Biopolyesters in Tissue Engineering Applications - Advances in Polymer Science - page 8

3-hydroxyhexanoate (3HH) [63]. P3HB-4HB and P3HB-3HH copolymers will
be described in Sects. 4 and 5 of this article. The effect of increasing 3HV con-
tent on the properties of P3HB-3HV is shown in Table 1 [64]. Crystallinities
of P3HB-3HV copolymers are high over the whole composition range due to
isodimorphism, i.e., cocrystallization of the two monomer units in either of
the homopolymer crystal lattices [65]. Molecular weight is one of the major
factors governing the mechanical properties of a polymer. Thus, the tensile
strength of P3HB-3HV has been reported to rapidly decrease below molecular
weights of about 100 000 [66] to 150 000 [67].
The mechanical properties of P3HB can also be improved by addition of
plasticizers [68-72] (Fig. 4a). For example, citric acid esters, which are con-
sidered to be nontoxic [73, 74], were demonstrated to be effective plasticizers
for P3HB-3HV [75-78]. However, triethyl citrate, which is commonly used
for plasticization, is highly water soluble resulting in fast leaching out of the
polymer matrix under physiological conditions leading to embrittlement of
initially plasticized polymer films [38, 79]. Recently, it has been shown that
esters of anti-inflammatory drugs such as salicylic acid, acetylsalicylic acid,
and ketoprofen have plasticizing effects on P3HB films comparable to those
of citric acid esters [71].
Blending with other degradable polyesters, such as atactic P3HB (at-
PHB) [38, 80-83], syndiotactic P3HB (st-PHB) [81, 82, 84], poly( -caprolacto-

ne) (PCL) [85-87], poly(6-hydroxyhexanoate) [88], P3HB-3HH [89, 90], and
P3HO-3HH [52], has been shown to increase the flexibility and elongation
at break of P3HB or P3HB-3HV. Some examples of P3HB blends are given
in Fig. 4b. In contrast, addition of polyesters commonly used in medical ap-
plications, such as PDLLA, PLLA, poly(d,l-lactide- co -glycolide) (PLGA), or
poly( p -dioxanone), does not improve the mechanical properties of P3HB due
to lack of miscibility.
Polymer films with improved mechanical properties can also be obtained
by hot-drawing of ultrahigh molecular weight P3HB [91] or P3HB/PLLA
blends [92]. If unmodified, films made from ultrahigh molecular weight P3HB

ε

Table 1 Thermal and mechanical properties of P3HB and P3HB-3HV [64]

Polymer T g T m Elastic Tensile Elongation

modulus strength at break

[ ◦ C] [ ◦ C] [GPa] [MPa] [%]

P3HB 9 175 3.8 45 4

P3HB-11%3HV 2 157 3.7 38 5

P3HB-20%3HV - 5 114 1.9 26 27

P3HB-28%3HV - 8 102 1.5 21 700

P3HB-34%3HV - 9 97 1.2 18 970