Biomedical Engineering Reference
In-Depth Information
Isenburg et al., 2004;
Isenburg et al., 2006
Tannic acid
Table 3. Some chemical agents used for the stabilization and fixation of biological tissues
4.1 Tissue crosslinking with glutaraldehyde
The procedure most studied and exploited in the manufacture of tissue valve includes the
crosslinking with glutaraldehyde, which is also widely used as tanning agent in the leather
industry. Glutaraldehyde is an important reagent in the biomedical field and has been used
as crosslinking agent in the preparation of collagen-rich biomaterials or for the
immobilization of enzymes or cell fixation.
Glutaraldehyde is an efficient agent for the crosslinking of collagen matrix because it react
relatively quickly and because is able to join separate protein molecules by means of the
amino groups abundantly present in collagen. Glutaraldehyde is a cheap and water soluble
five-carbon bifunctional aldehyde that in aqueous solution consists of a mixture of free
aldehyde, mono and dihydrated monomeric glutaraldehyde, monomeric and polymeric
cyclic hemiacetals and various α, β unsaturated polymers (Whipple & Ruta, 1974). This
means that glutaraldehyde itself forms a number of different reactive species and that these
species may also react in different ways, rendering a highly crosslinked network.
Glutaraldehyde crosslinking has been and is still applied to most of the experimental and
clinical bioprostheses. This process consists in blocking the ε-amino groups of lysine in the
protein through imino bond formation. The contribution of the glutaraldehyde as
sterilization and crosslinking agent is partly due to its hydrophobicity and hydrophilicity,
allowing it to penetrate both aqueous media and in the cell membrane. However, in the
manufacture of bioprostheses, the use of glutaraldehyde has led to many disadvantages
associated with the residual free aldehyde groups. Table 4 shows some of the problems
associated with glutaraldehyde tissue crosslinking and some solutions that have been
suggested to solve them.
In aqueous solution, the glutaraldehyde is presented as a mixture of free aldehyde, mono
and dihydrate glutaraldehyde monomer, monomeric and polymeric cyclic hemiacetals, and
several alpha or beta unsaturated polymers (Monsan et al., 1975). In turn, this heterogeneity
of chemical species leads to a heterogeneous crosslinking. In addition, high concentration of
glutaraldehyde promotes rapid surface crosslinking in the tissue (Olde-Damink et al., 1995),
creating a barrier that impedes or prevents the diffusion of more glutaraldehyde within the
biomaterial. In order to avoid this, the use of low concentrations has been suggested (Khor,
1997). It has also been proposed glutaraldehyde protection as a monomer by the formation
of di-acetals, between glutaraldehyde and alcohols in acidic medium (Giossis et al., 1998).
