Biomedical Engineering Reference
In-Depth Information
6.3
Biodegradability
In Vitro Degradation
Solution-cast films of P3HO-3HH (82%3HO)werefoundtobeveryresis-
tant to hydrolysis after 20 weeks of incubation in buffer solution at pH 10 and
37
◦
C. The slow decrease in molecular weight was associated with a strong in-
crease in the crystallinity. Addition of PDLLA oligomer led to an acceleration
of the P3HO-3HH degradation in these experiments [282]. No accelerating
effect by blending with PLLA, PDLLA, or PEG was reported from a subse-
quent study. However, the introduction of polar carboxylic groups in side
chains resulted in a significant increase in the degradation rate of P3HO-
3HH due to enhanced water penetration into the polymer matrix [171].
The very slow hydrolysis of P3HO-3HH (97%3HO)wasconfirmedinstud-
ies investigating the influence of enzymes on the P3HO-3HH degradation
process. It was found that acid phosphatase and
-glucuronidase, both as-
sociated with the foreign-body reaction, are not involved in the polymer
degradation [283]. Long-term in vitro studies on P3HO-3HH solution-cast
films (97% HO) in either water (pH 7.0) or buffer solution (pH 7.4) at 37
◦
C
revealed a simple hydrolytic degradation process characterized by low wa-
ter absorption, slow gradual molecular weight loss, and negligible mass loss
after 24 months of incubation [54]. At that time, the molecular weight of the
films reached approximately 25% of the initial value in both water and buffer
solution [53].
The polymer stability was also tested in terms of a suitable sterilization
method. It was reported that the molecular weight
M
w
is decreased by 5%
or 17% after sterilization by ethylene oxide or gamma-irradiation. Significant
changes in both the physicochemical and tensile properties were observed
after gamma-irradiation [284].
β
In Vivo Degradation
The degradation of P3HO-3HH in vivo was found to be slower in compari-
son to in vitro conditions in buffer solution, with less than 30%lossinthe
molecular weight 6 months after implantation s.c. in rats [53]. A molecular
weight loss of less than 30% was also reported from in vivo experiments using
porous P3HO-3HH heart valve scaffolds 17 weeks [280] or 24 weeks [268]
after implantation in sheep. The degradation rate was found to be unaltered
by increasing the surface area with porous P3HO-3HH scaffolds in compari-
son to dense films [268]. P3HO-3HH samples implanted s.c. in mice showed
a 50% decrease in the molecular weight 40 weeks after implantation. No sig-
nificant differences between the molecular weights at the surface and in the
interior of the samples were observed in these experiments [3]. Based on the
in vivo data it can be expected that the time for complete resorption of P3HO-
3HH is in the order of magnitude of that for P3HB.
