Biomedical Engineering Reference
In-Depth Information
in aqueous solution and the intermediate O-acyl isourea is extremely unstable producing a
low crosslinking.
The crosslinking density and the shrinkage temperature of bovine perichardium treated
with EDAC had values lower that a control of bovine perichardium fixed with
glutaraldehyde (Mendoza-Novelo & Cauich-RodrÃguez, 2009). However, the use of the N-
hydroxysuccinimide (NHS) during crosslinking with EDAC improved the stabilization of
tissue due to the formation of a stable intermediate compound after reaction of the NHS
with carboxylic groups or isourea O-acyl intermediate (Lee et al., 1996). Such is the case
reported for porcine aortic valves crosslinked by a two-step method. These steps included
the blocking of the free primary amino groups of collagen with butanal and the crosslinking
with Jeffamines
TM
of different molecular weights by activating the carboxylic acid groups
with EDAC and NHS. This process led to a decrease in calcification (subcutaneous
implantation in rats) of engineered tissue (Everaerts et al., 2004).
The appearance of bovine perichardial tissue crosslinked with glutaraldehyde and EDAC is
shown in figure 7.
4.3 Tissue crosslinking with epoxy compounds
The chemistry of epoxy groups, cyclic ethers of three members, has also been explored and
applied in the fixation of tissue. Polyepoxide compounds or epoxy bifunctional polyether
react with amino groups from collagen opening the terminal epoxide ring (Tu et al., 1993;
Lee at al., 1994; Khorn, 1997). This reaction is nucleophilic and can be carried out under
acidic conditions (highly reactive protonated epoxy) or alkaline (amine at its most
nucleophilic). In this case, the modification of swine tendons with ethylene glycol diglycidyl
ether has been reported for the repair of cruciate ligaments (Sung et al., 1996). The 1,4-
butanediol diglycidyl ether (BD) has been reported as a crosslinking agent in the
preparation of bioprosthetic valves (Zeeman et al., 2000). However, the fixation of porcine
valves with BD caused immune response, foreign body reaction (proliferation of
lymphocytes and macrophages) and calcification of implanted tissue using rats as animal
model to levels similar to glutaraldehyde-fixed tissue, although low levels of cytotoxicity
were reported (van Wachem et al., 2000). The combined treatment of BD and EDAC-
dicarboxylic acid or detergents led to a reduction in calcification (implantation in rats) but
not at significant levels (van Wachem et al., 1994). Therefore, it was concluded that the
treatment with BD did not represents an alternative to glutaraldehyde to reduce the
calcification of bioprosthetic valves (van Wachem et al., 1994). However, in another report
the crosslinking of bovine perichardium and porcine aortic valves with triglycidylamine, a
molecule of high polarity and solubility in water, resulted an improvement in
biocompatibility (assessed using bovine aortic valve interstitial cells, human umbilical
endothelial cells and rats artery smooth muscle cells) and resistance to calcification
(subcutaneous implantation in rats) compared with glutaraldehyde-fixed tissues (Connolly
et al., 2005). Furthermore, triglycidylamine-fixed tissues showed stable mechanical
properties (Sacks et al., 2007) and optimal reduction of calcification when treatments
included mercapto-aminobisphosphonate (Rapoport et al., 2007). It was hypothesized that
the difference between these two results, which explored the chemistry of epoxy in the
crosslinking of tissue, may be due to differences in water solubility, chemical heterogeneity
and contamination with used epoxy residual reactants (Connolly et al., 2005).
