Biomedical Engineering Reference
In-Depth Information
Studies on the dependence of the degrading activity upon enzyme concentration
indicate that activity increased to a saturation value that remained constant when
an excess of the enzyme was present [29]. This observation contrasts with the
decrease in activity reported for depolymerases with a similar two-domain struc-
ture (e.g., polyhydroxyalkanoate depolymerase) [37]. It has been suggested that
both domains of polyurethanases are either located in three- dimensionally close
positions or separated by a fl exible linker. In the former case, the catalytic domain
can access the polymer substrate even if the surface is saturated with the maximum
number of enzymes molecules per unit surface. It might be possible to obtain new
solid polyester degrading enzymes by adding new binding domains to estearases,
which are ineffective in solid substrate degradation.
Unlike polyester derivatives, polyether-based PURs are quite resistant to degra-
dation by microorganisms [32].
Sthaphylococcus epidermidis
was reported to degrade
some kinds of polyether derivatives although the degradation rate was very slow.
This feature was interpreted according to a degradation mechanism involving an
exo-type depolymerization that differed from the endo-type depolymerization
typical of polyester-based PURs [38]. Despite this, polyether urethane materials are
known to be susceptible to a degradative phenomenon involving crack formation
and propagation, which is considered environmental stress cracking [39]. This
seems to be the result of a residual polymer surface stress introduced during
fabrication and not suffi ciently reduced by subsequent annealing.
6.3
Applications of Biodegradable Polyurethanes
Nowadays PURs play a dominant role in the design of medical devices with excel-
lent performance in life-saving areas. PURs are highly interesting for internal
(
in vivo
) uses, particularly for short-term applications like catheters or long- term
applications like implants. External (
in vitro
) uses like controlled drug delivery
systems must also be considered. Biodegradable properties are only required for
some of their biomedical applications.
6.3.1
Scaffolds
Degradation characteristics are of special interest for design of scaffolds for
in vivo
tissue engineering. The advantages of these devices lie in that they do not have to
be removed surgically once they are no longer needed, and that problems such as
stress shielding may be avoided by adapting the degradation rate to the specifi c
application. Scaffolds can be prepared by a wide range of well-established tech-
niques such as salt leaching/freeze drying, thermally induced phase separation,
and even electrospinning. Features like suitable mechanical properties, overall
porosity, pore size, and interconnectivity are basic to develop materials for scaffold
applications. Thus, literature data indicate that a correct cell in-growth requires a
