Biomedical Engineering Reference
In-Depth Information
regions in addition to amorphous regions [46]. In the absence of an enzyme, water
penetrates the surface of a nanofiber and preferentially attacks the amorphous
regions first, converting the long polymer chains into shorter and eventually water-
soluble species. Since the crystalline regions are still intact, the nanofiber does not
fall apart. As hydrolysis continues, the nanofiber eventually starts to disintegrate
and disappear. In the presence of an enzyme, the nanofiber can be digested by the
enzyme, resulting in a rapid loss of mass [47].
When assembled into a scaffold, the structure of the scaffold also plays a
very important role in determining the degradation profile of nanofibers. When
compared with a thin film cast from the same polymer, a scaffold made of
electrospun nanofibers has a higher porosity and therefore the degradation product
will be able to diffuse away more quickly. Otherwise, the accumulation of acidic
degradation products will act as a catalyst to make the degradation process faster.
As a result, a scaffold based on electrospun nanofibers would require a longer time
to degrade than a bulk film of the same mass due to the difference in porosity
[48]. Some researchers have also attributed the slow degradation rates of nanofiber
scaffolds to the increase in chain orientation and thus higher crystallinity [49], as
the strong electric field involved in an electrospinning process tended to align the
polymer chains parallel to the field [50].
The porosity of a nanofiber-based scaffold is a key factor in controlling the
degradation profile. A number of methods have been developed for manipulating
the porosity of a nanofiber scaffold, including those based on variation of the size
of nanofibers, salt leaching, cryogenic electrospinning, and removal of a sacrificial
component. These methods will be discussed in Section 9.2.6, as cell infiltration is
also affected by the porosity.
9.2.5
Mechanical Properties
The mechanical properties of a nanofiber-based scaffold depend on a number of
parameters, including the composition, molecular structure, and size of individual
nanofibers, as well as the alignment and density of the nanofibers [51]. For
example, scaffolds made of PLGA nanofibers can be 10 times stiffer than scaffolds
made of PCL nanofibers [52]. Ramakrishna and co-workers have found that the
rotating speed of a mandrel was a dominant parameter in inducing a highly ordered
molecular structure in an electrospun PLLA fiber, which consequently led to higher
tensile modulus and strength [53]. Leong and co-workers reported an increase in
both strength and stiffness as the fiber diameter was reduced from
mto
∼200 nm [54]. Encapsulation of different drugs may also exert different impacts on
the mechanical properties of a single nanofiber. An increase inmechanical strength
was reported when 10-20 wt% retinoic acid was encapsulated whereas an opposite
trend was observed when 10-20 wt% bovine serum albumin (BSA) was added [54].
Alignment of nanofibers results in significant stiffening in the direction of
alignment and increased scaffold anisotropy [55]. This is an important feature to
mimic when engineering anisotropic load-bearing tissues such as tendons, annulus
∼
5
μ
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