Biomedical Engineering Reference
In-Depth Information
when nanofibers were present. However, increasing the thickness of the nanofiber layer
reduced the infiltration of cells into the matrices [27].
The pore size of a fibrous structure can be controlled with melt-plotted strands from
computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies. In
this method, the melted polymer is plotted with a 250-mm dispensing needle tip, laid down
layer by layer. The resulting fabricated matrices have smooth strand surfaces and large pore
size between the strands. These characteristics limit the initial cell adhesion. To overcome
this disadvantage, micro-/nanofiber electrospun with PCL were layered between microsized
PCL strands [28]. The cell attachment was further improved by two natural biomaterials
(small intestinal submucosa (SIS) and silk fibroin). Bone-marrow-derived rat MSCs revealed
an incredible increase in initial cell attachment and cell expansion on the three-dimensional
hierarchical PCL fibrous matrices modified with two natural biomaterials [29].
Poly(
l
-Lactic Acid)
Poly(
l
-lactic acid) is a biodegradable, biocompatible polymer and it has better thermal pro-
cessibility than other biopolymers such as poly(ethylene glycol) (PEG) and PCL. However,
unmodified PLLA has limited applications in tissue engineering due to its poor toughness,
very slow degradation (more than 3 years), relative hydrophobicity, which results in low cell
affinity, and lack of reactive side-chain groups [30].
One modification for PLLA is to fabricate PLLA nanofibers using a liquid-liquid phase
separation method [31]. This method can create nanofibrous matrices with controlled and
carefully designed macroporous architecture. However, the nanofiber diameter is not
adjustable in this method. An electrospinning method has been exploited to produce PLLA
nanofibers with variable diameters. A liquid-surface separation method may produce nano-
fibrous matrices with soft surface, while electrospinning methods may result in nanofibers
with increased surface roughness. Results have shown that the NSC attachment was better
on a surface fabricated using the electrospinning method [32, 33], since the greater the
roughness, the better the cell adhesion [34, 35]. However, increasing the roughness will also
lead to an increase in the hydrophobicity, which impels the nutrients from pores.
Micro- and nanosized PLLA fibrous matrices have also been fabricated in order to study
the effect of architectural characteristics on cell spreading, migration, and proliferation.
The microfibrous meshes with a large pore size enhanced cell aggregation while small-pore
nanofiber structures presented a spread, spindle-shape morphology. Cell attachment may
be higher on the nanofiber scaffolds because the fibers were highly packed in matrices. By
increasing the fiber diameter size, cells were aggregated within the larger interfiber distance/
pore space rather than spread across multiple fibers [36].
Poly(
l
-lactic acid) nanofibers have also been fabricated to have either aligned or ran-
domly oriented structures. Human tendon stem/progenitor cells (hTSPCs) were seeded onto
both nanofibers. They adhered very well and the cell number was increased about threefold
in 14 days for both nanofibers. However, histological staining and confocal images
(Figure 14.3) showed that hTSPCs were spindle-shaped and well orientated on the aligned
nanofibers [37], demonstrating that nanofibrous-aligned structure can influence stem-cell
morphology and orientation.
Poly(
d
,
l
-Lactide-co-Glycolide)
Poly(
d
,
l
-lactide-co-glycolide) is a biocompatible and biodegradable linear copolymer that
can be prepared at different ratios between its constituent monomers, lactic and glycolic
acid. Poly(
d
,
l
-lactide-co-glycolide) offers superior control on its degradation by varying the
