fabricated using an array of metallic beads as the
collector (Figure 9.3d) [69]. Whereas fibers deposited on the beads were randomly
oriented, those deposited across the gaps between adjacent beads were uniaxially
aligned. The resultant non-woven mat had concave microwells at the positions
corresponding to the beads. The size of the microwells and the distance between
adjacent microwells can both be tailored to accommodate different applications.
Dorsal root ganglia (DRG) cultured in the microwells extended neurites to adjacent
microwells and formed neural networks on the entire scaffold [69]. This type
of scaffold containing aligned nanofibers and patterned microwells may be a
useful platform for research related to cell-cell communication and cell microarray
assays.
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