Polyhydroxyalkanoates: Structure, Properties and Sources - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 41

the limiting nutrient to trigger PHA production has been shown to have
severe limitations, and, although sucient, not a necessary condition for
achievement of high productivity, PHA content in cells and biopolymer
yields, as well for microorganisms of Cupriavidus genus, as for the mixed
cultures.
The thermo-chemical properties of PHA have been shown to be strongly
related to their structure. Thus, PHA SCL are mainly brittle thermoplastics
with high melting and glass transition temperatures, whereas PHA SCL are
mainly elastomers. Biocompatibility is another interesting property of PHAs.
In this case, PHA MCL have been shown to be more biocompatible than
PHA SCL , however, they are also more hydrophobic and the hydrophobicity
may be an important drawback for such types of applications. Thus, post-
fermentation modifications of PHAs were also applied to modify their
structure. Those modifications consist either of monomeric modifications,
i.e. grafting,

d n 2 r 4 n g | 8

functionalization etc., or surface treatment (UV, basic

treatments).
Finally, the more successful enhancement of the biocompatibility prop-
erties than of the thermo-mechanical properties achieved in recent years,
allows us to foresee a switch in applications, from packaging and low-added
value applications to biomedical applications. This switch can also imply
another one, in the field of the carbon sources used for PHA production.
Indeed, until now, low-added value applications have driven the studies of
cheap carbon sources, such as waste, even if in some cases (e.g. glycerol), the
actual economical gain is doubtful. Yet biomedical applications require ex-
treme purity of biomaterials; in these cases, the use of waste carbon sources,
possibly containing micropollutants that are dicult to remove, can be
an issue.
Thus, the extremely strong relationship between sources, structure and
properties requires an overall consideration of the goal for the future de-
velopment of the field. A global approach, considering that the carbon
source influences the cost and structure of PHAs, which influence the pos-
sible applications, which in turn influence the choice of the initial carbon
source used for the production of the biopolymer, then seems necessary for
further improvements and the widespread use of PHAs.

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