Vascular tissue engineering - Biomaterials and Devices for the Circulatory System - page 317

Pre-treatment involves removing cellular material followed by cross-
linking the residual structural ECM proteins. A combination of mechanical,
enzymatic and chemical methods are available to optimise cell removal,
leaving the ECM scaffold for further processing (Gilbert et al. , 2006). ECM
in which cells are well embedded requires mechanical means, such as freez-
ing, direct pressure, sonication and agitation to disrupt ECM integrity,
permitting chemical and enzymatic cell treatment. Vascular conduits are
usually decellularised using enzymatic digestion and detergents (Dahl et al. ,
2003; Ketchedjian et al. , 2005; McFetridge et al. , 2004b). Consistent and
effective decellularisation is essential in reducing infl ammation and the
host's immune-mediated tissue rejection of allergenic and xenogenic grafts.
Decellularisation techniques may affect the three-dimensional confi gura-
tion of the conduits as the cells no longer function as a strut within the
composite structure. Substances employed to lyse cells are not always very
specifi c, causing collateral damage to the surrounding ECM (Gilbert et al. ,
2006). Despite pre-treatment, cellular material may still remain in the bio-
logical scaffold which may induce an immune response on implantation of
an allogenic or xenogenic graft (Yu et al. , 2008). Decellularised biological
scaffolds retain their collagen and elastin fi bre networks which respectively
contribute to the structural integrity and elasticity of the vessel wall.
After decellularisation, explanted blood vessels are further processed
by cross-linking prior to their use as scaffolds (Courtman et al. , 2001;
McFetridge et al. , 2004b; Schmidt and Baier, 2000). Cross-linking is
employed to stabilise the collagen fi bre network of the graft's ECM, thus
making it more resistant to enzymatic degradation once implanted into its
new environment (Schmidt and Baier, 2000). Hydroxylysine and hydroxy-
proline, derived from the hydroxylation of lysine and proline residues
respectively, are crucial to the structure and function of collagen microfi -
brils (Ottani et al. , 2002). Classically, cross-linking employs glutaraldehyde,
which reacts with the -amino group of lysine residues within the collagen

proteins via an unknown mechanism (Nimni et al. , 1987; Schmidt and Baier,
2000). This interaction between collagen lysine residues and glutaraldehyde
further stabilises the scaffold (Nimni et al. , 1987). Vascular grafts, pre-
treated using glutaraldehyde, do not maintain their patency as well as
autologous grafts over the long term, but they do have a longer patency
rate than synthetic grafts (Dardik, 1995; Teebken et al. , 2001). Although
glutaraldehyde cross-linking is benefi cial, this pre-treatment may induce
calcifi cation, increased rigidity, altered mechanical responsiveness and
cytotoxicity (Chanda et al. , 1998a; Eybl et al. , 1989; Gendler et al. , 1984;
Golomb et al. , 1987; Schmidt and Baier, 2000; Yu et al. , 2008). Glutaralde-
hyde-induced calcifi cation in graft ECM cross-linked with this chemical
may be concentration dependent (Sanchez et al. , 2007; Zilla et al. , 2000).
Alternate techniques employed to cross-link the protein scaffold of

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