New Trends in Polymers - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 195

The PHB biodegradation in the enriched culture obtained from soil on the me-
dium used to cultivate denitrifying bacteria (Gil'tai medium) has been also studied.
The dominant bacterial species, Pseudomonas À uorescens and Pseudomonas stutzeri ,
have been identi¿ ed in this enrichment culture. Under denitrifying conditions, PHB
¿ lms were completely degraded for seven days. Both the ¿ lm weight and M w of PHB
decreased with time. In contrast to the data of Doi et al. [21] who found that M w of
PHB remained unchanged upon enzymatic biodegradation in an aquatic solution of
PHB depolymerase from Alcaligenes faecalis, in our experiments, the average viscos-
ity molecular weight of the higher and lower molecular polymers decreased gradually
from 1540 to 580 kDa and from 890 to 612 kDa, respectively. The “exo”-type cleav-
age of the polymer chain, that is a successive removal of the terminal groups, is known
to occur at a higher rate than the “endo”-type cleavage, that is, a random breakage of
the polymer chain at the enzyme binding sites. Thus, the former type of polymer deg-
radation is primarily responsible for changes in its average molecular weight. How-
ever the “endo”-type attack plays the important role at the initiation of biodegradation,
because at the beginning, a few polymer chains are oriented so that their ends are
accessible to the effect of the enzyme [50]. Biodegradation of the lower molecular
polymer, which contains a higher number of terminal groups, is more active, probably,
because the “exo”-type degradation is more active in lower than in higher molecular
polymer [48, 51].
16.2.4 Biodegradation of PHB in vivo in Animal Tissues
The first scientific works on biodegradation of PHB in vivo in animal tissues were car-
ried out 15-20 years ago by Miller N. D. et al. and Saito T. et al. [17, 19]. They are high
qualitative researches that disclosed many important characteristics of this process. As
it was noted that the both enzymatic and non-enzymatic processes of biodegradation
of PHB in vivo can occur simultaneously under normal conditions. But it does not
mean that polymer biodegradation in vivo is a simple combination of non-enzymatic
hydrolysis and enzymatic degradation. Moreover, in vivo the biodegradation (decrease
of molecular weight and mass loss) of PHB is a controversial subject in the literature.
As it was noted for in vitro PHB hydrolysis, the main reason for the controversy is the
use of samples made by various processing technologies and the incomparability of
different implantation and animal models. The most of researches on PHB biodegra-
dation was carried out with use of prototypes of various medical devices on the base
of PHB: solid films and plates [10, 13, 24, 52], porous patches [11, 12], nonwoven
patches consisted of fibers [53-57], screws [24], cylinders as nerve guidance channels
and conduits [13, 15, 16], monofilament sutures [17, 18], microspheres [19, 58]. In
vivo biodegradation researches were carried out on various laboratory animals: rats
[11, 16-19, 24], mice [13, 58], rabbits [10, 52, 59], minipigs [12], cats [15], calves
[53], sheep [54-56], and patients [57]. It is obvious that these animals differ in level
of metabolism are very much: for example, only weight of these animals differs from
10-20 g (mice) to 50 kg (calves). The implantation of devices from PHB was carried
out through different ways: subcutaneously [10, 13, 17, 18, 24, 59], intraperitoneally
on a bowel [11], subperiostally on the osseus skull [12, 52], nerve wraparound [14-16],
intramuscularly [58, 59], into the pericardium [54-57], into the atrium [53], and intra-