Biomedical Engineering Reference
In-Depth Information
The fixation reaction was carried out by the exposure of the tissue balanced with
glutaraldehyde acetals solutions to triethylamine vapours. This process allowed the
diffusion of the non-reactive glutaraldehyde into the tissue, minimized the formation of
polymeric glutaraldehyde and reduced the waterproofing (hydrophobicity) at the tissue
surface (Yoshioka & Giossis, 2008).
The conditions of the crosslinking reaction (pressure for instance) have been varied with the
aim of improving the biomechanical properties of bovine perichardium. The crosslinking of
bovine perichardium with glutaraldehyde at a pressure of 4 mm Hg (low pressure) both
statically and dynamically (1.2 Hz) has been reported. By comparing the properties of
crosslinked bovine perichardium, the dynamically crosslinked tissue showed a very similar
extensibility to native biomaterial (non-crosslinked) in contrast to statically crosslinked
tissue, which showed a higher extensibility, while no differences were reported in other
mechanical properties (Duncan & Boughner, 1998). The bovine perichardial fixation with
glutaraldehyde under biaxial static pressures (~225 and ~1875 mmHg) has been proposed.
The bovine perichardium treated at high pressure showed an increase in stiffness and
almost isotropic behaviour, while low pressure-treated bovine perichardium preserved the
anisotropy exhibited by the native tissue (Langdon, et al., 1999). Porcine valves have also
been subjected to crosslinking at high pressure (80 mm Hg), low and zero pressure. In this
case, it was reported an increase in the rigidity of the leaflets fixed under low pressure and
the preservation of geometric corrugations and undulations of the native tissue when the
leaflet were fixed without pressure (Lee et al., 1984).
Heat treatment during glutaraldehyde fixation has also been reported. The thermal
treatment at 50ºC showed an anti-calcifying effect which was attributed to structural
changes in collagen or lipid extraction by heat treatment (Carpentier et al., 2001).
4.1.1 Post-treatments after glutaraldehyde fixation
The residual unbounded aldehyde groups that remain in the tissue after glutaraldehyde
fixation process have been associated with degenerative phenomenum on different
bioprosthesis. The grafting of different molecules on collagenous tissues treated with
glutaraldehyde has been an answer to these disadvantages.
The grafted molecules are incorporated in order to block free aldehyde groups and thus to
reduce or to neutralize both cytotoxicity and calcification. Some surface modification
procedures of crosslinked collagenous tissues are described in table 5.
It is known that nitric oxide releasing compounds can improve the biocompatibility of
blood-contacting medical devices (Frost et al., 2005; Masters et al., 2005). Two common nitric
oxide generating substances immobilized on synthetic polymers are diazeniumdiolates and
S-nitrosothiols (Frost et al., 2005). In the same line of thought, surface modification of
polymeric materials, such as PET or PU, with thiol compounds is interesting as it might
exchange nitric oxide with endogenous donors such as S-nitrosothiols that already circulate
in blood (Gappa-Fahlenkamp et al., 2004; Gappa-Fahlenkamp & Lewis, 2005).
The thiol groups on the polymer allowed the exchange reaction with S-nitroso serum
albumin and then, the release of nitric oxide to inhibit platelet adhesion on the polymeric
surfaces (Duan & Lewis, 2002). This approach has been proposed in perichardial tissue
biomaterial by using L-cysteine as thiol compound (Mendoza-Novelo & Cauich-Rodríguez,
2009). One additional advantage of L-cysteine grafting on glutaraldehyde-crosslinked
perichardial tissue is that free aldehyde groups will be diminished or even eliminated on the
tissue allowing its detoxification. A schematic representation of grafting of collagenous
tissue with L-cysteine is described in the figure 5. Similar approaches with other amino
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