Biomedical Engineering Reference
In-Depth Information
dom chain scission with a more accelerated hydrolysis of the 3HV component
was also found in these experiments. An inhomogeneous degradation with
accelerated hydrolysis of the core was observed in thicker P3HB-7%3HV
films [186]. The accelerated hydrolysis of the core, similar to that found in
poly(lactide)s, was attributed to the accumulation of acidic degradation prod-
ucts in the polymer bulk.
In a study on P3HB-14%3HV fibers incubated in buffer solution (pH 7.4,
37
◦
C), the molecular weight
M
n
started to decrease after an induction period
of about 80 days, and reached 64% of its initial value after 6 months. As
confirmed in the accelerated test, the tensile strength and mass started to de-
crease at molecular weights
M
n
of about 70 000 and 17 000, respectively [187].
Textiles made from melt-extruded P3HB fibers showed a molecular weight
decrease to about 90% of the initial values after 6 months of incubation
(pH 7.4, 37
◦
C). The single fibers, however, degraded at a faster rate, reach-
ing molecular weights
M
w
of 84%after12weeksinvitro,whichcouldbe
further accelerated after pretreatment by electron irradiation [105]. P3HB
“wool” made from solution-spun fibers have been tested under accelerated
conditions (pH 10.6, 70
◦
C), confirming the influence of polymer purity and
processing on the hydrolysis rate [188].
Porous scaffolds made from P3HB-3%3HV and P3HB-3%3HV/wollastonite
composites showed a continuous fast decrease in their mass and molecular
weight when stored for up to 15 weeks in buffer solution (pH 7.4, 37
◦
C).
For example, unmodified P3HB-3HV scaffolds lost about 12% of their ini-
tial mass and about 90% of their initial molecular weight at the end of the
incubation period. Increasing the amount of wollastonite in the composite
resulted in increased mass loss due to wollastonite dissolution but delayed
polymer hydrolysis, which was explained by the buffering effect of acidic
hydrolysis products by alkaline ions dissolved from the wollastonite. This
was confirmed by pH measurements showing a drop during the incuba-
tion of unmodified P3HB-3HV, but almost constant values when testing the
composites [189]. Porous P3HB scaffolds tested in another study showed
amasslossofonly3% after 50 days of incubation in buffer solution (pH 7.4,
37
◦
C) [124]. Mass loss of P3HB-8%3HV porous scaffolds started after an in-
duction period of 120 days and reached about 40% after 180 days in vitro
(pH 7.4, 37
◦
C) [126, 127].
The degradation behavior of P3HB was also studied for the selection of
suitable sterilization methods. It was found that with steam sterilization and
with gamma-irradiation in particular a molecular weight reduction and dete-
rioration of mechanical properties takes place, while sterilization with ethy-
lene oxide or formaldehyde gas has no affect on the polymer properties [190].
Significant loss in the molecular weight together with embrittlement have
been observed for P3HB films, but not P3HB-3HV films, in a screening
study testing a variety of sterilization methods [102]. The irradiation-induced
degradation of P3HB or P3HB-3HV was studied and confirmed also by other
