Biomedical Engineering Reference
In-Depth Information
conditions are used [28]. Degradation processes can be roughly classifi ed into
those involving urethane bonds and those involving the macrodiol units of both
polyester and polyether types [24] .
It is well known that low-molecular-weight urethanes may be easily degraded
by some microorganisms, hydrolysis being catalyzed by enzymes with an estearase
activity [29]. Although cleavage of urethane bonds has also been reported for poly-
mers [30], it is not clear whether these bonds were hydrolyzed directly or after a
fi rst degradation step, resulting in lower molecular weight compounds.
Degradation of polyester-based PURs by microorganisms mainly occurs by
hydrolysis of their ester bonds. It has been stated that aliphatic polyesters used in
the synthesis of PURs (e.g., polyethylene adipate or poly(caprolactone)) are easily
degraded by microorganisms or estereolytic enzymes like lipase [31]. It has also
been reported that PURs prepared from high-molecular-weight polyesters degrade
faster than those prepared from low-molecular weight polyesters [32].
Experiments show that a large variety of fungi can be highly effective in degrad-
ing PURs [32, 33]. Systematic studies on the effects of fungi are relatively scarce
but point to a remarkable infl uence of the specifi c diisocyanate used in the syn-
thesis, as well as an improvement of resistance to degradation by the presence of
side chains in the polyester segment. In general, degradation by fungi requires
the addition of several nutrients such as gelatin. A degradation mechanism of
polyester PURs, based on extracellular estearases, has been proposed: a synergic
effect is obtained by random action throughout the polymer chain of endoenzymes
and successive monomer scission from the chain ends by exoenzymes [34].
Both Gram-positive and Gram-negative bacteria have been reported as PUR
degraders, although few detailed works have been performed until now. Kay
et al.
[35] investigated the ability of 16 kinds of bacteria to degrade polyester PURs fol-
lowing their burial in soil for 28 days. In all cases, IR led to determining that the
ester segments were the main site of attack because of the hydrolytic cleavage of
the ester bonds. The bacterial attack usually proceeded by the binding of cells to
the polymer surface with subsequent fl oc formation and degradation of the sub-
strate to metabolites. Estearase and/or protease activities were identifi ed and two
kinds of enzymes were observed: (i) a cell-associated membrane bound polyuretha-
nase and (ii) an extracellular polyurethanase [36] (Figure 6.3). The former provides
cell-mediated access to the hydrophobic polymer surface, and must consequently
be characterized by both a surface-binding domain and a catalytic domain. Note
that enzyme molecules can easily attack water-soluble substrates, resulting in a
high degradation rate. However, when the substrate is insoluble, it seems neces-
sary to improve the contact between the enzyme and the substrate by means of a
binding domain. Adherence of the bacteria enzyme to the polymer substrate must
be followed by hydrolysis to soluble compounds, which will then be metabolized
by the cell. This mechanism would decrease competition between degrading bac-
teria and other cells, as well as allowing adequate access to metabolites. The soluble
extracellular enzymes should stick on the polymer surface and also hydrolyze the
polymer into smaller units, facilitating the metabolization of soluble products and
providing easy access of enzymes to the partially degraded polymer.
