Biomedical Engineering Reference
In-Depth Information
measurements were used to determine the rela-
tive hydrophilicity of the scaffolds. The blend
scaffold of chitosan and PCL was shown to
exhibit a significant decrease in hydrophobicity
and degradation rates compared to the pure PCL
fiber scaffold. The blend fibers were also opti-
mally mixed to produce the most bioactive struc-
tures, resulting in a larger percentage of chitosan
being used to sustain cells more favorably.
Studies have also been conducted demon-
strating that optimizing biocompatibility does
not cause severe losses of mechanical integrity
[111]
. The natural polymer chitosan was again
combined with the synthetic polymer PCL. The
ultimate goal was to produce a synergistic blend
of the two polymers that had the mechanical
integrity of PCL and the biocompatibility and
bioactivity of chitosan.
Composite nanofibers are a product of ceram-
ics or metals added to polymers to increase fiber
mechanical properties and enhance bioactivity.
Hydroxyapatite or some other form of calcium
phosphate or bioglass is a commonly used
ceramic in bioscaffolds because of its chemical
similarity to natural mineral components. The
increased osteoconductivity, the ability to sup-
port bone formation, obtained by adding ceram-
ics aids in the formation of natural bone.
Although it is simple to add the mineral compo-
nent to the polymer solution to be electrospun,
this typically results in the mineral being embed-
ded or encapsulated in the fibers, rendering it
ineffective. To remedy this problem, several
researchers have deposited the mineral on the
fibers as a post-fabrication treatment. Incubation
in simulated body fluid allows the pores and
fiber surfaces to get sufficiently covered in min-
erals; this process can take several hours to
weeks. To shorten the time it takes to mineralize
a scaffold, electrodeposition technology was
used to coat the fibers, accomplishing minerali-
zation in less than an hour. The resulting fibers
had the same morphology compared to simu-
lated body fluid (SBF) incubated fibers as well
as similar mechanical strength
[43]
.
7.2.2.1.3 Centrifugal electrospinning
Centrifugal electrospinning is another modified
electrospinning technique that produces highly
aligned nanofibers. This process involves load-
ing a spinneret and nozzle onto a circular disk
that is attached to a rotating axle. A metallic
cylindrical shell is then placed around the disk
and grounded to serve as the collector. Centrifu-
gal action on the polymer solution provides a
uniform distribution of stress, which stretches
the polymer into a long fiber if the solution
viscosity is ample. This process also allows
lower-molecular-weight polymer solutions to
be electrospun.
Polymer solution viscosity is a result of the
friction between polymer chains. The frictional
forces are dependent on the speed of the centri-
fuge: as the speed increases, the frictional force
increases and consequently the solution viscos-
ity increases. The electrocentrifugal technique
produces fibers that are better aligned compared
to electrospinning, due to its ability to reduce
the bending stability of the polymer jet. These
fibers, however, exhibit only marginal improve-
ments to mechanical properties
[112]
.
7.2.2.1.4 Coaxial electrospinning
Coaxial or core-shell electrospinning is another
common modification to the traditional electro-
spinning technique to obtain multifunctional
nanofibrous scaffolds. The fibers can be spun
from many different polymers and polymeric
combinations, and a variety of material (syn-
thetic or natural) can be placed in the core, all
aimed at efficient tissue formation. Most often,
biomolecules, which easily lose their bioactivity
in the harsh solvents used to dissolve polymers,
are encapsulated as the core. The polymer shell
surrounding the biomolecule core has the prop-
erty of tunable degradability, depending on its
composition, which allows these biochemical
agents to be released over a favorable time
period. The ability to optimize the temporal
release of the molecules promotes more effica-
cious behavior of the biomolecules
[113, 114]
.
