Biopolyesters in Tissue Engineering Applications - Advances in Polymer Science - page 23

in the human body in the hydrolysis of P3HB was also discussed in another
study in which a mass loss of more than 10%in41dayswasobservedafter
incubation of polymer films in the presence of lysozyme [103]. Microbial li-
pases, on the other hand, had no effect on the P3HB degradation [170, 173].
The occurrence of low molecular weight P3HB in the human body may sug-
gest that specific enzymes are involved in the polymer synthesis and its
depolymerization in vivo. Moreover, it is worth noting that a Ser .. His .. Asp
triad constitutes the active center of the catalytic domain of both P3HB
depolymerase [174] and pancreatic lipase [175]. The serine is part of the
pentapeptide Gly-X 1 -Ser-X 2 -Gly, which is located in all known P3HB depoly-
merases as well as lipases, esterases, and serine proteases [174]. However, the
occurrence of P3HB-specific enzymes in the human body and their contribu-
tion to the hydrolysis of implanted P3HB needs still further clarification.
The hydrolytic degradation of P3HB-3HV copolymers with low hydroxy-
valerate content (up to 20%) has been extensively studied. A comparison with
the P3HB homopolymer is possible because the low 3HV content has only
little influence on the hydrolysis rate [176] while the molecular weight, crys-
tallinity, porosity, and any additives are of significant importance. P3HB-3HV
samples showed an increased degradation rate with decreased initial molecu-
lar weights [176-178]. The degree of crystallinity depends on the processing
so that an acceleration of P3HB degradation is in the order of decreasing crys-
tallinity: injection molding < melt pressing < solution casting [177]. P3HB
samples produced by cold compression degrade even faster than the solu-
tion cast ones [177, 178]. The addition of hydrophilic polysaccharides [179]
resulted in a strong acceleration of the hydrolysis rate of P3HB-3HV, whereas
the addition of hydrophobic PCL [180] showed only little influence on the
hydrolysis rate. The degradation of P3HB and P3HB-3HV films can also be
accelerated by incorporation of basic molecules [181, 182].
P3HB-5%3HV samples did not show any mass loss after 16 weeks in buffer
solution (pH 7.4, 37 ◦ C), while the decreasing viscosity indicated a molecu-
lar weight reduction [99]. The degradation times of P3HB and high molecular
weight PLLA, as well as PCL, were comparable in this study. P3HB-7%3HV
showed, after an initial period, a continual decrease in molecular weight
to nearly half of its initial value after 2 years in buffer solution (pH 7.4,
37 ◦ C) [183]. However, using the same material, a continual decline in the mo-
lecular weight without induction period was detected in another study [184].
P3HB-3HV (7%, 14%, 22% 3HV) tested in an accelerated degradation test
(pH 7.4, 70 ◦ C) for up to 220 days showed increasing mass loss with increasing
3HV content [100].
Films made of P3HB-6%3HVshowedatwo-stephydrolysis(pH7,60 ◦ C)
with continual molecular weight decrease in the first step [185]. The begin-
ning of mass loss in the second step coincides with a decrease in the pH of
the solution to a value of about 4. An autocatalytic effect of released hydroxy
acids was confirmed by an increasing degradation rate. Moreover, a nonran-