Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 318

explanted scaffolds include chemical means by using polyepoxy compounds
and catalysis by dye-mediated photo-oxidation.
Glycerol polyglycidyl ether and polyethylene glycol diglycidyl ether are
part of the polyepoxy group of chemicals. Unlike glutaraldehyde, polye-
poxy compounds react with amide, carboxyl, phenol and alcohol groups of
proteins (Tu et al. , 1993). Conduits pre-treated with polyepoxy compounds
undergo less calcifi cation and thrombosis than grafts treated with glutaral-
dehyde. However cytotoxity still remains an issue with polyepoxy com-
pound-treated grafts and the effect on the conduits' mechanical properties
have been suboptimal (Mazzucotelli et al. , 1995). Polyepoxy compound was
used to coat human cadaveric ureters prior to their implantation into a
rabbit carotid model. Ureters coated with this compound had signifi cantly
better patency rates when compared to saphenous veins treated with poly-
epoxy compound and ureters pre-treated with 1% glutaraldehyde (Uematsu
and Okada, 1996).
Photo-oxidation encourages protein cross-linking without employing
chemicals and reagents, apart from a photo-sensitiser such as methylene
blue. Visible light oxidises amino acids such as tryptophan, tyrosine and
methionine within the ECM proteins of the conduit's wall by irradiating
them in the presence of the photosensitiser. Collagen responds positively
to photo-oxidation, becoming more resistant to denaturation, enzymatic
degradation and calcifi cation with less cytotoxicity and similar mechanical
characteristics to autologous grafts (Li et al. , 2003). Explanted veins that
have undergone photo-oxidation have demonstrated short-term resis-
tance to the development of intimal hyperplasia, an important cause of
medium- to long-term graft failure (Chanda et al. , 1998b; Liu et al. , 1999).
Interestingly, autologous cell attachment to allogenic ovine carotid arter-
ies, decellularised using dye-mediated photo-oxidation, was enhanced
when the porosity of the grafts' ECM was increased using a Ti-sapphire
laser. The enhanced 'repopulation' of these porous, pre-treated grafts
encouraged remodelling and integration of the allograft (Bergmeister
et al. , 2005).
Untreated xenogenic tissue undergoes degradation, which is less pro-
nounced in allogenic tissue which instead is susceptible to calcifi cation.
Pre-treatment of xenogenic tissue with glutaraldehyde stifl es degradation,
but this cross-linking reagent does make the tissue more prone to calcifi ca-
tion (Huma et al. , 2002). The calcifi cation observed within the ECM of
explanted and processed tissue has been investigated in order to reduce its
incidence and hence the failure rate of this tissue once implanted. The
application of a combination of ethanol and ethylenediaminetetraacetic
acid (EDTA) on a collagen-elastin matrix model identifi ed that elastin
acted as an important regulator of calcifi cation within ECM. First treating
the collagen-elastin matrix model with ethanol and then EDTA in a water

￿ ￿ ￿ ￿ ￿