Biomedical Engineering Reference
In-Depth Information
4.4 Tissue crosslinking with diisocyanate
Bifunctional molecules capable of crosslinked proteins by urea bond formation after reaction
between terminal isocyanate groups and -amino group of lysine residue have been
explored. Such is the case of crosslinking of extracellular matrix proteins (elastin and
fibronectin) with hexamethylene diisocyanate in dimethyl sulfoxide (Tirrell and Nowatzki,
2004). Similarly, the crosslinking of ovine skin collagen with hexamethylene diisocyanate
has been reported. This crosslinking procedure was carried out in an aqueous medium
including surfactants to increase solubility and promote the penetration of diisocyanate into
the tissue (Olde Damink et al., 1995). Futhermore, the effects of the tissue crosslinking with
hexamethylene diisocyanate and the effects of mixtures of water/isopropanol (50/50 and
0/100) as solvent on the thermal and biomechanical properties of bovine perichardium have
been reported (Naimark et al., 1995). On the other hand, the stabilization of porcine
perichardium has been achieved by the interaction of polyurethane oligomers containing
isocyanate end groups (Loke et al., 1996). The interaction in organic media between
perichardial tissue and polyurethane oligomers resulted in the increase of the denaturation
temperature, a reduction in the content of lysine and a poor diffusion of polyurethane
oligomers into the tissue (H&E staining). The crosslinking of bovine perichardium with
polyurethane oligomers, EDAC and diphenyl phosphoric azide showed less cytotoxicity
(assessed by a direct cytotoxicity test or Homsy test) than the tissue crosslinking with
glutaraldehyde (Jorge-Herrero et al., 2005).
After these results, it is clear that diisocyanates are an alternative to glutaraldehyde in the
preparation of bioprostheses. However, protein fixation with isocyanates has the
disadvantage of using organic solvents. In addition, during the fixation in aqueous media,
the crosslinking degree can be reduced due to competition of hydrolysis reactions.
Therefore, the blocking reaction of isocyanate with bisulphite salts is an alternative in the
preparation of water soluble isocyanates (Petersen, 1949). The protein crosslinking process
with blocking isocyanates has the advantages of the use of aqueous media and reduced
isocyanate toxicity (Mata-Mata et al., 2008). In this regard, the treatment of perichardial
tissue with the carbamoylsulphonate blocked polyurethane prepolymers resulted in an
increase of the in vitro tissue biostability (Mendoza-Novelo, 2011). The coating of collagen
fiber network of perichardial tissue with polyurethane is shown in figure 8.
Fig. 8. SEM micrographs for bovine perichardium (a) native and treated with polyurethane
prepolymers
