Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 315

polysaccharide has protein-like properties which have made it attractive as
a possible scaffold for tissue engineering applications. Concerns were high-
lighted about chitosan's mechanical properties when an infl exible, porous
scaffold demonstrated improved pliability once hydrated but was weaker,
with a lower elastic modulus, than non-porous constructs (Madihally and
Matthew, 1999). Although vascular cells adhere to chitosan, the effective-
ness of the cell-chitosan bond appears tenuous as does the reliability of
cellular proliferation. In order to improve this Achilles heel, chitosan has
been combined with a number of substances including glycosaminoglycans
and gelatine which have demonstrated disappointing degrees of improve-
ment (Chupa et al. , 2000; Huang et al. , 2005).
A scaffold's degree of porosity appears to infl uence cell attachment
and proliferation in static in vitro models (Bu et al. , 2009; Turner et al. ,
2004). The consequence of greater and less scaffold porosity were exam-
ined in both Hyaff-11s, a scaffold containing hyaluronan (Turner et al. ,
2004) and a scaffold comprising fi bronectin, fi brinogen and laminin (Bu
et al. , 2009). Fibronectin, fi brinogen and laminin possess the RGD cell
adhesive peptide sequence which is an attachment site recognised by inte-
grins found on most cell surfaces. Both scaffold types were compressed
by applying a pressure of 1 N to the unpressed scaffold. In the combined
fi bronectin, fi brinogen and laminin scaffold, the applied pressure reduced
pore size from 1.5 to 1.0 m. The attachment of ECs (Turner et al. , 2004)

and endothelial progenitor cells (Bu et al. , 2009) was enhanced in com-
pressed scaffolds in a time-dependent manner. In the case of the endo-
thelial progenitor cells seeded onto compressed scaffolds, proliferation
was signifi cantly increased in the fi rst seven days after seeding, but then
plateaued when compared with uncompressed scaffolds. In addition, cells
seeded onto the upper surface of compressed scaffolds maintained their
position whereas cells seeded onto uncompressed scaffolds migrated to its
lower surface (Bu et al. , 2009; Turner et al. , 2004). In the case of the
Hyaff-11s scaffold, ECs appeared to proliferate regardless of the scaf-
fold's degree of porosity and secreted a subendothelial matrix (Turner
et al. , 2004). The matrix comprised laminin, fi brinogen, fi bronectin, and
types IV and VIII collagen. Scaffolds that are less porous have denser
constitution and hence cells maintain their position on the upper surface
where they are originally seeded. ECs are required to line the luminal
surface of a vascular conduit, and would thus be expected to maintain the
position in which they are originally seeded. Compressed scaffolds tend
to a smoother and more even surface which would encourage less turbu-
lent blood fl ow in vivo . Although compression is considered to delay scaf-
fold degradation, degeneration of pressed Hyaff-11s scaffolds have been
reported at 14 days in vitro (Milella et al. , 2002). Delayed degradation
would benefi t vascular tissue engineered conduits as their integrity would

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