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Natural polymers
are the ideal preference when generating tissue engineered scaffolds due to
their excellent biological performances as they do no activate chronic
inflammation of toxicity. Original attempts at creating tissue engineered vascular
grafts were carried out using natural polymers, primarily collagen and fibrin.
In 1986, Weinberg and Bell reported the fabrication of the first TEVG made of a
collagen gel cultured with smooth muscle cells (SMCs) 31. However, the addition of a Dacron mesh was required to make up for
the low burst pressures of 120-180 mmHg associated with these grafts. Despite
the limitations associated with early grafts, over the last 20 years
significant advancements in research has enhanced the potential for development
of natural-based TEVGs. However, in general scaffolds generated from natural
polymers lack the mechanical performance required of TEVGs and the mechanical
performance is uncontrollable. Electrospun tropoelastin scaffolds fabricated by
KcKenna et al illustrated a low mechanical strength when compared to native
blood vessels, resulting in an ultimate tensile strength of 0.34 ±
0.14 MPa and a burst pressure of 485 ±
25 mmHg 32.

polymers, in comparison to natural polymers, show controllable physical and
mechanical properties but lack in biological performances. In addition,
synthetic polymers can be produced in large amounts. PCL is a synthetic biodegradable
polymer which is commonly used for tissue engineering purposes due to its
favourable mechanical properties, specifically its high elongation and strength,
and good biocompatibility 33. It is important for the synthetic polymer to be biodegradable as
the graft must slowly degrade whilst being replaced by functional vascular
tissue. Creating vascular scaffolds using PCL has shown promising results to
date. A study by De Valence et al in 2012 investigated the electrospinning of 2
layers of PCL with different porosity onto a 2mm cylindrical rotating
translating mandrel 34. The
bi-layered scaffolds produced were evaluated in vivo in a rat model and found
to exhibit excellent patency with no thrombosis and favourable vascularisation.
This study illustrates the potential for the three layers of the native blood
vessel to be replicated using PCL as a synthetic polymer. Recent studies have
proposed the fabrication of hybrid scaffolds from synthetic and natural
polymers in an attempt to combine the favourable mechanical properties of
synthetic polymers with the excellent biological behaviour of natural polymers 33, 35. For
the purpose of this report, synthetic polymers will be investigated to
replicate the distinctive layers of the native blood vessel as this must be
achieved before hybrid scaffolds can be created.

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TEVGs without Scaffolds

Although the
majority of studies on TEVG to date have been based around creating suitable
tubular scaffolds, there have been studies which have focused on the
fabrication of completely biological TEVGs without the use of scaffolds. From
these studies, two main approaches have been developed. The first approach
involves the use of the body as a bioreactor whereby a silicone compound tubing
was inserted into the body to enable the growth of a fibrous capsule
surrounding the implanted tube. After 2-3 weeks, the tubing was removed from
the body and reversed to produce a tubing capable of vessel replacement.
However, this process is limited in that it requires an additional site of
surgery and the problems associated with this 36.

The other main
approach at developing a TEVG without the use of a scaffold is the cell
sheet-based approach. This process involves the in vitro stimulation of cells
in the presence of ascorbic acid in the culture medium to produce high levels
of extracellular matrix proteins 37. These sheets are then detached from the culture flasks and
assembled around a support mandrel following a maturation period. The
fabrication of these constructs is illustrated in Figure 2.2 below where a
single cell sheet of SMCs and fibroblasts are used to fabricate the media and
adventitia of the native vessel. However, although the results using this
strategy have been promising thus far, the process is severely limited in that
they require very long culture times (up to 24 weeks) and have a high cost
associated with them which makes them inefficient for large scale production 38.

Figure 2.2: Illustration of
the cell sheet-based approach to fabricate a tissue-engineered vascular media
(TEVM) and a tissue-engineered vascular adventitia (TEVA) by rolling a single
cell sheet of SMCs or fibroblasts 30. 

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