Biomedical Engineering Reference
In-Depth Information
Molecular ordering crystallization may be favored by subjecting a PUR chain to
stress [23]. Thus, at a moderate extension (e.g., 250%) macrodiols of the soft
segment become partially aligned and crystallized. When the extension is increased,
further crystallization occurs and hard segments turn into the direction of elonga-
tion and form paracrystalline layer lattice crystals (Figure 6.2).
6.2
Biodegradation Mechanisms of Polyurethanes
Susceptibility of PURs to biodegradation is an inherent feature of their chemistry
[24, 25] . It was detected by the industrial manufacturing community before sys-
tematic biodegradation studies were conducted in the 1980s. In fact, degradation
of PURs may initiate during fabrication due to high temperatures, the presence
of liquids, and the diffi culty to completely remove moisture from the reaction
mixture [26] .
Microorganisms can be easily grown in appropriate cellular media following
well-established technologies that allow using enzymes segregated outside cells,
even in industrial applications. Biodegradation is governed by organism type,
polymer characteristics, and the pretreatment performed on the sample. During
degradation, the polymer is fi rst converted into its monomers, which should then
be mineralized. It is clear that polymers are too large to pass through cell mem-
branes, so they must fi rst be depolymerized into smaller compounds which may
then be absorbed and biodegraded within microbial cells [27] (Figure 6.3). Com-
plete mineralization can thus be achieved, the end products being biomass, CO
2
,
and water when aerobic microorganisms are involved, plus CH
4
when anaerobic
Figure 6.3
Proposed model for the degradation of PURs by the action of a cell-associated
enzyme and extracellular enzymes.
