Biomedical Engineering Reference
In-Depth Information
by electrospinning different polymers for the separate layers.
54
,
55
Finally, Li
et al.
19
showed that scaffolds could be constructed where
the direction of fiber orientation systematically rotates by 90
◦
. This
micron-scaleorientationoffibersmimicsthealternatingorientation
of collagen fibrils in the discrete lamallae that comprise the annulus
fibrosus.
15.5 Composite Scaffolds and the Spatial Heterogeneity
Composite scaffolds—consisting of two or more ingredients—are
frequently attractive over homogeneous scaffolds because the com-
bination of materials can lead to superior mechanical and biological
properties. In addition, this heterogeneity can be controlled at the
nano-,micro-, andmillimeter-length scales.
The simplest approach for introducing heterogeneity into elec-
trospun scaffolds is to combine various ingredients in a single spin-
neret and to electrospin this mixture to form composite filaments.
For example synthetic and natural polymers have been blended
together to improve the mechanical properties,
56
bioactivity,
57
,
58
and electrospinnability.
59
Various particles and small molecules
have also been incorporated into electrospun fibers, including
ceramics,
32
carbon nanotubes,
33
pharmaceutics (e.g., rifampin,
paclitaxel, and doxorubicin
60
,
61
), and biologicallyactiveproteins.
51
Additionally, heterogeneity can be incorporated by electrospin-
ning two different solutions from separate spinnerets. If the two
spinnerets are placed in close proximity with opposite polarities,
the two polymers can attract one another to form an interpen-
etrating mixture with heterogeneity at the micron-length scale
(Fig. 15.3). When the spinnerets are offset
37
or have the same
polarity, the polymers deposit around different centers and pro-
duce a mesh with a spatial gradient of properties on the order
of millimeters. Finally, if one spinneret is placed within the
second, then heterogeneous fibers with distinct core and shell
chemistries can be fabricated.
62
,
63
This coaxial electrospinning
approachmayalsobeameanstocontrolthebiologicalandmechan-
ical properties of electrospun meshes
63
or to tune the drug release
rate.
64
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