Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 314

and elastin, as is the case for native arteries, the two ECM components
have been combined using techniques including electrospinning and
freeze-drying (Boland et al. , 2004; Buijtenhuijs et al. , 2004; Buttafoco et al. ,
2006).
The successful engineering of a vessel-like conduit has been reported
using a single peripheral blood sample from which fi brin and cells were
obtained (Aper et al. , 2007). Fibrin, an insoluble protein lattice, forms
blood clot when its fi brinogen precursor is catalysed by thrombin, an
enzyme. Clotting is an essential component of early wound healing, forming
a matrix which achieves haemostasis and encourages infl ammatory cell
infi ltration into the injured area. Once this versatile matrix has served its
purpose it easily degrades through fi brinolysis. Peripheral blood will clot in
vitro , forming almost ideal three-dimensional tissue engineering scaffolds
which are completely autologous, thus avoiding a foreign body immune
response. Helpfully, growth factors, such as fi broblast growth factor (FGF),
are stored and released from the fi brin lattice in vitro , and encourage cel-
lular infi ltration (Bhang et al. , 2007; Jeon et al. , 2005). As fi brin is biomi-
metic and biodegradable it does not release deleterious material and does
not require combining with other substances to increase its effi ciency. Pre-
mature degradation of an implanted fi brin scaffold, through fi brinolysis,
can be prevented using aprotinin, a protease inhibitor (Jockenhoevel
et al. , 2001). Vascular cells, seeded onto fi brin scaffolds, demonstrated sat-
isfactory adherence and proliferation and manufactured ECM components
(Aper et al. , 2004). ECs seeded onto fi brin coated synthetic conduits were
not signifi cantly disturbed by the shear stress of laminar fl ow (Sreerekha
and Krishnan, 2006). Although incorporation of cells in the fi brin scaffold
signifi cantly improved burst strength from 20 to 90 mm Hg, the conduit's
integrity is still far less than that of other constructs (Aper et al. , 2007).
Fibrin hydrogel impaction and the scaffold's ability to cope with tensile
stress is reportedly inversely proportional to thrombin concentration
without an effect on seeded cells (Rowe et al. , 2007). A fi brin conduit
seeded with ECs and SMCs in vitro was successfully manufactured and
implanted into an ovine jugular vein model. After 15 weeks, constructs had
maintained their integrity in this low pressure environment and had similar
physical characteristics to native veins (Swartz et al. , 2005). Fibrin therefore
has obvious benefi ts as a scaffold which are offset by its disappointing
mechanical integrity. The ideal role of this biopolymer may therefore be as
a 'cell carrier' which is employed in conjunction with synthetic scaffolds
such as PET, ePTFE and poly(L/D)lactide (Mol et al. , 2005; Sreerekha and
Krishnan, 2006; Tschoeke et al. , 2009).
Chitin, a ubiquitous glucose derivative, is the main component of
many structures, including the exoskeleton of anthropods such as crusta-
ceans. Chitosan emanates from chitin. Because of its amine groups, this

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