New Trends in Polymers - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 196

venously [19]. The terms of implantation were also different: 2.5 hr, 24 hr, 13 days,
and 2 months [19], 7, 14, and 30 days [16], 2, 7, 14, 21, 28, 55, 90, and 182 days [17],
1, 3, and 6 months [10, 13, 14], 3, 6, and 12 months [53], 6 and 12 months [15], 6 and
24 months [57], and 3, 6, 9, 12, 18, and 24 months [56] (Table 2).
In the entire study of PHB in vivo biodegradation was ful¿ lled by Gogolewski S.
et al. and Qu X. H. et al. [10, 13]. It was shown that PHB lost about 1.6% (injection
molded ¿ lm, 1.2 mm thick, M w of PHB = 130 kDa) [13] and 6% (solvent casting
¿ lm, 40 —m thick, M w = 534 kDa) [10] of initial weight after 6 months of implanta-
tion. But the observed small weight loss was partially due to the leaching out of low
molecular weight fractions and impurities present initially in the implants. The M w of
PHB decreased from 130 kDa to 74 kDa (57% of initial M w ) [13] and from 534 kDa
to 216 kDa (40% of initial Mw) [10] after 6 months of implantation. The polydisper-
sity of PHB polymers narrowed following implantation. The PHB showed a constant
increase in crystallinity (from 60.7 to 64.8%) up to 6 months [13] or an increase (from
65.0 to 67.9%) after 1 month and then fall again (to 64.5%) after 6 month of implan-
tation [10] which suggests the degradation process had not affected the crystalline
regions. This data is in accordance with data of PHB hydrolysis [20] and enzymatic
PHB degradation by lipases in vitro [11] where M w decrease was observed. The initial
biodegradation of amorphous regions of PHB in vivo is similar to PHB degradation by
depolymerase [39].
Thus, the observed biodegradation of PHB showed coexistence of two different
degradation mechanisms in hydrolysis in the polymer: enzymatically or non-enzymat-
ically catalyzed degradation. Although non-enzymatical catalysis occurred randomly
in homopolymer, indicated by M w loss rate in PHB, at some point in a time, a criti-
cal molecular weight is reached whereupon enzyme-catalyzed hydrolysis accelerated
degradation at the surface because easier enzyme/polymer interaction becomes pos-
sible. However considering the low weight loss of PHB, the critical molecular weight
appropriate for enzymes predominantly does not reach, yet resulting low molecular
weight and crystallinity in PHB could provide some sites for the hydrolysis of en-
zymes to accelerate the degradation of PHB [10, 13]. Additional data revealing the
mechanism of PHB biodegradation in animal tissues were obtained by Kramp B. et al.
in long term implantation experiments. A very slow, clinically not recordable degra-
dation of ¿ lms and plates was observed during 20 month (much more than in experi-
ments mentioned). A drop in the PHB weight loss evidently took place between the
20 th and 25 th month. Only initial signs of degradation were to be found on the surface
of the implant until 20 months after implantation but no more test body could be de-
tected after 25 months [52]. The complete biodegradation in vivo in the wide range
from 3-30 months of PHB was shown by other researches [53, 55-57, 60], whereas
almost no weight loss and surface changes of PHB during 6 months of biodegradation
in vivo was shown [13, 17]. Residual fragments of PHB implants were found after 30
months of the patches implantation [54, 56]. A reduction of PHB patch size in 27%
was shown in patients after 24 months after surgical procedure on pericardial closure
with the patch [57]. Signi¿ cantly more rapid biodegradation in vivo was shown by
other researches [10, 15, 18, 37, 53]. It was shown that 30% mass loss of PHB sutures
occurred gradually during 180 days of in vivo biodegradation with minor changes in