New Trends in Polymers - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 184

molecular weight [8, 9]. In spite of that PHB application in vitro and in vivo has been
intensively investigated, the most of the available data are often incomplete and even
contradictory. The presence of conflicting data can be partially explained by the fact
that biotechnologically produced PHB with standardized properties is relatively rare
and is not readily available due to ɚ wide variety of PHB biosynthesis sources and dif-
ferent PHB manufacturing processes.
Contradictoriness can be explained also by excess applied trend in PHB degra-
dation research. At most of the chapters observed in this review, PHB degradation
process has been investigated in the narrow framework of development of speci¿ c
medical device on the base of PHB. Depending on applied biomedical purposes bio-
degradation of PHB was investigated under different geometry: ¿ lms and plates with
various thickness [10-13], cylinders [13-16], mono¿ lament threads [17, 18], and
microspheres [19]. At these experiments PHB was used from various sources, with
different molecular weight and crystallinity. Besides, different technologies of PHB
devices manufacturing affect such important characteristics as polymer porosity and
surface structure [11, 12]. The reports regarding the complex theoretical research of
mechanisms of hydrolysis, enzymatic degradation, and biodegradation in vivo of PHB
processes are relatively rare [10, 11, 13, 20-22] that attaches great value and impor-
tance to these investigations. Nevertheless, the effect of thickness, size, and geometry
of PHB device, molecular weight, and crystallinity of PHB on the mechanism of PHB
hydrolysis and biodegradation were not yet well clari¿ ed.
16.2 PHB HYDROLYSIS AND BIODEGRADATION
16.2.1 Nonenzymatic Hydrolysis of PHB in vitro
Examination of hydrolytic degradation of natural PHB in vitro is a very important step
for understanding of PHB biodegradation. There are several very profound and careful
examinations of PHB hydrolysis that were carried out 10-15 years [20-23]. Hydrolytic
degradation of PHB was usually examined under standard experimental conditions
simulating internal body fluid: in buffered solutions with pH = 7.4 at 37°C but at some
cases the higher temperature (55°C, 70°C, and more) and other values of pH (from 2
to 11) were selected.
The classical experiment for examination of PHB hydrolysis in comparison with
hydrolysis of other widespread biopolymer, polylactic acid (PLA), was carried out by
Koyama N. and Doi Y. [20]. They selected ¿ lms (10 × 10 mm size, 50 —m thickness, 5
mg initial mass) from PHB (M n = 300 kDa, M w = 650 kDa) and PLA (M n = 9 kDa, M w
= 21 kDa) prepared by solvent casting and aged for 3 weeks to reach equilibrium crys-
tallinity. It was shown that hydrolytic degradation of natural PHB is very slow process.
The mass of PHB ¿ lm remained unchanged at 37°C in 10 mm phosphate buffer (pH
= 7.4) over a period of 150 days, while the mass of the PLA ¿ lm rapidly decreased
with time and reached 17% of the initial mass after 140 days. The rate of decrease in
the M n of the PHB was also much slower than the rate of decrease in the M n of PLA.
The M n of the PHB decreased approximately to 65% of the initial PHB M n after 150
days, while the M n of the PLA decreased to 20% (2 kDa) of the initial PLA M n at the
end of same time point. As PLA used at this research was with low molecular weight