Biomedical Engineering Reference
In-Depth Information
Table 5.3 Amino acid analysis of bovine pericardial tissues
Amino acid
Fresh tissue
Glut-fixed tissue
Explanted tissue
Glycine
257
334
295
Lysine
31
8
15
Lys/Gly ratio
0.121
0.024
0.051
% Crosslinked
0%
83%
58%
Data from: Schoen et al. (1986).
radioactive compounds are unavailable or cost-prohibitive, amino acid analysis
is an alternative approach. As shown in Table 5.3, fresh tissue has Lys/Gly ratio
of about 0.12, or 31 lysine residues/257 glycine residues. After glutaraldehyde
crosslinking, this value drops to about 0.02, or 8 lysine residues/334 glycine
residues. Thus at completion of the reaction, 83% of the lysines in the tissue
have reacted with glutaraldehyde adducts.
Interestingly, he also evaluated explanted tissues, which demonstrate a Lys/
Gly ratio of 0.05. Assuming the tissue was made from the same initial lot, this
leads to the conclusion that only 58% of the lysines in the implanted tissue are
still reacted with glutaraldehyde adducts. This implies that, after implantation in
the body, some glutaraldehyde reactions with tissue are not permanent, and thus
can reverse their reaction with lysine. In order to prevent glutaraldehyde
reversibility, the double bond formed in the reaction must be reduced to a stable,
single, carbon±carbon bond with sodium borohydride (Connolly et al., 2004).
Typically this is not done in commercial processes, so there is the potential for
some glutaraldehyde reversibility. If the data are typical, we can assume that it is
a relatively small amount of glutaraldehyde overall, although it represents a high
percentage of the crosslinks initially formed. That is, while the Lys/Gly ratio
increases by 0.03 after implantation, this represents about 30% of the crosslinks
initially formed. In a similar manner, amino acid analysis can be used as a
measure of tissue stability during the shelf-life of the product.
5.7.5 Bioburden
For products manufactured from animal tissues which have been sourced from
an abattoir, the tissues are not sterile. The amount of viable microorganisms on
the tissue is called bioburden. Many things influence bioburden, including
cleanliness, chemicals, staff, and environmental conditions. As a tissue valve
manufacturer, it is your goal to process non-sterile starting materials in such a
way as to produce sterile, functional heart valves without damaging the delicate
tissue structure or components.
It is an important mindset to always consider the bioburden and how it affects
your processes, and how your processes affect the bioburden. For example, it is a
