Biomedical Engineering Reference
In-Depth Information
nanofiber formation [64] . These micellar struc-
tures are typically cylindrical in shape due to
the assembly of the amphiphiles that have an
overall conical shape. Once the cylindrical
micelles aggregate, they form nanofibrous struc-
tures [64] .
Synthetic polymers can also be used to fabri-
cate self-assembled nanofibers. The process begins
with synthesizing a block or a block copolymer of
more than two polymers in order to introduce
different regions of functionality into the back-
bone. These polymers often include positively
and negatively charged domains, which, once
introduced into an aqueous solution, interact to
form nanofibers.
Currently, there are a variety of commercially
available extracellular matrix products that aid
in the regeneration of human tissue. These extra-
cellular matrix scaffolds come from human auto-
graft, human autogenous, and allogeneic sources
and include tendon, fascia, ligament, and der-
mis (e.g., Alloderm TM ). The xenogeneic sources
of extracellular matrix used to manufacture scaf-
folds include porcine, equine, and bovine tis-
sues. The specific tissues processed from the
xenogeneic sources include heart valves, dermis,
pericardium, and parts of the intestine. Depend-
ing on the processing conditions, the host bio-
logical response can be tailored. A collagen-based
bioscaffold from the small intestine submucosa
(SIS) has been shown to induce site-specific
regeneration in numerous tissues, including
blood vessels, tendons, hernias, ligaments, skin,
urinary bladder, musculoskeletal repair, and
dural substitute [119-123] .
The human response to extracellular matrix
scaffolds can range from the foreign body
encapsulation observed with permanent
implants to degradation and resorption, as well
as being populated with fibroblasts and vascu-
larized to support new tissue growth. Chemical
crosslinking is the most prominent way to influ-
ence the human body's response to extracellu-
lar matrix-based bioscaffolds (e.g., Contigen TM ).
Some researchers have used hexamethylene
diisocyanate (HMDI) and carbodiimide (CDI)
to chemically crosslink the scaffold to prevent
rapid degradation and also mitigate the immu-
nogenicity of the xenogeneic tissue derived
ECM products. This technique, however, results
in the chemical crosslinker remaining in the
final scaffold product-a potential detriment to
a host. To remedy this, 1-ethyl-3-(3-dimethyl-
aminopropyl) carbodiimide (EDC) was used,
and the EDC allowed the natural scaffold to be
crosslinked without the retention of the
crosslinker molecule. There have been negative
effects associated with unnatural crosslinking
of scaffolds, including the decreased binding
affinity of growth factors toward the modified
7.2.3 Special Scaffolds
7.2.3.1 Native-Tissue-Derived Scaffolds
Biological scaffolds, particularly extracellular
matrices derived from various animal tissues,
are an increasingly attractive option for tissue
engineering applications. The native extracellu-
lar matrix affords the researcher an intact 3D
bioscaffold that can effectively modulate the nec-
essary events for regeneration. In addition, there
is some degree of tailorability to the researcher
in constructing the bioscaffolds. The tissue from
which the matrix is derived can be excised from
an organism at various time points of develop-
ment. This dictates the chemical constituents
and, consequently, the mechanical properties of
the bioscaffold. There are also inherent differ-
ences in the extracellular matrix owing to ECM
variability between tissue types and species.
Together these differences contribute to varia-
tions in the microstructure, quantities of non-
collagenous proteins, glycosaminoglycans
(GAGs), and other factors, mechanical proper-
ties, and ratios of collagen fibers present. For
instance, fetal and neonatal bovine dermises
have 3-5 times more Type III collagen than that
of adult dermis, while as the calf develops, the
thickness of the dermis increases as well as the
diameter of the fibers [66] .
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